[Ecn-sane] [tsvwg] Comments on L4S drafts

[Luca Muscariello]

On Fri, Jun 7, 2019 at 8:10 PM Bob Briscoe <ietf at bobbriscoe.net> wrote:

>
>  I'm afraid there are not the same pressures to cause rapid roll-out at
> all, cos it's flakey now, jam tomorrow. (Actually ECN-DualQ-SCE has a much
> greater problem - complete starvation of SCE flows - but we'll come on to
> that in Q4.)
>
> I want to say at this point, that I really appreciate all the effort
> you've been putting in, trying to find common ground.
>
> In trying to find a compromise, you've taken the fire that is really aimed
> at the inadequacy of underlying SCE protocol - for anything other than FQ.
> If the primary SCE proponents had attempted to articulate a way to use SCE
> in a single queue or a dual queue, as you have, that would have taken my
> fire.
>
> But regardless, the queue-building from classic ECN-capable endpoints that
> only get 1 congestion signal per RTT is what I understand as the main
> downside of the tradeoff if we try to use ECN-capability as the dualq
> classifier.  Does that match your understanding?
>
> This is indeed a major concern of mine (not as major as the starvation of
> SCE explained under Q4, but we'll come to that).
>
> Fine-grained (DCTCP-like) and coarse-grained (Cubic-like) congestion
> controls need to be isolated, but I don't see how, unless their packets are
> tagged for separate queues. Without a specific fine/coarse identifier,
> we're left with having to re-use other identifiers:
>
>    - You've tried to use ECN vs Not-ECN. But that still lumps two large
>    incompatible groups (fine ECN and coarse ECN) together.
>    - The only alternative that would serve this purpose is the flow
>    identifier at layer-4, because it isolates everything from everything else.
>    FQ is where SCE started, and that seems to be as far as it can go.
>
> Should we burn the last unicorn for a capability needed on "carrier-scale"
> boxes, but which requires FQ to work? Perhaps yes if there was no
> alternative. But there is: L4S.
>
>
I have a problem to understand why all traffic ends up to be classified as
either Cubic-like or DCTCP-like.
If we know that this is not true today I fail to understand why this should
be the case in the future.
It is also difficult to predict now how applications will change in the
future in terms of the traffic mix they'll generate.
I feel like we'd be moving towards more customized transport services with
less predictable patterns.

I do not see for instance much discussion about the presence of RTC traffic
and how the dualQ system behaves when the
input traffic does not respond as expected by the 2-types of sources
assumed by dualQ.

If my application is using simulcast or multi-stream techniques I can have
several video streams in the same link,  that, as far as I understand,
will get significant latency in the classic queue. Unless my app starts
cheating by marking packets to get into the priority queue.

In both cases, i.e. my RTC app is cheating or not, I do not understand how
the parametrization of the dualQ scheduler
can cope with traffic that behaves in a different way to what is assumed
while tuning parameters.
For instance, in one instantiation of dualQ based on WRR the weights are
set to 1:16.  This has to necessarily
change when RTC traffic is present. How?

Is the assumption that a trusted marker is used as in typical diffserv
deployments
or that a policer identifies and punishes cheating applications?

BTW I'd love to understand how dualQ is supposed to work under more general
traffic assumptions.

Luca
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