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Re: [tsvwg] L4S DSCP (was: L4S drafts: Next Steps)

Jonathan Morton <chromatix99@gmail.com> Wed, 24 March 2021 22:23 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
In-Reply-To: <6f0ac4bf-bd1a-65cd-1d40-a97d4aa71aab@bobbriscoe.net>
Date: Thu, 25 Mar 2021 00:23:46 +0200
Cc: "Black, David" <David.Black@dell.com>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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To: Bob Briscoe <in@bobbriscoe.net>
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Subject: Re: [tsvwg] L4S DSCP (was: L4S drafts: Next Steps)
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> On 25 Mar, 2021, at 12:10 am, Bob Briscoe <in@bobbriscoe.net> wrote:
> 
> Can someone explain for the record how using a DSCP as an L4S identifier would solve the CE ambiguity problem?

It doesn't.  It only allows containing the resulting harm to participating networks, thereby protecting (most) innocent bystanders.  It's enough, I think, to conduct a field experiment to discover just how bad the problem really is in practice.

This is predicated on the notion that the L4S-ID DSCP(s) are bleached or dropped at the participating networks' border, and in the case of border bleaching, that L4S endpoints interpret CE marks *not* carrying the L4S-ID DSCP as per RFC-3168 or RFC-8511.

I refer you also to my recent post titled "L4S, DSCP, and RFC-4774 Option 2" in which I give a more detailed explanation of a two-DSCP proposal.  The short version is that a version of L4S using that scheme would have *some* positive confidence whether or not they were receiving L4S or conventional marking, without the need for heuristic probing or monitoring.

 - Jonathan Morton

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