Katz StoicaF04TCP Congestion Control SummaryMeasure available bandwidthSlow start fast hard on networkAIMD slow gentle on networkDetecting congestionTimeout based on RTT ID: 158117
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Katz, StoicaF04EECS 122:Introduction to Computer Networks TCPComputer Science DivisionDepartment of Electrical Engineering and Computer SciencesUniversity of California, BerkeleyBerkeley, CA 94720-1776Katz, StoicaF04The big picture (with timeouts)TimecwndTimeoutSlowStartAIMDssthreshTimeoutSlowStartSlowStartAIMDKatz, StoicaF04Congestion Detection RevisitedWait for Retransmission Time Out (RTO)RTO kills throughputIn BSD TCP implementations, RTO is usually more than 500ms-The granularity of RTT estimate is 500 ms-Retransmission timeout is RTT + 4 * mean_deviationSolution: Dont wait for RTO to expire Katz, StoicaF04Fast RetransmitsResend a segment after 3 duplicate ACKs-Duplicate ACK means that an out-of sequence segment was receivedNotes:-ACKs are for next expected packet -Packet reordering can cause duplicate ACKs-Window may be too small to get enough duplicate ACKs21cwnd = 1cwnd = 2234cwnd = 4456733 duplicateACKs444Katz, StoicaF04Fast Recovery: After a Fast Retransmitssthresh= cwnd/ 2cwnd= ssthresh -instead of setting cwndto 1, cut cwnd in half (multiplicative decrease)For each dup ack arrivaldupack++-MaxWindow = min(cwnd+ dupack, AdvWin) -indicates packet left network, so we may be able to send moreReceive ack for new data (beyond initial dup ack)dupack= 0-exit fast recoveryBut when RTO expires still do cwnd= 1Katz, StoicaF04Fast Retransmit and Fast RecoveryRetransmit after 3 duplicated acks-Prevent expensive timeoutsReduce slow startsAt steady state, cwndoscillates around the optimal window sizeTimecwndSlow StartAI/MDFast retransmit Katz, StoicaF04TCP Congestion Control SummaryMeasure available bandwidth-Slow start: fast, hard on network-AIMD: slow, gentle on networkDetecting congestion-Timeout based on RTTRobust, causes low throughput-Fast Retransmit: avoids timeouts when few packets lostCan be fooled, maintains high throughputRecovering from loss-Fast recovery: dont set cwnd=1 with fast retransmitsKatz, StoicaF04TCP Flavors TCP-Tahoe-cwnd =1 whenever drop is detectedTCP-Reno-cwnd =1 on timeout-cwnd = cwnd/2 on dupackTCP-newReno-TCP-Reno + improved fast recoveryTCP-Vegas, TCP-SACKKatz, StoicaF04TCP-SACKSACK = Selective AcknowledgementsACK packets identify exactly which packets have arrivedMakes recovery from multiple losses much easier10Katz, StoicaF04TCP VegasImproved timeout mechanismDecrease cwnd only for losses sent at current rate-avoids reducing rate twiceCongestion avoidance phase:-compare Actual rate (A) to Expected rate (E)-if E-A � , decrease cwnd linearly-if E-A , increase cwnd linearly-rate measurements ~ delay measurements-see textbook for details!11Katz, StoicaF04Standards?How can all these algorithms coexist?Dont we need a single, uniform standard?What happens if Im using Reno and you are using Tahoe, and we try to communicate?12Katz, StoicaF04Equation-Based CCSimple scenario-Assume a drop every kth RTT (for some large k)-w, w+1, w+2, ...w+k-1 DROP (w+k-1)/2, (w+k-1)/2+1,... Observations:-In steady state: w= (w+k-1)/2 so w=k-1-Average window: 1.5(k-1)-Total packets between drops: 1.5k(k-1)-Drop probability: p = 1/[1.5k(k-1)]Throughput: T ~ (1/RTT)*sqrt(3/2p) 13Katz, StoicaF04Equation-Based CCIdea:-Forget complicated increase/decrease algorithms-Use this equation T(p) directly!Approach:-Measure drop rate (dont need ACKs for this)-Send drop rate p to source-Source sends at rate T(p)Good for streaming audio/video that cant tolerate the high variability of TCPs sending rate14Katz, StoicaF04Question!Why use the TCP equation?Why not use any equation for T(p)?15Katz, StoicaF04CheatingThree main ways to cheat:-Increasing cwnd faster than 1 per RTT-Using large initial cwnd-Opening many connections16Katz, StoicaF04Increasing cwnd Faster Cxy&#x/Pre; ict;&#xor 1; /C;&#xolum;&#xns 9;å&#x/Pre; ict;&#xor 1; /C;&#xolum;&#xns 9;å17Katz, StoicaF04Increasing cwnd Faster10203040506028558210913616319021724427129832535237940643346048718Katz, StoicaF04Larger Initial cwnd 19Katz, StoicaF04Open Many ConnectionsAssume A starts 10 connections to BD starts 1 connection to EEach connection gets about the same throughputThen A gets 10 times more throughput than D20Katz, StoicaF04Cheating and Game Theory15, 1535, 1010, 3522, 22 \n\r \n\rD Increases by 1 Increases by 5Individual incentives: cheating paysSocial incentives: better off without cheatingClassic Prisoner Dilemma: resolution depends on accountability !!21Katz, StoicaF04Lossy LinksTCP assumes that all losses are due to congestionWhat happens when the link is lossy?Recall that Tput ~ 1/sqrt(p) where p is loss prob.This applies even for non-congestion losses22Katz, StoicaF04Example1020304050601265176101126151176201226251276301326351376401426451476