Re: [tsvwg] Follow-up to your DSCP and ECN codepoint comments at tsvwg interim

Jonathan Morton <chromatix99@gmail.com> Thu, 12 March 2020 13:07 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
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Date: Thu, 12 Mar 2020 15:07:14 +0200
Cc: Lars Eggert <lars@eggert.org>, tsvwg@ietf.org
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To: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Subject: Re: [tsvwg] Follow-up to your DSCP and ECN codepoint comments at tsvwg interim
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> On 12 Mar, 2020, at 2:19 pm, Gorry Fairhurst <gorry@erg.abdn.ac.uk> wrote:
> 
>>> Is it simple?

>> Yes.  I could summarise the technicalities on two slides - or one, if I really had to.  Implementing SCE also does not require very intrusive or extensive changes to existing network stacks; a year ago, we got a basic proof of concept running in about a man-week.

> Yes, we should capture those two slides for SCE! We will also ask the same for L4S.

> One slide on how SCE offers benefit will also be useful. Again we should obtain the same for L4S.

We'll bear that in mind when preparing.

> ... Or rewrite ECT(1),  as I understand for some existing tunnels/encaps. I recall from the early L4S discussion that this consideration was also be important, and hence we charted work to try and improve ECN-marking in the IP layers.

I think we will need to have a focused discussion about tunnels at some point.  For the moment, I'll just note that if the SCE signal is erased (on either the forward or feedback path), then the flow will revert to conventional RFC-3168 behaviour, which is conservatively safe.

>> SCE enabled middleboxes are easy to detect, by passing ECT(0) traffic through it and seeing if ECT(1) marks appear alongside (or before) CE marks. If CE marks or an AQM-like (or tail-drop-like) pattern of drops appears but ECT(1) does not, it's not an SCE enabled middlebox.  This is probably sufficient to reclaim the spare ECN codepoint.

> Probably worth writing down. I've found it hard to test for ECN-marking on operational paths, but insight in this is always going to be welcome.

Granted, there is the corner case where the middlebox in question is not a bottleneck on the path under test - so SCE or ECN functionality may be present but not visible.  That would appear to be the chief difficulty with achieving full probe coverage.  But I think enough confidence could be obtained by such tests to permit reclaiming the codepoint for another experiment in a reasonable timeframe.  We can try paying more rigorous attention to this question if the WG deems it necessary.

> This was to be an important topic of the face-to-face meeting in Vancouver, where we have been planning to allow time for different viewpoints to be aired and the consensus of the group to be sought. We still plan to do that, and are considering how best to proceed.
> 
> Comments in-line to show the expected direction of travel, but please do avoid growing this discussion thread for a week or so, until we have a plan to conclude it (see end of email).

> The TSVWG Chairs are now aware that we need to plan to make progress and will be in touch in the next week or so, with a plan move forward in the absence of an IETF face-to-face meeting.

Noted.

 - Jonathan Morton