[tsvwg] explanation of option 3 choice
Tom Henderson <tomh@tomh.org> Sun, 17 May 2020 17:06 UTC
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From: Tom Henderson <tomh@tomh.org>
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Date: Sun, 17 May 2020 10:06:46 -0700
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Subject: [tsvwg] explanation of option 3 choice
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I support the L4S goals to deploy scalable congestion controls and achieve low latency while maintaining high throughput when possible. I believe that a clear input signal to the network will be needed, so I lean towards ECT(1) because it is the most likely to survive tunnel encapsulation and avoid bleaching. However, the discussion has raised several performance and architectural concerns that I believe that the WG should address, so I believe that option 3 best aligns with my current view. Selecting between L4S and SCE involves tradeoffs, but the decision seems to be clouded by skepticism about the viability of deploying a low-latency service even if we had enough bits to support two clear input signals and two output signals, and space to carry these different signals back to the sender. How robust will the congestion controls be with a shallow threshold in the presence of jitter or burstiness, which is likely for multiple access wireless networks? How low can the marking threshold be configured, and what are the tradeoffs (and how does the performance degrade when assumptions are violated)? For a given latency threshold, what limits on RTT, burstiness, or bottleneck link rate may exist, and should the congestion controllers try to detect those conditions with heuristics? If one were to try to design policers to prevent abuse, what would be the conformant flow behavior to police, and what if flows cannot be distinguished due to tunneling, or if in-network aggregation makes the flow non-conformant? Assuming that the long-term issues inherent with a low-threshold ECN mark can be worked out even in the presence of enough signaling bits, then both the L4S and SCE proposals present further questions. For L4S, aside from RFC 3168 detection, how does the lack of an overload signal (other than packet loss) inhibit the design of a congestion controller, and how does slow start to congestion avoidance transition occur? For SCE, the reliance on DSCP-based classification will cause legacy/classic and low latency flows to end up more often in the same queue, and SCE-enabled flows could end up with worse performance than legacy/classic flows. Can SCE results be presented that show the performance of competing flows (legacy/classic and SCE) in shared queues, but with the SCE classification disabled? My suggestion to move forward is for the WG to better define the desired properties, behavior, and scope of low-latency ECN semantics assuming (in the long run) two inputs and two outputs, and to expand the evaluation framework to take into account the suggestions that others have made to include short flows, bidirectional traffic, asymmetry, jitter and network aggregation, and tunnels. Constraints introduced by SCE and L4S could also be evaluated in the same framework. If the WG were to reach consensus that, with enough bits, the objective design works well enough, but the constraints do not, then perhaps more effort could be devoted to finding more bits. Jake Holland made one such proposal (ECT(1) to ECT(0), plus existing CE). If low latency service is within reach but is blocked by some existing decapsulation rules that could be revised, why shouldn’t the WG try for that? Appendix B of the AccECN draft hypothesizes that there may be creative ways to send both a low-level congestion and overload signal back to the sender. - Tom
- [tsvwg] explanation of option 3 choice Tom Henderson
- Re: [tsvwg] explanation of option 3 choice Jonathan Morton
- Re: [tsvwg] explanation of option 3 choice Ingemar Johansson S
- Re: [tsvwg] explanation of option 3 choice Rodney W. Grimes
- Re: [tsvwg] explanation of option 3 choice Tom Henderson
- Re: [tsvwg] explanation of option 3 choice Ingemar Johansson S
- Re: [tsvwg] explanation of option 3 choice Ingemar Johansson S