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Chapter 3Transport Layer

Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2007J.F Kurose and K.W. Ross, All Rights Reserved

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Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of reliable data transfer

3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection

management

3.6 Principles of congestion control

3.7 TCP congestion control

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TCP Connection EstablishmentThree-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 SYN+ACK segment server allocates buffers specifies server initial

seq. #Step 3: client receives

SYN+ACK, replies with ACK segment, which may contain data

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ACK

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TCP segment structure

source port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

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TCP Connection Termination

Step 1: client end system sends TCP FIN control segment to server

Step 2: server receives FIN, replies with ACK.

When server is done sending all data, it closes connection by sending FIN.

Step 3: client receives FIN, replies with ACK.

Enters “timed wait” - 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

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TCP Connection Management (cont)

TCP clientlifecycle

TCP serverlifecycle

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SYN

SYN + ACK

ACK

FIN

clientFIN

server

ACK

ACK

FIN

close

close

closed

tim

ed

wait

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TCP state-transition diagram

CLOSED

LISTEN

SYN_RCVD SYN_SENT

ESTABLISHED

CLOSE_WAIT

LAST_ACKCLOSING

TIME_WAIT

FIN_WAIT_2

FIN_WAIT_1

Passive open Close

Send/SYNSYN/SYN + ACK

SYN + ACK/ACK

SYN/SYN + ACK

ACK

Close/FIN

FIN/ACKClose/FIN

FIN/ACKACK + FIN/ACKTimeout after two segment lifetimes

FIN/ACK

ACK

ACK

ACK

Close/FIN

Close

CLOSED

Active open/SYN

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Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of reliable data transfer

3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection

management

3.6 Principles of congestion control

3.7 TCP congestion control

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Principles of Congestion Control

Congestion: informally: “too many sources sending too

much data too fast for network to handle” the load on the network is more than its capacity

different from flow control! manifestations:

lost packets (buffer overflow at routers) long delays (queueing in router buffers)

a top-10 problem!

9/13/0510

Network Performance

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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 explicit rate sender

should send at

Two broad approaches towards congestion control:

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Network-Assisted Congestion Control

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TCP Congestion Control

Approach Limit the rate at which sender sends data as

a function of perceived congestion in the network

Issues How does it perceive that there is

congestion in the network? What algorithm does it use to change the

sending rate as a function of the perceived level of congestion?