Re: flow control and DATAGRAM

Ian Swett <ianswett@google.com> Mon, 29 October 2018 12:32 UTC

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From: Ian Swett <ianswett@google.com>
Date: Mon, 29 Oct 2018 08:32:24 -0400
Message-ID: <CAKcm_gMDOyJC0AwOo2knN6AMxVbySjGsrLvjcC9A8UA9xcvcWw@mail.gmail.com>
Subject: Re: flow control and DATAGRAM
To: Martin Thomson <martin.thomson@gmail.com>
Cc: Tommy Pauly <tpauly@apple.com>, IETF QUIC WG <quic@ietf.org>
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Good catch Martin, I missed that in the draft as well, and I also think
it's impossibly with the proposed design.

And yes, I think Martin's proposed solution is likely the only practical
one.  In my implementation, the frame is passed up to the application
immediately, so technically QUIC processed it, and it's the application's
job to decide what to do with it.

On Mon, Oct 29, 2018 at 1:16 AM Martin Thomson <martin.thomson@gmail.com>;
wrote:

> Hi Tommy,
>
> Your slides - <
> https://github.com/quicwg/wg-materials/blob/master/ietf103/IETF103-QUIC-Datagram.pdf
> >
> - say that DATAGRAM frames respect connection-level flow control.  I
> missed that in the draft, and I don't know how they can do that in the
> face of packet loss, especially when you don't necessarily retransmit
> lost DATAGRAM frames.
>
> For that to work, you would need a bunch more machinery to make the
> connection-level flow control sync between endpoints in the case that
> packets are lost.  A disagreement about how much flow control is used
> causes things to break down badly.  Ian and I discussed this point at
> the last meeting and quickly agreed that while it might be nice to
> have flow control for this stuff, the increase in complexity is
> considerable and (at the time) we thought it wouldn't be worth it.
>
> The problem that introduces is that you could end up having too many
> DATAGRAM frames arrive.  The receiver has to drop something at the
> point that it can't handle them.  And we say that when you acknowledge
> something, you processed it.  That's tricky.
>
> It might be easier to say that a QUIC acknowledgment for a DATAGRAM
> frame doesn't mean that it was received and processed by an
> application.  An endpoint might discard these frames before passing
> them on to applications if it doesn't have space.  In other words,
> acknowledgment of DATAGRAM means that QUIC got it, not that the
> application got it.  Sadly, that means that the QUIC acknowledgment
> machinery doesn't help the application that uses DATAGRAM all that
> much.  Also, the lower bound on reliability is 0, which isn't the best
> thing ever.
>
> Hard choices, I know.  I don't have a good design for maintaining
> connection-level flow control (or any back pressure mechanism with
> equivalent properties) that doesn't add both complexity and overhead.
>
> Cheers,
> Martin
>
>