Re: [tsvwg] L4S status: #17 Interaction w/ FQ AQMs

Pete Heist <pete@heistp.net> Mon, 16 September 2019 08:05 UTC

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From: Pete Heist <pete@heistp.net>
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Date: Mon, 16 Sep 2019 10:05:39 +0200
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References: <8321f975-dfe7-694c-b5cc-09fa371b9b61@mti-systems.com> <B58A5572-510E-42C7-8181-42A0BE298393@gmail.com> <D2E12331-F504-4D5F-B8E7-A1A5E98DDF7E@cablelabs.com> <2275E6A5-C8F8-477F-A24A-3E6168917DDF@gmail.com> <55F724CD-6E74-40D9-8416-D1918C2008DD@cablelabs.com> <BBE7C7A9-0222-4D84-BF27-8D5CAE2F995E@gmail.com> <6f189711-ffa0-90f4-fd16-3464ba4df3ce@mti-systems.com> <4A706B11-3239-4DAC-BE85-0B4BFF2D8FF8@heistp.net> <85653C73-449A-4FB0-B15A-F3617B398D29@heistp.net> <06F55720-52AF-4AD8-9DE1-2ACF8D694239@gmx.de>
To: Sebastian Moeller <moeller0@gmx.de>
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Subject: Re: [tsvwg] L4S status: #17 Interaction w/ FQ AQMs
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> On Sep 16, 2019, at 9:27 AM, Sebastian Moeller <moeller0@gmx.de> wrote:
> 
> Hi Pete,
> 
> Excellent, thanks.
> 
> So it looks like my fear that L4S-flows would completely crowd out standard compliant flows in the post bottleneck FQ-AQM scenario was unfounded!

If bottleneck 3 were a single queue (‘flows 1’ parameter to fq_codel), that may lead to unfairness, similar to what happens in two-flow competition in Scenario 3:
https://www.heistp.net/downloads/sce-l4s-bakeoff/bakeoff-2019-09-13T045427-r1/l4s-s3-2/batch-l4s-s3-2-cubic-vs-prague-50Mbit-10ms_fixed.png

Note also that in scenario 5-2 for Cubic vs Prague:
https://www.heistp.net/downloads/sce-l4s-bakeoff/bakeoff-2019-09-13T045427-r1/l4s-s5-2/batch-l4s-s5-2-cubic-vs-prague-50Mbit-80ms_fixed.png

the Prague flow exits slow start early, which is at least party influenced by Linux’s default pacing parameters. The early exit from slow start prevents the latency spike on startup of the Prague flow from occurring. On a run of the same test with ss_ratio=100 and ca_ratio=40, which we typically test with for SCE, the early exit from slow start doesn’t happen and results on startup of the Prague flow look different:

https://www.heistp.net/downloads/sce-l4s-bakeoff/bakeoff-2019-09-12T021200-pacing-100/l4s-s5-2/batch-l4s-s5-2-cubic-vs-prague-50Mbit-80ms_fixed.png