Re: [tcpm] WGLC for draft-ietf-tcpm-hystartplusplus-04

Yuchung Cheng <ycheng@google.com> Tue, 03 May 2022 20:03 UTC

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From: Yuchung Cheng <ycheng@google.com>
Date: Tue, 3 May 2022 13:02:28 -0700
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To: Neal Cardwell <ncardwell=40google.com@dmarc.ietf.org>
Cc: Michael Tuexen <tuexen@fh-muenster.de>, tcpm IETF list <tcpm@ietf.org>
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Subject: Re: [tcpm] WGLC for draft-ietf-tcpm-hystartplusplus-04
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Small suggestions to be more precise on actual implementation

5. Deployments and Performance Evaluations
As of the time of writing, HyStart++ as described in draft versions 01
through 04 was default enabled for all TCP connections in the Windows
operating system for over three years *with an actual L = 8*. The original
Hystart has been default-enabled for all TCP connections in the Linux
operating system using the default congestion control module CUBIC
([RFC8312]) for a decade *with an infinite L*.

Personally I think having the "L" ABC constraint would hinder hystart
performance.It's maybe too late to modify the actual algorithm hence I ask
the implementation description can be more precise.

On Mon, May 2, 2022 at 8:00 AM Neal Cardwell <ncardwell=
40google.com@dmarc.ietf.org> wrote:

> On Wed, Apr 13, 2022 at 4:19 PM Michael Tuexen <tuexen@fh-muenster.de>
> wrote:
> >
> > Dear all,
> >
> > this e-mail starts the working group last call for
> draft-ietf-tcpm-hystartplusplus-04.
> >
> > The WGLC runs until Friday, May 6th 2022.
> >
> > Please send any comments, including indications to support this document,
> > to the TCMP mailing list by then.
> >
> > The ID is available at
> > https://www.ietf.org/archive/id/draft-ietf-tcpm-hystartplusplus-04.html
> >
> > Best regards
> > Michael
>
> I am generally supportive of this draft advancing, but do want to put
> a word in about the application-limited issue Randall mentioned again
> earlier in this thread. After the algorithm exits slow start, it seems
> like it is likely to oscillate between:
>
> (a) application-limited periods that cause low RTT samples and a
> re-entry into slow start ("if current round's minRTT drops below
> baseline indicating that HyStart exit was spurious"), and then
>
> (b) non-application-limited periods that will cause a re-exit of
> slow-start and re-entry into CSS.
>
> It seems like the algorithm should either be adjusted to avoid this,
> or this expected oscillation dynamic should be documented.
>
> best regards,
> neal
>
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