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Introduction to Computer Networks

University of ilam

Dr. Mozafar Bag-Mohammadi

Transport Layer

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Outline

Connection Establishment/Termination

Sliding Window Revisited Flow Control Adaptive Timeout

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End-to-End Protocols Underlying best-effort network

drop messages re-orders messages delivers duplicate copies of a given message limits messages to some finite size delivers messages after an arbitrarily long delay

Common end-to-end services guarantee message delivery deliver messages in the same order they are sent deliver at most one copy of each message support arbitrarily large messages support synchronization allow the receiver to flow control the sender support multiple application processes on each host

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Transport Layer Function(s)

Multiplexing/demultiplexing between network application processes.

Others? Error Detection within a segment Reliability Flow Control. Congestion control. Connection Management.

Difference between Error detection and reliability? Difference between flow control and congestion control?

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Data Link Versus Transport

Potentially connects many different hosts (logical connection) need explicit connection establishment and termination

Potentially different RTT need adaptive timeout mechanism

Potentially reordering packets. (How far?), Maximum segment life time. Currently 120 sec.

Potentially long delay in network need to be prepared for arrival of very old packets Delay X bandwidth? Buffer size

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Data Link Versus Transport (cont) Potentially different capacity at destination

need to accommodate different node capacity Potentially different network capacity

need to be prepared for network congestion

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Simple Demultiplexor (UDP) Unreliable and unordered datagram service Adds multiplexing No flow control Endpoints identified by ports

servers have well-known ports see /etc/services on Unix

Header format

Optional checksum pseudo header + UDP header + data

SrcPort DstPort

Checksum Length

Data

0 16 31

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Using UDP

Non-standard protocols can be implemented on top of UDP.» Non-standard = non-TCP in practice

» use the port addressing provided by UDP

» implement their own reliability, flow control, ordering, congestion control

Examples:» remote procedure calls

» multimedia

» distributed computing communication libraries

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TCP Overview

Connection-oriented Byte-stream

app writes bytes TCP sends segments app reads bytes

Application process

Writebytes

TCPSend buffer

Segment Segment Segment

Transmit segments

Application process

Readbytes

TCPReceive buffer

…

… …

Full duplex Flow control: keep sender from

overrunning receiver Congestion control: keep sender

from overrunning network

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High level TCP Characteristics

Connection-oriented reliable byte-stream protocol.» Used for file transfers, telnet, web access, ….

Two way connections.» control information for one direction piggy-backed on

data flow in other direction

» header fields fall in three classes: general, forward flow, opposite flow

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Data delivery in TCP

TCP is byte oriented, however transmit data in segments (messages).

How TCP knows the time for delivery? Maximum Segment Size (MSS) Pushed by the application. Like telnet. Timer

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Segment Format

Options (variable)

Data

Checksum

SrcPort DstPort

HdrLen 0 Flags

UrgPtr

AdvertisedWindow

SequenceNum

Acknowledgment

0 4 10 16 31

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Segment Format (cont) Each connection identified with 4-tuple:

(SrcPort, SrcIPAddr, DsrPort, DstIPAddr) Sliding window + flow control

acknowledgment, SequenceNum, AdvertisedWinow

Checksum pseudo header + TCP header + data

Sender

Data (SequenceNum)

Acknowledgment +AdvertisedWindow

Receiver

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Segment Format (cont) Flags

SYN, FIN, RESET, PUSH, URG, ACK SYN and FIN for establish and tear down connection.

ACK indicates the Ack field is valid. URG shows some part of data is urgent.(Up to UrgPtr).

PUSH shows the sender had push operation.

RESET shows confusion in receiver.

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Connection Establishment and Termination

Active participant(client)

Passive participant(server)

SYN, SequenceNum = x

SYN + ACK, SequenceNum = y,

ACK, Acknowledgment = y + 1

Acknowledgment = x + 1

Client does active connection.Server must do passive connection first.

Closing is symmetric, bothside must tear down theconnection.

Three way handshaking.Connection parameter are exchanged first.Acknowledgment shows the next expected sequence number.

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Sliding Window Revisited

Sending side LastByteAcked < = LastByteSent

LastByteSent < = LastByteWritten

buffer bytes between LastByteAcked and LastByteWritten

Sending application

LastByteWrittenTCP

LastByteSentLastByteAcked

Receiving application

LastByteReadTCP

LastByteRcvdNextByteExpected

Receiving side LastByteRead < NextByteExpected

NextByteExpected < = LastByteRcvd +1

buffer bytes between LastByteRead and LastByteRcvd

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Flow Control Send buffer size: MaxSendBuffer Receive buffer size: MaxRcvBuffer Receiving side

LastByteRcvd - LastByteRead < = MaxRcvBuffer AdvertisedWindow = MaxRcvBuffer - (NextByteExpected -

LastByteRead) Sending side

LastByteSent - LastByteAcked < = AdvertisedWindow EffectiveWindow = AdvertisedWindow - (LastByteSent -

LastByteAcked) LastByteWritten - LastByteAcked < = MaxSendBuffer block sender if (LastByteWritten - LastByteAcked) + y >

MaxSenderBuffer Always send ACK in response to arriving data segment Persist when AdvertisedWindow = 0

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Adaptive Retransmission(Original Algorithm)

Measure SampleRTT for each segment/ ACK pair Compute weighted average of RTT

EstRTT = x EstRTT + x SampleRTT where + = 1 between 0.8 and 0.9 between 0.1 and 0.2

Set timeout based on EstRTT TimeOut = 2 x EstRTT

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Karn/Partridge Algorithm

Do not sample RTT when retransmitting Double timeout after each retransmission

Sender Receiver

Original transmission

ACK

Sam

pleR

TT

Retransmission

Sender Receiver

Original transmission

ACK

Sam

pleR

TT

Retransmission

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Jacobson/ Karels Algorithm

New Calculations for average RTT Diff = SampleRTT - EstRTT EstRTT = EstRTT + ( x Diff) Dev = Dev + ( |Diff| - Dev)

where is a factor between 0 and 1 Consider variance when setting timeout value TimeOut = x EstRTT + x Dev

where = 1 and = 4 Notes

algorithm only as good as granularity of clock (500ms on Unix) accurate timeout mechanism important to congestion control

(later)