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1
Transport Layer 3-1
Chapter 3 Transport LayerOur goals
understand principles behind transport layer services
multiplexingdemultiplexingreliable data transferflow controlcongestion control
learn about transport layer protocols in the Internet
UDP connectionless transportTCP connection-oriented transportTCP congestion control
Transport Layer 3-2
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
2
Transport Layer 3-3
Transport services and protocolsprovide logical communicationbetween app processes running on different hoststransport protocols run in end systems
send side breaks app messages into segments passes to network layerrcv side reassembles segments into messages passes to app layer
more than one transport protocol available to apps
Internet TCP and UDP
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-4
Transport vs network layer
network layer logical communication between hoststransport layer logical communication between processes
relies on enhances network layer services
Household analogy12 kids sending letters to 12
kidsprocesses = kidsapp messages = letters in envelopeshosts = housestransport protocol = Ann and Billnetwork-layer protocol = postal service
3
Transport Layer 3-5
Internet transport-layer protocols
reliable in-order delivery (TCP)
congestion control flow controlconnection setup
unreliable unordered delivery UDP
no-frills extension of ldquobest-effortrdquo IP
services not available delay guaranteesbandwidth guarantees
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-6
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
4
Transport Layer 3-7
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-8
How demultiplexing workshost receives IP datagrams
each datagram has source IP address destination IP addresseach datagram carries 1 transport-layer segmenteach segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
5
Transport Layer 3-9
Connectionless demultiplexing
Create sockets with port numbers
DatagramSocket mySocket1 = new DatagramSocket(99111)
DatagramSocket mySocket2 = new DatagramSocket(99222)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment
checks destination port number in segmentdirects UDP segment to socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-10
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
clientIP A
P1P1P3
serverIP C
SP 6428DP 9157
SP 9157DP 6428
SP 6428DP 5775
SP 5775DP 6428
SP provides ldquoreturn addressrdquo
6
Transport Layer 3-11
Connection-oriented demux
TCP socket identified by 4-tuple
source IP addresssource port numberdest IP addressdest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets
each socket identified by its own 4-tuple
Web servers have different sockets for each connecting client
non-persistent HTTP will have different socket for each request
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
clientIP A
P1P2P4
serverIP C
SP 9157DP 80
SP 9157DP 80
P5 P6 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
7
Transport Layer 3-13
Connection-oriented demux Threaded Web Server
ClientIPB
P1
clientIP A
P1P2
serverIP C
SP 9157DP 80
SP 9157DP 80
P4 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
Transport Layer 3-14
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
8
Transport Layer 3-15
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquoInternet transport protocolldquobest effortrdquo service UDP segments may be
lostdelivered out of order to app
connectionlessno handshaking between UDP sender receivereach UDP segment handled independently of others
Why is there a UDPno connection establishment (which can add delay)simple no connection state at sender receiversmall segment headerno congestion control UDP can blast away as fast as desired
Transport Layer 3-16
UDP more
often used for streaming multimedia apps
loss tolerantrate sensitive
other UDP usesDNSSNMP
reliable transfer over UDP add reliability at application layer
application-specific error recovery
source port dest port
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength in
bytes of UDPsegmentincluding
header
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
2
Transport Layer 3-3
Transport services and protocolsprovide logical communicationbetween app processes running on different hoststransport protocols run in end systems
send side breaks app messages into segments passes to network layerrcv side reassembles segments into messages passes to app layer
more than one transport protocol available to apps
Internet TCP and UDP
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-4
Transport vs network layer
network layer logical communication between hoststransport layer logical communication between processes
relies on enhances network layer services
Household analogy12 kids sending letters to 12
kidsprocesses = kidsapp messages = letters in envelopeshosts = housestransport protocol = Ann and Billnetwork-layer protocol = postal service
3
Transport Layer 3-5
Internet transport-layer protocols
reliable in-order delivery (TCP)
congestion control flow controlconnection setup
unreliable unordered delivery UDP
no-frills extension of ldquobest-effortrdquo IP
services not available delay guaranteesbandwidth guarantees
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-6
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
4
Transport Layer 3-7
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-8
How demultiplexing workshost receives IP datagrams
each datagram has source IP address destination IP addresseach datagram carries 1 transport-layer segmenteach segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
5
Transport Layer 3-9
Connectionless demultiplexing
Create sockets with port numbers
DatagramSocket mySocket1 = new DatagramSocket(99111)
DatagramSocket mySocket2 = new DatagramSocket(99222)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment
