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Re: [TLS] Efficiency of ACKing scheme

Eric Rescorla <ekr@rtfm.com> Thu, 09 April 2020 13:20 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 09 Apr 2020 06:20:15 -0700
Message-ID: <CABcZeBOd44CL-8kjwntS9fMg9NgzpgXhkXNi6Lsc70BwAqaxwQ@mail.gmail.com>
To: Thomas Fossati <Thomas.Fossati@arm.com>
Cc: Hanno Becker <Hanno.Becker@arm.com>, Rob Sayre <sayrer@gmail.com>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Efficiency of ACKing scheme
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Assuming I understand Hanno's proposal, I do not believe that this is in
fact an improvement, as it does not cover the important case where the
record containing the SH is lost and then the rest of the messages from the
server are uninterpretable.

-Ekr


On Thu, Apr 9, 2020 at 4:27 AM Thomas Fossati <Thomas.Fossati@arm.com>
wrote:

> Hey Hanno,
>
> On 08/04/2020, 15:11, "Hanno Becker" <Hanno.Becker@arm.com> wrote:
> > As far as I see, tail loss indication involves a timer in both cases:
> >
> > - As it stands, tail loss recovery is triggered by the ACK resulting
> >   from the 'lack of progress' indicator of disruption, described in
> >   the second bullet point of draft-ietf-tls-dtls13-37#section-7.1
> >   In particular, it only occurs after a 'short' timer triggered on the
> >   receiver, where by 'short' I mean that is has smaller threshold than
> >   the ordinary retransmission timer from DTLS 1.2, marking the bottom
> >   line recovery time we want to improve upon.
> >
> > - Likewise, there's short timer based recovery in the new proposal,
> >   but mirrored: The sender retransmits upon noticing a gap in the
> >   ACKs, which too can be detected by a short timer as in the current
> >   proposal.
>
> You are right, this wouldn't have worse tail-loss recovery than what is
> currently specified.  So, all things considered it looks like a real
> improvement compared to dtls13-37.
>
> Could collect the text from this thread in a PR against Section 7?  This
> way folks that haven't followed the discussion closely can see how your
> proposal looks overall.
>
> cheers, thanks!
>
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