Lecture 5 Transport [Compatibility Mode]

8/14/2019 Lecture 5 Transport [Compatibility Mode]

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Advanced Computer Networks

Lecture 5

The Transport Layer, TransmissionControl Protocol (TCP), and User

Datagram Protocol (UDP)

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Outline

The Transport Layer

The TCP Protocol

TCP Characteristics TCP Connection setup

TCP Segments

TCP Sequence Numbers

TCP Slidin Window

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Timeouts and Retransmission

(Congestion Control and Avoidance)

The UDP Protocol


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The Transport Layer

What characteristics might it have? Reliable delivery

Flow control

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Review of the transport layerLeland.Stanford.edu Athena.MIT.edu

Application Layer

Nick Dave

Network La er

Transport Layer

O.S. O.S.HeaderData HeaderData

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Link Layer

HD

HD

HD

HD HD


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Layering: The OSI ModelApplication Application

layer-to-layer communication

Session

Network

Presentation

Transport

Network Network

Transport

Session

Presentation

Network

Peer-layer communication

Router Router

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4

5

6

3

4

5

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Link

PhysicalPhysicalPhysical

Link LinkLink

Physical1

2

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Layering: Our FTP Example

ApplicationFTP

Network

Transport

Presentation

SessionTransport

Network

ASCII/Binary

IP

TCP

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LinkLin

Physical

The 7-layer OSI Model The 4-layer Internet Model

Ethernet


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TCP Characteristics TCP is connection-oriented.

3-way handshake used for connection setup.

TCP provides a stream-of-bytesservice. TCP is reliable:

Acknowledgements indicate delivery of data. Checksums are used to detect corrupted data. Sequence numbers detect missing, or mis-sequenced data. Corrupted data is retransmitted after a timeout. Mis-se uenced data is re-se uenced.

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(Window-based) Flow control prevents over-run of receiver.

TCP uses congestion controlto share networkcapacity among users. Well study this in the nextlecture.

TCP is connection-oriented

(Active)Client

(Passive)Server

(Active)Client

(Passive)Server

Syn + Ack

Ack

(Data +) Ack

Fin

Ack

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Connection Setup3-way handshake

Connection Close/Teardown2 x 2-way handshake


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TCP supports a stream of

bytes serviceHost A

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Host B

which is emulated using TCPsegments

Host A

TCP Data Segment sent when:1. Segment full (MSS bytes),2. Not full, but times out, or3. Pushed by application.

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Host BTCP Data


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The TCP Segment Format

IP HdrIP Data

TCP HdrTCP Data

Src port Dst port

Sequence #

Ack Sequence #

HLEN4

RSVD6

URG

ACK

PSH

RST

SYN

FIN

FlagsWindow Size

0 15 31

TCP Headerand Data + IP

Addresses

Src/dst port numbersand IP addresses

uniquely identify socket

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(TCP Options)

TCP Data

Sequence NumbersHost A

ISN (initial sequence number)

TCP Data TCPHDR

Sequencenumber = 1st

byte Ack sequencenumber = nextexpected byte

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Host BTCP Data TCP

HDR


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Initial Sequence Numbers

(Active)Client

(Passive)Server

A

Syn + Ack +ISNB

Ack

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Connection Setup3-way handshake

TCP Sliding Window

How much data can a TCP sender haveoutstan ing in t e network?

How much data should TCP retransmitwhen an error occurs? Just selectivelyrepeat the missing data?

How does the TCP sender avoid over-

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running the receivers buffers?


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TCP Sliding WindowWindow Size

OutstandingUn-ackd data

Data OKto send

Data not OKto send yet

Data ACKd

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Window is meaningful to the sender. Current window size is advertised by receiver

(usually 4k 8k Bytes when connection set-up). TCPs Retransmission policy is Go Back N.

TCP Sliding Window

Round-trip time

Round-trip time

Host A

Host B

???

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(1) RTT > Window size (2) RTT = Window size


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TCP: Retransmission andTimeoutsRound-trip time (RTT) Retransmission TimeOut (RTO)

Host A

Host B

ACK ACK

Estimated RTT

Data1 Data2

GuardBand

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TCP uses an adaptive retransmission timeout value:Congestion

Changes in RoutingRTT changesfrequently

TCP: Retransmission and Timeouts

Picking the RTO is important: Pick a values thats too big and it will wait too long to

retransmit a acket Pick a value too small, and it will unnecessarily retransmit

packets.

The original algorithm for picking RTO:

1. EstimatedRTTk= EstimatedRTTk-1 + (1 - ) SampleRTT2. RTO = 2 * EstimatedRTT

Determined

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Characteristics of the original algorithm: Variance is assumed to be fixed. But in practice, variance increases as congestion increases.


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TCP: Retransmission and Timeouts There will be some (unknown) distribution

of RTTs. We are trying to estimate an RTO to

Router queues grow when there is moretraffic, until they become unstable.

As load grows, variance of delay growsminimize t e pro a i ity o a a se timeout.

Probability

AverageQueueingDelay

Variancegrows rapidly

with load

rap y.

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RTTmean

variance

Load(Amount of traffic

arriving to router)

TCP: Retransmission and Timeouts

Newer Algorithm includes estimate of variance in RTT:

Difference = SampleRTT - EstimatedRTT EstimatedRTTk = EstimatedRTTk-1 + (*Difference)

Deviation = Deviation + *( |Difference| - Deviation )

RTO = * EstimatedRTT + * Deviation

Same as

before

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1

4


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TCP: Retransmission and TimeoutsKarns Algorithm

Host A Host B Host A Host B

Retransmission

Wrong RTTSample

Retransmission

Wrong RTTSample

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Problem:How can we estimate RTT when packets are retransmitted?Solution:

On retransmission, dont update estimated RTT (and double RTO).

User Datagram Protocol (UDP)Characteristics

UDP is a connectionless datagram service.

any time.

UDP packets are self-contained. UDP is unreliable:

No acknowledgements to indicate delivery of data. Checksums cover the header, and only optionally cover the data. Contains no mechanism to detect missing or mis-sequenced

ackets.

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No mechanism for automatic retransmission. No mechanism for flow control, and so can over-run the

receiver.


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User-Datagram Protocol (UDP)A1 A2 B1 B2

App App App App

UDP

OS

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IP

Like TCP, UDP usesport number todemultiplex packets

SRC port DST port

User-Datagram Protocol (UDP)Packet format

checksum length

DATA

Wh do we have UDP?

,covers the header.

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It is used by applications that dont need reliable delivery, or Applications that have their own special needs, such asstreaming of real-time audio/video.

Documents

Lecture 5 Transport [Compatibility Mode]