DNT 1013DATA COMMUNICATIONS

------------------------------------------CHAPTER 5:

TRANSPORT LAYER

Prepared By:Mdm Noor Suhana Bt SulaimanFKMT-NT, TATiUC

PROCESS TO PROCESS DELIVERY:

TCP, UDP, SCTP

Transport LayerOur goals: understand principles

behind transport layer services:multiplexing/

demultiplexingreliable data transferflow controlcongestion control

learn about transport layer protocols in the Internet:UDP: connectionless

transportTCP: connection-oriented

transportTCP congestion control

Transport services and protocols

provide logical communication between app processes running on different hosts

transport protocols run in end systems send side: breaks app

messages into segments, passes to network layer

received side: reassembles segments into messages, passes to app layer

more than one transport protocol available to appsInternet: TCP and UDP

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysicalnetwork

data linkphysical

logical end-end transport

Transport vs. Network Layernetwork layer: logical communication

between hostsPDU: DatagramDatagram’s may be lost, duplicated,

reordered in the Internet – “best effort” service

transport layer: logical communication between processes relies on, enhances, network layer servicesPDU: Segmentextends “host-to-host” communication to

“process-to-process” communication

Multiplexing/Demultiplexing

Use same communication channel between hosts for several logical communication processes

How does Mux/DeMux work?Sockets: doors between process & hostUDP socket: (dest. IP, dest. Port)TCP socket: (src. IP, src. port, dest. IP, dest. Port)

TransportLayer

NetworkLayer

TransportLayer

NetworkLayer

HTTP

FTP

Telnet

Connectionless demuxUDP socket identified by two-tuple:

(dest IP address, dest port number)

When host receives UDP segment:checks destination port number in segmentdirects UDP segment to socket with that port number

IP datagrams with different source IP addresses and/or source port numbers directed to same socket

Connection-oriented demuxTCP socket identified by

4-tuple: source IP addresssource port numberdest IP addressdest port number

recv host uses all four values to direct segment to appropriate socket

Server host may support many simultaneous TCP sockets:each socket identified by

its own 4-tupleWeb servers have

different sockets for each connecting clientnon-persistent HTTP will

have different socket for each request

UDP: User Datagram ProtocolWhy use UDP?

No connection establishment cost (critical for some applications, e.g., DNS)

No connection stateSmall segment headers (only 8 bytes)Finer application control over data

transmission

UDP Segment Structureoften used for streaming

multimedia apps loss tolerantrate sensitive

other UDP usesDNSSNMP

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

error recovery!

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,including

header

UDP checksum

Sender: treat segment contents as

sequence of 16-bit integers

checksum: addition (1’s complement sum) of segment contents

sender puts checksum value into UDP checksum field

Receiver:compute checksum of

received segmentcheck if computed

checksum equals checksum field value:NO - error detectedYES - no error detected.

Goal: detect “errors” (e.g., flipped bits) in transmitted segment

Internet Checksum ExampleNote: When adding numbers, a carryout

from the most significant bit needs to be added to the result

Example: add two 16-bit integers

Weak error protection? Why is it useful?

1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1

wraparound

sumchecksum

What is SCTP ?SCTP ( Stream Control Transmission Protocol ) - PSTN signaling messages over IP Networks

Similar to TCP and UDP - transport layer

- reliable / unreliable transmission - congestion control - ordered / unordered transmission

SCTP Feature SummaryStart with TCP:

reliable (retransmissions) congestion controlled

connection oriented

Add:4-way handshake

to reduce vulnerability to DOS attacksframing

preserve message boundaries multistreaming

instead of one ordered stream, up to 64K independent ordered streams

multihoming instead of one IP address per endpointa set of IP addresses per endpoint

SYN floodA SYN flood is a form of denial-of-service attack in

which an attacker sends a succession of SYN requests to a target's system.

When a client attempts to start a TCP connection to a server, the client and server exchange a series of messages which normally runs like this:

The client requests a connection by sending a SYN (synchronize) message to the server.

The server acknowledges this request by sending SYN-ACK back to the client.

The client responds with an ACK, and the connection is established.

SYN

victim Flooded!!

SYN Flooding Attack

TCB

TCB

TCB

TCB

TCB

• There is no ACK in response to the SYN-ACK, hence connection remains half-open• Other genuine clients cannot open connections to the victim• The victim is unable to provide service

attackers

128.3.4.5

192.10.2.8

221.3.5.10 SYN

SYN

190.13.4.1

228.3.14.5

130.2.4.15

Unavailable, reserved resources

Message BoundariesUDP honors message boundaries

Each app message becomes a datagram

TCP does not honor message boundariesApp messages become part of a byte stream

SCTP maintains message boundariesEach app message is maintained as one or more

data chunks

Chunks in SCTP

Source Port Destination Port

Verification Tag

Checksum

Chunk 1

Chunk N

Common Header

Building blocks of an SCTP PDUTwo kinds – control chunks and data chunksdata chunks are smallest atomic data units

