Fall 2005By: H. Veisi

Computer networks course

Olum-fonoon Babol

Chapter 6

The Transport Layer

Computer networking, Olum-Fonoon Babol H. VeisiH. Veisi Fall 2005 Page 2

Overview

It’s task is to provide efficient, reliable and cost-effective transport from the source to destination.

Hardware and/or Software within transport layer that does the work is called Transport entity.

Transport layer services: Connectionless Connection-oriented

Very similar to Network layer! So why we need this layer?! Transport layer runs on user’s machine, Network layer

mostly runs on routers and result inadequate services.

Provides Primitive functions to application layer for programmers.

Computer networking, Olum-Fonoon Babol H. VeisiH. Veisi Fall 2005 Page 3

Network layer: logical communication between hosts

Transport layer: logical communication between processes relies on, enhances,

network layer services

University analogy:

7 students sending letters to 7 students

processes = students app messages = letters in envelopes hosts = universities transport protocol = post office of

universities network-layer protocol = postal

service

Transport vs. Network Layer

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Transport service primitives (1)

The primitives for a simple transport service:

Nesting of TPDU (Transport Protocol Data Unit), packets and frames:

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Transport service primitives (2)

Connection/Transmission scenario:

Host A Host B

ListenConnect

Connection Request TPDUConnection Accept TPDU

Send

Receive

Send

Receive

Data

Disconnect

Disconnect TPDU

Send

Disconnect

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Addressing

Transport Service Access Pint = Port No.

Network Service Access Pint = IP No.

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The Internet Transport layer protocols

Two types: UDP (User Datagram Protocol)

Connectionless transport, Like IP! unreliable, unordered delivery

TCP: Transmission Control Protocol Connection-oriented transport reliable, in-order delivery (TCP) Congestion control Flow control Connection setup

Computer networking, Olum-Fonoon Babol H. VeisiH. Veisi Fall 2005 Page 8

UDP (1)

No connection establishment (which can add delay) Simple: no connection state at sender, receiver Small segment header No congestion control: UDP can blast away as fast as desired UDP segments may be:

lost delivered out of order to app

Applications: RPC: Remote Procedure Call

In DNS servers RTP: Real-Time Transport Protocol

Audio, video, …

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Reliable transfer over UDP: add reliability at application layer application-specific error recovery!

UDP segment format

source port # dest port #

32 bits

Applicationdata

(message)

length checksumLength, in

bytes of UDPsegment,including

header

UDP (2)

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TCP (1)

Computer networking, Olum-Fonoon Babol H. VeisiH. Veisi Fall 2005 Page 11

Full duplex data: bi-directional data flow in

same connection Connection-oriented:

handshaking (exchange of control msgs) init’s sender, receiver state before data exchange

Flow controlled: sender will not

overwhelm receiver

Point-to-point: one sender, one receiver

Reliable, in-order byte steam: no “message boundaries”

Pipelined: TCP congestion and flow

control set window size send & receive buffers

Processwrites data

Processwrites data

TCPsendbuffer

TCPsendbuffer

SocketSocket

Processreads data

Processreads data

TCPreceivebuffer

TCPreceivebuffer

SocketSocket

segment segment

TCP (1)

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TCP (2)

A TCP entity accepts user data stream and breaks them to segments.

For obtaining TCP services, both sender and receiver creating end points called sockets.

Socket No.= IP address of machine+ 16 bits local add. (Port No.)

Port No.s bellow 1024 are called Well-known ports

TCP does not support multicasting or broadcasting

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TCP (3)

Some standard ports :

Port Protocol Use21 FTP File transfer23 Telnet Remote login25 SMTP E-mail69 TFTP Trivial File Transfer Protocol79 Finger Lookup info about a user80 HTTP World Wide Web

110 POP-3 Remote e-mail access119 NNTP USENET news

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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(generally not used)

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

commands)

# bytes rcvr willingto accept

Internetchecksum

(as in UDP)

window scaling factor,Time-stamping,maximum segment length,…RFCs: 854, 1323

[4Bytes]

seq # is byte-stream number of first data

byte in segment

TCP (4)

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TCP (5)

Connection Establishment Tree-way handshaking

Use Sliding Window Protocol Go-Back-N and Selective Repeat

Host BHost A