Leccongestion.ppt

Leccongestion.ppt

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Spring2002,CS461,1,CongestionControl,OutlineQueuingDisciplineReactingtoCongestionAvoidingCongestion,Spring2002,CS461,2,Issues,Twosidesofthesamecoinpre-allocateresourcessoattoavoidcongestioncontrolcongestionif（andwhen）isoccursTwopointsofimplementationhostsattheedgesofthenetwork（transportprotocol）routersinsidethenetwork（queuingdiscipline）Underlyingservicemodelbest-effort（assumefornow）multiplequalitiesofservice（later）,Spring2002,CS461,3,Framework,Connectionlessflowssequenceofpacketssentbetweensource/destinationpairmaintainsoftstateattheroutersTaxonomyrouter-centricversushost-centricreservation-basedversusfeedback-basedwindow-basedversusrate-based,Spring2002,CS461,4,Evaluation,FairnessPower（ratioofthroughputtodelay）,Spring2002,CS461,5,QueuingDiscipline,First-In-First-Out（FIFO）doesnotdiscriminatebetweentrafficsourcesFairQueuing（FQ）explicitlysegregatestrafficbasedonflowsensuresnoflowcapturesmorethanitsshareofcapacityvariation:

weightedfairqueuing（WFQ）Problem?

Spring2002,CS461,6,FQAlgorithm,SupposeclocktickseachtimeabitistransmittedLetPidenotethelengthofpacketiLetSidenotethetimewhenstarttotransmitpacketiLetFidenotethetimewhenfinishtransmittingpacketiFi=Si+PiWhendoesrouterstarttransmittingpacketi?

ifbeforerouterfinishedpacketi-1fromthisflow,thenimmediatelyafterlastbitofi-1（Fi-1）ifnocurrentpacketsforthisflow,thenstarttransmittingwhenarrives（callthisAi）Thus:

Fi=MAX（Fi-1,Ai）+Pi,Spring2002,CS461,7,FQAlgorithm（cont）,FormultipleflowscalculateFiforeachpacketthatarrivesoneachflowtreatallFisastimestampsnextpackettotransmitisonewithlowesttimestampNotperfect:

cantpreemptcurrentpacketExample,Spring2002,CS461,8,TCPCongestionControl,Ideaassumesbest-effortnetwork（FIFOorFQrouters）eachsourcedeterminesnetworkcapacityforitselfusesimplicitfeedbackACKspacetransmission（self-clocking）Challengedeterminingtheavailablecapacityinthefirstplaceadjustingtochangesintheavailablecapacity,Spring2002,CS461,9,AdditiveIncrease/MultiplicativeDecrease,Objective:

adjusttochangesintheavailablecapacityNewstatevariableperconnection:

CongestionWindowlimitshowmuchdatasourcehasintransitMaxWin=MIN（CongestionWindow,AdvertisedWindow）EffWin=MaxWin-（LastByteSent-LastByteAcked）Idea:

increaseCongestionWindowwhencongestiongoesdowndecreaseCongestionWindowwhencongestiongoesup,Spring2002,CS461,10,AIMD（cont）,Question:

howdoesthesourcedeterminewhetherornotthenetworkiscongested?

Answer:

atimeoutoccurstimeoutsignalsthatapacketwaslostpacketsareseldomlostduetotransmissionerrorlostpacketimpliescongestion,Spring2002,CS461,11,AIMD（cont）,Inpractice:

incrementalittleforeachACKIncrement=（MSS*MSS）/CongestionWindowCongestionWindow+=Increment,AlgorithmincrementCongestionWindowbyonepacketperRTT（linearincrease）divideCongestionWindowbytwowheneveratimeoutoccurs（multiplicativedecrease）,Spring2002,CS461,12,AIMD（cont）,Trace:

sawtoothbehavior,Spring2002,CS461,13,SlowStart,Objective:

determinetheavailablecapacityinthefirstIdea:

beginwithCongestionWindow=1packetdoubleCongestionWindoweachRTT（incrementby1packetforeachACK）,Source,Destination,Spring2002,CS461,14,SlowStart（cont）,Exponentialgrowth,butslowerthanallatonceUsedwhenfirststartingconnectionwhenconnectiongoesdeadwaitingfortimeoutTraceProblem:

loseuptohalfaCongestionWindowsworthofdata,Spring2002,CS461,15,FastRetransmitandFastRecovery,Problem:

coarse-grainTCPtimeoutsleadtoidleperiodsFastretransmit:

useduplicateACKstotriggerretransmission,Spring2002,CS461,16,Results,FastrecoveryskiptheslowstartphasegodirectlytohalfthelastsuccessfulCongestionWindow（ssthresh）,Spring2002,CS461,17,CongestionAvoidance,TCPsstrategycontrolcongestiononceithappensrepeatedlyincreaseloadinanefforttofindthepointatwhichcongestionoccurs,andthenbackoffAlternativestrategypredictwhencongestionisabouttohappenreduceratebeforepacketsstartbeingdiscardedcallthiscongestionavoidance,insteadofcongestioncontrolTwopossibilitiesrouter-centric:

DECbitandREDGatewayshost-centric:

TCPVegas,Spring2002,CS461,18,DECbit,AddbinarycongestionbittoeachpacketheaderRoutermonitorsaveragequeuelengthoverlastbusy+idlecyclesetcongestionbitifaveragequeuelength1attemptstobalancethroughoutagainstdelay,Spring2002,CS461,19,EndHosts,DestinationechoesbitbacktosourceSourcerecordshowmanypacketsresultedinsetbitIflessthan50%oflastwindowsworthhadbitsetincreaseCongestionWindowby1packetIf50%ormoreoflastwindowsworthhadbitsetdecreaseCongestionWindowby0.875times,Spring2002,CS461,20,RandomEarlyDetection（RED）,Notificationisimplicitjustdropthepacket（TCPwilltimeout）couldmakeexplicitbymarkingthepacketEarlyrandomdroprather