TO 3-7-06 p. 1 Spring 2006 EE 5304/EETS 7304 Internet Protocols Tom Oh Dept of Electrical Engineering [email protected] Lecture 13 Transport Layer

TO 3-7-06 p. 1

Spring 2006

EE 5304/EETS 7304 Internet Protocols

Tom OhDept of Electrical Engineering

[email protected]

Lecture 13

Transport Layer

TO 3-7-06 p. 2

Administrative Issues

I will pass out the Test #2 next week.

For in-class students, we will have Test #3(Final) on May 9, 2006, 6:30PM.

I will post Homework 4 on our course website tomorrow.

TO 3-7-06 p. 3

Outline (Comer, Ch. 25)

Transport layer functions

General considerations

Types of networks

Transport functions for type A network

Transport functions for type B network

Transport functions for type C network

TO 3-7-06 p. 4

Transport Layer (TCP/IP View)

TransportTransport

Internet

IPIP

Network/Network/physicalphysical

ApplicationApplication

TransportTransport

IPIP

Network/Network/physicalphysical


Host Host

Host-to-host protocol to hide details of network from application

TO 3-7-06 p. 5

Transport Layer (cont)

Provides host-to-host communication service to application

Applications do not have to worry about details of network

Can often enhance network services (e.g, TCP/IP)

Ensure error recovery if network layer is unreliable Ensure end-to-end flow control Ensure multiplexing and demultiplexing sessions onto

same connection

Applications can call on transport layer without having to worry about these things

TO 3-7-06 p. 6


TransportTransport


TransportTransport


Host Host

Application may see reliable, flow controlled, multiplexed communication transport-layer service

Underlying network may be unreliable

TO 3-7-06 p. 7


Transport layer can be simpler if network is good

TCP/IP is extreme case

IP layer is intentionally simple as possible - best-effort and unreliable

Transport layer (TCP) is complicated

TO 3-7-06 p. 8

Connection-Oriented or Connectionless?

Transport layer offers connection-oriented or connectionless service to application?

Commonly connection-oriented, eg, TCP

Requires connection set-up and disconnect phases Most appropriate if hosts want reliable, flow controlled,

sequential transfer of long stream of data Network layer can be connection-oriented or

connectionless If connectionless, transport layer must do more work to

appear connection-oriented (e.g, TCP)

TO 3-7-06 p. 9


TransportTransport


TransportTransport


Host Host

Application sees a virtual connection: packets are delivered in sequential order

Underlying network may be connectionless or connection-

oriented

TO 3-7-06 p. 10

Connection-Oriented? (cont)

Some transport layer protocols are connectionless, eg, UDP

Sometimes connection set-up is unjustified, eg, when data is short/bursty or application does not need reliable delivery

Network layer can be connection-oriented or connectionless (but makes practical sense only for connectionless network)

TO 3-7-06 p. 11

Quality of Service Needed by Application?

Application may request a desired or minimum QoS

QoS can be characterized by specific parameters, eg:

Connection establishment delay: time to set up a new connection

Connection blocking probability: chance of failing to set up new connection

Throughput: number of data bytes transferred over time Transit delay: time from packet transmission to delivery

TO 3-7-06 p. 12

QoS (cont)

QoS parameters (cont):

Availability: how often service is unavailable Reliability: percentage of lost or errored messages that are

uncorrected by network Relative priority: priority of connection compared to other

connections

Transport layer QoS may be limited by network QoS

Although unreliable network can be made to appear more reliable, transport layer cannot overcome some limitations of the network (bandwidth, delays)

TO 3-7-06 p. 13

OSI Types of Network Service

A: reliable packet delivery, rare signaled network failures (that cause reported but uncorrected connection loss or packet loss)

A-1: sequential delivery, arbitrary packet size A-2: non-sequential delivery, arbitrary packet size (e.g.

datagram) A-3: non-sequential delivery, maximum packet size

B: reliable delivery, rate of signaled failures can be unacceptable, e.g. X.25

C: unreliable delivery (lost/duplicated packets), e.g, IP

TO 3-7-06 p. 14

Type A-1 Network Service

Assumes reliable network, sequential packet delivery, arbitrary packet size

Important transport layer functions:

