1. ICN01 Colmar, France July 10, 20011 A Performance Study of Explicit Congestion Notification (ECN) with Heterogeneous TCP Flows Robert Kinicki and Zici Zheng Worcester Polytechnic Institute Computer Science Department Worcester, MA 01609 USA

2. ICN01 Colmar, France July 10, 20012 Outline Motivation for Studying ECNMotivation for Studying ECN Performance MetricsPerformance Metrics Random Early Detection (RED) and ECN RoutersRandom Early Detection (RED) and ECN Routers Simulation Topology and Experimental ProceduresSimulation Topology and Experimental Procedures Results and AnalysisResults and Analysis ConclusionsConclusions

3. ICN01 Colmar, France July 10, 20013 Motivation for Studying ECN Congestion is still an Internet problem.Congestion is still an Internet problem. Researchers advocate Active Queue Management (AQM) techniques such as RED and ECN for congestion control.Researchers advocate Active Queue Management (AQM) techniques such as RED and ECN for congestion control. RED has been shown to be difficult to tune.RED has been shown to be difficult to tune. RED can be unfair to heterogeneous flows.RED can be unfair to heterogeneous flows.

4. ICN01 Colmar, France July 10, 20014 Motivation for Studying ECN Researchers believe ECN is better after a few RED versus ECN comparison studies.Researchers believe ECN is better after a few RED versus ECN comparison studies. The differences between RED and ECN behavior is not well understood.The differences between RED and ECN behavior is not well understood. Is ECN also unfair to heterogeneous flows?Is ECN also unfair to heterogeneous flows? What happens when there are many flows?What happens when there are many flows? Can ECN be adapted to perform better?Can ECN be adapted to perform better?

5. ICN01 Colmar, France July 10, 20015 Performance Metrics throughput (Mbps) - the aggregate rate of packets generated by all sources.throughput (Mbps) - the aggregate rate of packets generated by all sources. goodput (Mbps) - the rate at which packets arrive at the receiver. Goodput differs from throughput in that retransmissions are excluded from goodput.goodput (Mbps) - the rate at which packets arrive at the receiver. Goodput differs from throughput in that retransmissions are excluded from goodput. delay (sec) - the time required to transmit a packet from source node to receiver node.delay (sec) - the time required to transmit a packet from source node to receiver node.

6. ICN01 Colmar, France July 10, 20016 Performance Metrics Jain’s fairnessJain’s fairness –For any given set of user throughputs (x1, x2,…xn ), the fairness index to the set is defined: f(x1, x2, …, xn) = f(x1, x2, …, xn) = max-min fairnessmax-min fairness –A flow rate x is max-min fair if any rate x cannot be increased without decreasing some y which is smaller than or equal to x. To satisfy the min-max fairness criteria, the smallest throughput rate must be as large as possible. “visual” max-min fairness“visual” max-min fairness –the visual gap between the smallest and the largest goodput

7. ICN01 Colmar, France July 10, 20017 RED Routers Random Early Detection (RED) detects congestion “early” by maintaining an exponentially-weighted average queue size.Random Early Detection (RED) detects congestion “early” by maintaining an exponentially-weighted average queue size. RED probabilistically drops packets before the queue overflows to signal congestion to TCP sources.RED probabilistically drops packets before the queue overflows to signal congestion to TCP sources. RED attempts to avoid global synchronization and bursty packet drops.RED attempts to avoid global synchronization and bursty packet drops.

8. ICN01 Colmar, France July 10, 20018 ECN Routers Explicit Congestion Notification (ECN) is a RED extension that marks packets to signal congestion.Explicit Congestion Notification (ECN) is a RED extension that marks packets to signal congestion. ECN must be supported by both TCP senders and receivers.ECN must be supported by both TCP senders and receivers. ECN-compliant TCP senders initiate their congestion avoidance algorithm after receiving marked ACK packets from the TCP receiver.ECN-compliant TCP senders initiate their congestion avoidance algorithm after receiving marked ACK packets from the TCP receiver. Packets from non-ECN flows are treated by the RED mechanism in the ECN router.Packets from non-ECN flows are treated by the RED mechanism in the ECN router.

