Frame Relay Congestion ControlOverviewFrame Relay Congestion TechniquesDiscard ControlSlide 5Slide 6Slide 7Leaky Bucket AlgorithmFECNBECNBECN (Cont.)Implicit Congestion ControlSummaryRaj JainThe Ohio State University1Frame Relay Frame Relay Congestion ControlCongestion ControlRaj Jain Professor of Computer and Information SciencesThe Ohio State UniversityColumbus, OH [email protected] slides are available at http://www.cse.ohio-state.edu/~jain/cis777-00/Raj JainThe Ohio State University2OverviewOverviewCongestion avoidance vs recoveryDiscard controlExplicit forward/backward congestion notificationImplicit notificationRaj JainThe Ohio State University3Frame Relay Frame Relay Congestion TechniquesCongestion TechniquesDiscard Control (DE Bit)Backward Explicit Congestion NotificationForward Explicit Congestion NotificationImplicit congestion notification (sequence numbers in higher layer PDUs)Raj JainThe Ohio State University4Discard ControlDiscard ControlCommitted Information Rate (CIR)Committed Burst Size (Bc): Over measurement interval T T = Bc/CIRExcess Burst Size (Be)Between Bc and Bc + Be Mark DE bitOver Be DiscardRaj JainThe Ohio State University5BitsBc + BeBcT0T0 + TTimeDE = 1DE = 0Access rateCIRDiscardFrames 1 3All frames with CIR2Raj JainThe Ohio State University6BitsB + BBccT0T + T0TimeDE = 1DE = 0Access rateCIRDiscardeOne Frame marked DEFrames 1 32 4Raj JainThe Ohio State University7BitsB + BBccT0T + T0TimeDE = 1DE = 0Access rateCIRDiscardeOne Frame marked DE; one frame discardedFrames 1 32 4Raj JainThe Ohio State University8Leaky Bucket AlgorithmLeaky Bucket AlgorithmBeC = counter; increment with incoming dataDecrement C by min{C, Bc} every T Time unitsCIR = Bc /TLimit C to Bc + BeDiscard any incoming data while C is at its thresholdBcRaj JainThe Ohio State University9FECNFECNForward Explicit Congestion NotificationSource sets FECN = 0Networks set FECN if avg Q >1Dest tells source to inc/dec the rate (or window)Start with R = CIR (or W=1)If more than 50% bits set decrease to 0.875 × R (or 0.875W)If less than 50% bits set increase to 1.0625 × R (or min{W+1, Wmax})If idle for a long time, reset R = CIR (or W=1)S DFrame Relay SwitchFECNRaj JainThe Ohio State University10S DFrame Relay SwitchBECNBECNBECNBackward Explicit Congestion NotificationSet BECN bit in reverse traffic or send Consolidated Link-Layer Management (CLLM) message to sourceOn first BECN bit: Set R = CIROn further "S" BECNs: R=0.675 CIR, 0.5 CIR, 0.25 CIROn S/2 BECNs clear: Slowly increase R = 1.125 RIf idle for long, R = CIRRaj JainThe Ohio State University11 BECN (Cont.)BECN (Cont.)For window based control:S = One frame intervalStart with W=1First BECN W = max(0.625W,1)Next S BECNs W = max(0.625W,1)S/2 clear BECNs W = min(W+1, Wmax)CLLM used if no reverse trafficCLLM = XID message on maintenance DLCI = 1007 (decimal)CLLM contains a list of congested DLCIsRaj JainThe Ohio State University12Implicit Congestion Implicit Congestion ControlControlDecrease window on frame lossIncrease window slowlyDecrease by 1, Decrease to Wmin, Decrease by a factor Increase by 1 after N framesIncrease by 1 after W framesRaj JainThe Ohio State University13SummarySummaryDiscard strategy: Leaky bucketForward explicit congestion notificationBackward Explicit congestion notificationImplicit congestion
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