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1、 TCP拥塞控制拥塞控制机制机制TCP Congestion Controlp Challengen determining the available capacity in the first place(Without additional protocols or APIs)n adjusting to changes in the available capacity(Adjustments must be made quickly since a large window may already be out on the network)p Implementationn inc
2、rease CongestionWindow when congestion goes down(slowly)n decrease CongestionWindow when congestion goes up(quickly)p Question:how does the source determine whether or not the network is congested?TCP Congestion Control:detailsp sender limits transmission:LastByteSent-LastByteAcked CongWinp Roughly,
3、p CongWin is dynamic,function of perceived network congestionHow does sender perceive congestion?p loss event=timeout or 3 duplicate acksp TCP sender reduces rate(CongWin)after loss eventthree mechanisms:n AIMDn slow startn conservative after timeout eventsTCP congestion control:additive increase,mu
4、ltiplicative decreaserApproach:increase transmission rate(window size),probing for usable bandwidth,until loss occursmadditive increase:increase CongWin(congestion window)by 1 MSS(Maximum Segment Size)every RTT until loss detected MSS=MTU-IP-TCP mmultiplicative decrease:cut CongWin in half after los
5、s Saw toothbehavior:probingfor bandwidthTCP Slow Startp When connection begins,CongWin=1 MSSn Example:MSS=500 bytes&RTT=200 msecn initial rate=20 kbpsp available bandwidth may be MSS/RTTn desirable to quickly ramp up to respectable ratep When connection begins,increase rate exponentially fast until
6、first loss eventTCP Slow Start(more)p When connection begins,increase rate exponentially until first loss event:n double CongWin every RTTn done by incrementing CongWin for every ACK receivedp Summary:initial rate is slow but ramps up exponentially fastRefinement:inferring lossp After 3 dup ACKs:n C
7、ongWin is cut in halfn window then grows linearlyp But after timeout event:n CongWin instead set to 1 MSS;n window then grows exponentiallyn to a threshold,then grows linearlyq 3 dup ACKs indicates network capable of delivering some segmentsq timeout indicates a“more alarming”congestion scenarioPhil
8、osophy:RefinementQ:When should the exponential increase switch to linear?A:When CongWin gets to 1/2 of its value before timeout.Implementation:p Variable Threshold p At loss event,Threshold is set to 1/2 of CongWin just before loss eventTCP Congestion Controlp When CongWin is below Threshold,sender
9、in slow-start phase,window grows exponentially.p When CongWin is above Threshold,sender is in congestion-avoidance phase,window grows linearly.p When a triple duplicate ACK occurs,Threshold set to CongWin/2 and CongWin set to Threshold.p When timeout occurs,Threshold set to CongWin/2 and CongWin is set to 1 MSS.TCP sender congestion controlTCP Congestion Window TraceSummaryPrinciples of TCP congestion controlCongestion window adjustment
计算机网络英文版课件：3-12 TCP拥塞控制机制
