A Self-Configuring RED Gateway Wu-chang Feng, Dilip Kandlur, Debanjan Saha, Kang Shin INFOCOM ‘99

Outline Motivation Background (TCP, Drop-tail, RED queue management) RED, packet loss, and ECN Adaptive RED Conclusion

Motivation Exponential increase in network demand –Rise in packet loss rates 17% loss rates reported [Paxson97] –Decrease in link utilization and goodput –Two essential ingredients for congestion collapse [Jacobson88]

Congestion Control Today TCP –Instrumental in preventing congestion collapse –Limits transmission rate at the source –Window-based rate control Increased and decreased based on implicit signals from the network (acknowledgments and packet loss) Slow-start Fast-retransmit, Fast-recovery Congestion avoidance

Example of TCP Windowing RTT W W+1 2W Congestion avoidance Fast Retransmit/Recovery Slow-start

Random Early Detection Drop-tail queue management –Default queue management mechanism –Packets dropped upon queue overflow –Global synchrony –Poor link utilization –Excess packet loss due to late congestion notification –Potentially large queuing delay RED –Randomize congestion notification –Early detection of incipient congestion

RED Queue Management RED (Random Early Detection) [Floyd93] –Keep EWMA of queue length (Q ave ) –Increase in EWMA triggers random drops Basic algorithm P drop 0 1 max p min th max th Q ave

This Work Without ECN –RED has minimal effect on packet loss With ECN –RED has a hard time Avoiding packet loss Avoiding under-utilization Avoiding global synchrony Controlling queuing delay Problem –RED is not adaptive to congestion A solution –Adaptive RED

RED and Packet Loss Impact of RED on loss rates minimal Experiment –64 connections over full-duplex Ethernet links –Simulated in ns –Verified over smaller FreeBSD/ALTQ testbed 100 Mbs 10 Mbs 100 Mbs

RED and Packet Loss Loss rates are a first order function of TCP

TCP Revisited RTT W W+1 2W 1/p = W + (W+1) + … + 2W = 3W 2 /4 W/2 RTTs RTT*(W/2) RTT*sqrt(p) N BW = MSS*(3W 2 /4) = MSS*C = L p = N * MSS * C L * RTT 2

Comments on Model Reducing N - [Balakrishnan98] Increasing RTT - [Villamizar94] Decreasing MSS - [Feng98] Loss rates as a function of N between linear and quadratic –Fair share assumption (L/N) - [Morris97] –No retransmission timeouts - [Padhye98] p = N * MSS * C L * RTT 2

ECN Without ECN, packet loss rates will remain high IETF ECN WG (1998) RFC January 1999 (Experimental standard) –2-bits in “DS Field” of IPv4/IPv6 headers (ECT, CE) –2-bits in “TCP Flags” field of TCP (CWR, ECN Echo)

RED and Packet Loss Even with ECN, RED does not eliminate packet loss Problem –RED is not adaptive to congestion level –max p constant Congestion notification vs. number of connections –N = number of connections –Offered load reduced by [1 - (1/2N)] per notification

RED Experiments 100 Mbs 10 Mbs 45 Mbs RED queue (min th =20K, max th =80K, Q size =80K) 8 or 32 TCP sources using ECN Conservative vs. aggressive early detection Simulated in ns –Aggressive detection: max p = –Conservative detection: max p = 0.016

Aggressive Early Detection 8 sources32 sources

Conservative Early Detection 8 sources32 sources

Conservative Early Detection 32 sources, Q len = 120KB

Adaptive RED Adapt max p based only on queue behavior (not N) Increase max p when Q ave crosses above max th Decrease max p when Q ave crosses below min th Freeze max p after changes to prevent oscillations P drop 0 1 max p min th max th Q ave

Evaluation Workload varied between 8 and 32 sources 100 Mbs 10 Mbs 45 Mbs RED queue (min th =20K, max th =80K, Q size =120K) 1ms 5ms 100ms 5ms

Static Early Detection AggressiveConservative

Adaptive RED Queue lengthmax p

Implementation FreeBSD ALTQ Ascend, Cisco 100 Mbs 10 Mbs 200 MHz 32 MB 233 MHz 128 MB 200 MHz 64 MB 50 KB RED queue 166 MHz 32 MB

Adaptive RED Performance Loss ratesLink utilization

Conclusion Without ECN –RED does not impact packet loss rates With ECN –RED still has trouble adapting to workload Adaptive RED can improve performance Future work –Deployment issues –Tuning additional RED parameters –Eliminating loss with SubTCP –Influence on fairness extensions to RED –