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

Bob Briscoe <> Thu, 04 July 2019 11:55 UTC

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To: Luca Muscariello <>, "Holland, Jake" <>
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From: Bob Briscoe <>
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Subject: Re: [tsvwg] [Ecn-sane] Comments on L4S drafts
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On 19/06/2019 14:02, Luca Muscariello wrote:
> Jake,
> Yes, that is one scenario that I had in mind.
> Your response comforts me that I my message was not totally unreadable.
> My understanding was
> - There are incentives to mark packets  if they get privileged 
> treatment because of that marking. This is similar to the diffserv 
> model with all the consequences in terms of trust.
[BB] I'm afraid this is a common misunderstanding. We have gone to great 
lengths to ensure that the coupled dualQ does not give any privilege, by 
separating out latency from throughput, so:

  * It solely isolates traffic that gives /itself/ low latency from
    traffic that doesn't.
  * It is very hard to get any throughput advantage from the mechanism,
    relative to a FIFO (see further down this email).

The phrase "relative to a FIFO" is important. In a FIFO, it is of course 
possible for flows to take more throughput than others. We see that as a 
feature of the Internet not a bug. But we accept that some might disagree...

So those that want equal flow rates can add per-flow bandwidth policing, 
e.g. AFD, to the coupled dualQ. But that should be (and now can be) a 
separate policy choice.

An important advance of the coupled dualQ is to cut latency without 
interfering with throughput.

> - Unresponsive traffic in particular (gaming, voice, video etc.) has 
> incentives to mark. Assuming there is x% of unresponsive traffic in 
> the priority queue, it is non trivial to guess how the system works.
> - in particular it is easy to see the extreme cases,
>                (a) x is very small, assuming the system is stable, the 
> overall equilibrium will not change.
>                (b) x is very large so the dctcp like sources fall back 
> to cubic like and the systems behave almost like a single FIFO.
>                (c) in all other cases x varies according to the 
> unresponsive sources' rates.
>                     Several different equilibria may exist, some of 
> which may include oscillations. Including oscillations of all 
> fallback  mechanisms.
> The reason I'm asking is that these cases are not discussed in the I-D 
> documents or in the references, despite these are very common use cases.
[BB] This has all already been explained and discussed at length during 
various IETF meetings. I had an excellent student (Henrik Steen) act as 
a "red-team" guy. His challenge was: Can you contrive a mis-marking 
strategy with unresponsive traffic to cause any more harm than in a 
FIFO? We wanted to make sure that introducing a priority scheduler could 
not be exploited as a significant new attack vector.

Have you looked at his thesis - the [DualQ-Test 
reference at the end of this subsection of the Security Considerations 
in the aqm-dualq-coupled draft:
4.1.3. Protecting against Unresponsive ECN-Capable Traffic 
(we ruled evaluation results out of scope of this already over-long 
draft - instead giving references).

Firstly, when unresponsive traffic < link rate, counter-intuitively it 
doesn't matter which queue it classifies itself into. Any responsive 
traffic in either or both queues still shares out the remaining capacity 
as if the unresponsive traffic had subtracted from the overall capacity 
(like a FIFO).

Beyond that, Henrik tested whether the persistent overload mechanism 
that switches off any distinction between the queues (code in the 
reference Linux implementation 
pseudocode and explanation in Appendix A.2 
left any room for mis-marked traffic to gain an advantage before the 
switch-over. There was a narrow region in which unresponsive traffic 
mismarked as ECN could strengthen its attack relative to the same attack 
on the Classic queue without mismarking.

I presented a one-slide summary of Henrik's experiment here in 2017 in 
IETF tcpm 
I tried to make the legends self-explanatory as long as you work at it, 
but shout if you need it explained.
Each column of plots shows attack traffic at increasing fractions of the 
link rate; from 70% to 200%.

Try to spot the difference between the odd columns and the even columns 
- they're just a little different in the narrow window either side of 
100% - a sharp kink instead of a smooth kink.
I included log-scale plots of the bottom end of the range to magnify the 

Yes, the system oscillates around the switch-over point, but you can see 
from the tcpm slide that the oscillations are also there in the 3rd 
column (which emulates the same switch-over in a FIFO). So we haven't 
added a new problem.

In summary, the advantage of mismarking was small and it was hard for 
the attacker not to trip the dualQ into overload state when it applies 
the same drop level in either queue. And that was when the victim 
traffic was just a predictable long-running flow. With normal less 
predictable victim traffic, I cannot think how to get this attack to be 

> If we add the queue protection mechanism, all unresponsive flows that 
> are caught cheating are registered in a blacklist and always scheduled 
> in the non-priority queue.
1/ Queue protection is an alternative to overload protection, not an 

  * The Linux implementation solely uses the overload mechanism, which
    is sufficient to prevent the priority scheduler amplifying a
    mismarking attack (whether ECN or DSCP).
  * The DOCSIS implementation use per-flow queue protection instead.

2/ Aligned incentives

The coupled dualQ with just overload protection ensures incentives are 
aligned so that, normal developers won't intentionally mismark traffic. 
As explained at the start of this email:

    the DualQ solely isolates traffic that gives /itself/ low latency
    from traffic that doesn't. Low latency solely depends on the
    traffic's own behaviour. Traffic doesn't /get/ anything from the low
    latency queue, so there's no point mismarking to get into it.

However, incentives only address rational behaviour, not accidents and 
malice. That's why DOCSIS operators asked for Q protection - to protect 
against something accidentally or deliberately introducing bursty or 
excessive traffic into the low latency queue.