checks destination port number in segmentdirects UDP segment to socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-10
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
clientIP A
P1P1P3
serverIP C
SP 6428DP 9157
SP 9157DP 6428
SP 6428DP 5775
SP 5775DP 6428
SP provides ldquoreturn addressrdquo
6
Transport Layer 3-11
Connection-oriented demux
TCP socket identified by 4-tuple
source IP addresssource port numberdest IP addressdest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets
each socket identified by its own 4-tuple
Web servers have different sockets for each connecting client
non-persistent HTTP will have different socket for each request
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
clientIP A
P1P2P4
serverIP C
SP 9157DP 80
SP 9157DP 80
P5 P6 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
7
Transport Layer 3-13
Connection-oriented demux Threaded Web Server
ClientIPB
P1
clientIP A
P1P2
serverIP C
SP 9157DP 80
SP 9157DP 80
P4 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
Transport Layer 3-14
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
8
Transport Layer 3-15
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquoInternet transport protocolldquobest effortrdquo service UDP segments may be
lostdelivered out of order to app
connectionlessno handshaking between UDP sender receivereach UDP segment handled independently of others
Why is there a UDPno connection establishment (which can add delay)simple no connection state at sender receiversmall segment headerno congestion control UDP can blast away as fast as desired
Transport Layer 3-16
UDP more
often used for streaming multimedia apps
loss tolerantrate sensitive
other UDP usesDNSSNMP
reliable transfer over UDP add reliability at application layer
application-specific error recovery
source port dest port
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength in
bytes of UDPsegmentincluding
header
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
3
Transport Layer 3-5
Internet transport-layer protocols
reliable in-order delivery (TCP)
congestion control flow controlconnection setup
unreliable unordered delivery UDP
no-frills extension of ldquobest-effortrdquo IP
services not available delay guaranteesbandwidth guarantees
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-6
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
4
Transport Layer 3-7
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-8
How demultiplexing workshost receives IP datagrams
each datagram has source IP address destination IP addresseach datagram carries 1 transport-layer segmenteach segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
5
Transport Layer 3-9
Connectionless demultiplexing
Create sockets with port numbers
DatagramSocket mySocket1 = new DatagramSocket(99111)
DatagramSocket mySocket2 = new DatagramSocket(99222)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment
checks destination port number in segmentdirects UDP segment to socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-10
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
clientIP A
P1P1P3
serverIP C
SP 6428DP 9157
SP 9157DP 6428
SP 6428DP 5775
SP 5775DP 6428
SP provides ldquoreturn addressrdquo
6
Transport Layer 3-11
Connection-oriented demux
TCP socket identified by 4-tuple
source IP addresssource port numberdest IP addressdest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets
each socket identified by its own 4-tuple
Web servers have different sockets for each connecting client
non-persistent HTTP will have different socket for each request
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
clientIP A
P1P2P4
serverIP C
SP 9157DP 80
SP 9157DP 80
P5 P6 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
7
Transport Layer 3-13
Connection-oriented demux Threaded Web Server
ClientIPB
P1
clientIP A
P1P2
serverIP C
SP 9157DP 80
SP 9157DP 80
P4 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
Transport Layer 3-14
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
8
Transport Layer 3-15
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquoInternet transport protocolldquobest effortrdquo service UDP segments may be
lostdelivered out of order to app
connectionlessno handshaking between UDP sender receivereach UDP segment handled independently of others
Why is there a UDPno connection establishment (which can add delay)simple no connection state at sender receiversmall segment headerno congestion control UDP can blast away as fast as desired
Transport Layer 3-16
UDP more
often used for streaming multimedia apps
loss tolerantrate sensitive
other UDP usesDNSSNMP
reliable transfer over UDP add reliability at application layer
application-specific error recovery
source port dest port
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength in
bytes of UDPsegmentincluding
header
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
4
Transport Layer 3-7
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-8
How demultiplexing workshost receives IP datagrams
each datagram has source IP address destination IP addresseach datagram carries 1 transport-layer segmenteach segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
5
Transport Layer 3-9
Connectionless demultiplexing
Create sockets with port numbers
DatagramSocket mySocket1 = new DatagramSocket(99111)
DatagramSocket mySocket2 = new DatagramSocket(99222)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment
checks destination port number in segmentdirects UDP segment to socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-10
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
clientIP A
P1P1P3
serverIP C
SP 6428DP 9157
SP 9157DP 6428
SP 6428DP 5775
SP 5775DP 6428
SP provides ldquoreturn addressrdquo
6
Transport Layer 3-11
Connection-oriented demux
