lecture 7: transport control protocollecture 7: transport control protocol! tcp connection...

CSE 123: Computer NetworksAaron Schulman

Lecture 7:Transport Control Protocol

! TCP Connection Managementu Starting a TCP connectionu Graceful, symmetric closeu The TIME_WAIT state

! Connecting links together to form networksu Hubs/repeatersu Switching

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Lecture 7 Overview

CSE 123 – Lecture 7: TCP

Connection Establishment! Both sender and receiver must be ready before we

start to transfer the datau Sender and receiver need to agree on a set of parametersu Most important: sequence number space in each directionu Lots of other parameters: e.g., the Maximum Segment Size

! Handshake protocols: setup state between two oblivious endpointsu Need to deal with delayed and reordered packets

3CSE 123 – Lecture 7: TCP

Two-way handshake?

Active participant(client)

Passive participant(server)

SYN, SequenceNum = x

SYN, SequenceNum = y

+data

What’s wrong here?


Two-way handshake?



Old SYN, SequenceNum = x

SYN, SequenceNum = y

+data

Delayed old SYNNew SYN, SequenceNum = q

Rejected


Client and server do not agree they are starting the same connection

Three-Way Handshake! Opens both directions for transfer




SYN + ACK, SequenceNum = y,

ACK, Acknowledgment = y + 1

Acknowledgment = x + 1

+data


Again, with States







+data

LISTEN

SYN_RCVD

SYN_SENT

ESTABLISHED

ESTABLISHED


Three-Way Handshake! Opens both directions for transfer







+data

8CSE 123 – Lecture 5: Flow Control

Some Comments! We could abbreviate this setup, but it was chosen to

be robust, especially against delayed duplicatesu Three-way handshake from Tomlinson 1975

! Choice of changing initial sequence numbers (ISNs) minimizes the chance of hosts that crash getting confused by a previous incarnation of a connection

! How to choose ISNs?u Maximize period between reuseu Minimize ability to guess (why?)

9CSE 123 – Lecture 6: Transport Protocols

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/ACK

Timeout after two segment lifetimesFIN/ACK

ACK

ACK

ACK

Close /FIN

Close

CLOSED

Active open/SYN

TCP State Transitions


Again, with States







+data

LISTEN

SYN_RCVD

SYN_SENT

ESTABLISHED

ESTABLISHED


Connection Teardown! Orderly release by sender and receiver when done

u Delivers all pending data and “hangs up”

! Cleans up state in sender and receiver

! TCP provides a “symmetric” closeu Both sides shutdown independently


TCP Connection Teardown

Server Client

FIN

ACK

ACK

FIN

FIN_WAIT_1

CLOSE_WAIT

LAST_ACKFIN_WAIT_2TIME_WAIT

CLOSEDCLOSED…


The TIME_WAIT State! We wait 2*MSL (maximum segment) before

completing the close! Maximum segment lifetime is usually 60 seconds

(longest period of time packet can be stuck in network)u Why?

! ACK might have been lost and so FIN will be resentu Could interfere with a subsequent connection

! Real life: Abortive closeu Don’t wait for 2*MSL, simply send Reset packet (RST)u Why?


Lots of Icky Details! Window probes! Silly Window Syndrome! Nagle’s algorithm! PAWS! Etc…

! Steven’s books “TCP/IP Illustrated (vol 1,2)” is a great source of information on this


Lots of Icky Details! Nagle’s algorithm! Window probes! Silly Window Syndrome! Nagle’s algorithm! PAWS! Etc…

! Steven’s books “TCP/IP Illustrated (vol 1,2)” is a great source of information on this


Jumping back down

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HTTP

TCP

IP

Ethernetinterface

HTTP

TCP

IP

Ethernetinterface

IP IP

Ethernetinterface

Ethernetinterface

SONETinterface

SONETinterface

host host

router router

Application Layer

Transport Layer

Link Layer

Network Layer

CSE 123 – Lecture 6: Transport Protocols

! One shared link (a bus) limits scale in terms of:u Distance (e.g., 2500 m for Ethernet)u Number of nodes (1024 nodes)u Performance (Capacity shared across all nodes)

! Often called a Local Area Network (LAN)

! A better alternative is to have multiple bussesu Each bus is of a limited size, scale, number of hosts, etc.

! Need the ability to connect multiple busses togetheru In other words move frames from one wire to another

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So far, just one link

CSE 123 – Lecture 7: TCP

For next time…

! Read P&D 3-3.1


lecture 7: transport control protocollecture 7: transport control protocol! tcp connection...

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