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Transport Layer

Computer Networks

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Where are we?

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Recall

Network LayerProvides host-to-host communicationSource and destination addresses identify

host interfacesMachine-to-machine networking

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Transport Protocols

Provide application-to-application communication

Need extended addressing mechanism to identify applications

Called end-to-endOptionally provide:

ReliabilityFlow ControlCongestion Control

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Example Transport Layer:Transmission Control Protocol (TCP)

Standardized by IETF as RFC 793Most popular layer 4 protocolConnection-oriented protocolConceptually between applications and IPFull-duplex operationByte-stream interfaceOf utmost importance for this class!

The book: TCP/IP Illustrated, Volume I - W.R. StevensAlso see: http://condor.depaul.edu/~jkristof/tcp.html

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TCP Feature Summary

Provides a completely reliable (no data duplication or loss), connection-

oriented, full-duplex stream transport service that allows two application programs to form a connection, send data in either

direction and then terminate the connection.

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Relationship Between TCP and Other Protocols

TCP on one computer uses IP to communicate with TCP on another computer

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Apparent Contradiction

IP offers best-effort (unreliable) delivery

TCP uses IPTCP provides completely reliable

transferHow is this possible?

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Achieving Reliability

Reliable connection setupReliable data transmissionReliable connection shutdown

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Reliable Data Transmission

Positive AcknowledgementReceiver returns short message when data

arrivesCall an acknowledgement

RetransmissionSender starts timer whenever message is

transmittedIf timer expires before acknowledgement

arrives, sender retransmits message

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Retransmission Illustrated

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How Long Should TCP Wait Before Retransmitting?

Time for acknowledgement to arrive depends onDistance to destinationCurrent traffic conditions

Multiple connections can be open simultaneously

Traffic conditions change rapidly

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Important Point

The delay required for data to reach a destination and an acknowledgement to return depends on traffic in the internet as well as the distance to

the destination. Since it allows multiple application programs to communicate with

multiple destinations concurrently, TCP must handle a variety of delays that can change

rapidly.

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Solving the Retransmission Problem

Keep estimate of round trip time on each connection

Use current estimate to set retransmission timer

Known as adaptive retransmissionKey to TCP’s success

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Adaptive Retransmission Illustrated

Timeout depends on current round-trip estimate

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TCP Flow Control

ReceiverAdvertises available buffer spaceCalled the window

SenderCan send up to entire window before ACK

arrives

Also called a sliding window protocol

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Window Advertisement

Each acknowledgement carries new window informationCalled window advertisementCan be zero (called closed window)

Interpretation: I have received up through X and can take Y more octets

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Window Advertisement Illustrated

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Another View: Sliding Window Illustrated

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Startup and Shutdown

Connection StartupMust be reliable

Connection ShutdownMust be graceful

Difficult

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Why Startup/Shutdown is Difficult

Segments can belostduplicateddelayeddelivered out of ordereither side can crasheither side can reboot

Need to avoid duplicate shutdown "message” from affecting later connection

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TCP’s Startup Solution

Use three-message exchangeKnown as the 3-way handshakeNecessary and sufficient for

unambiguous, reliable startupSYN messages used for connection

establishment

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3-Way Handshake Illustrated

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TCP’s Shutdown Illustrated

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Byte Stream Sequencing

Segments are labeled with a sequence number

Protects from out-of-order delivery32-bit numberLimited size of byte stream?Initial Sequence Numbers (ISNs) must be

exchanged at TCP connection establishment

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More Complete Illustration of the 3-Way Handshake

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Application Multiplexing

Cannot extend IP addressNo unused bits

Cannot use OS dependent quantityProcess IDTask numberJob name

Must work on all computer systems

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Application Multiplexing Illustrated

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Protocol Ports

Each application assigned a unique integer

ServerFollows standardAlways uses same port numberUsually uses lower port numbers

ClientObtains unused port from protocol softwareUsually uses higher port numbers

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Protocol Port Example

Web server application is assigned port 80Web client application obtains port 32938TCP segment sent from client to server has

source port number 32938destination port number 80

When web server responds, TCP segment hassource port number 80destination port number 32938

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Standard Protocol Ports

See http://www.iana.org for standard protocol port assignmentsSee /etc/services in UNIX systems and \winnt\system32\drivers\etc\services in Windows NT

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TCP Segment Format

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Example Transport Layer: User Datagram Protocol (UDP)

Unreliable message deliveryConnectionless protocolNo flow control (no window)No error recovery (no ACKs)Provides application multiplexingError detection optional (checksum

field)

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UDP Message Format