Re: [Ecn-sane] ect(1) queue selector question

[Pete Heist <pete@heistp.net>]

Sorry for reviving an old thread as I haven't been on this list in a
while:

> > SCE proposes to use ect(1) as an indicator of some congestion and
does
> > not explictly
> > require a dscp codepoint in a FQ'd implementation.
> 	Pretty much. I do think that a demonstration using an
> additional DSCP to create a similar HOV lane for SCE would have gone
> miles in convincing people in the WG that L4S might really not be as
> swell as its proponents argue, IMHO it won the day more with its
> attractive promise of low latency for all instead of what it
delivers.

On that, I don't think any of us knows how things will end up or how
long it will take to get there...

I do agree that the interim meeting leading up to the codepoint
decision could have gone better. Everything went great until it came to
how to deploy SCE in a small number of queues. We had dismissed the
idea of using DSCP, because we thought it would be panned for its poor
traversal over the Internet. That may still have been the case, but it
also may have worked if sold right. We thought that AF alone might be
enough to get past that part, but it wasn't.

We already implemented a two-queue design that uses DSCP, but either
there wasn't much interest, or we didn't sell it enough. Plus, for
those who demand a two queue solution that requires no flow awareness
at all, DSCP alone may not get you there, because you still need some
reasonably fair way to schedule the two queues. So that might have been
the next line of criticism. Scheduling in proportion to the number of
flows each queue contains is one effective way to do that, but that
requires at least some concept of a flow. Perhaps there's another way
that doesn't introduce too much RTT unfairness, but I'm not sure.

In our defense, there was already a lot of support built up for L4S,
and stepping in front of that was like stepping in front of a freight
train no matter what we did. I think we've made a decent argument in
the most recent version of the SCE draft that ECN is a "network
feature" which carries higher risks than drop-based signaling, and
warrants the requirement for unresponsive flow mitigation, for
starters. That of course requires some level of flow awareness, which
then makes various queueing designs possible. And, there may still be
deployment possibilities with DSCP, as Rodney mentioned.

Anyway, there's progress being made on SCE, with some new ideas and
improvements to testing tools coming along.

Pete