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

Bob Briscoe ietf at bobbriscoe.net
Mon Mar 11 11:31:27 EDT 2019


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

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.

==In summary==

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

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.

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.

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.

==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 
where we've written:

    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  <https://tools.ietf.org/html/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.

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

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.

==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.

==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."

==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 



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.
> _______________________________________________
> Bloat mailing list
> Bloat at lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/bloat

Bob Briscoe                               http://bobbriscoe.net/

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