[Bloat] Fair queuing detection for congestion control

[Maximilian Bachl]

Building upon the ideas and advice I received, I simplified the whole
concept and updated the preprint (https://arxiv.org/abs/2206.10561). The
new approach is somewhat similar to what you propose in point 3). True
negative rate (correctly detecting the absence of FQ) is now >99%; True
positive rate is >95% (correctly detecting the presence of FQ (fq_codel and
fq)). It can also detect if the bottleneck link changes during a flow from
FQ to non-FQ and vice versa.

A new concept is that each application can choose its maximum allowed delay
independently if there's FQ. A cloud gaming application might choose to not
allow more than 5 ms to keep latency minimal, while a video chat
application might allow 25 ms to achieve higher throughput. Thus, each
application can choose its own tradeoff between throughput and delay. Also,
applications can measure how large the base delay is and, if the base delay
is very low (because the other host is close by), they can allow more
queuing delay. For example, if the base delay between two hosts is just 5
ms, it could be ok to add another 45 ms of queuing to have a combined delay
of 50 ms. Because the allowed queuing delay is quite high, throughput is
maximized.



On Sun, Jul 3, 2022 at 4:49 PM Dave Taht <dave.taht at gmail.com> wrote:

> Hey, good start to my saturday!
>
> 1) Apple's fq_"codel" implementation did not actually implement the
> codel portion of the algorithm when I last checked last year. Doesn't
> matter what you set the target to.
>
> 2) fq_codel has a detectable (IMHO, have not tried) phase where the
> "sparse flow optimization" allows non queue building flows to bypass
> the queue building
> flows entirely. See attached. fq-pie, also. Cake also has this, but
> with the addition of per host FQ.
>
> However to detect it, requires sending packets on an interval smaller
> than the codel quantum. Most (all!?) TCP implementations, even the
> paced ones, send 2 1514 packets back to back, so you get an ack back
> on servicing either the first or second one. Sending individual TCP
> packets paced, and bunching them up selectively should also oscillate
> around the queue width. (width = number of queue building flows,
> depth, the depth of the queue). The codel quantum defaults to 1514
> bytes but is frequently autoscaled to less at low bandwidths.
>
> 3) It is also possible, (IMHO), to send a small secondary flow
> isochronously as a "clock" and observe the width and depth of the
> queue that way.
>
> 4) You can use a fq_codel RFC3168 compliant implementation to send
> back a CE, which is (presently) a fairly reliable signal of fq_codel
> on the path. A reduction in *pacing* different from what the RFC3168
> behavior is (reduction by half), would be interesting.
>
> Thx for this today! A principal observation of the BBR paper was that
> you cannot measure for latency and bandwidth *at the same time* in a
> single and you showing, in a FQ'd environment, that you can, I don't
> remember seeing elsewhere (but I'm sure someone will correct me).
>
> On Sun, Jul 3, 2022 at 7:16 AM Maximilian Bachl via Bloat
> <bloat at lists.bufferbloat.net> wrote:
> >
> > Hi Sebastian,
> >
> > Thank you for your suggestions.
> >
> > Regarding
> > a) I slightly modified the algorithm to make it work better with the
> small 5 ms threshold. I updated the paper on arXiv; it should be online by
> Tuesday morning Central European Time. Detection accuracy for Linux's
> fq_codel is quite high (high 90s) but it doesn't work that well with small
> bandwidths (<=10 Mbit/s).
> > b) that's a good suggestion. I'm thinking how to do it best since also
> every experiment with every RTT/bandwidth was repeated and I'm not sure how
> to make a CDF that includes the RTTs/bandwidths and the repetitions.
> > c) I guess for every experiment with pfifo, the resulting accuracy is a
> true negative rate, while for every experiment with fq* the resulting
> accuracy is a true positive rate. I updated the paper to include these
> terms to make it clearer. Summarizing, the true negative rate is 100%, the
> true positive rate for fq is >= 95% and for fq_codel it's also in that
> range except for low bandwidths.
> >
> > In case you're interested in reliable FQ detection but not in the
> combination of FQ detection and congestion control, I co-authored another
> paper which uses a different FQ detection method, which is more robust but
> has the disadvantage of causing packet loss (Detecting Fair Queuing for
> Better Congestion Control (https://arxiv.org/abs/2010.08362)).
> >
> > Regards,
> > Max
> > _______________________________________________
> > Bloat mailing list
> > Bloat at lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/bloat
>
>
>
> --
> FQ World Domination pending:
> https://blog.cerowrt.org/post/state_of_fq_codel/
> Dave Täht CEO, TekLibre, LLC
>
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