Connection setup/termination Multiplexing/demultiplexing Flow control

TO 3-7-06 p. 15

Type A-1 Network: Connection Setup

Set-up verifies dest. host is ready, negotiates optional parameters (TPDU size, window size, QoS), and allows allocation of resources (buffer space)

TPDU = transport protocol data unit (transport layer packet)

Simple 2-way handshake for connection set-up is sufficient

closedclosed

host Bhost A

passive open (listen)

active openSYN

SYNestablished

established

TO 3-7-06 p. 16

Type A-1 Network: Connection Setup (cont)

Connection request can be initiated by OPEN command from user → send RFC, wait to receive RFC (connection accepted) or CLOSE (connection refused)

If receive RFC → OPEN state (connection established)

If receive RFC (connection request) in LISTEN state → send RFC to accept, enter OPEN state (connection established)

If receive RFC in IDLE state → notify user of connection request, then send RFC and enter OPEN state if user accepts

TO 3-7-06 p. 17

Type A-1 Network: Connection Setup (cont)

If both hosts initiate RFC about same time → no confusion

Connection termination works in same way Initiated by either side, return to IDLE state

closedclosed

host Bhost A

SYN SYNactive open active open

established established

TO 3-7-06 p. 18

Type 1-A Network: Multiplexing

Multiplexing → multiple applications running on same host can use same transport protocol

Applications are identified by port numbers carried in transport layer packet header

Transport layer can multiplex data from multiple applications (identified by ports) to send via network layer

Can receive data from network layer and demultiplex to appropriate application

Well known ports: TCP 80 = HTTP; UDP 53 = DNS; TCP 25 = SMTP

TO 3-7-06 p. 19

Multiplexing (cont)

TransportTransport


TransportTransport


Host Host

Port 80


Port 35


Port 26 Port 18

TO 3-7-06 p. 20

Type 1-A Network: Flow Control

Like data link layer, flow control is to prevent sender host A from overwhelming dest. host B

Dest. host B has options:

Do nothing• If receive buffers overflow, data is lost• A will time-out and resend → makes congestion worse

Refuse to accept data from network layer• Relies on backpressure from flow control in network to slow

down A• Slow: backpressure may take long time to reach A• Coarse control: other connections may be also effected

TO 3-7-06 p. 21

Type 1-A Network: Flow Control (cont)

Use sliding window

Need ACKs and sequence numbers for TPDUs B will withhold ACKs to slow down A (A will not time-out

and resend because packet delivery is assumed reliable) Works well, but may not work well if network is unreliable

(A will time-out and resend if ACKs are too slow or lost)

Use credits

Separates ACKs from flow control: can ACK without granting credit & vice versa

Also works well for unreliable networks

TO 3-7-06 p. 22


Assumes reliable network, non-sequential packet delivery, arbitrary packet size

Non-sequential delivery → TPDU sequence numbers are required for resequencing at dest. host

Already saw sequence numbers are useful for flow control

Transport protocol must keep track of control TPDUs

Possible confusion for flow control

TO 3-7-06 p. 23

Type A-2 Network Service (cont)

New credit value might overtake old credit value → sender gets wrong message

Need to sequentially number credit messages to avoid confusion

packet N

host Bhost A

credit 0

credit MAck

waiting for datawaiting for credit

TO 3-7-06 p. 24


Possible confusion in connection set-up

Data packets could arrive before a CONNECTION_ACCEPT

• Should queue these TPDUs in expectation of a CONNECTION_ACCEPT

Data packets could arrive after a CONNECTION_RELEASE

• CONNECTION_RELEASE should contain number of last TPDU

TO 3-7-06 p. 25


RFC

host Bhost A

RFC

2

Ack

data arrives before connection is accepted

SYN

1

CLS

host Bhost A

N

data arrives after connection is released

N-1

CLS

TO 3-7-06 p. 26


Assumes reliable network, non-sequential packet delivery, limited packet size

Transport service is stream oriented (user data is treated as continuous) or block oriented

If block oriented, blocks are segmented into TPDUs and reassembled

Maybe number the blocks & number TPDUs within each block, but TPDUs have sequence numbers

• Only need End of Block flag

TO 3-7-06 p. 27

Type B Network Service

Assumes reliable network, maybe non-sequential packet delivery, network failures are possible

TPDUs can be lost but reported to transport entities

Transport entity must handle known lost data and/or lost connection

TO 3-7-06 p. 28

Type B Network Service (cont)

