Re: [quicwg/base-drafts] Flow/congestion control and rejected 0-RTT (#390)

[hardie <[e-mail]>]

On Wed, Mar 15, 2017 at 10:11 PM, Martin Thomson <[e-mail]>
wrote:

> Yes, this all binds to the 0-RTT credentials, so that places an upper
> bound on how long this is valid for.
>
> I'm going to concede straight up that the lifetime of things like "how
> much memory I am going to commit" is 100% not the same as the credential
> lifetime. But the client doesn't have anything else to go on.
>
I agree that it doesn't have anything else to go on now.  We could, in
theory, create methods to give it more to go on (e.g. a separate transport
parameter lifetime provided by the server), but I seriously doubt that's
worth it.

That assessment makes me wonder, though, how the success rate of the 0RTT
connections with defaults will compare with that of connections using
remembered parameters.  If the remembered parameter connections succeed as
often or more often, then they are clearly double plus good; if they
succeed less often, then the delta is the trade-off.  I don't,
unfortunately, have any data on what the delta is going to be in different
network conditions, and I distrust my own instincts here (I fear, but
without data, that they will fail more often in the cases where the
failures hurt worst).

> Ultimately, when the new session is created with a 0-RTT offer, the server
> can always choose to reject 0-RTT if it can't live up to that promise.
> That's all it has, and we can ever hope for.
>
Well, that and clear signaling that this was why it was closed.

> Given the dynamics involved, this is going to be a per-connection decision.
>
> Sure, that means that you have a bunch of pressures that eventually end up
> with a flat success/fail decision, and you might be sad about that, but I'm
> far more sad about the current design lacking any real control or
> determinism. At least this way the server can tune in the values it
> provides so that it gets the best result (for whatever metrics the server
> cares about).
>
I think the control surface is very hard to use.  You can't the credential
life time, so you can provide a value that is "generally useful", but no
more.  That may be better than nothing, but I don't think it stands much
tuning.

Ted



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