TCP socket identified by 4-tuple
source IP addresssource port numberdest IP addressdest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets
each socket identified by its own 4-tuple
Web servers have different sockets for each connecting client
non-persistent HTTP will have different socket for each request
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
clientIP A
P1P2P4
serverIP C
SP 9157DP 80
SP 9157DP 80
P5 P6 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
7
Transport Layer 3-13
Connection-oriented demux Threaded Web Server
ClientIPB
P1
clientIP A
P1P2
serverIP C
SP 9157DP 80
SP 9157DP 80
P4 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
Transport Layer 3-14
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
8
Transport Layer 3-15
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquoInternet transport protocolldquobest effortrdquo service UDP segments may be
lostdelivered out of order to app
connectionlessno handshaking between UDP sender receivereach UDP segment handled independently of others
Why is there a UDPno connection establishment (which can add delay)simple no connection state at sender receiversmall segment headerno congestion control UDP can blast away as fast as desired
Transport Layer 3-16
UDP more
often used for streaming multimedia apps
loss tolerantrate sensitive
other UDP usesDNSSNMP
reliable transfer over UDP add reliability at application layer
application-specific error recovery
source port dest port
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength in
bytes of UDPsegmentincluding
header
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
5
Transport Layer 3-9
Connectionless demultiplexing
Create sockets with port numbers
DatagramSocket mySocket1 = new DatagramSocket(99111)
DatagramSocket mySocket2 = new DatagramSocket(99222)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment
checks destination port number in segmentdirects UDP segment to socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-10
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
clientIP A
P1P1P3
serverIP C
SP 6428DP 9157
SP 9157DP 6428
SP 6428DP 5775
SP 5775DP 6428
SP provides ldquoreturn addressrdquo
6
Transport Layer 3-11
Connection-oriented demux
TCP socket identified by 4-tuple
source IP addresssource port numberdest IP addressdest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets
each socket identified by its own 4-tuple
Web servers have different sockets for each connecting client
non-persistent HTTP will have different socket for each request
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
clientIP A
P1P2P4
serverIP C
SP 9157DP 80
SP 9157DP 80
P5 P6 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
7
Transport Layer 3-13
Connection-oriented demux Threaded Web Server
ClientIPB
P1
clientIP A
P1P2
serverIP C
SP 9157DP 80
SP 9157DP 80
P4 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
Transport Layer 3-14
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
8
Transport Layer 3-15
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquoInternet transport protocolldquobest effortrdquo service UDP segments may be
lostdelivered out of order to app
connectionlessno handshaking between UDP sender receivereach UDP segment handled independently of others
Why is there a UDPno connection establishment (which can add delay)simple no connection state at sender receiversmall segment headerno congestion control UDP can blast away as fast as desired
Transport Layer 3-16
UDP more
often used for streaming multimedia apps
loss tolerantrate sensitive
other UDP usesDNSSNMP
reliable transfer over UDP add reliability at application layer
application-specific error recovery
source port dest port
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength in
bytes of UDPsegmentincluding
header
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
6
Transport Layer 3-11
Connection-oriented demux
TCP socket identified by 4-tuple
source IP addresssource port numberdest IP addressdest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets
each socket identified by its own 4-tuple
Web servers have different sockets for each connecting client
non-persistent HTTP will have different socket for each request
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
clientIP A
P1P2P4
serverIP C
SP 9157DP 80
SP 9157DP 80
P5 P6 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
7
Transport Layer 3-13
Connection-oriented demux Threaded Web Server
ClientIPB
P1
clientIP A
P1P2
serverIP C
SP 9157DP 80
SP 9157DP 80
P4 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
Transport Layer 3-14
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
8
Transport Layer 3-15
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquoInternet transport protocolldquobest effortrdquo service UDP segments may be
lostdelivered out of order to app
connectionlessno handshaking between UDP sender receivereach UDP segment handled independently of others
Why is there a UDPno connection establishment (which can add delay)simple no connection state at sender receiversmall segment headerno congestion control UDP can blast away as fast as desired
Transport Layer 3-16
UDP more
often used for streaming multimedia apps
loss tolerantrate sensitive
other UDP usesDNSSNMP
reliable transfer over UDP add reliability at application layer
application-specific error recovery
source port dest port
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength in
bytes of UDPsegmentincluding
header
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
7
Transport Layer 3-13
Connection-oriented demux Threaded Web Server
ClientIPB
P1
clientIP A
P1P2
serverIP C
SP 9157DP 80
SP 9157DP 80
P4 P3
D-IPCS-IP AD-IPC
S-IP B
SP 5775DP 80
D-IPCS-IP B
Transport Layer 3-14
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
8
Transport Layer 3-15