In connection reset (eg, X.25 RESET), maybe some TPDUs will be lost

Transport entity sends control TPDU to other end to ACK reset condition and gives number of last received TPDU

Wait to send new TPDUs until receive corresponding reset control TPDU from other end

If network connection is lost without reset (eg, X.25 RESTART), new connection must be requested

Transport entity sends control TPDU to other end to identify new network connection

TO 3-7-06 p. 29

Type C Network Service

Assumes unreliable network, non-sequential packet delivery, eg, IP

Transport layer functions:

Retransmission Duplicate detection Flow control Connection setup/clear Crash recovery

TO 3-7-06 p. 30

Type C Network - Retransmission Strategy

TPDUs can be lost or errored

Requires ACKs to sender

Sender has timers and timeouts to resend

If timeout too short → unnecessary retransmissions If timeout too long → slow to respond to lost TPDU Should be somewhat longer than (variable) roundtrip delay

Fixed timeouts cannot adapt

Adaptive timeout: no known best solutions (although used in TCP)

TO 3-7-06 p. 31

Type C Network - Duplicate Detection

No confusion for lost TPDUs that are retransmitted

But if ACK lost, receiver might get duplicate TPDUs

If a duplicate TPDU arrives before connection close,

Receiver assumes ACK was lost & it ACKs duplicate Sender should not be confused by multiple ACKs for same

TPDU Range of sequence numbers should be large enough not

to repeat (wrap around) during connection

TO 3-7-06 p. 32

Type C Network - Duplicate Detection (cont)

If a duplicate TPDU arrives after connection close,

TPDU from old connection could arrive during new connection and be mistaken for new TPDU

Could use continuous sequence numbers, transport connection ID,...

RFCs contain initial sequence number

TO 3-7-06 p. 33

Type C Network - Duplicate Detection (cont)

host Bhost A

duplicate TPDU at B, no confusion

N

N

host Bhost A

ACK

duplicate ACKs at A, no confusion

N

N

ACK

ACK

host Bhost A

duplicate TPDU arrives during new connection, confused for new TPDU

N

N

CLS

CLS

CLS

CLS

SYN

SYN

TO 3-7-06 p. 34

Type C Network - Flow Control

Credits work well

Eg, (ACK N, CREDIT M) acks all TPDUs up to N and grants credit for TPDUs N+1 through N+M

If ACK is lost, future ACKs will resync. protocol

Sender will timeout and resend, and generate new ACK

But (ACK N, CREDIT 0) can close window, if next (ACK N, CREDIT M) is lost → deadlock

Window timer is reset with each ACK If timeout, entity must send ACK even if duplicates earlier

ACK

TO 3-7-06 p. 35

Type C Network - Flow Control (cont)

N

host Bhost A

Ack N, credit 0

Ack N, credit M

waiting for datawaiting for credit

N

host Bhost A

Ack N, credit 0

Ack N, credit M

window timer

Ack N, credit M

TO 3-7-06 p. 36

Type C Network - Connection Setup

RFCs (request or confirm) can be lost or delayed in normal 2-way handshake

Use retransmit-RFC timer → receiver may get duplicate RFCs (if CONNECTION_CONFIRM lost)

1. ignore duplicate if connection already set up 2. confusing if arrives after connection clear

• Mistaken for new request, real new request is discarded

TO 3-7-06 p. 37

Type C Network - Connection Setup (cont)

SYN i

host Bhost A

obsolete SYN arrives

B discards duplicate SYN

B rejects TPDU

A sends new SYNSYN j B respondsSYN k

A opens connection

A sends TPDU k+1

TO 3-7-06 p. 38


If CONNECTION_REQUEST is delayed, sender may get duplicate RFCs

Ignore duplicate if connection already up

3-way handshake: ACK both the RFC and sequence number

Old RFC causes a reset (rejection) at sender Old CONNECTION_CONFIRMED is discarded

TO 3-7-06 p. 39


host Bhost A

normal

SEQ i, ACK j

SYN i

host Bhost A

old SYN, connection is rejected by A

SYN j, ACK i

host Bhost A

delayed SYN/ACK, connection is rejected by A, new connection is accepted

RST , ACK j

old SYN i

SYN j, ACK i

SEQ i, ACK j

SYN i

SYN j, ACK i

old SYN k, ACK m

RST, ACK k

TO 3-7-06 p. 40

Spring 2006



[email protected]