9. ICN01 Colmar, France July 10, 20019 RED and ECN Router Parameters avg : average queue sizeavg : average queue size avg = (1-wq) * avg + wq * instantaneous queue size avg = (1-wq) * avg + wq * instantaneous queue size w q : weighting factor 0.001 <= wq <= 0.004w q : weighting factor 0.001 <= wq <= 0.004 min_th : average queue length threshold for triggering probabilistic drops/marks.min_th : average queue length threshold for triggering probabilistic drops/marks. max_th : average queue length threshold for triggering forced dropsmax_th : average queue length threshold for triggering forced drops max_p : maximum dropping/marking probabilitymax_p : maximum dropping/marking probability pb = max_p * (avg – min_th) / (max_th – min_th) pb = max_p * (avg – min_th) / (max_th – min_th) pa = pb / (1 – count * pb) pa = pb / (1 – count * pb) buffer_size: the size of the router queue in packetsbuffer_size: the size of the router queue in packets

10. ICN01 Colmar, France July 10, 200110 RED/ECN Router Mechanism 1 0 Average Queue Length Min-threshold Max-threshold Dropping/Marking Probability Queue Size max_p

11. ICN01 Colmar, France July 10, 200111 Simulation Topology and Experimental Procedures three sets of heterogeneous flowsthree sets of heterogeneous flows –Fragile flows, Robust flows, Average flows –flows delineated by distance from congested router two ECN variantstwo ECN variants –ECN: ECN with Drop after max_th –ECNM: ECN with Mark after max_th

12. ICN01 Colmar, France July 10, 200112 Simulation Topology 10mbps, 5ms 5ms5ms 45ms : Source : Sink Router F1F1 FmFm A1A1 AmAm R1R1 RmRm.................. 145ms 600 Mbps RTTs: (300ms, 100ms, 20ms)

13. ICN01 Colmar, France July 10, 200113 Experimental Procedures and Parameter Settings Experimental Procedures and Parameter Settings 100 second ns-2 simulations100 second ns-2 simulations n flows divided equally among three flow types (n = 3m)n flows divided equally among three flow types (n = 3m) input demand, i.e, aggregate flow capacity fixed at 600 Mbpsinput demand, i.e, aggregate flow capacity fixed at 600 Mbps staggered start of half the flows (0 sec, 2 sec)staggered start of half the flows (0 sec, 2 sec) fixed RED/ECN and TCP parameters for all runsfixed RED/ECN and TCP parameters for all runs –wq = 0.001 –min_th = 5 –buffer_size = 50 packets –TCP max_window_size = 30 packets

14. ICN01 Colmar, France July 10, 200114 Figure 2: RED and ECN Goodput min_th = 5, max_th = 30

15. ICN01 Colmar, France July 10, 200115 Figure 3: RED and ECN Delay min_th = 5, max_th =30, max_p =0.5

16. ICN01 Colmar, France July 10, 200116 Figure 4: Goodput with 30 flows min_th = 5

17. ICN01 Colmar, France July 10, 200117 Figure 5: Goodput with 120 flows min_th = 5

18. ICN01 Colmar, France July 10, 200118 Figure 6: RED and ECN Fairness min_th = 5, max_th = 30

19. ICN01 Colmar, France July 10, 200119 Figure 7: Goodput Distribution with 30 flows min_th = 5, max_th = 30, max_p = 0.2

20. ICN01 Colmar, France July 10, 200120 Figure 8: Goodput Distribution with 30 flows min_th = 5, max_th = 30, max_p = 0.8

21. ICN01 Colmar, France July 10, 200121 Figure 9: Goodput Distribution with 120 flows min_th = 5, max_th = 30, max_p = 0.8

22. ICN01 Colmar, France July 10, 200122 Figure 10: Throughput Distribution with 120 flows min_th = 5, max_th = 30, max_p = 0.8

23. ICN01 Colmar, France July 10, 200123 Figure 11: ECN and ECNM Goodput with 120 flows min_th = 5

24. ICN01 Colmar, France July 10, 200124 Conclusions ECN provides higher goodput than RED.ECN provides higher goodput than RED. Both RED and ECN are unfair to heterogeneous flows. ECN is fairer in some situations.Both RED and ECN are unfair to heterogeneous flows. ECN is fairer in some situations. ECN performs better with a more aggressive max_p setting. This is more pronounced when the number of flows generating the demand is high.ECN performs better with a more aggressive max_p setting. This is more pronounced when the number of flows generating the demand is high.

25. ICN01 Colmar, France July 10, 200125 Conclusions For fixed demand, as the number of flows increase the performance of both RED and ECN decrease.For fixed demand, as the number of flows increase the performance of both RED and ECN decrease. –This may be due to buffer contention at the router and flow lockout. When there are many flows, increasing max_th improves ECN goodput.When there are many flows, increasing max_th improves ECN goodput.

26. ICN01 Colmar, France July 10, 200126 Conclusions An adaptive version of ECN that varies max_p and max_th appears to be promising.An adaptive version of ECN that varies max_p and max_th appears to be promising. An adaptive ECN mechanism that varies max_p with flow type should significantly improve fairness.An adaptive ECN mechanism that varies max_p with flow type should significantly improve fairness.