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquoInternet transport protocolldquobest effortrdquo service UDP segments may be
lostdelivered out of order to app
connectionlessno handshaking between UDP sender receivereach UDP segment handled independently of others
Why is there a UDPno connection establishment (which can add delay)simple no connection state at sender receiversmall segment headerno congestion control UDP can blast away as fast as desired
Transport Layer 3-16
UDP more
often used for streaming multimedia apps
loss tolerantrate sensitive
other UDP usesDNSSNMP
reliable transfer over UDP add reliability at application layer
application-specific error recovery
source port dest port
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength in
bytes of UDPsegmentincluding
header
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
8
Transport Layer 3-15
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquoInternet transport protocolldquobest effortrdquo service UDP segments may be
lostdelivered out of order to app
connectionlessno handshaking between UDP sender receivereach UDP segment handled independently of others
Why is there a UDPno connection establishment (which can add delay)simple no connection state at sender receiversmall segment headerno congestion control UDP can blast away as fast as desired
Transport Layer 3-16
UDP more
often used for streaming multimedia apps
loss tolerantrate sensitive
other UDP usesDNSSNMP
reliable transfer over UDP add reliability at application layer
application-specific error recovery
source port dest port
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength in
bytes of UDPsegmentincluding
header
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
9
Transport Layer 3-17
UDP checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-18
Internet Checksum ExampleNote
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
10
Transport Layer 3-19
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-20
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
11
Transport Layer 3-21
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-22
Principles of Reliable data transferimportant in app transport link layerstop-10 list of important networking topics
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
12
Transport Layer 3-23
Reliable data transfer getting started
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to
deliver to receiver upper layer
udt_send() called by rdtto transfer packet over
unreliable channel to receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to upper
Transport Layer 3-24
Reliable data transfer getting started
Wersquollincrementally develop sender receiver sides of reliable data transfer protocol (rdt)consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquonext state uniquely determined by next
event
eventactions
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
13
Transport Layer 3-25
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliableno bit errorsno loss of packets
separate FSMs for sender receiversender sends data into underlying channelreceiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-26
Rdt20 channel with bit errors
underlying channel may flip bits in packetchecksum to detect bit errors
the question how to recover from errorsacknowledgements (ACKs) receiver explicitly tells sender that pkt received OKnegative acknowledgements (NAKs) receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10)error detectionreceiver feedback control msgs (ACKNAK) rcvr-gtsender
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
14
Transport Layer 3-27
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Λ
Transport Layer 3-28
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
15
Transport Layer 3-29
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampampisNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Λ
Transport Layer 3-30
rdt20 has a fatal flaw
What happens if ACKNAK corruptedsender doesnrsquot know what happened at receivercanrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current pkt if ACKNAK garbledsender adds sequence numberto each pktreceiver discards (doesnrsquot deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
16
Transport Layer 3-31
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait forcall 1 from
above
Wait for ACK or NAK 1
ΛΛ
Transport Layer 3-32
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamphas_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
17
Transport Layer 3-33
rdt21 discussion
Senderseq added to pkttwo seq rsquos (01) will suffice Whymust check if received ACKNAK corrupted twice as many states
state must ldquorememberrdquowhether ldquocurrentrdquo pkt has 0 or 1 seq
Receivermust check if received packet is duplicate
state indicates whether 0 or 1 is expected pkt seq
note receiver can notknow if its last ACKNAK received OK at sender
Transport Layer 3-34
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs onlyinstead of NAK receiver sends ACK for last pkt received OK
receiver must explicitly include seq of pkt being ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
18
Transport Layer 3-35
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||
isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0sender FSM
fragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)receiver FSM
fragment
Λ
Transport Layer 3-36
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs)
checksum seq ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost)
retransmission will be duplicate but use of seq rsquos already handles thisreceiver must specify seq of pkt being ACKed
requires countdown timer
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
19
Transport Layer 3-37
rdt30 sendersndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
Λrdt_rcv(rcvpkt)
ΛΛ
Λ
Transport Layer 3-38
rdt30 in action