Chunks

SCTP

PDU

SCTP Chunk FormatType Flags Length

Chunk Data

•Type – e.g. Data, Init, SACK

•Flags – bit meanings depend on type

•Length – includes type, flags, length, and data/parameters

Some Chunk Types0x00 DATA User data

0x01 INIT ~ SYN

0x02 INIT-ACK

0x03 SACK Selective ACK

0x04 HEARTBEAT Keep-alive message

0x05 HEARTBEAT-ACK

0x07 SHUTDOWN ~FIN

0x08 SHUTDOWN-ACK

Chunk Bundling in SCTP

Multiple chunks in one SCTP PDUControl chunks bundled before data chunksChunk boundary cannot cross SCTP PDU boundaryOptional at sender, but receiver has to support

Source Port Destination Port

Verification Tag

Checksum

Chunk 1

Chunk N

Common Header

Bundling

SCTP

PDU

CONGESTION CONTROL & QUALITY of SERVICE

What is congestion?Impact of congestion

Packet queues at links start to grow…

Packets start dropping

Sources start re-transmitting

After a while only re-transmissions occupy the network

Network resources start getting utilized in useless work (packets in queues that get timed out and re-transmitted)

“Goodput” goes to nearly zero

Max capacity

Congestion controls try to avoid getting into this situation

Congestion ControlWhat is congestion control?How is it done in example networks ?

Bus LANSwitched LANsInternet Telephone network

Congestion controlIs done in some form at all layers

Flow control b/w source and destn.Network layer congestion control is still

needed. (Why?)Can be done at various time-scales

Congestion control and QoSPre-QoS: Everything “best-effort”

E.g. TCP/IP networks, congestion control is left to TCP, i.e. TCP is a “well-behaved” source, that adapts to congestion

Post QoS-Integrated Services: Congestion control should be different for different sources Different for file-transfer/e-mail Different for real-time-sensitive apps, e.g. voice,

video Different based on what type of coding is used for

these apps

Quality of ServiceQuality parameters that define the

performance needs of a “flow” (i.e. a stream of packets belonging to a particular connection)Reliability – Probability of delivering

packets correctlyDelayJitter – Variation in Packet delayBandwidth

QoS Requirements

Jitter Control

(a) High jitter. (b) Low jitter.

BufferingSmoothing the output stream by buffering

packets.

General Principles of Congestion ControlMonitor the system .

detect when and where congestion occurs.

Pass information to where action can be taken.

Adjust system operation to correct the problem.

Congestion control time-scalesLong Term: Network Resource Provisioning

(sizing the network correctly)Connection duration

Connection (call) admission control: In connection oriented networks, decide whether to admit connection or not

Round Trip propagation time: Explicit forward congestion signaling

Packet Insertion Level: Traffic shaping, policing, selective discarding

Congestion Prevention PoliciesPolicies that affect congestion.

5-26

Routing around congestion

(a) A congested subnet. (b) A redrawn subnet, eliminates congestion and a virtual circuit from A to B.

Mechanisms for Congestion Control

Implicit Congestion SignalingTransmission delay may increase with congestionPacket may be discardedSource can detect these as implicit indications of

congestionUseful on connectionless (datagram) networks

e.g. IP based (TCP includes congestion and flow control)

Congestion SignalingNetwork alerts end systems of increasing

congestionEnd systems take steps to reduce offered loadBackwards

Congestion avoidance in opposite direction to packet required

Forwards Congestion avoidance in same direction as packet

required

Backward NotificationMark packets headed in the opposite

direction of the congestionTell source that packets transmitted on this

logical connection may encounter congestionSource can slow down

Forward notificationMarks packets going in the direction of

congestionTells the destination that these packets

experienced congestionDestination may alert source about

congestionAt network layerAt transport layer

Categories of Explicit SignalingBinary

A bit set in a packet indicates congestion

Credit based Indicates how many packets source may send Common for end to end flow control

Rate based Supply explicit data rate limit e.g. ATM

“Load Shedding”Drop packets when buffers are fullRouter can try to drop intelligently

Dropping older packets is better for multimedia streaming apps

Dropping newer packets is better for data apps (e.g. file transfer).

Receiver may discard out-of-order packets

Random Early DetectDrop packets before buffers are full, so

prevent congestion before it occursSources will react to packet drops and slow

down (e.g. TCP)

Sources may not be doing flow control (voice, video)Feedback is slow, (propagation time much greater

compared to transmission time)Diverse capacity requirements – may get penalized

unfairlyTraffic patterns very different Vastly different QoS requirementsVery high speed network wide fluctuations in reactive

controls

Issues to be addressed in broadband multi-service networks

Controlling SourcesConnection admission control

Based on some traffic descriptors, determine whether this connection can be admitted

Traffic Shaping Make sure the traffic has certain performance

attribute (shape) e.g. not burstyTraffic Policing

Make sure traffic sent by user is according to contract done during connection admission

And one truly last word…

ThAnKs