A Baker's dozen of TCP
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Linux Plumbers Conference discussion of TCP and Bufferbloat
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A Baker's dozen of TCP
- 1. Congestion Control vs Bufferbloat Stephen Hemminger shemminger@vyatta.com 7 Sept 2011
- 2. Agenda What is TCP congestion control? What are some of the alternatives? Problems in congestion control Speculation
- 3. TCP congestion control is the result of years of research.
- 4. Acronymns RTT → Round Trip Time BDP → Bandwidth Delay Product AIMD → Additive Increase Multiplicative Decrease
- 5. Goals of Congestion control? Utilization Fairness Adaptability What about latency?
- 6. Evolution of TCP Congestion Control
- 7. TCP Congestion Control Types Loss based Delay based RENO Vegas BIC, CUBIC HTCP Hybrid Illinois Veno Yeah
- 8. Traditional TCP (Reno) http://www.deneholme.net/tom/
- 9. What's wrong with RENO? Rate Recovery Time @ 200ms RTT 1Mbps 1.7s 10Mbps 10.7s 100Mbps 2min 1Gbps 28min 10Gbp 4 hrs 43min http://www.deneholme.net/tom/
- 11. CUBIC
- 12. Title:cubic2.eps Creator:gnuplot 4.0 patchlevel 0 CreationDate:Sun Jul 29 00:11:16 2007 King of the hill problem
- 13. HTCP Use Epoch as estimate of BDP
- 14. Performance with small Queues Title:util.eps Creator:gnuplot 4.0 patchlevel 0 CreationDate:Tue Jul 31 13:59:03 2007
- 15. Vegas
- 16. Delay based Issues Sample size Delayed ACK Short lived flows RTT Data quality Packet trains Timer resolution Not aggressive Works great in “well behaved” network
- 17. General Issues Middleboxes: broken, cheating Most connections too short No persistence Sender chooses, receiver loses
- 18. CC Rap Sheet Reno → Too slow to recover BIC → Unfair at short RTT or low speed CUBIC → Slow to converge HTCP → Slower with small queues Vegas → Too wimpy and sensitive Hybrid → Reverts to delay based
- 19. Ideas Research Hybrid congestion control Refine delay based portion Detect mega queues Best CC discovery Testing More real world testing Googlish – random testing
Editor's Notes
- Created congestion infrastructure Implemented (broke) several algorithms from papers Trying to stay CC neutral
- How the Formula behaves Using the above formula with an MSS of 1460 bytes, we can plot the throughput as a function of loss and RTT as shown in the chart below for the range RTT from 0.25ms (typical LAN RTTs) to 650ms (typical geostationary satellite speed). If one takes the speed of light in fibre as roughly 0.6c or msec = alpha * 100km where empirically alpha ~0.4 accounts for non direct paths, router delays etc. then the distances corresponding to 10, 50, 100, 250, 500, 1000, 2500, 5000, 10000, 25000km are 0.25, 1.25, 2.5, 6.25, 12.5, 25, 62.5, 125, 250, 625 msec.