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
20
Transport Layer 3-39
rdt30 in action
Transport Layer 3-40
Performance of rdt30
rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit = 8kbpkt109 bsec = 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender =
00830008
= 000027 L R RTT + L R
=
L (packet length in bits)R (transmission rate bps) =
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
21
Transport Layer 3-41
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender =
00830008
= 000027 L R RTT + L R
=
Transport Layer 3-42
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-be-
acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
22
Transport Layer 3-43
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender =
02430008
= 00008 3 L R RTT + L R
=
Increase utilizationby a factor of 3
Transport Layer 3-44
Go-Back-NSender
k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)
timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
23
Transport Layer 3-45
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)
start_timernextseqnum++
elserefuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum)
stop_timerelsestart_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Λ
Transport Layer 3-46
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq
may generate duplicate ACKsneed only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)default
rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Λ
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
24
Transport Layer 3-47
GBN inaction
Transport Layer 3-48
Selective Repeat
receiver individually acknowledges all correctly received pkts
buffers pkts as needed for eventual in-order delivery to upper layer
sender only resends pkts for which ACK not receivedsender timer for each unACKed pkt
sender windowN consecutive seq rsquosagain limits seq s of sent unACKed pkts
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
25
Transport Layer 3-49
Selective repeat sender receiver windows
Transport Layer 3-50
Selective repeat
data from above if next available seq in window send pkt
timeout(n)resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-1]
send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwiseignore
receiver
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
26
Transport Layer 3-51
Selective repeat in action
Transport Layer 3-52
Selective repeatdilemma
Example seq rsquos 0 1 2 3window size=3
receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
27
Transport Layer 3-53
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-54
TCP Overview RFCs 793 1122 1323 2018 2581
full duplex databi-directional data flow in same connectionMSS maximum segment size
connection-orientedhandshaking (exchange of control msgs) initrsquos sender receiver state before data exchange
flow controlledsender will not overwhelm receiver
point-to-pointone sender one receiver
reliable in-order byte steam
no ldquomessage boundariesrdquo
pipelinedTCP congestion and flow control set window size
send amp receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
28
Transport Layer 3-55
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
Receive windowUrg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-56
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquoof first byte in segmentrsquos data
ACKsseq of next byte expected from other sidecumulative ACK
Q how receiver handles out-of-order segments
A TCP spec doesnrsquot say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt oflsquoCrsquo echoes
back lsquoCrsquo
timesimple telnet scenario
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
29
Transport Layer 3-57
TCP Round Trip Time and Timeout
Q how to set TCP timeout valuelonger than RTT
but RTT variestoo short premature timeout
unnecessary retransmissions
too long slow reaction to segment loss
Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt
ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo
average several recent measurements not just current SampleRTT
Transport Layer 3-58
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT
Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
30
Transport Layer 3-59
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
iseco
nds)
SampleRTT Estimated RTT
Transport Layer 3-60
TCP Round Trip Time and Timeout
Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|
(typically β = 025)
Then set timeout interval
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
31
Transport Layer 3-61
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-62
TCP reliable data transfer
TCP creates rdt service on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer
Retransmissions are triggered by
timeout eventsduplicate acks
Initially consider simplified TCP sender
ignore duplicate acksignore flow control congestion control
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
32
Transport Layer 3-63
TCP sender eventsdata rcvd from app
Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unackedsegment)expiration interval TimeOutInterval
timeoutretransmit segment that caused timeoutrestart timer
Ack rcvdIf acknowledges previously unackedsegments
update what is known to be ackedstart timer if there are outstanding segments
Transport Layer 3-64
TCP sender(simplified)
NextSeqNum = InitialSeqNumSendBase = InitialSeqNum
loop (forever) switch(event)
event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)
start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)
event timer timeoutretransmit not-yet-acknowledged segment with
smallest sequence numberstart timer
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
end of loop forever
Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
33
Transport Layer 3-65
TCP retransmission scenariosHost A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92 t
imeo
ut
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
