15-441 Computer NetworkingOutlineAdditive Increase/DecreaseMuliplicative Increase/DecreaseWhat is the Right Choice?TCP Congestion ControlImplementation IssueACK ClockingAIMDCongestion AvoidanceCongestion Avoidance Sequence PlotCongestion Avoidance BehaviorPacket ConservationSlide 17Slide 18Reaching Steady StateSlow Start Packet PacingSlow Start ExampleSlow Start Sequence PlotReturn to Slow StartTCP Saw Tooth BehaviorSlide 25TCP PerformanceSlide 27Single TCP Flow Router without buffersSummary Unbuffered LinkSlide 30Slide 31Single TCP Flow Router with large enough buffers for full link utilizationSummary Buffered LinkTCP (Summary)Slide 3515-441 Computer NetworkingLecture 18 – TCP Performance11-01-07Lecture 19: TCP Congestion Control 2Outline•TCP congestion avoidance•TCP slow start•TCP modeling11-01-07Lecture 19: TCP Congestion Control 3Additive Increase/DecreaseT0T1Efficiency LineFairness LineUser 1’s Allocation x1User 2’s Allocation x2•Both X1 and X2 increase/ decrease by the same amount over time•Additive increase improves fairness and additive decrease reduces fairness11-01-07Lecture 19: TCP Congestion Control 4Muliplicative Increase/Decrease•Both X1 and X2 increase by the same factor over time•Extension from origin – constant fairnessT0T1Efficiency LineFairness LineUser 1’s Allocation x1User 2’s Allocation x211-01-07Lecture 19: TCP Congestion Control 5What is the Right Choice?•Constraints limit us to AIMD•Improves or keeps fairness constant at each step•AIMD moves towards optimal pointx0x1x2Efficiency LineFairness LineUser 1’s Allocation x1User 2’s Allocation x211-01-07Lecture 19: TCP Congestion Control 6TCP Congestion Control•Changes to TCP motivated by ARPANET congestion collapse•Basic principles•AIMD•Packet conservation•Reaching steady state quickly•ACK clocking11-01-07Lecture 19: TCP Congestion Control 7Implementation Issue•Operating system timers are very coarse – how to pace packets out smoothly?•Implemented using a congestion window that limits how much data can be in the network.•TCP also keeps track of how much data is in transit•Data can only be sent when the amount of outstanding data is less than the congestion window.•The amount of outstanding data is increased on a “send” and decreased on “ack”•(last sent – last acked) < congestion window•Window limited by both congestion and buffering•Sender’s maximum window = Min (advertised window, cwnd)11-01-07Lecture 19: TCP Congestion Control 8ACK Clocking•Congestion window helps to “pace” the transmission of data packets•In steady state, a packet is sent when an ack is received•Data transmission remains smooth, once it is smooth•Self-clocking behaviorPrPbArAbReceiverSenderAs11-01-07Lecture 19: TCP Congestion Control 9AIMD•Distributed, fair and efficient•Packet loss is seen as sign of congestion and results in a multiplicative rate decrease •Factor of 2•TCP periodically probes for available bandwidth by increasing its rateTimeRate11-01-07Lecture 19: TCP Congestion Control 10Congestion Avoidance•If loss occurs when cwnd = W•Network can handle 0.5W ~ W segments•Set cwnd to 0.5W (multiplicative decrease)•Upon receiving ACK•Increase cwnd by (1 packet)/cwnd•What is 1 packet? 1 MSS worth of bytes•After cwnd packets have passed by approximately increase of 1 MSS•Implements AIMD11-01-07Lecture 19: TCP Congestion Control 11Congestion Avoidance Sequence PlotTimeSequence NoPacketsAcks11-01-07Lecture 19: TCP Congestion Control 12Congestion Avoidance BehaviorTimeCongestionWindowPacket loss+ retransmitGrabbingback BandwidthCutCongestionWindowand Rate11-01-07Lecture 19: TCP Congestion Control 13Packet Conservation•At equilibrium, inject packet into network only when one is removed•Sliding window and not rate controlled•But still need to avoid sending burst of packets would overflow links•Need to carefully pace out packets•Helps provide stability •Need to eliminate spurious retransmissions•Accurate RTO estimation•Better loss recovery techniques (e.g. fast retransmit)11-01-07Lecture 19: TCP Congestion Control 17Outline•TCP congestion avoidance•TCP slow start•TCP modeling11-01-07Lecture 19: TCP Congestion Control 18Congestion Avoidance BehaviorTimeCongestionWindowPacket loss+ retransmitGrabbingback BandwidthCutCongestionWindowand Rate11-01-07Lecture 19: TCP Congestion Control 19Reaching Steady State•Doing AIMD is fine in steady state but slow…•How does TCP know what is a good initial rate to start with?•Should work both for a CDPD (10s of Kbps or less) and for supercomputer links (10 Gbps and growing)•Quick initial phase to help get up to speed (slow start)11-01-07Lecture 19: TCP Congestion Control 20Slow Start Packet Pacing•How do we get this clocking behavior to start?•Initialize cwnd = 1•Upon receipt of every ack, cwnd = cwnd + 1•Implications•Window actually increases to W in RTT * log2(W)•Can overshoot window and cause packet loss11-01-07Lecture 19: TCP Congestion Control 21Slow Start Example1One RTTOne pkt time0R21R342R56783R910111213141512 34 5 6 711-01-07Lecture 19: TCP Congestion Control 22Slow Start Sequence PlotTimeSequence No...PacketsAcks11-01-07Lecture 19: TCP Congestion Control 23Return to Slow Start•If packet is lost we lose our self clocking as well•Need to implement slow-start and congestion avoidance together•When retransmission occurs set ssthresh to 0.5w•If cwnd < ssthresh, use slow start•Else use congestion avoidance11-01-07Lecture 19: TCP Congestion Control 24TCP Saw Tooth BehaviorTimeCongestionWindowInitialSlowstartFast Retransmitand RecoverySlowstartto pacepacketsTimeoutsmay stilloccur11-01-07Lecture 19: TCP Congestion Control 25Outline•TCP congestion avoidance•TCP slow start•TCP modeling11-01-07Lecture 19: TCP Congestion Control 26TCP Performance•Can TCP saturate a link?•Congestion control•Increase utilization until… link becomes congested•React by decreasing window by 50%•Window is proportional to rate * RTT•Doesn’t this mean that the network oscillates between 50 and 100% utilization?•Average utilization = 75%??•No…this is *not* right!11-01-07Lecture 19: TCP Congestion Control 27TCP Congestion ControlOnly W packets may be outstandingRule for adjusting W•If an ACK is received: W ← W+1/W•If a packet is lost: W ← W/2Source DestmaxW2maxWtWindow size11-01-07Lecture 19: TCP Congestion Control 28Single TCP
View Full Document
Unlocking...