Re: Deadlocking in the transport

Mikkel Fahnøe Jørgensen <mikkelfj@gmail.com> Thu, 11 January 2018 00:09 UTC

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Subject: Re: Deadlocking in the transport
To: Martin Thomson <martin.thomson@gmail.com>
Cc: "Charles 'Buck' Krasic" <ckrasic@google.com>, Jana Iyengar <jri@google.com>, QUIC WG <quic@ietf.org>
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I’m not sure we disagree, but it is hard to formulate exact.

An example is a bus and 2 car lanes, and a bicycle lane. You don’t stop all
traffic to ensure a bus can get through, but you reserve some capacity for
priority.

My concern is that you create a presidential cortege where you close the
highway for all other traffic for some hard to predict interval. It might
solve the primary deadlock but could generate others, and have other
unintended side effects.


Kind Regards,
Mikkel Fahnøe Jørgensen


On 11 January 2018 at 00.59.48, Martin Thomson (martin.thomson@gmail.com)
wrote:

On Thu, Jan 11, 2018 at 9:56 AM, Mikkel Fahnøe Jørgensen
<mikkelfj@gmail.com>; wrote:
> I think that this needs to be in the main spec. Failing to document
> this sort of pitfall could be fatal. Does anyone disagree?
>
> I agree with the idea which I find it important. But I disagree with
strict
> priorities. It should be possible to for lower priorities to communicate
> with reduced throughput which is readily handled by a pacing algorithm.
> There need to be some guarantees for priority 0 though.

See what I mean when I said that talking about this in terms of
priorities is hazard? If you have a concrete dependency, then
reducing throughput doesn't solve the issue. At best, it might hide
the issue by reducing probability.

The language problem is that we can sometimes express a dependency in
a priority scheme, such as the one in h2. This is fundamentally
different to that particular scheme in that it isn't something that
can be fudged or ignored.