Lecture 13

UDP

TO 3-7-06 p. 41

Outline

UDP (comer, Ch. 24)

UDP header (Comer, Ch. 24)

TO 3-7-06 p. 42

UDP (User Datagram Protocol)

Provides unreliable datagram service with less overhead than TCP

Application has full responsibility for handling datagrams that are lost, duplicated, or out of order

UDP adds multiplexing on top of IP

Different applications on same host are identified by port numbers

An application is identified uniquely by <host address, port number>

TO 3-7-06 p. 43

UDP (cont)

TransportTransport


TransportTransport


Host Host

UDP port 80


UDP port 25


UDP port 26

UDP port 18

Unreliable connectionless service

TO 3-7-06 p. 44

UDP 8-byte Header

bits:

data

UDP source port

8

UDP destination port

message length checksum

8 8 8

Source port (16 bits): optional; allows replies to sender

Destination port (16 bits): identifies application at destination hostMessage length (16 bits):

bytes of data + 8 for UDP header

TO 3-7-06 p. 45

UDP Header (cont)

Checksum (2 bytes) = error detection over a pseudoheader + UDP datagram

PseudoheaderPseudoheader UDP datagramUDP datagram

TO 3-7-06 p. 46

UDP Header (cont)

Pseudoheader = 12 bytes constructed from IP header

Source and dest. IP addresses (4 bytes each) Protocol (1 byte) = 17 for UDP UDP length (2 bytes) = length of UDP datagram

(excluding pseudoheader)

bits:

source IP address

8

zero UDP length

8 8 8

protocol

destination IP address

TO 3-7-06 p. 47

UDP Header (cont)

IP address is used in checksum to verify correct destination

Does this checksum violate the layering principle?

Yes - because UDP uses info from IP layer below it (IP packet header fields)

TO 3-7-06 p. 48

Spring 2006



[email protected]

Lecture 13

TCP - Part 1

TO 3-7-06 p. 49

Outline (Comer, Ch. 25)

TCP

TCP header

TCP retransmissions

TCP duplicate detection

TCP connection set-up and close

TO 3-7-06 p. 50

TCP (Transmission Control Protocol)

TCP is predominant transport layer protocol to add end-to-end reliability above IP

Designed for reliable sequential byte stream delivery with no duplicates, no loss

Views application data as continuous byte stream, breaks into segments of 64-Kbyte max. length

Keeps track of each byte with a sequence number Segments are prefixed with TCP header and encapsulated

into IP packets

TO 3-7-06 p. 51

TCP (cont)

TCP dataTCP dataTCP headerTCP headerIP headerIP header

Sending application

Data• • • • • •

DataData

DataDataTCP headerTCP header

TCP segment

Receiving application

Data• • • • • •

DataData


TCP segment

IP packet

TO 3-7-06 p. 52

TCP (cont)

Provides connection-oriented service between applications on different hosts

An application is identified to TCP by port address

Application is completely identified by 16-bit port address & 32-bit IP address

TCP connection is between two endpoints, source <host address, port> and destination <host address, port>

TO 3-7-06 p. 53

TCP (cont)

TransportTransport


TransportTransport


Host Host

TCP port 80


TCP port 25


TCP port 26

TCP port 18

Reliable connection-oriented service with no duplicate, lost, misordered, or errored bytes

TO 3-7-06 p. 54

TCP (cont)

TCP assumes IP - a type C network - so has all of most complicated functions of transport protocol

Error control detects missing, errored, non-sequential, and duplicate packets

Uses sequence numbers and piggybacked ACKs, adaptive retransmissions

Flow control using credits

Connection control: 3-way handshake

Also, TCP assumes responsibility for congestion avoidance because IP has no congestion control

TO 3-7-06 p. 55

TCP Header

bits:

data

TCP source port

8

TCP destination port

checksum urgent pointer

8 8 8

sequence number

acknowledgement number

HLEN windowflagsRES

options

Source port (16 bits): optional; allows replies to sender

Destination port (16 bits): identifies application at destination host

TO 3-7-06 p. 56

TCP Header

bits:

data

TCP source port

8



8 8 8

sequence number


HLEN windowflagsRES

options

Checksum (16 bits): error detection over pseudoheader + TCP segment

TO 3-7-06 p. 57

TCP Header (cont)

