Re: [tsvwg] L4S vs SCE

"Scheffenegger, Richard" <rs.ietf@gmx.at> Wed, 20 November 2019 06:49 UTC

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To: Jonathan Morton <chromatix99@gmail.com>, G Fairhurst <gorry@erg.abdn.ac.uk>
Cc: "tsvwg@ietf.org" <tsvwg@ietf.org>, Ingemar Johansson S <ingemar.s.johansson@ericsson.com>, "tsvwg-chairs@ietf.org" <tsvwg-chairs@ietf.org>, "De Schepper, Koen (Koen)" <koen.de_schepper@nokia.com>, "4bone@gndrsh.dnsmgr.net" <4bone@gndrsh.dnsmgr.net>
References: <HE1PR07MB44250F3C4E6A744DDCC3DAFCC24C0@HE1PR07MB4425.eurprd07.prod.outlook.com> <ad7b763e-b3dd-36cf-a9c5-7de99476babb@mti-systems.com> <12ED7632-5E3E-4EB9-B65E-8A8324067C9A@akamai.com> <5DD4BB25.3060700@erg.abdn.ac.uk> <B804BC0F-1618-4C89-AAF9-0AD09C1DDF81@gmail.com>
From: "Scheffenegger, Richard" <rs.ietf@gmx.at>
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Date: Wed, 20 Nov 2019 14:49:17 +0800
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Subject: Re: [tsvwg] L4S vs SCE
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Gorry, Jonathan,


Am 20.11.2019 um 05:23 schrieb Jonathan Morton:
>> On 20 Nov, 2019, at 12:03 pm, G Fairhurst <gorry@erg.abdn.ac.uk> wrote:
>>
>> Would an SCE router likely preserve or change the queueing of ECT(1) L4S traffic? - is this behvaiour different to any existing ECN-marking router?
>
> SCE middleboxes don't queue or handle ECN(1)-marked or CE-marked traffic any differently from ECT(0) traffic.  The a-priori ECT(1) marking merely removes the option of applying an SCE mark.

Will a SCE middlebox keep a rolling update of the incoming fraction of
ECT(1) vs. the fraction it would want to (re-)mark with ECT(1)? Or is
"ships-in-the-night" behavior between two fractionally congested SCE
middleboxes sufficient with an overall remarking probability of
1-((1-p1)*(1-p2))

With regular RED, ships-in-the-night was acepted, but the typical
marking ratios for 3168-TCP would be an order of magnitude lower,
limiting the deviation compared to "correct" max(p1, p2) marking.

As SCE has a target marking probability (ect0 -> ect1) from close to 50%
under steady state, such couplings between consecutive bottlenecks would
be much more prononounced.

>> And would SCE traffic arriving at the L4S queue meet L4S expectations in terms of responsiveness?
>
> Yes, since the MD response to CE is actually more severe/conservative than what L4S expects.

Well, SCE would be starved out due to the high p of CE marks in a L4S
queue, even if not sharing the Q.

[...]

> Is that sufficient, or does a more detailed analysis need to be organised?

A good analytical model of the sce cc response function would be good to
have always.