1 chapter 16 protocols and protocol layering. 2 protocol agreement about communication specifies ...
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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
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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