Pseudoheader is constructed from IP packet header including IP source/destination addresses, protocol field (=6 for TCP), length of TCP segment

Ensures that IP addresses are correct

Like UDP, this violates layering principle of OSI model

bits:

source IP address

8

zero TCP length

8 8 8

protocol

destination IP address

TO 3-7-06 p. 58

TCP Header

bits:

data

TCP source port

8



8 8 8

sequence number


HLEN windowflagsRES

options

Sequence number (32 bits): number of first data byte, except if SYN=1; data bytes are numbered sequentially, to reconstruct sender’s byte stream

TO 3-7-06 p. 59

TCP Header (cont)

Sending application

Byte n+1Byte n • • •

DataData


Number of first byte = sequence number

Receiving application

DataData

Byte n+2 Byte n+1Byte n • • •Byte n+2

Sequence number tells where this segment belongs in reconstructed byte stream

TO 3-7-06 p. 60

TCP Header

bits:

data

TCP source port

8



8 8 8

sequence number


HLEN windowflagsRES

options

Acknowledgement (32 bits): piggybacked ACK tells sender the next byte that is expected; ACKs are cumulative and refers to end of contiguous received data; additional received data, if not contiguous, triggers a duplicate ACK

TO 3-7-06 p. 61

TCP Header (cont)

Sending application

DataData

Segment ASEQ = 400

Receiver’s buffer

Byte 399

DataData

DataData

Segment BSEQ = 600

Segment CSEQ = 800

DataData

Segment Breceived first

ACK 400

bytes

TO 3-7-06 p. 62

TCP Header (cont)

Sending application

DataData

Segment ASEQ = 400

Receiver’s buffer

Byte 399

DataData

DataData

Segment BSEQ = 600

Segment CSEQ = 800

DataData

Segment Creceived second

ACK 400

duplicate

bytes

TO 3-7-06 p. 63

TCP Header (cont)

Sending application

DataData

Segment ASEQ = 400

Receiver’s buffer

Byte 999

DataData

DataData

Segment BSEQ = 600

Segment CSEQ = 800

DataData

Segment Areceived third

ACK 1000

bytes

TO 3-7-06 p. 64

TCP Header (cont)

bits:

data

TCP source port

8



8 8 8

sequence number


HLEN windowflagsRES

options

Header length (4 bits): in units of 4 bytes; header is 20 bytes (value = 5) + options (if any)

Reserved (6 bits): all zeros

TO 3-7-06 p. 65

TCP Header (cont)

bits:

data

TCP source port

8



8 8 8

sequence number


HLEN windowflagsRES

options

Flags (6 bits): URG: tells if Urgent pointer is usedACK: tells if Acknowledgement field is used PUSH: forces immediate transmission at senderRST: tells receiver to abort and reset connectionSYN: segments for 3-way handshake to set up connectionFIN: segments for 3-way handshake to terminate connection

TO 3-7-06 p. 66

TCP Header (cont)

bits:

data

TCP source port

8



8 8 8

sequence number


HLEN windowflagsRES

options

URG flag: tells if Urgent pointer is used

Urgent pointer (16 bits): used if URG=1

TO 3-7-06 p. 67

TCP Header (cont)

Urgent pointer (2 bytes): points to number of first byte after urgent data in segment

If URG flag =1, data up to urgent pointer is urgent data to be processed immediately; rest of data is regular (not urgent)

Allows "out of band" data (to be processed immediately, out of sequence)


Urgent pointer

Urgent data

Regular data

TO 3-7-06 p. 68

TCP Header (cont)

Push function:

Normally, TCP accumulates data from sender before transmitting a segment

If sender issues a “push”, TCP will send the ready data, even if segment will be short (e.g., 1 byte of data)

TO 3-7-06 p. 69

TCP Header (cont)

bits:

data

TCP source port

8



8 8 8

sequence number


HLEN windowflagsRES

options

Window (16 bits): piggybacked credit advertised by receiver; for flow control of sender

Documents

TO 3-7-06 p. 1 Spring 2006 EE 5304/EETS 7304 Internet Protocols Tom Oh Dept of Electrical Engineering [email protected] Lecture 13 Transport Layer