Re: [tsvwg] Comments on L4S drafts

"Holland, Jake" <> Fri, 14 June 2019 17:40 UTC

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From: "Holland, Jake" <>
To: Bob Briscoe <>, "" <>
CC: "" <>
Thread-Topic: Comments on L4S drafts
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Date: Fri, 14 Jun 2019 17:39:12 +0000
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Subject: Re: [tsvwg] Comments on L4S drafts
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Hi Bob,

Thanks for your response, I think it helped clarify some important things

for me.

The point about starvation especially was a good one I hadn't fully

considered, and I agree if SCE-based implementations can’t demonstrate a

solution, that would be a major problem with the SCE approach for signaling.

And sorry for my slow response, I ended up restarting a few times to try to

dodge ratholes.  (Plus some day-job duties, apologies...)

I found it a bit challenging to avoid the ratholes effectively, so I'm

thinking maybe the right move is to set up a testbed.  Maybe playing with

that (very cool-looking!) L4SDemo tool can either ease my concerns, or

provide some more specific and detailed scenarios to address.

I see that the source code is published now at (thanks Olivier!).  So I’ll try to

bring that up at some point, time permitting, in hopes it makes the

comments and questions more productive.

One meta-point I wanted to make:

  "In trying to find a compromise, you've taken the fire that is really

  aimed at the inadequacy of underlying SCE protocol - for anything

  other than FQ.  If the primary SCE proponents had attempted to

  articulate a way to use SCE in a single queue or a dual queue, as you

  have, that would have taken my fire."

I think "fire" here is a potentially harmful metaphor--I don't take your

comments as an attack or this discussion as a battle, but rather a

collaborative attempt to reach a common goal of a better internet.

I hope my comments on this are received the same way, even where we don't

see eye to eye yet.  While both ideas can't be the best use of ECT(1) at

the same time, I take this discussion as an effort to reach a common and

complete understanding of the issues at hand, so that we can hopefully

agree on the best approach in the end (or if we can't get there, maybe we

can at least agree on the underlying reasons we don't agree).

With that said, a few brief points I think really should be raised:

1. "non-problem" is an unreasonably strong conclusion to reach from a

snapshot failure to detect any single-queue marking AQMs.

We know that tc-pie exists in widely deployed systems, supports ECN, and

could be turned on at any moment by anybody, and we also know there's an

increased interest in ECN since Apple and Linux got it turned on on

endpoints.  Even if we measure everything today, it’s hard to be sure this

wouldn’t impact an in-progress rollout that someone has been working toward

for their network with proper due diligence, and following IETF advice


I think if the intent is really to deploy this experiment under the claim

that's a non-problem, it should be called out in the docs as a risk factor,

and consensus should probably be explicitly checked on that point.  It also

probably would be polite to update RFC 7567's advice in section 4, since it

seems like this position would invalidate (or at least add nuance) to

several of the SHOULDs given there, recommending the use of ECN.

2. “does not starve a classic flow, but can be highly unequal” is also

perhaps too low a bar to consider a non-problem, and also seems like maybe

it deserves to be called out as a risk factor.

3. One more meta-point: the sales-y language makes the drafts hard to

read for me, so please forgive some of my confusion.  I'm having a hard

time distinguishing the claims that are well-supported by test results in a

realistic experimental design from some of the claims that are more forward-

looking or speculative.

(4. There’s one other point I’ll mention in response to Ingemar’s comment,

about performance being sufficient to drive adoption, and the difference

between what’s achievable with classic ECN and what’s achievable with L4S,

but that thread is perhaps a better venue for discussing it.)

Best regards,