Re: [tcpm] Review (Re: WGLC for draft-ietf-tcpm-rack-08)

Yuchung Cheng <ycheng@google.com> Sun, 26 July 2020 17:12 UTC

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From: Yuchung Cheng <ycheng@google.com>
Date: Sun, 26 Jul 2020 10:11:53 -0700
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To: Theresa Enghardt <ietf@tenghardt.net>
Cc: tcpm IETF list <tcpm@ietf.org>
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Subject: Re: [tcpm] Review (Re: WGLC for draft-ietf-tcpm-rack-08)
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On Fri, Jul 24, 2020 at 5:10 PM Theresa Enghardt <ietf@tenghardt.net> wrote:
>
> Hi Yuchung,
>
> See inline:
>
> On 2020-07-24 16:45, Yuchung Cheng wrote:
>
> >> On 2020-07-13 15:06, Yuchung Cheng wrote:
> >> […]
> >> Reading the new Section 3.3.2, I'm wondering about the following
> >> though:
> >>
> >> "If the sender has not observed any reordering since the
> >> connection was established, then the RACK reordering window
> >> SHOULD be zero in either of the following cases: […]
> >>
> >> 1.  After learning that three segments have been delivered out of
> >> order (e.g. receiving 3 DUPACKs per [RFC5681]); in turn, this
> >> will cause the RACK loss detection logic to trigger fast
> >> recovery.
> >>
> >> 2.  During fast recovery or RTO recovery."
> >>
> >> Does this SHOULD mean that the implementation should set the
> >> reordering
> >> window to 0 under either of the listed conditions?
> > yes
> >
> >> Because I fail to see how the two listed conditions mean that that
> >> "the
> >> sender has not observed any reordering".
> >> Or SHOULD the sender set the reordering window to 0 if either it
> >> hasn't
> >> observed any reordering, or one of the listed conditions are true.
> >
> > agree this can be better clarified. do you think the following is more
> > clear:
> >
> > 1. The reordering window SHOULD be zero if no reordering has been
> > observed on the connection so far, plus either (a)
> > three segments have been delivered out of order since the last
> > recovery or (b) the sender is already in fast or RTO recovery.
> > Otherwise the reordering window SHOULD start from a small fraction of
> > the
> > round trip time, or zero if no round trip time estimate is available.
> >
> > 2.  The RACK reordering window SHOULD adaptively increase if the
> > sender receives the Duplicate Selective Acknowledgement (DSACK)
> > [RFC2883],
> > suggesting the window is too small which causes spurious
> > retransmission.
> >
> > 3.  The RACK reordering window MUST be bounded and this bound SHOULD
> > be SRTT.
> >
> > (The text after this needs to be adjusted for the new bullet numbers).
>
> This is much clearer, thanks!
>
> Additionally, I suggest to change "The reordering window SHOULD be zero"
> to "The reordering window SHOULD be set to zero", or to prefix the
> entire list with a sentence like "More specifically, the sender sets the
> reordering window as follows:".
>
>
> >> In Section 3.3.2, I fail to understand or parse the following
> >> sentence:
> >> The RACK reordering window SHOULD leverage that to
> >> adaptively
> >> estimate the duration of reordering events, if the receiver
> >> uses
> >> Duplicate Selective Acknowledgement (DSACK) [RFC2883].
> >>
> >> The RACK reordering window SHOULD leverage what? (If "that" refers to
> >> the sentence before, SRTT, then the connection to DSACK is not obvious
> >> to me. So I think it's good to substitute "that" here.)
> > sorry that=DSACK. sentence got cut out accidentally. Does the proposed
> > new text in my earlier comment look better?
>
> Yes, this is clearer to me now.
>
> I would suggest the following slight changes to this sentence though:
>
> OLD:
> "The RACK reordering window SHOULD adaptively increase if the sender
> receives the Duplicate Selective Acknowledgement (DSACK) [RFC2883],
> suggesting the window is too small which causes spurious
> retransmission."
>
> NEW:
> "The RACK reordering window SHOULD adaptively increase if the sender
> receives a Duplicate Selective Acknowledgement (DSACK) [RFC2883],
> suggesting the window is too small, which causes spurious
> retransmission."
Sure we'll revise accordingly.

>
> Best,
> Theresa