计算机网络英文版课件：slide 07 The Network Layer – Part 3

TheNetworkLayer–Part3ContentsCongestionControlAlgorithms(Section5.3)GeneralPrinciplesofCongestionControlCongestionPreventionPoliciesCongestionControlinVirtual-CircuitSubnetsQualityofService(Section5.4)QoSRequirementsTechniquesforAchievingGoodQualityofServiceInternetQoSArchitectureIntegratedServicesDifferentiatedServicesLabelSwitchingandMPLSCongestionWhentoomuchtrafficisoffered,congestionsetsinandperformancedegradessharply.Ifallofasudden,streamsofpacketsbeginarrivingonthreeorfourinputlinesandallneedthesameoutputline,aqueuewillbuildup.GeneralPrinciplesofCongestionControlManyproblemscomplexsystems,suchascomputernetworks,canbeviewedfromacontroltheorypointofviewopenloop:attempttosolvetheproblembygooddesign;makedecisionswithoutregardtothecurrentstateofthenetworkclosedloop:basedontheconceptofafeedbackloop

Theclosedloopsolutions:Monitorthesystem:detectwhenandwherecongestionoccurs.Passinformationtowhereactioncanbetaken.Adjustsystemoperationtocorrecttheproblem.Avarietyofmetricscanbeusedtomonitorthesubnetforcongestion.thepercentageofallpacketsdiscarded;theaveragequeuelengths;thenumberofpacketsthattimeoutandareretransmitted;theaveragepacketdelay;thestandarddeviationofpacketdelayGeneralPrinciplesofCongestionControl(2)Ataxonomyforcongestioncontrolalgorithmsopenloopalgorithms:theopenloopalgorithmsthatactatthesource;theopenloopalgorithmsthatactatthedestination

closedloopalgorithms:explicitfeedbackalgorithms:packetsaresentbackfromthepointofcongestiontowarnthesourceimplicitfeedbackalgorithms:thesourcededucestheexistenceofcongestionbymakinglocalobservations,suchasthetimeneededforacknowledgementstocomebackCongestionPreventionPoliciesPoliciesthataffectcongestion.5-26CongestionControlinVirtual-CircuitSubnets(a)Acongestedsubnet.(b)Aredrawnsubnet,eliminatescongestionandavirtualcircuitfromAtoB.QoSRequirementsHowstringentthequality-of-servicerequirementsare.5-30TechniquesforAchievingGoodQualityofServiceOverprovisioningBuffering:Flowscanbebufferedonthereceivingsidebeforebeingdelivered.TrafficShaping:isaboutregulatingtheaveragerate(andburstiness)ofdatatransmissionMonitoringatrafficflowiscalledtrafficpolicingTheLeakyBucketAlgorithm,TokenBucketAlgorithmResourceReservation:Oncewehaveaspecificrouteforaflow,itbecomespossibletoreserveresourcesalongthatroutetomakesuretheneededcapacityisavailableThreedifferentkindsofresourcescanpotentiallybereserved:1)Bandwidth;2)Bufferspace;3)CPUcyclesTechniquesforAchievingGoodQualityofServiceAdmissionControl:whethertoadmitorrejectaflow?ProportionalRouting:tosplitthetrafficforeachdestinationovermultiplepaths.PacketSchedulingProcessingpacketsintheorderoftheirarrivalmeansthatanaggressivesendercancapturemostofthecapacityoftheroutersitspacketstraverse,reducingthequalityofserviceforothers.fairqueueingalgorithm:Theessenceofthealgorithmisthatroutershaveseparatequeuesforeachoutputline,oneforeachflow.Whenalinebecomesidle,therouterscansthequeuesroundrobin,takingthefirstpacketonthenextqueue.BufferingSmoothingtheoutputstreambybufferingpackets.TheLeakyBucketAlgorithm(a)Aleakybucketwithwater.(b)aleakybucketwithpackets.TheTokenBucketAlgorithm(a)Before.(b)After.5-34AdmissionControlAnexampleofflowspecification.5-34PacketScheduling(a)ArouterwithfivepacketsqueuedforlineO.(b)Finishingtimesforthefivepackets.InternetQoSArchitectureIntegratedService(IntServ)flow-basedalgorithmsRSVP(TheResourcereSerVationProtocol):usedformakingthereservationsDifferentiatedService(DiffDServ)class-basedalgorithmsexpeditedForwarding:Theexpeditedpacketsshouldbeabletotransitthesubnetasthoughnootherpacketswerepresentassuredforwarding:specifiesthatthereshallbefourpriorityclasses,eachclasshavingitsownresources.Inaddition,itdefinesthreediscardprobabilitiesforpacketsthatareundergoingcongestion:low,medium,andhigh.MPLS(MultiProtocolLabelSwitching)ExpeditedForwardingExpeditedpacketsexperienceatraffic-freenetwork.AssuredForwardingApossibleimplementationofthedataflowforassuredforwarding.LabelSwitchingandMPLSTransmittingaTCPsegmentusingIP,M