The Transport Layer

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

This critical layer provides communications services directly to distributed processes running on different hosts.

The Most Frequently Used Internet Transport Protocols

Transmission Control Protocol (TCP)

User Datagram Protocol (UDP)

TCP/IP Protocol Data Units (PDUs)

TCP/IP Concepts

TCP/IP Concepts – Data Flow

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

TCP Applications

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTPSMTP

SMTPSMTP

SMTPSMTPPC runningPC running

ExplorerExplorer

Server Server runningrunning

Apache WebApache WebServerServer

Mac runningMac runningNavigatorNavigator

http request

http re

quest

http response

http re

sponse

Email

Web browsing

File transfer

The transport level addressing

Network Service Access Point (NSAP): it is between the network and transport levels

Transport Service Access Point (Transport address, TSAP): it is between the transport and the application layers

Port Number = Application Selection Address (ASA)

Port

An abstraction, which is used by the transport protocols to take the difference among each addressee in a given host

The TCP/IP protocol identify the port with a small integer

The port equivalent to the Transport Selector (TSEL) in the transport layer of the OSI model

Assigned Port or Socket Number

Decimal Octal Description0-63 0-77 Network Wide Standard Function64-127 100-177 Hosts Specific Functions128-223 200-337 Reserved for Future Use224-255 340-377 Any Experimental Function

Some well-known port numbers

Port number Application

20 FTP control

21 FTP data

23 Telnet

25 SMTP (mail)

37 Timeserver

53 DNS (Domain Name Service)

80 HTTP

119 NNTP (Network News Transfer Protocol)

520 RIP/UDP routing information

Transport services and protocols

provide logical communication between application processes running on different hosts

transport protocols run in end systems

transport vs. network layer services: network layer: data transfer

between end systems (hosts) transport layer: data transfer

between processes Transport layer relies on

and enhances network layer services

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysicalnetwork

data linkphysical

logical end-end transport

The source and destination port in the client-server communication

Clienthost

Transport levelprotocol messages Server

hostSource port=xDest. port=80

Source port=80Dest. port=x

Transportlevel

protocol entity

Transportlayer

protocol entity

Socket, the realization of the port in the programming

socketdoor

TC Psend buffer

TC Preceive buffer

socketdoor

se g m e n t

applicationwrites data

applicationreads data

TCP is a transport level protocol, see later

applicationtransportnetwork

MP2

applicationtransportnetwork

Multiplexing/demultiplexing

Recall: segment - unit of data exchanged between transport layer entities aka TPDU: transport

protocol data unitreceiver

HtHn

Demultiplexing: delivering received segments to correct app layer processes

segment

segment Mapplicationtransportnetwork

P1M

M MP3 P4

segmentheader

application-layerdata

Transport level multiplexing

Internet

Host A

PA1, PA2, PA3 application processes Port1 Port2 Port3

Transport prot. entity

IP protocol entity

Network access protocol entity

Host C

PC1 application processes

Port1

Transport prot. entity

IP protocol entity

Network access protocol entity

Host B

PB1, PB2 application processes Port1 Port2

Transport prot. entity

IP protocol entity

Network access protocol entity

Multiplexing/demultiplexing: examples

host A server Bsource port: xdest. port: 23

source port:23dest. port: x

port use: simple telnet app

Web clienthost A

Webserver B

Web clienthost C

Source IP: CDest IP: B

source port: x

dest. port: 80

Source IP: CDest IP: B

source port: y

dest. port: 80

port use: Web server

Source IP: ADest IP: B

source port: x

dest. port: 80

Transmission Control Protocol (TCP)

Connection-oriented End-to-end reliability

Reliability: positive acknowledging It is intended for use as a highly reliable host-

to-host protocol between hosts in packet-switched computer communication networks,

and in interconnected systems of such networks

Flow control with sliding window protocol-adaptation to:

• round-trip time (RTT) or• available transmission velocity

higher-levelTCP

internet protocolcommunication network

Protocol Layering:Protocol Layering:

TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581

Point-to-point: one sender, one receiver

Reliable transport using sliding window protocol

Flow controlled and congestion controlled

User Datagram Protocol (UDP)

The UDP gives the transport mechanism with the possible smallest overhead

Connection-less It carry out multiplexing/demultiplexing and

error check, only The delivery is not reliable The delivery is not ordered

Relation With Other Protocols

Application Layer(SMTP, FTP, HTTP, etc)

Transport Layer(TCP, UDP)

Internet Layer(IP, ICMP, ARP)

Network Interface Layer(Ethernet, X25, SLIP, PPP)

PHYSICAL NETWORKPHYSICAL NETWORK

TCP/IPSTACK

UDP: User Datagram Protocol [RFC 768]

“best effort” service, UDP segments may be: lost delivered out of order

to app connectionless:

no handshaking between UDP sender, receiver

each UDP segment handled independently of others

Why is there a UDP? no connection

establishment (which can add delay)

simple: no connection state at sender, receiver

small segment header no congestion control:

UDP can blast away and send data as fast as desired

UDP: more

often used for streaming multimedia apps loss tolerant rate sensitive

other UDP uses: DNS SNMP

reliable transfer over UDP: add reliability at application layer application-specific

error recover!

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,including

header

UDP-based Error Check If sum of the whole 16-bit words of the received

UDP datagram equals to FFFFh, the datagram is probably error free

Calculation of the control sum: E.g. let the next 16-bit words: 0110111001110110

00101010101010100001111000101011

Their sum: 1011011101001011 (1st complement: all 0 are exchanged to 1 and vice versa) So we get the control sum: 0100100010110100 At the receiver side the sum of the whole 16-bit word plus

the control sum (possible carry bit is eliminated):1111111111111111=FFFFh

This error check is not very safe

Features of the Application of UDP

Small header overhead Non-controlled sending speed No connection establishing No connection state Lack of error control Lack of congestion protection Message carried by UDP restricted to

512 bytes, not counting the IP or UDP headers Longer message are truncated

Services provided by Internet transport protocols

TCP service: connection-oriented: setup

required between client, server

reliable transport between sending and receiving process

flow control: sender won’t overwhelm receiver

congestion control: throttle sender when network overloaded

does not provide: timing, minimum bandwidth guarantees

UDP service: unreliable data transfer

between sending and receiving process

does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee

Q: Why is there a UDP?Why is there a UDP?

Application of the Transport Protocols

Application Application level protocol Transport protocol

routing protocol e.g. RIP UDP

domain name service DNS UDP

network management SNMP UDP

remote file service NFS UDP

file transfer FTP TCP

remote terminal access Telnet TCP

electronic-mail SMTP UDP, TCP

Internet based telephoning application dependent UDP

World Wide Web (WWW) HTTP TCP

streaming media handling application dependent UDP