The Linux code is sufficient under normal circumstances though. There 
are already other mechanisms that deal with the worms, trojans, etc. 
that might launch these attacks.

3/ DOCSIS Q protection does not black-list flows.

It redirects certain /packets/ from those flows with the highest queuing 
scores into the Classic queue, only if those packets would otherwise 
risk a threshold delay for the low latency queue being exceeded.

If a flow has a temporary wobble, some of its packets get redirected to 
protect the low latency queue, but if it gets back on track, then 
there's just no further packet redirection.

> It that happens unresponsive flows will get a service quality that is 
> worse than if using a single FIFO for all flows.
4/ Slight punishment is a feature, not a bug

If an unresponsive flow is well-paced and not contributing to queuing, 
it will accumulate only a low queuing score, and experience no 
redirected packets.

If it is contributing to queuing and it is mismarking itself, then Q 
Prot will redirect some of its packets, and the continual reordering 
will (intentionally) give it worse service quality. This deliberate 
slight punishment gives developers a slight incentive to mark their 
flows correctly.

I could explain more about the queuing score (I think I already did for 
you on these lists), but it's all in Annex P of the DOCSIS spec 
<>. and I'm 
trying to write a stand-alone document about it at the moment.

> Using a flow blacklist brings back the complexity that dualq is 
> supposed to remove compared to flow-isolation by flow-queueing.
> It seems to me that the blacklist is actually necessary to make dualq 
> work under the assumption that x is small,
[BB] As above, the Linux implementation works and aligns incentives 
without Q Prot, which is merely an optional additional protection 
against accidents and malice.

(and there's no flow black-list).

> because in the other cases the behavior
> of the dualq system is unspecified and likely subject to 
> instabilities, i.e. potentially different kind of oscillations.

I do find the tone of these emails rather disheartening. We've done all 
this work that we think is really cool. And all we get in return is 
criticism in an authoritative tone as if it is backed by experiments. 
But so far it is not. There seems to be a presumption that we are not 
professional and we are somehow not to be trusted to have done a sound job.

Yes, I'm sure mistakes can be found in our work. But it would be nice if 
the tone of these emails could become more constructive. Possibly even 
some praise. There seems to be a presumption of disrespect that I'm not 
used to, and I would rather it stopped.

Sorry for going silent recently - had too much backlog. I'm working my 
way backwards through this thread. Next I'll reply to Jake's email, 
which is, as always, perfectly constructive.



> Luca
> On Tue, Jun 18, 2019 at 9:25 PM Holland, Jake < 
> <>> wrote:
>     Hi Bob and Luca,
>     Thank you both for this discussion, I think it helped crystallize a
>     comment I hadn't figured out how to make yet, but was bothering me.
>     I’m reading Luca’s question as asking about fixed-rate traffic
>     that does
>     something like a cutoff or downshift if loss gets bad enough for long
>     enough, but is otherwise unresponsive.
>     The dualq draft does discuss unresponsive traffic in 3 of the sub-
>     sections in section 4, but there's a point that seems sort of swept
>     aside without comment in the analysis to me.
>     The referenced paper[1] from that section does examine the question
>     of sharing a link with unresponsive traffic in some detail, but the
>     analysis seems to bake in an assumption that there's a fixed amount
>     of unresponsive traffic, when in fact for a lot of the real-life
>     scenarios for unresponsive traffic (games, voice, and some of the
>     video conferencing) there's some app-level backpressure, in that
>     when the quality of experience goes low enough, the user (or a qoe
>     trigger in the app) will often change the traffic demand at a higher
>     layer than a congestion controller (by shutting off video, for
>     instance).
>     The reason I mention it is because it seems like unresponsive
>     traffic has an incentive to mark L4S and get low latency.  It doesn't
>     hurt, since it's a fixed rate and not bandwidth-seeking, so it's
>     perfectly happy to massively underutilize the link. And until the
>     link gets overloaded it will no longer suffer delay when using the
>     low latency queue, whereas in the classic queue queuing delay provides
>     a noticeable degradation in the presence of competing traffic.
>     I didn't see anywhere in the paper that tried to check the quality
>     of experience for the UDP traffic as non-responsive traffic approached
>     saturation, except by inference that loss in the classic queue will
>     cause loss in the LL queue as well.
>     But letting unresponsive flows get away with pushing out more classic
>     traffic and removing the penalty that classic flows would give it
>     seems
>     like a risk that would result in more use of this kind of unresponsive
>     traffic marking itself for the LL queue, since it just would get lower
>     latency almost up until overload.
>     Many of the apps that send unresponsive traffic would benefit from low
>     latency and isolation from the classic traffic, so it seems a mistake
>     to claim there's no benefit, and it furthermore seems like there's
>     systematic pressures that would often push unresponsive apps into this
>     domain.
>     If that line of reasoning holds up, the "rather specific" phrase in
>     section 4.1.1 of the dualq draft might not turn out to be so specific
>     after all, and could be seen as downplaying the risks.
>     Best regards,
>     Jake
>     [1]
>     PS: This seems like a consequence of the lack of access control on
>     setting ECT(1), and maybe the queue protection function would address
>     it, so that's interesting to hear about.
>     But I thought the whole point of dualq over fq was that fq state
>     couldn't
>     scale properly in aggregating devices with enough expected flows
>     sharing
>     a queue?  If this protection feature turns out to be necessary,
>     would that
>     advantage be gone?  (Also: why would one want to turn this
>     protection off
>     if it's available?)
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Bob Briscoe