Re: [tmrg] [Tmrg] Is it possible that IW doesn't effect other traffic
Bob Briscoe <rbriscoe@jungle.bt.co.uk> Fri, 29 October 2010 19:44 UTC
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Date: Fri, 29 Oct 2010 20:12:25 +0100
To: Matt Mathis <mattmathis@google.com>
From: Bob Briscoe <rbriscoe@jungle.bt.co.uk>
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Subject: Re: [tmrg] [Tmrg] Is it possible that IW doesn't effect other traffic
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Matt, Going back >2months (my mulling-over time constant ;) At 22:55:58 -0400 22/08/2010 Matt Mathis wrote: >I suspect that our inability to detect the intuitively expected >result, that IW10 hurts IW3, may actually be a symptom of a deeper >theoretical result that it really doesn't matter, because the system >is memoryless. ? [snipped the rest] Surely the *queue* is memoryless but the *system* is not. In the classic design of TCP/IP, the congestion control's memory is all in the end-systems. So when a queue drains, the queue forgets everything. But TCP hosts hold on to the memory of what their congestion windows were before the queue drained as they gradually probe the capacity again. In fact, hosts remember their congestion windows and evolve them over long periods always depending on all the losses they saw previously (at least since the last idle period) to gauge what the available capacity probably is now. And over longer time periods they hold on to ssthresh too. For proof, if you look at the critical path analysis of a session transaction (e.g. [Barford00]), a loss early on in a flow can "ruin your whole day" (so to speak), having far greater effect on session completion time than a loss later in the session. If the *system* were memoryless, this just wouldn't be able to happen. True, if all the hits from one IW=10 spike impact just one flow (*if* it's a typical TCP), it will forget this after 1RTT and treat it like one loss. But, if a number of losses impact a number of flows, the memory of the difference between IW=10 & IW=3 will last longer. According to Appendix B of [Padlesny06], it takes about 7 increase-decrease cycles for TCP's AIMD to reach 98% fairness index (their own, not Jain's) after being upset from equality by a loss. Let's not forget too that competing flows might not be TCP. Adaptive media (e.g. over RTP/RTCP) can have a long memory. RTCP counts losses irrespective of whether they occur within 1RTT or not, and can use this count to adapt its rate in future. In addition, inelastic congestion controls tend to be conservative, being slow to increase after a decrease because user experience is generally better with a stable low rate than a fluctuating medium rate. A different sort of memory in the end-system: A VoIP flow can cover up loss of a small number of packets in a row by smudging the audio over the gap using its memory of the recent audio. But a spike of 10 large packets might cause an AQM queue to knock out more than 10 small (VoIP) packets. That would be harder to cover up, and continual IW=10 spikes could disrupt audio considerably. Nothing here is really evidence for or against IW=10, and I'm open to be convinced that IW=10 makes more sense than IW=3. I just don't want to see this superficially convincing memorylessness argument take hold if it's bogus. [Barford00] Barford, P. & Crovella, M., "Critical Path Analysis of TCP Transactions," Proc. ACM SIGCOMM'00, Computer Communication Review 30(4):127--138 (October 2000) <<http://doi.acm.org/10.1145/347057.347416>http://doi.acm.org/10.1145/347057.347416> [Podlesny06] Multimodal Congestion Control for Low Stable-State Queuing Podlesny, Maxim; Gorinsky, Sergey Washington Uni CS Dept Tech Report WUCSE-2006-41 <http://www.arl.wustl.edu/~gorinsky/pdf/WUCSE-TR-2006-41.pdf> Bob ________________________________________________________________ Bob Briscoe, BT Innovate & Design