[Bloat] [Codel] The "Some Congestion Experienced" ECN codepoint - a new internet draft -

Dave Taht dave.taht at gmail.com
Mon Mar 11 12:34:16 EDT 2019

On Mon, Mar 11, 2019 at 7:14 AM Bob Briscoe <research at bobbriscoe.net> wrote:
> Dave,
> L4S is far from dead. It's merely been working differently from how you're used to. Those working on an L4S AQM (at least those in the cable industry) had to have a private WG for the last ~18 months, but now we're allowed to publish and talk openly again. Similarly, there's work under the covers on an L4S AQM in switch hardware.

After traffic essentially vanished from the various ietf mailing
lists, and specs kept dropping, I assumed there was work being done
somewhere. I'm very glad that it's now more open

We announced the ecn-sane project and it's goals last august. If you
and/or you group cannot participate under those house rules please
suggest changes. And/or resurrect an appropriate list within the ietf.


>And I see external signs of work under covers on DSL access equipment (covers that I am not under any longer).

It has certainly been my hope that the DSL folk would at least wake up
and implement BQL in their lowest level firmware for about 8 years
now. Free.fr basically did that in 2012 when they shipped fq_codel on
their revolution series of modems.

> Nonetheless, I think you will see updated Linux code for an L4S DualQ Coupled AQM built against the mainline tree appear on netdev list today.

I am beyond delighted to finally have a chance to evaluate this. Have
you run any flent related tests through it yet?

Regrettably since this code posting is so close to netdevconf it's
going to be very difficult for me to do a comprehensive evaluation in
time to form an objective opinion. I'm busy on something else.

> ==In summary==
> The problem that the SCE draft identifies with TCP's sharp multiplicative decrease is also the primary problem that L4S identified.

Yes, we have long been in agreement that some congestion signal should
be an earlier notification than drop.

> Like L4S, SCE requires changes to network, sender and receiver (see comment later about the rcv-window approach hinted at in the SCE draft). But SCE is just starting on its journey. Having to change end systems and network together is really tough and takes many years.

Not really, the AQM portion of SCE could roll out very quickly across
all of the linux and freebsd universe, if consensus is achieved that
it's a good idea.

> It seems you're trying to do the same thing as L4S, but by slightly different means. Before splitting the people involved in this into two factions, can you say what you didn't like about the L4S approach in the first place? We've been very careful to specify L4S broadly enough so that it can encompass many different approaches within it.

I've read the 100+ of spec now multiple times over the years (and all
your work on ecn in general), and I hope, that once we get a bit of
time, we can do a detailed comparison of the two approaches.

But, honestly, based on the total inactivity on the tcp prague mailing
list that it had died, until recently.

> The only thing stated against L4S I can find is that it's taking a long time. Starting an identically difficult approach now is going to restart the clock, and take a lot lot longer.

SCE and the modifications to the relevant already IETF approved AQMs
are extremely straightforward, backward compatible approaches to
extending RFC3168 for all existing transports.

It's not an identically difficult approach at all.

Aside from some more detailed analysis of transport effects and the
inevitable debate in netdevconf and ietf, rolling out SCE, at least in
openwrt and linux, could be happen by about.... june.  Particularly
with the now more readily available source code to compare for the two
approaches, independent experts should be able to leap in and provide

> ==2 output values vs. 2 input values.==
> We considered schemes where the AQM can use a second marking as a lower strength /output/ (like VCP, my own QV and now SCE). But we worked out that there were a wider range of advantages and much more significant performance improvements from the sender using a second marking to distinguish how it will behave (i.e. a second /input/ to the classifier in front of the AQM).
> Don't get me wrong. It's useful that you guys are putting the work in on SCE. Then everyone can compare the two approaches (again), as a check on whether that decision was correct. That's important, cos ECT(1) is the last useful codepoint in the IP header. See: "Notification of Less Severe Congestion than CE" at https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-05#appendix-C.2 where we've written:

Yep. I am glad that both of our cards are on the table.

>    Before assigning ECT(1) as an identifer for L4S, we must carefully
>    consider whether it might be better to hold ECT(1) in reserve for
>    future standardisation of rapid flow acceleration, which is an
>    important and enduring problem [RFC6077].
> ==FQ-only vs. FQ or DualQ==
> One of the problems with the 2 outputs approach (SCE etc.) is that it is only possible with per-flow queuing. I doubt you'll get the last useful codepoint in the IP header for just that. It's sort-of obvious that, if you try to implement SCE in a FIFO, you can only have one queue length for all the flows. Then legacy TCP flows that don't understand SCE would push the queue deeper to the CE threshold, ruining it for the flows that support SCE.
> We worked out that the 2 inputs approach (L4S) is more generic - ie. it can be used with FQ or DualQ (multiple or just 2 queues).
> For instance, you can modify fq_CoDel for senders that uses ECT(1) to indicate that they support a small multiplicative decrease (L4S senders). You only need the following: Include the last bit of the ECN field with the flow ID when you do the classification for sfq. Then in the queues with ECN==X1, you instantiate a shallow threshold ECN AQM. This could be CoDel with a shallow 'target', but you also want it to respond immediately (zero 'interval'), so even a simple step at about 1ms will work, but a random RED-like ramp on the /instantaneous/ queue is much better.
> ==Re-purposing the Receive Window?===
> Receiver congestion control using the receive window may seem like a useful stop-gap, but it runs counter to how nearly all today's transport protocols are intended to work (except, I know of a LEDBAT-like example from Microsoft Research). So you will have your work cut out proving that it is stable and that the two ends don't fight, etc. if you think L4S is taking years, you will find that takes longer. There is current research on this that I can point you to, if you want.

