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TCP擁塞控制機(jī)制

TCPCongestionControlChallengedeterminingtheavailablecapacityinthefirstplace(WithoutadditionalprotocolsorAPIs)adjustingtochangesintheavailablecapacity(Adjustmentsmustbemadequicklysincealargewindowmayalreadybeoutonthenetwork)ImplementationincreaseCongestionWindowwhencongestiongoesdown(slowly)decreaseCongestionWindowwhencongestiongoesup(quickly)Question:howdoesthesourcedeterminewhetherornotthenetworkiscongested?TCPCongestionControl:detailssenderlimitstransmission:LastByteSent-LastByteAckedCongWinRoughly,CongWinisdynamic,functionofperceivednetworkcongestionHowdoessenderperceivecongestion?lossevent=timeoutor3duplicateacksTCPsenderreducesrate(CongWin)afterlosseventthreemechanisms:AIMDslowstartconservativeaftertimeouteventsTCPcongestioncontrol:additiveincrease,multiplicativedecreaseApproach:

increasetransmissionrate(windowsize),probingforusablebandwidth,untillossoccursadditiveincrease:increaseCongWin

(congestionwindow)

by1MSS(MaximumSegmentSize

)everyRTTuntillossdetectedMSS=MTU-IP-TCPmultiplicativedecrease:cutCongWininhalfafterlossSawtoothbehavior:probingforbandwidthTCPSlowStartWhenconnectionbegins,CongWin=1MSSExample:MSS=500bytes&RTT=200msecinitialrate=20kbpsavailablebandwidthmaybe>>MSS/RTTdesirabletoquicklyrampuptorespectablerateWhenconnectionbegins,increaserateexponentiallyfastuntilfirstlosseventTCPSlowStart(more)Whenconnectionbegins,increaserateexponentiallyuntilfirstlossevent:doubleCongWineveryRTTdonebyincrementingCongWinforeveryACKreceivedSummary:initialrateisslowbutrampsupexponentiallyfastRefinement:inferringlossAfter3dupACKs:CongWiniscutinhalfwindowthengrowslinearlyButaftertimeoutevent:CongWininsteadsetto1MSS;windowthengrowsexponentiallytoathreshold,thengrowslinearly

3dupACKsindicates

networkcapableof

deliveringsomesegmentstimeoutindicatesa“morealarming”congestionscenarioPhilosophy:RefinementQ:Whenshouldtheexponentialincreaseswitchtolinear?A:WhenCongWingetsto1/2ofitsvaluebeforetimeout.Implementation:VariableThresholdAtlossevent,Thresholdissetto1/2ofCongWinjustbeforelosseventTCPCongestionControlWhenCongWinisbelowThreshold,senderinslow-startphase,windowgrowsexponentially.WhenCongWinisaboveThreshold,senderisincongestion-avoidancephase,windowgrowslinearly.WhenatripleduplicateACKoccurs,ThresholdsettoCongWin/2andCongWinsettoThreshold.Whentimeoutoccurs,ThresholdsettoCongWin/2andCongWinisse

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