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Re: [LOOPS] How important is congestion detection

Spencer Dawkins at IETF <spencerdawkins.ietf@gmail.com> Tue, 23 July 2019 14:45 UTC

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From: Spencer Dawkins at IETF <spencerdawkins.ietf@gmail.com>
Date: Tue, 23 Jul 2019 10:45:11 -0400
Message-ID: <CAKKJt-fknF=ieOZDTBjsdNCJST7KnUZzXmL=CD55GoMDag74uw@mail.gmail.com>
To: Michael Welzl <michawe@ifi.uio.no>
Cc: Carsten Bormann <cabo@tzi.org>, loops@ietf.org
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Subject: Re: [LOOPS] How important is congestion detection
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FWIW,

On Mon, Jul 22, 2019 at 6:51 PM Michael Welzl <michawe@ifi.uio.no> wrote:

>
>
> > On Jul 22, 2019, at 6:17 PM, Carsten Bormann <cabo@tzi.org> wrote:
> >
> > Sorry for typing a message from a heated BOF where I was representing a
> WG that needs a specific outcome…
> >
> >>> Bob pointed out that some end host implementations today interpret CE
> as definite congestion events, while applying congestion detection to loss
> signals, so this conversion might be counterproductive.
> >>
> >> The last sentence - I don't understand it (“interpret CE as definite…
> while applying detection to loss” - huh?). What does it mean? Or, maybe:
> what’s the issue here?
> >
> > What I think Bob meant: An end-host noticing a loss might make its own
> determination (a.k.a. congestion detection) whether that is a reason to
> slow down or not.
> > An end-host getting a CE might simply assume this is indeed congestion,
> and always slow down.  So, CE > loss, and a simple replacement of loss by
> CE worsens throughput.
>
> Ok, thanks!  now I understand that the statement is indeed what I thought,
> but it still makes zero sense to me. See below:
>
>
> >> I thought the whole point is to signal CE *as a replacement* for the
> loss that LOOPS repairs. So, then… ?
> >
> > If CE > loss, replacing loss by CE is not enough, we need to vet the
> losses via a LOOPS-side congestion detection that is as good as the one as
> the end-host would do, or (preferably) better.
>
> RFC 3168 says CE = loss. RFC 8311 finally allows us to diverge from this
> rule, which we used in RFC to state: “CE < loss”.
> L4S of course also does “CE < loss”, albeit in much more elaborate ways.
>
> However: "CE > loss” is truly weird to me - perhaps this IS now allowed by
> RFC 8311  :-D    but if it’s spec’ed then it must be an April 1 RFC.
>
> What am I missing, Bob?
>

I was hoping CE > loss was a typo, myself.

Spencer


>
> Cheers,
> Michael
>
> --
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