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Chapter 16Chapter 16

Protocols and

Protocol Layering

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ProtocolProtocol

Agreement about communicationSpecifies

Format of messages (syntax)Meaning of messages (semantics)Rules for exchangeProcedure for handling problems

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Need for ProtocolsNeed for Protocols

Hardware is low levelMany problems can occur

Bits corrupted or destroyedEntire packet lostPacket duplicatedPackets delivered out of order

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Need for Protocols (continued)

Need for Protocols (continued)

Need mechanisms to distinguish amongMultiple computers on a networkMultiple applications on a computerMultiple copies of a single application on a

computer

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Set of ProtocolsSet of Protocols

Work togetherEach protocol solves part of communication

problemKnown as

Protocol suiteProtocol family

Designed in layers

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Plan for Protocol DesignPlan for Protocol Design

Intended for protocol designersDivides protocols into layersEach layer devoted to one subproblemExample: ISO 7-layer reference model

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Illustration of the 7-Layer Model

Illustration of the 7-Layer Model

Defined earlyNow somewhat datedDoes not include internet layer!

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ISO LayersISO Layers

Layer 1: PhysicalUnderlying hardware

Layer 2: Data Link (media access)Hardware frame definitions

Layer 3: NetworkPacket forwarding

Layer 4: TransportReliability

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ISO Layers (continued)ISO Layers (continued)

Layer 5: SessionLogin and passwords

Layer 6: PresentationData representation

Layer 7: ApplicationServices for common applications

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TCP/IP protocol suiteTCP/IP protocol suite

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Layers and Protocol Software

Layers and Protocol Software

Protocol software follows layering modelOne software module per layerModules cooperateIncoming or outgoing data passes from one

module to anotherEntire set of modules known as stack

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Illustration of StacksIllustration of Stacks

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Layers and Packet Headers

Layers and Packet Headers

Each layerPrepends header to outgoing packetRemoves header from incoming packet

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Example of encapsulationExample of encapsulation

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Scientific Layering Principle

Scientific Layering Principle

Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source

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Illustration of Layering Principle

Illustration of Layering Principle

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Protocol TechniquesProtocol Techniques

For bit corruptionParityChecksumCRC

For out-of-order deliverySequence numbers

DuplicationSequence numbers

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Protocol Techniques (continued)

Protocol Techniques (continued)

For lost packetsPositive acknowledgement and retransmission

For replay (excessive delay)Unique message ID

For data overrunFlow control

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Flow ControlFlow ControlNeeded because

Sending computer system faster than receiving computer

Sending application faster than receiving application

Related to bufferingTwo forms

Stop-and-goSliding window

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Stop-And-Go Flow ControlStop-And-Go Flow Control

Sending SideTransmits one packetWaits for signal from receiver

Receiving sideReceives and consumes packetsTransmits signal to sender

Inefficient

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Sliding Window Flow Control

Sliding Window Flow Control

Receiving sideEstablishes multiple buffers and informs sender

Sending sideTransmits packets for all available buffersOnly waits if no signal arrives before

transmissionReceiving side

Sends signals as packets arrive

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Illustration of SlidingWindow on Sending Side

Illustration of SlidingWindow on Sending Side

Window tells how many packets can be sentWindow moves as acknowledgements arrive

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PerformancePerformance

Stop-and-goSlowUseful only in special cases

Sliding windowFastNeeded in high-speed network

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Comparison of Flow Control

Comparison of Flow Control

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Why Sliding Window?Why Sliding Window?

SimultaneouslyIncrease throughputControl flow

Speedup

Tw = min(B, TG * W)where

B is underlying hardware bandwidthTW is sliding window throughputTG is stop-and-go throughputW is the window size

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CongestionCongestion

Fundamental problem in networksCaused by traffic, not hardware failureAnalogous to congestion on a highwayPrinciple cause of delay

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Illustration of ArchitectureThat Can Experience

Congestion

Illustration of ArchitectureThat Can Experience

Congestion

Multiple sourcesBottleneck

Dealing with CongestionDealing with Congestion

Congestion results in filled buffers in packet switches => packets will be discarded => retransmission => more packets…..

SolutionsIncreasing buffer spaceReducing the amount of packets in the network

Packet switches inform senders, or Senders use packet loss as an indication

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Congestion and LossCongestion and Loss

Modern network hardware works well; most packet loss results from congestion, not from hardware failure