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Re: [tsvwg] New rev of draft-ietf-tsvwg-ecn-l4s-id-17

Jonathan Morton <chromatix99@gmail.com> Mon, 24 May 2021 11:08 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
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Date: Mon, 24 May 2021 14:08:18 +0300
Cc: Bob Briscoe <ietf@bobbriscoe.net>, tsvwg@ietf.org
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To: Sebastian Moeller <moeller0@gmx.de>
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Subject: Re: [tsvwg] New rev of draft-ietf-tsvwg-ecn-l4s-id-17
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> On 24 May, 2021, at 1:54 pm, Sebastian Moeller <moeller0@gmx.de> wrote:
> 
>> BTW, the C-queue target has never been 25ms. I noticed JM said that incorrectly as well recently.
> 
> 	[SM] Fair enough (it seems I ms-attributed the https://github.com/L4STeam/linux/commit/a2ef76f8da1c9d1b13fa941f55607f3e60d4112eprague_rtt_target with the typical_rtt), but that only changes the numbers slightly the issue is still he same...
> 
> 
>> * A default C queue delay target of 15ms has always been recommended in aqm-dualq-coupled.
> 
> 	[SM] Not quite "always", https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-aqm-dualq-coupled-00 stated:
> "target = 20ms"
> But from https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-aqm-dualq-coupled-01 on it has been and still is set to 15ms...
> 
> Which reminds me, I still would like to see the stability analysis and optimization work that lead to that number.

Checking the code in sch_dualpi2.c, I find that the default target delay is indeed 15ms.  However, this still results in a *median* delay of 25ms when used with CUBIC.  Not 99+%ile, but MEDIAN.  It's not my place to figure out how that happens, precisely, just to observe that it does.  So there is basically 25ms of delay *difference* between the L and C queues, and that is what I based my analysis on.

 - Jonathan Morton

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