I kind of wish we'd cut that from the draft. My proposal was
considerably different, and a bit longer in the prior paragraph.

> That's why we chose an approach that had a pre-existing widely deployed existence proof (DCTCP) to start from.

I have never had a good number for DCTCP's actual deployment. In the
cloud services I use, it's all (paced) cubic and BBR nowadays.

> IETF groups like rmcat explicitly decided early on to require the approach where the receiver is a dumb reflector, then new sender congestion control algorithms can be deployed unilaterally. The argument was that the feedback function can be thought of as a sub-layer below the congestion control function. The ongoing addition of accurate ECN feedback to TCP and to QUIC also take the dumb reflector approach. And RTCP already does it that way.

I do think there is more work to do here.

> ==ECN feedback problems===
> Over the last decades, we've made sure that the ECN feedback schemes for TCP, QUIC, RTP (but not SCTP yet) can all feed back ECT(1) as well as CE, in case a scheme like SCE came along.
> However, the solution in the TCP case [draft-ietf-tcpm-accurate-ecn] is still problematic for SCE if you're impatient. The base scheme overloads 3 bits in the TCP header, which it uses to feed back CE only. To feed back ECT(1) we had to add a TCP option. That's not going to get through middleboxes for many years. The TCP option is also optional to implement. Two of the main TCP developers are currently saying they will probably not implement it, at least not initially.

One thing I've not understood about L4S is why didn't you just pick a
new NNN codepoint for it and not fiddled with the ECT(1) at all.

> ==Tunnels and lower layers==
> Over the years I've maintained a fairly lonely activity to make sure that the ECN propagation behaviour of tunnels and layer 2 protocols will treat ECT(1) as either a stronger output signal (as in SCE) or as an alternative input signal to an AQM (as in L4S). Theoretically, this allows either the SCE or the L4S approach.
> HOWEVER, you would probably not be surprised at how many people read the spec [RFC6040], and say "Ah, no router alters ECT(0) to ECT(1) today, so I'm not going to implement that unnecessary extra line of code in my tunnel decap."

For the record I have made sure your efforts to do ecn as right as
possible made it into new things - notably wireguard "does it right",
now - and mosh has a wonderful response to ecn that we put in there
years ago.

I am grateful for your work on clarifying these things.
> ==Wider benefit: Relaxing link ordering==
> By overloading the ECT(1) marking to mean "the sender uses time for loss detection" a link can relax the reordering requirement on ECT(1) packets today. You can do that with L4S, cos the sender is selecting the marking. You can't do that when the AQM is selecting the marking (as with SCE).
> If transport protocols detect loss in time units without tying it to any marking (as in RACK on its own), a link cannot use this to relax the ordering requirement until it is sure that all the legacy non-RACK transports have decayed out of the network. That would be measured in decades.

There's an awful lot here worthy of discussion that I cannot respond
to today, and I've got this other, open, mailing list for it. You are
welcome to join.

> Bob
> On 11/03/2019 10:11, Dave Taht wrote:
> Everybody, calm down. I put this out merely to get comment before we
> submitted the first of several drafts. That draft is now submitted and
> we've asked for a talk slot in the tsvwg for it. I cc'd the world to
> get quick initial feedback, and I want to shut this overbroad
> conversation down and move it to just the ecn-sane mailing list.
> The l4s mailing list is dead, and the debates on the AQM mailing list and here,
> unhelpful - for decades. So, back in august I started a new working
> group here, under house rules that I thought would be more productive,
> and asked that people that wanted to debate ecn more sanely, join. few
> did.
> And jon and I have been working for months (and largely not on the
> list) to try and create a compromise proposal of which y'all just saw
> the first output. There's more in the bufferbloat-rfcs repo.
> The rules for joining the ecn-sane list are simple - take the time to
> step back and write a write a short position paper, and join (or
> create) a team. You needn't do that immediately. If you disagree with
> the rules of operation of the ecn-sane working group, submit a pull
> request or file a bug on the web site. where we can discuss it.
> Ironically our ssl cert just expired and I don't remember how to fix it.
> Please join the ecn-sane mailing list for discussing this stuff and
> stop cc-ing the whole bufferbloat.net  world on it, please.
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> --
> ________________________________________________________________
> Bob Briscoe                               http://bobbriscoe.net/


Dave Täht
CTO, TekLibre, LLC
Tel: 1-831-205-9740

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