37 Networking Sol

7/27/2019 37 Networking Sol

1/17

Transport Layer: UDP, TCP


2/17

Internet Transport ProtocolsTCP

Connection-oriented

setup required between client

and server

Reliable transport

Flow control

Wont overwhelm receiver

Congestion control Wont overwhelm network

Does not provide

Timing and throughputguarantees, security

UDP

Unreliable data transfer

Does not provide Connection setup, reliability, flow

control, congestion control,timing and throughputguarantee, or security

Copyright : Nahrstedt, Angrave, Abdelzaher, Kravets, Gupta, Caccamo


3/17

UDP Services


User Datagram Protocol Service

OSI Transport Layer

Provides a thin layer over IP

16-bit port space (distinct from TCPports) allows multiple recipients on a

single host


4/17

UDP Services


Unit of Transfer

Datagram (variable length packet)

Packet boundaries are preserved

when receiving data (a readoperation yields packet payload as itwas originally sent.

Unreliable

No guaranteed delivery

Drops packets silently Unordered

No guarantee of maintained order ofdelivery

Unlimited Transmission

No flow control

Postal Mail

Unreliable

Not necessarily in-order

Letters sentindependently

Must address each reply


5/17

UDP Packet structure


Quiz: by locking at UDP packet structure, can youguess what services are provided by UDP?

bits 0-15 16-310

32

64

Source port Destination port

Length Checksum

Data


6/17

UDP Packet structure


UDP provides

application multiplexing (via port numbers)

integrity verification (via checksum) of the header and payload

packets are sent individually and error checked only if they arrive

if any kind of transmission reliability is needed, it must beimplemented at application level

bits 0-15 16-310

32

64

Source port Destination port

Length Checksum

Data


7/17

When should I use UDP?


Why should I use UDP? After all, TCP is a betteralgorithm!


8/17

When should I use UDP?


Why should I use UDP? After all, TCP is a betteralgorithm!

Reliability of TCP comes with some costs TCP needs 3 packets to set up a connection

UDP header has 8 bytes instead of 20 (like TCP header)

UDP is a better choice for interactive (soft real-time) voice

and video traffic (like Skype). During real-time streaming, it is better to occasionally lose

few packets (slight QoS degradation) rather thanintroducing large delays if lost packets are retransmitted.


9/17

TCP Connections Transmission Control Protocol (TCP) Service

TCP is a protocol running on OSI Transport Layer

Service Model

Byte stream (interpreted by application); packetboundaries cannot be distinguished by application

16-bit port space allows multiple connections on a singlehost

Connection-oriented

Set up connection before communicating

Tear down connection when doneCopyright : Nahrstedt, Angrave, Abdelzaher, Kravets, Gupta, Caccamo


10/17

TCP Service


Reliable Data Transfer

Guaranteed delivery

Exactly once

Sequenced Data Transfer In-order delivery

Regulated Data Flow

Monitors network and adjusts transmissionappropriately

Data Transmission

Full-Duplex byte stream


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TCP Connection Establishment


3-Way Handshake

Sequence Numbers

J,K Message Types

Synchronize (SYN)

Acknowledge (ACK)

Passive Open

Server listens for connectionfrom client

Active Open

Client initiates connection toserver

Client Server

Time flows down

listen

Passive open

Active open


12/17

TCP Connection Termination


Either client or server caninitiate connection teardown (4-way handshake)

Message Types

Finished (FIN)

Acknowledge (ACK)

3-way handshake is alsopossible. The sequence is FIN J,ACK J+1 & FIN K (as singlepacket), ACK K+1

Client Server

Time flows down

Active close

Passive close


13/17

TCP Reliable transmission TCP uses a sequence number to identify each byte of data.

When establishing a connection, the 3-way handshake allows remotehosts to exchange and ACKan initial sequence number (ISN).

Exploiting sequence numbers, data can be reconstructed in order,regardless of fragmentation, lack of ordering, or packet loss

Sequence number is incremented for each payload byte that is sent.

Receiver uses cumulative acknowledgment

For example, if host A sends 10 bytes with sequence numbers 100 to 109,then the receiver transmits ACK 110 since next expected byte should havesequence number 110



14/17

TCP Segment Format

IP header TCP header TCP data

Sequence number (32 bits)

DATA

20 bytes 20 bytes

0 15 16 31

Source Port Number Destination Port Number

Acknowledgement number (32 bits)

window sizeheaderlength

0 Flags

Options (if any)

TCP checksum urgent pointer

20bytes

TCP segments have a 20 byte header with >= 0 bytes of data.

Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher


15/17

TCP header fields

TCP

IP

Applications

23 10480Ports:

TCP

IP

Applications

7 1680 Ports:

Port number A port number identifies the endpoint of a connection A pair identifies one endpoint of a

connection Two pairs and identify a TCP connection



16/17

TCP header fields Sequence Number (SeqNo):

Sequence number is 32 bits long.

Each sequence number identifies a byte in thebyte stream

Initial Sequence Number (ISN) of a connection is

set during connection establishment



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TCP header fieldsAcknowledgement Number (AckNo):

Acknowledgements are piggybacked

A host uses the AckNo field to sendacknowledgements. (If AckNo has a valid value,then the ACK flag is set)

The AckNo contains the next SeqNo that a host

(receiver) expects to receiveExample: The acknowledgement for a segmentwith sequence numbers 0-1500 is AckNo=1501

C i ht J Li b h 98 M difi d ith i i Abd l h

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37 Networking Sol

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