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Internet Engineering

University of ilam

Dr. Mozafar Bag-Mohammadi

Transport Layer

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Course Materials

Course Web page– visit regularly

Textbook– مهندسی اینترنت، احسان ملکیان، انتشارات نص– “Computer Networks, A system approach”. Peterson & Davie

3rd edition Grading

– Homework 10%– Project 15%– Midterm 30%– Final 50%

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Prerequisition

General knowledge in Computer. Fluency in English, specially reading. C++ programming. Knowledge of UNIX (LINUX) system and

programming.

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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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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

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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 Protocol has evolved over time and will continue to do

so.– Nearly impossible to change the header– Uses options to add information to the header– Change processing at endpoints– Backward compatibility is what makes it TCP

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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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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)– Timer– Pushed by the application. Like telnet.

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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, both side must tear down the connection.

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

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State Transition Diagram

CLOSED

LISTEN

SYN_RCVD SYN_SENT

ESTABLISHED

CLOSE_WAIT

LAST_ACKCLOSING

TIME_WAIT

FIN_WAIT_2

FIN_WAIT_1

Passive open Close

Send/ SYNSYN/SYN + ACK

SYN + ACK/ACK

SYN/SYN + ACK

ACK

Close /FIN

FIN/ACKClose /FIN

FIN/ACKACK + FIN/ACK

Timeout after two segment lifetimes

FIN/ACK

ACK

ACK

ACK

Close /FIN

Close

CLOSED

Active open /SYN

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State Transition (cont)

Sliding window is in ESTABLISHED. All connections start with CLOSED. Arcs are tagged with event/action. Triggering a transition.

– Arriving a segment from a peer.– Envoking by an application.

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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

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

MaxSenderBuffer

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

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

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Protection Against Wrap Around

32-bit SequenceNum

Bandwidth Time Until Wrap AroundT1 (1.5 Mbps) 6.4 hoursEthernet (10 Mbps) 57 minutesT3 (45 Mbps) 13 minutesFDDI (100 Mbps) 6 minutesSTS-3 (155 Mbps) 4 minutesSTS-12 (622 Mbps) 55 secondsSTS-24 (1.2 Gbps) 28 seconds

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Keeping the Pipe Full

16-bit AdvertisedWindow

Bandwidth Delay x Bandwidth Product

T1 (1.5 Mbps) 18KB

Ethernet (10 Mbps) 122KB

T3 (45 Mbps) 549KB

FDDI (100 Mbps) 1.2MB

STS-3 (155 Mbps) 1.8MB

STS-12 (622 Mbps) 7.4MB

STS-24 (1.2 Gbps) 14.8MB

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TCP Extensions

Implemented as header options Store timestamp in outgoing segments Extend sequence space with 32-bit

timestamp (PAWS) Shift (scale) advertised window

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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)