Re: Deadlocking in the transport

Martin Thomson <martin.thomson@gmail.com> Thu, 11 January 2018 00:11 UTC

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From: Martin Thomson <martin.thomson@gmail.com>
Date: Thu, 11 Jan 2018 11:11:01 +1100
Message-ID: <CABkgnnVHH0L3Mdg1Hbv8HvAVULM_oSmRrTpfeUDzPaZuFWvETw@mail.gmail.com>
Subject: Re: Deadlocking in the transport
To: =?UTF-8?Q?Mikkel_Fahn=C3=B8e_J=C3=B8rgensen?= <mikkelfj@gmail.com>
Cc: "Charles 'Buck' Krasic" <ckrasic@google.com>, Jana Iyengar <jri@google.com>, QUIC WG <quic@ietf.org>
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I don't think that those analogies are accurate in this context.
Maybe a better analogy would be a truck and the oversized trailer it
pulls.  You can't put the oversized trailer into three adjacent lanes
before the truck.

On Thu, Jan 11, 2018 at 11:08 AM, Mikkel Fahnøe Jørgensen
<mikkelfj@gmail.com>; wrote:
> 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.