Re: Deadlocking in the transport

Martin Thomson <martin.thomson@gmail.com> Wed, 10 January 2018 22:08 UTC

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From: Martin Thomson <martin.thomson@gmail.com>
Date: Thu, 11 Jan 2018 09:08:00 +1100
Message-ID: <CABkgnnU3CQkvd7m+G80sCOPJfzb_=HonbRDSQJC8wqD_uWoj0w@mail.gmail.com>
Subject: Re: Deadlocking in the transport
To: Jana Iyengar <jri@google.com>
Cc: "Charles 'Buck' Krasic" <ckrasic@google.com>, QUIC WG <quic@ietf.org>
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On Thu, Jan 11, 2018 at 7:32 AM, Jana Iyengar <jri@google.com> wrote:
>> What would this advice be?  "Consume *all data* from higher priority
>> streams before consuming data from lower priority streams?"
>
> Yes, to avoid deadlock.

Priorities in h2 were always guidelines.  You (and I think Buck as
well) are suggesting that a dependency would be something that the
transport could be bound by.  That creates a dangerous schism between
interpretation of priorities.

In h2, if Y depends on X, a server can still send Y first.  It will
likely do so if Y is ready to go before X.  That is, generally, h2
favours forward progress over strict adherence to priority ordering.

I agree that the case that we're talking about at the transport layer
is substantively different, but if we don't have other language for
what we're doing, we create the potential for some really bad
confusion about this mechanism.  This is exacerbated by the fact that
QUIC doesn't have any concrete priority mechanism built in.  QUIC
depends on the application protocol to feed it priority information,
and that naturally tends toward having hq (and h2) dictate priority.
hq will likely use the h2 idiom, which won't fit well with this
notion.