timeSe
q=92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-66
TCP retransmission scenarios (more)Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
34
Transport Layer 3-67
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Immediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment startsat lower end of gap
Transport Layer 3-68
Fast Retransmit
Time-out period often relatively long
long delay before resending lost packet
Detect lost segments via duplicate ACKs
Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKsfor the same data it supposes that segment after ACKed data was lost
fast retransmit resend segment before timer expires
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
35
Transport Layer 3-69
event ACK received with ACK field value of y if (y gt SendBase)
SendBase = yif (there are currently not-yet-acknowledged segments)
start timer
else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)
resend segment with sequence number y
Fast retransmit algorithm
a duplicate ACK for already ACKed segment
fast retransmit
Transport Layer 3-70
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
36
Transport Layer 3-71
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate
app process may be slow at reading from buffer
sender wonrsquot overflowreceiverrsquos buffer by
transmitting too muchtoo fast
flow control
Transport Layer 3-72
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)spare room in buffer
= RcvWindow= RcvBuffer-[LastByteRcvd -
LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow
guarantees receive buffer doesnrsquot overflow
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
37
Transport Layer 3-73
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
Transport Layer 3-74
TCP Connection ManagementRecall TCP sender receiver
establish ldquoconnectionrdquo before exchanging data segmentsinitialize TCP variables
seq sbuffers flow control info (eg RcvWindow)
client connection initiatorSocket clientSocket = new Socket(hostnameport
number)
server contacted by clientSocket connectionSocket = welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server
specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffersspecifies server initial seq
Step 3 client receives SYNACK replies with ACK segment which may contain data
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
38
Transport Layer 3-75
TCP Connection Management (cont)
Closing a connection
client closes socketclientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closedti
med
wai
t
Transport Layer 3-76
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
39
Transport Layer 3-77
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 3-78
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
40
Transport Layer 3-79
Principles of Congestion Control
Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations
lost packets (buffer overflow at routers)long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-80
Causescosts of congestion scenario 1
two senders two receiversone router infinite buffers no retransmission
large delays when congestedmaximum achievable throughput
unlimited shared output link buffers
Host Aλin original data
Host B
λout
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
41
Transport Layer 3-81
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A λin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-82
Causescosts of congestion scenario 2always (goodput)
ldquoperfectrdquo retransmission only when loss
retransmission of delayed (not lost) packet makes larger (than perfect case) for same
λin
λout=
λin
λoutgtλ
inλout
ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt
R2
R2λin
λ out
b
R2
R2λin
λ out
a
R2
R2λin
λ out
c
R4
R3
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
42
Transport Layer 3-83
Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit
λin
Q what happens as and increase λ
in
finite shared output link buffers
Host Aλin original data
Host B
λout
λin original data plus retransmitted data
Transport Layer 3-84
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted
Host A
Host B
λou
t
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
43
Transport Layer 3-85
Approaches towards congestion control
End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP
Network-assisted congestion controlrouters provide feedback to end systems
single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-86
Case study ATM ABR congestion control
ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo
sender should use available bandwidth
if senderrsquos path congested sender throttled to minimum guaranteed rate
RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)
NI bit no increase in rate (mild congestion)CI bit congestion indication
RM cells returned to sender by receiver with bits intact
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
44
Transport Layer 3-87
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus minimum supportable rate on path
EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell
Transport Layer 3-88
Chapter 3 outline
31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer
35 Connection-oriented transport TCP
segment structurereliable data transferflow controlconnection management
36 Principles of congestion control37 TCP congestion control
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
45
Transport Layer 3-89
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs
additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-90
TCP Congestion Control details
sender limits transmissionLastByteSent-LastByteAcked
le CongWin
Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestionloss event = timeout or 3 duplicate acksTCP sender reduces rate (CongWin) after loss event
three mechanismsAIMDslow startconservative after timeout events
rate = CongWinRTT Bytessec
