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

1

2

4

RTTRTT RTT

WW+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 (Qave)

– Increase in EWMA triggers random drops

• Basic algorithm

Pdrop

0

1

maxp

minth maxth

Qave

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

1

2

4

RTTRTT

WW+1

2W

1/p = W + (W+1) + … + 2W = 3W2/4

W/2 RTTs

RTT*(W/2) RTT*sqrt(p) NBW = MSS*(3W2/4) = 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 2481 - 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

– maxp 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 45 Mbs

RED queue (minth=20K, maxth=80K, Qsize=80K)

• 8 or 32 TCP sources using ECN• Conservative vs. aggressive early detection• Simulated in ns

– Aggressive detection: maxp = 0.250

– Conservative detection: maxp = 0.016

Aggressive Early Detection

8 sources 32 sources

Conservative Early Detection

8 sources 32 sources

Conservative Early Detection

32 sources, Qlen = 120KB

Adaptive RED

• Adapt maxp based only on queue behavior (not N)

• Increase maxp when Qave crosses above maxth

• Decrease maxp when Qave crosses below minth

• Freeze maxp after changes to prevent oscillations

Pdrop

0

1

maxp

minth maxth

Qave

Evaluation

• Workload varied between 8 and 32 sources

100 Mbs

10 Mbs 45 Mbs 45 Mbs

RED queue (minth=20K, maxth=80K, Qsize=120K)

1ms

5ms

100ms

5ms 5ms 5ms

Static Early Detection

Aggressive Conservative

Adaptive RED

Queue length maxp

Implementation

• FreeBSD 2.2.6 + ALTQ• Ascend, Cisco

100 Mbs

100 Mbs

10 Mbs

200 MHz32 MB

233 MHz128 MB

200 MHz64 MB50 KB RED queue

166 MHz32 MB

Adaptive RED Performance

Loss rates Link 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

– http://www.eecs.umich.edu/~wuchang/ared/