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Blue: An Alternative Approach to Active Queue Management

Blue: An Alternative Approach to Active Queue Management. Wu-chang Feng June 23, 2001. Outline. Motivation Congestion control and queue management today (TCP, Drop-tail, RED) Blue Conclusion. Motivation. Exponential increase in network demand Rise in packet loss rates

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Blue: An Alternative Approach to Active Queue Management

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  1. Blue: An Alternative Approach to Active Queue Management Wu-chang Feng June 23, 2001

  2. Outline • Motivation • Congestion control and queue management today (TCP, Drop-tail, RED) • Blue • Conclusion

  3. Motivation • Exponential increase in network demand • Rise in packet loss rates • 17% loss rates reported [Paxson97] • Decrease in link utilization and goodput • Potential for congestion collapse [Jacobson88] • Goal • Queue management algorithms for maximizing network efficiency in times of heavy congestion • 0% packet loss, 100% link utilization, low queuing delay

  4. 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

  5. Drop-tail queue management • Default queue management mechanism • Packets dropped upon queue overflow • Problems • Global synchrony (poor utilization) • Late congestion notification (packet loss) • Solution • Randomize • Early detection of incipient congestion

  6. RED queue management • RED (Random Early Detection) [Floyd93] • Keep EWMA of queue length (Qave) using weight wq • Increase in EWMA triggers random drops • Basic algorithm 1 Pdrop maxp 0 maxth minth Qave

  7. Problems with RED • Hard to tune • Does not adapt well to traffic • [Feng99], [Anjum99], [Christiansen2000], [May99], [Ott99], …. • Why? • Wrong control variable being used • Queue length indicates congestion not severity

  8. Blue • Class of fundamentally different queue management algorithms • Decouple congestion management from queue length • Rely only on queue and link history • Example • Increase aggressiveness when loss rates high • Decrease aggressiveness when link underutilized

  9. Sending rate increases above L Mbs Sinks generate DupACKs/ECN DupACKs/ECN travel back Rate > L Mbs Rate > L Mbs Rate > L Mbs Rate > L Mbs Sources detect congestion Sources see sustained CN Rate < L Mbs Queue increases Queue increases, EWMA increases to trigger RED Queue increases some more Queue increases some more Queue overflows, maxth triggered Queue clears, but under-utilization imminent RED example L Mbs Sources Sinks A B

  10. Ideal example (Blue) L Mbs Sources Sinks A B Sinks generate DupACKs/ECN Rate = L Mbs Queue drops and/or ECN marks at steady rate Rate = Exactly what will keep sources at L Mbs

  11. Example Blue algorithm • Single dropping/marking probability • Increase upon packet loss • Decrease when link underutilized • Freeze value upon changing Upon packet loss: if ((now - last_update) > freeze_time) then Pmark = Pmark + d1 last_update = now Upon link idle: if ((now - last_update) > freeze_time) then Pmark = Pmark - d2 last_update = now

  12. Blue parameter selection • freeze_time: set to prevent multiple updates on a single round-trip (~10ms-100ms) • d1/d2: set to allow pm to range from 0 to 1 on the order of seconds • [Paxson97], [Balakrishnan98]

  13. Evaluation • 1000 and 4000 Pareto on/off TCP sources • Router queue sizes 100KB (17.8ms) to 1000KB (178ms) • RED (wq = 0.0002, 0.002, 0.02, 0.2) • Blue • freeze_time = 10ms, 100ms • d1 = 0.02, 0.0025 ; d2 = 0.002, 0.00025 100 Mbs 100 Mbs 45 Mbs 45 Mbs 10ms 10ms 1ms, 5ms, 20ms 1ms, 5ms, 20ms

  14. Blue evaluation • 1000 sources

  15. Blue evaluation • 4000 sources

  16. Understanding Blue • Experiment • 50 sources added every 10 seconds • Queue length plots Blue RED

  17. Understanding Blue • Marking behavior RED Blue

  18. Understanding Blue w/ECN timeouts • Lack of ECN timeouts allows packet loss with pm=1 • With ECN timeouts Blue similarly outperforms RED RED Blue

  19. Understanding Blue w/ECN timeouts • Marking behavior RED Blue

  20. Blue evaluation w/ECN timeouts • 1000 sources

  21. Blue evaluation w/ECN timeouts • 4000 sources

  22. Implementation • FreeBSD 2.2.7 + ALTQ IBM PC 365 (200 MHz/64 MB) Winbook XL (233 MHz/32 MB) 100 Mbs 10 Mbs 100 Mbs Intellistation Mpro (400 MHz/128 MB) Intellistation Zpro (200 MHz/64 MB) Thinkpad 770 (266 MHz/64 MB) IBM PC 360 (150 MHz/64 MB)

  23. Blue Evaluation Loss rates Link utilization

  24. Conclusion

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