[Ecn-sane] [Bloat] [iccrg] Fwd: [tcpPrague] Implementation and experimentation of TCP Prague/L4S hackaton at IETF104

Sat Mar 16 18:05:20 EDT 2019

Yeah, great question.

I don't want to answer for the L4S guys, I don't have a good feel for
what they might think.  But it does concern me that there seems to be at
least one tuning parameter that was picked for Reno, which I think I
mentioned on the tsvwg list:
https://tools.ietf.org/html/draft-ietf-tsvwg-aqm-dualq-coupled-08#section-2.1

For SCE, I would assume they'll want to make use of it at some point,
and so they'll have to write a draft for how BBR will handle it.

I think there's an open question of what exactly the rate of SCE
markings would look like for a SCE-capable AQM, and presumably this also
needs to be nailed down before it can be useful.  My initial instinct is
a probabilistic SCE setting based on current queue length, either when
forwarded or when enqueued, but I think this will take some more
thought, and I'm not sure that's best.

But whatever the most informative schedule we can figure out is, if that
info can get back to sender, it can essentially do whatever it thinks is
right, according to the cc it’s running, is my understanding.

-Jake

From: Vint Cerf <vint at google.com>
Date: 2019-03-16 at 14:57
To: "Holland, Jake" <jholland at akamai.com>
Cc: Mikael Abrahamsson <swmike at swm.pp.se>, "David P. Reed" <dpreed at deepplum.com>, "ecn-sane at lists.bufferbloat.net" <ecn-sane at lists.bufferbloat.net>, bloat <bloat at lists.bufferbloat.net>
Subject: Re: [Ecn-sane] [Bloat] [iccrg] Fwd: [tcpPrague] Implementation and experimentation of TCP Prague/L4S hackaton at IETF104

where does BBR fit into all this?

v

On Sat, Mar 16, 2019 at 5:39 PM Holland, Jake <jholland at akamai.com<mailto:jholland at akamai.com>> wrote:
On 2019-03-15, 11:37, "Mikael Abrahamsson" <swmike at swm.pp.se<mailto:swmike at swm.pp.se>> wrote:
    L4S has a much better possibility of actually getting deployment into the
    wider Internet packet-moving equipment than anything being talked about
    here. Same with PIE as opposed to FQ_CODEL. I know it's might not be as
    good, but it fits better into actual silicon and it's being proposed by
    people who actually have better channels into the people setting hard
    requirements.

    I suggest you consider joining them instead of opposing them.

Hi Mikael,

I agree it makes sense that fq_anything has issues when you're talking
about the OLT/CMTS/BNG/etc., and I believe it when you tell me PIE
makes better sense there.

But fq_x makes great sense and provides real value for the uplink in a
home, small office, coffee shop, etc. (if you run the final rate limit
on the home side of the access link.)  I'm thinking maybe there's a
disconnect here driven by the different use cases for where AQMs can go.

The thing is, each of these is the most likely congestion point at
different times, and it's worthwhile for each of them to be able to
AQM (and mark packets) under congestion.

One of the several things that bothers me with L4S is that I've seen
precious little concern over interfering with the ability for another
different AQM in-path to mark packets, and because it changes the
semantics of CE, you can't have both working at the same time unless
they both do L4S.

SCE needs a lot of details filled in, but it's so much cleaner that it
seems to me there's reasonably obvious answers to all (or almost all) of
those detail questions, and because the semantics are so much cleaner,
it's much easier to tell it's non-harmful.

<aside regarding="non-harmful">
The point you raised in another thread about reordering is mostly
well-taken, and a good counterpoint to the claim "non-harmful relative
to L4S".

To me it seems sad and dumb that switches ended up trying to make
ordering guarantees at cost of switching performance, because if it's
useful to put ordering in the switch, then it must be equally useful to
put it in the receiver's NIC or OS.

So why isn't it in all the receivers' NIC or OS (where it would render
the switch's ordering efforts moot) instead of in all the switches?

I'm guessing the answer is a competition trap for the switch vendors,
plus "with ordering goes faster than without, when you benchmark the
switch with typical load and current (non-RACK) receivers".

If that's the case, it seems like the drive for a competitive advantage
caused deployment of a packet ordering workaround in the wrong network
location(s), out of a pure misalignment of incentives.

RACK rates to fix that in the end, but a lot of damage is already done,
and the L4S approach gives switches a flag that can double as proof that
RACK is there on the receiver, so they can stop trying to order those
packets.

So point granted, I understand and agree there's a cost to abandoning
that advantage.
</aside>

But as you also said so well in another thread, this is important.  ("The
last unicorn", IIRC.)  How much does it matter if there's a feature that
has value today, but only until RACK is widely deployed?  If you were
convinced RACK would roll out everywhere within 3 years and SCE would
produce better results than L4S over the following 15 years, would that
change your mind?

It would for me, and that's why I'd like to see SCE explored before
making a call.  I think at its core, it provides the same thing L4S does
(a high-fidelity explicit congestion signal for the sender), but with
much cleaner semantics that can be incrementally added to congestion
controls that people are already using.

Granted, it still remains to be seen whether SCE in practice can match
the results of L4S, and L4S was here first.  But it seems to me L4S comes
with some problems that have not yet been examined, and that are nicely
dodged by a SCE-based approach.

If L4S really is as good as they seem to think, I could imagine getting
behind it, but I don't think that's proven yet.  I'm not certain, but
all the comparative analyses I remember seeing have been from more or
less the same team, and I'm not convinced they don't have some
misaligned incentives of their own.

I understand a lot of work has gone into L4S, but this move to jump it
from interesting experiment to de-facto standard without a more critical
review that digs deeper into some of the potential deployment problems
has me concerned.

If it really does turn out to be good enough to be permanent, I'm not
opposed to it, but I'm just not convinced that it's non-harmful, and my
default position is that the cleaner solution is going to be better in
the long run, if they can do the same job.

It's not that I want it to be a fight, but I do want to end up with the
best solution we can get.  We only have the one internet.

Just my 2c.

-Jake

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