Embed Size (px)
Citation preview
Transport Layer 10-1
2010 session 1TELE3118 Network
Technologies
Week 10 Transport LayerTCP
Some slides have been taken fromComputer Networking A Top Down Approach Featuring the Internet 3rd edition Jim Kurose Keith Ross Addison-Wesley July 2004 All material copyright 1996-2004 JF Kurose and KW Ross All Rights Reserved
Transport Layer 10-2
TCP Connection Management Setup
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control
info (eg RcvWindow) client connection initiator
connect() server contacted by client
accept()
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 buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 10-3
TCP Connection Management TeardownClosing a connection
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
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
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 10-4
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 10-5
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 overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 10-6
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 segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 10-7
Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 10-8
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers 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 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-2
TCP Connection Management Setup
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control
info (eg RcvWindow) client connection initiator
connect() server contacted by client
accept()
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 buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 10-3
TCP Connection Management TeardownClosing a connection
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
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
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 10-4
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 10-5
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 overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 10-6
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 segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 10-7
Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 10-8
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers 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 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-3
TCP Connection Management TeardownClosing a connection
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
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
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 10-4
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 10-5
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 overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 10-6
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 segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 10-7
Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 10-8
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers 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 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-4
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
Transport Layer 10-5
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 overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 10-6
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 segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 10-7
Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 10-8
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers 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 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-5
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 overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 10-6
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 segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 10-7
Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 10-8
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers 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 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-6
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 segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 10-7
Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 10-8
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers 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 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-7
Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 10-8
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers 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 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-8
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers 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 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-9
TCP Congestion Control
end-end control (no network assistance)
sender limits transmission LastByteSent-LastByteAcked
CongWin Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-10
TCP AIMD
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
multiplicative decrease cut CongWin in half after loss event
additive increase increase CongWin by 1 MSS every RTT in the absence of loss events probing
Long-lived TCP connection
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-11
TCP Slow Start When connection begins
CongWin = 1 MSS Egbull MSS = 500 bytesbull RTT = 200 msecbull initial rate = 20 kbps
available bandwidth may be gtgt MSSRTT Ramp-up takes too long
When connection begins increase rate exponentially until first loss event double CongWin every RTT done by incrementing
CongWin for every ACK received
Summary initial rate slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-12
Refinement After 3 dup ACKs
CongWin is cut in half window then grows linearly
But after timeout event CongWin instead set to 1 MSS window then grows exponentially to a threshold then grows linearly
bull 3 dup ACKs indicates network capable of delivering some segmentsbull timeout before 3 dup ACKs is ldquomore alarmingrdquo
Philosophy
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-13
Refinement (more)
Q When should the exponential increase switch to linear
A When CongWin gets to 12 of its value before timeout
Implementation Variable Threshold At loss event Threshold
is set to 12 of CongWin just before loss event
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-14
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 CongWin2 and CongWin is set to 1 MSS
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-15
TCP sender congestion control
Event State TCP Sender Action Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
ACK receipt for previously unacked data
CongestionAvoidance (CA)
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
Timeout SS or CA Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
Duplicate ACK
SS or CA Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-16
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 occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-17
TCP Futures
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-18
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 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-19
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 10-20
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel cnctions between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
