Re: adjusted_rtt

Ian Swett <ianswett@google.com> Wed, 21 April 2021 14:01 UTC

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From: Ian Swett <ianswett@google.com>
Date: Wed, 21 Apr 2021 10:01:35 -0400
Message-ID: <CAKcm_gM7twJZsQWjqfa9iMSm9HhfhUbmodchmFnETTonpRLqPw@mail.gmail.com>
Subject: Re: adjusted_rtt
To: Lars Eggert <lars@eggert.org>
Cc: Timo Völker <timo.voelker@fh-muenster.de>, IETF QUIC WG <quic@ietf.org>
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Good catch Timo, I think you're correct.

PR sent: https://github.com/quicwg/base-drafts/pull/4861

On Thu, Apr 8, 2021 at 4:15 AM Lars Eggert <lars@eggert.org> wrote:

> I created https://github.com/quicwg/base-drafts/issues/4860 to track this.
>
> Thanks,
> Lars
>
>
> > On 2021-4-7, at 20:22, Timo Völker <timo.voelker@fh-muenster.de> wrote:
> >
> > Hi,
> >
> > I noticed in a simulation with a constant RTT that the smoothed_rtt
> value includes the ack_delay. It looks like my implementation follows the
> recovery draft. By looking closer, I believe I found a tiny error in the
> draft.
> >
> > Section 5.3 contains this sentence.
> >
> > "MUST NOT subtract the acknowledgment delay from the RTT sample if the
> resulting value is smaller than the min_rtt."
> >
> > I interpret this as do not subtract ack_delay from latest_rtt if
> >
> >  latest_rtt - ack_delay < min_rtt
> >
> > Or to say the inverse, substract ack_delay from latest_rtt if
> >
> >  latest_rtt - ack_delay >= min_rtt
> >  <==> latest_rtt >= min_rtt + ack_delay
> >  <==> min_rtt + ack_delay <= latest_rtt
> >
> > The pseudo code in Section 5.3 and in the appendix use a smaller instead
> of an equal or smaller in the if statement.
> >
> >  if (min_rtt + ack_delay < latest_rtt):
> >
> > It should be <= or did I miss something?
> >
> > Timo
>
>