CPSC 826 Internetworking. Congestion Control Approaches Outline. Router-Based Congestion Control Approaches. Router-Based Approaches Papers

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1 1 CPSC 826 Internetworking Router-Based Congestion Control Approaches Michele Weigle Department of Computer Science Clemson University October 25, Congestion Control Approaches Outline End-to-End» TCP Loss-Based (Reno) details» TCP Error Recovery: NewReno, SACK (Floyd/Fall paper)» Delay-Based TCP Vegas Sync-TCP Router-Assisted» ECN» AQM RED PI REM DCN» 2 Router-Based Approaches Papers ECN - K.K. Ramakrishnan and Sally Floyd. A proposal to add explicit congestion notification (ECN) to IP. RFC 2481, January RED - Sally Floyd and Van Jacobson. Random early detection gateways for congestion avoidance. IEEE/ACM Transactions on Networking, 1(4): , August PI - C.V. Hollot, V. Misra, W.B. Gong, and D. Towsley. On Designing Improved Controllers for AQM Routers Supporting TCP Flows. INFOCOM REM - S. Athuralia, V.H. Li, S.H. Low and Q. Yin. REM: Active Queue Management, IEEE Network, Vol 15, No 3, May DCN - L. Le, J. Aikat, K. Jeffay, and F.D. Smith. Differential Congestion Notification: Taming the Elephants. ICNP D. Katabi, M. Handley, and C. Rohrs. Congestion Control for High Bandwidth-Delay Product Networks. SIGCOMM Explicit Congestion Notification ECN allows router to notify sender when congestion present in the network ECN at the router is mechanism, not policy Upon receiving ECN congestion experienced notification, TCP sender will react by reducing cwnd by 50%» sender then will set congestion window reduced flag in packet header 3 4

2 5 Explicit Congestion Notification (ECN) Active Queue Management Two bits in IP header» ECN-Capable Transport (ECT)» Congestion Experienced (CE) Two bits in TCP header» ECN-Echo flag» Congestion Window Reduced (CWR) flag Router Set a bit in a packet s header and forward towards the ultimate destination A receiver recognizes the marked packet and sets a corresponding bit in the next outgoing ACK When a sender receives an ACK with ECN it invokes a response similar to that for packet loss. How do routers know when to set ECN congestion experienced bit? Monitor the queue» RED» Adaptive/Gentle RED» PI» REM» DCN - doesn t need ECN Sender Receiver 6 RED Algorithm Adaptive/Gentle RED Average router Min Forced drop early mark No mark Average router 2* Min Forced drop gentle drop early mark No mark 100% max p min max Weighted Average Queue Length 7 100% max p min max 2*max Weighted Average Queue Length 8

3 9 Proportional Integral controller PI Random Exponential Marking controller REM Instantaneous Instantaneous Queue reference PI samples instantaneous at fixed intervals Mark/Drop probability:» p(kt) = a x (q(kt) q ref ) b x (q((k-1)t) - q ref ) + p((k-1)t) a, b, and T depend on link capacity, maximum RTT and the number of flows at a router Queue reference Mark/Drop probability depends on» mismatch between input rate and output rate» difference between the instantaneous and its target» p(t) = max(0, p(t 1) + γ x ( α x (q(t) q ref ) ) + x(t) c) )» prob(t) = 1 φ -p(t) REM is similar to PI (though differs in details) 10 Comparison across AQMs With packet drops at 90% load Comparison across AQMs With ECN at 90% load 11 12

4 13 Do AQM Schemes Work? Evaluation of ARED, PI, and REM Evaluation of ARED, PI, and REM Experimental Results 98% Load The Effects of Active Queue Management on Web Performance [SIGCOMM 2003]. When HTTP user response times are important performance metrics:» Without ECN, PI results in a modest performance improvement over drop-tail and other AQM schemes» With ECN, both PI and REM provide significant performance improvement over drop-tail Performance gain of PI over drop-tail Performance gain with ECN 14 Discussion 1. Why does ARED perform so poorly? Discussion 2. Why does ARED not benefit from ECN? Router queue length Router queue length ARED bases mark/drop probability on the (weighted) average PI, REM use instantaneous measures of ARED s reliance on the average limits its ability to react effectively in the face of bursty traffic Router 2 Min Mark/Drop Probability Forced drop gentle drop early mark/drop No mark/drop ARED drops marked packets when average queue size is above max th This is done to deal with potentially non-responsive flows We believe this policy is a premature optimization 15 16

5 17 Discussion 3. Why does ECN improve REM more than PI? Without ECN, REM drops more packets than PI REM causes more flows to experience multiple losses within a congestion window» Loss recovered through timeout rather than fast recovery REM Performance w/, w/o ECN at 90% Load In general ECN allows more flows to avoid timeouts» Thus ECN is ameliorating a design flaw in REM Future work: Differential congestion notification» Don t signal short flows that can t adapt Differential Congestion Notification DCN How to identify packets belonging to long-lived, high bandwidth flows with minimal state?» Adopt the Estan & Varghese flow filtering scheme developed for traffic accounting [SIGCOMM 2002] How to determine when to signal congestion (by dropping packets)?» Use a PI-like scheme [Infocom 2001] Differential treatment of flows an old idea:» FRED» SRED» CHOKe» SFB» AFD» RED-PD» RIO-PS» 18 DCN - Classifying Flows A score-boarding approach HB SB DCN Evaluation Experimental plan Scheduler Use two hash tables (hash keys are formed by IP addressing 4-tuple plus protocol):» A suspect flow table HB ( high-bandwidth ) and» A per-flow packet count table SB ( scoreboard ) Arriving packets from flows in HB are subject to dropping P 1 P 2 P 3 Low-bandwidth flows PI-like Controller Arriving packets from other flows are inserted into SB and tested to determine if the flow should be considered high-bandwidth» Use a simple packet count for this determination Flow classifier High-bandwidth flows uncongested drop-tail DCN AFD PI 80% 90% 98% 105% loss rate utilization response times completed requests RIO-PS Run experiments with DCN, AFD, RIO-PS, and PI at different offered loads» PI always uses ECN, test AFD and RIO-PS with and without ECN» DCN always signals congestion via drops Compare DCN results against» The better of PI, AFD, and RIO-PS (the performance to beat)» The uncongested network (the performance to approximate) 19 20

6 21 Experimental Results 90% Load Comparison of all schemes Experimental Results 98% Load Comparison of all schemes All schemes give comparable performance and significantly outperform drop-tail DCN outperforms PI/ECN and RIO-PS/ECN 22 explicit Congestion Protocol Something totally different What if the designers of the Internet knew then what we know now? Have routers tell senders how fast to send data Decouple congestion control from fairness control More than 1 bit of congestion information» get information about degree of congestion» similar idea to Sync-TCP, but asks routers directly Parameters are constant independent of environment» very important 23 24

7 25 Congestion Header H_cwnd» sender s current cwnd H_rtt» sender s rtt estimate H_feedback» modified by router -- adjustment to cwnd Router has efficiency controller and fairness controller» EC - maximize link utilization while minimizing drop rate and persistent queues» FC - apportion feedback to individual packets to achieve fairness There s still a delay» router assigns change in cwnd on data packet, feedback copied to ACK» sender doesn t adjust until ACK received 26