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
46
Transport Layer 3-91
TCP Slow Start
When connection begins CongWin = 1 MSS
Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps
available bandwidth may be gtgt MSSRTT
desirable to quickly ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-92
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event
double CongWin every RTTdone by incrementing CongWin for every ACK received
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
47
Transport Layer 3-93
RefinementQ When should the
exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event
Transport Layer 3-94
Refinement inferring loss
After 3 dup ACKsCongWin is cut in halfwindow then grows linearly
But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly
3 dup ACKs indicates network capable of delivering some segments
timeout indicates a ldquomore alarmingrdquocongestion scenario
Philosophy
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
48
Transport Layer 3-95
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2and CongWin is set to 1 MSS
Transport Layer 3-96
TCP sender congestion control
SS or CA
SS or CA
SS or CA
CongestionAvoidance (CA)
Slow Start (SS)
State
CongWin and Threshold not changed
Increment duplicate ACK count for segment being acked
Duplicate ACK
Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Timeout
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Loss event detected by triple duplicate ACK
Additive increase resulting in increase of CongWin by 1 MSS every RTT
CongWin = CongWin+MSS (MSSCongWin)
ACK receipt for previously unackeddata
Resulting in a doubling of CongWin every RTT
CongWin = CongWin + MSS If (CongWin gt Threshold)
set state to ldquoCongestion Avoidancerdquo
ACK receipt for previously unackeddata
CommentaryTCP Sender Action Event
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
49
Transport Layer 3-97
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT
Ignore slow start
Let W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-98
TCP Futures
Example 1500 byte segments 100ms RTT want 10 GbpsthroughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate
L = 210-10 WowNew versions of TCP for high-speed needed
LRTTMSSsdot221
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
50
Transport Layer 3-99
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-100
Why is TCP fairTwo competing sessions
Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ecti
on 2
thr
ough
put
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
51
Transport Layer 3-101
Fairness (more)Fairness and UDP
Multimedia apps often do not use TCP
do not want rate throttled by congestion control
Instead use UDPpump audiovideo at constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 cnctions
new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2
Transport Layer 3-102
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced byTCP connection establishmentdata transmission delayslow start
Notation assumptionsAssume one link between client and server of rate RS MSS (bits)O object size (bits)no retransmissions (no loss no corruption)
Window sizeFirst assume fixed congestion window W segmentsThen dynamic window
modeling slow start
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
52
Transport Layer 3-103
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-104
Fixed congestion window (2)
Second caseWSR lt RTT + SR wait for ACK after sending windowrsquos worth of data sent
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
53
Transport Layer 3-105
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
RS
RSRTTP
RORTTLatency P )12(2 minusminus⎥⎦
⎤⎢⎣⎡ +++=
where P is the number of times TCP idles at server
1min minus= KQP
- where Q is the number of times the server idlesif the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-106
TCP Delay Modeling Slow Start (2)
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
54
Transport Layer 3-107
TCP Delay Modeling (3)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
idleTimeRTTRO
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
minusminus+++=
minus+++=
++=
minus
=
=
sum
sum
th window after the timeidle 2 1 kRSRTT
RS k =⎥⎦
⎤⎢⎣⎡ minus+
+minus
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time=+ RTTRS
window kth the transmit totime2 1 =minus
RSk
RTT
initiate TCPconnection
requestobject
first window= SR
second window= 2SR
third window= 4SR
fourth window= 8SR
completetransmissionobject
delivered
time atclient
time atserver
Transport Layer 3-108
TCP Delay Modeling (4)
⎥⎥⎤
⎢⎢⎡ +=
+ge=
geminus=
ge+++=
ge+++=minus
minus
)1(log
)1(logmin
12min
222min222min
2
2
110
110
SO
SOkk
SOk
SOkOSSSkK
k
k
k
L
L
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
55
Transport Layer 3-109
HTTP ModelingAssume Web page consists of
1 base HTML page (of size O bits)M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in seriesResponse time = (M+1)OR + (M+1)2RTT + sum of idle times
Persistent HTTP2 RTT to request and receive base HTML file1 RTT to request and receive M imagesResponse time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connectionsSuppose MX integer1 TCP connection for base fileMX sets of parallel connections for imagesResponse time = (M+1)OR + (MX + 1)2RTT + sum of idle times
Transport Layer 3-110
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission timePersistent connections only give minor improvement over parallelconnections
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
56
Transport Layer 3-111
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybullbandwidth networks
Transport Layer 3-112
Chapter 3 Summaryprinciples behind transport layer services
multiplexing demultiplexingreliable data transferflow controlcongestion control
instantiation and implementation in the Internet
UDPTCP
Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo
Recommended