[Bloat] Beyond Bufferbloat: End-to-End Congestion Control Cannot Avoid Latency Spikes

[Bjørn Ivar Teigen]

Jonathan Newton over at Vodafone Group made some interesting observations
about what happens when two of the optimal congestion controllers interact
through a shared FIFO queue:

If we take two flows E and F;  E is 90% of bandwidth and F 10%;  the time
> for the congestion signal to reach the sender for each flow is dE and dF
> where dE is 10ms and dF 100ms.   We assume no prioritisation so they must
> share the same buffer at X, and therefore share the same peak transient
> delay.
>
> We have an event at t0 where the bandwidth is halved.
>
> For time t0 to t0+dE (the first 10ms), the total rate transmitted by both
> sources is twice (90%*10%)/50% the output rate, so the component of the
> peak delay of this part is (2-1)*dE = 10ms
>
> For time t0+dE to t0+Fd (the next 90ms), the total rate transmitted by
> both sources is (45%+10%)/50% of the output rate, so the component of the
> peak delay of this part is (1.1-1)*dF-dE = 9ms
>
> Making the peak transient delay 19ms.
>

This implies that moving some senders closer to the edge (e.g. CDNs) might
help reduce lag spikes for everyone. It also implies that slow-responding
flows can have a very big impact on the peak transient latency seen by
rapidly responding flows. If the bandwidth sharing ratio in the above
example is 50/50, then the peak transient delay will be 55 ms, seen by both
flows. For flow E that's a big increase from the 10 ms we'd expect if flow
E was alone. For C=3 the increase is even worse, with flow E going from
20ms to 100ms when sharing the link with flow F.


- Bjørn

On Tue, 2 Nov 2021 at 14:08, Dave Taht <dave.taht at gmail.com> wrote:

> I am very pre-coffee. Something that could build on this would involve
> FQ. More I cannot say, til more coffee.
>
> On Tue, Nov 2, 2021 at 3:56 AM Bjørn Ivar Teigen <bjorn at domos.no> wrote:
> >
> > Hi everyone,
> >
> > I've recently published a paper on Arxiv which is relevant to the
> Bufferbloat problem. I hope it will be helpful in convincing AQM doubters.
> > Discussions at the recent IAB workshop inspired me to write a detailed
> argument for why end-to-end methods cannot avoid latency spikes. I couldn't
> find this argument in the literature.
> >
> > Here is the Arxiv link: https://arxiv.org/abs/2111.00488
> >
> > A direct consequence is that we need AQMs at all points in the internet
> where congestion is likely to happen, even for short periods, to mitigate
> the impact of latency spikes. Here I am assuming we ultimately want an
> Internet without lag-spikes, not just low latency on average.
> >
> > Hope you find this interesting!
> >
> > --
> > Bjørn Ivar Teigen
> > Head of Research
> > +47 47335952 | bjorn at domos.no | www.domos.no
> > WiFi Slicing by Domos
> > _______________________________________________
> > Bloat mailing list
> > Bloat at lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/bloat
>
>
>
> --
> I tried to build a better future, a few times:
> https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org
>
> Dave Täht CEO, TekLibre, LLC
>


-- 
Bjørn Ivar Teigen
Head of Research
+47 47335952 | bjorn at domos.no <name at domos.no> | www.domos.no
WiFi Slicing by Domos
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.bufferbloat.net/pipermail/bloat/attachments/20211103/7f567245/attachment.html>