[Bloat] [bbr-dev] Re: Are we heading towards a BBR-dominant Internet?

Sun Apr 2 10:02:42 EDT 2023

On Sun, Apr 2, 2023 at 8:14 AM Sebastian Moeller <moeller0 at gmx.de> wrote:

> Hi Ayush,
>
> > On Mar 28, 2023, at 11:36, Ayush Mishra via Bloat <
> bloat at lists.bufferbloat.net> wrote:
> >
> > Hey Neal,
> >
> > I was revisiting this thread before presenting this paper in iccrg
> tomorrow - and I was particularly intrigued by one of the motivations you
> mentioned for BBR:
> >
> > "BBR is not trying to maintain a higher throughput than CUBIC in these
> kinds of scenarios with steady-state bulk flows. BBR is trying to be robust
> to the kinds of random packet loss that happen in the real world when there
> are flows dynamically entering/leaving a bottleneck."
>
> But isn't "when there are flows dynamically entering" actually a bona fide
> reason for the already established flows to scale back a bit, to give the
> new-commers some room to establish themselves?
>

Yes, I agree that "when there are flows dynamically entering" is actually a
bona fide reason for the already established flows to scale back to give
the newcomers some room to establish themselves. I'm not arguing against
scaling back to give the newcomers some room to establish themselves. I'm
arguing against the specific way that Reno and CUBIC behave to try to
accomplish that. :-)

> > BBRv1 essentially tried to deal with this problem by doing away with
> packet loss as a congestion signal and having an entirely different
> philosophy to congestion control. However, if we set aside the issue of
> buffer bloat, I would imagine packet loss is a bad congestion signal in
> this situation because most loss-based congestion control algorithms use it
> as a binary signal with a binary response (back-off or no back-off). In
> other words, I feel the blame must be placed on not just the congestion
> signal, but also on how most algorithms respond to this congestion signal.
>
>         Fair enough, but even if we assume a capacity based loss we really
> do not know:
> a) did the immediate traffic simply exceed the bottleneck's queue
> (assuming a fixed egress capacity/rate)
> b) did the immediate traffic simply exceed the bottleneck's egress
> capacity (think variable rate link that just dropped in rate, while traffic
> rate was constant)
>
> In case a) we might be OK with doing a gentle reduction (and take a bit to
> do so) in case b) we probably should be doing a less gentle reduction and
> preferably ASAP.
>

Agreed. And that's the approach that BBRv2 takes; it would behave
differently in the two cases. In case (a) it would essentially notice that
packets are being dropped and yet the delivery rate remains high, so would
infer that in-flight is too high but the estimated bandwidth seems OK, so
it would immediately reduce the cwnd slightly but maintain the pacing rate.
In case (b) it would notice that the loss rate is high and delivery rate
has reduced substantially, so would immediately and substantially reduce
both the cwnd and pacing rate.

> >
> > On a per-packet basis, packet loss is a binary signal. But over a
> window, the loss percentage and distribution, for example, can be a rich
> signal. There is probably scope for differentiating between different kinds
> of packet losses
>
>         Sure, as long as a veridical congestion detection is still timely
> enough not to make case b) above worse...
>

Agreed.

> > (and deciding how to react to them) when packet loss is coupled with the
> most recent delay measurement too.
>
>         Hmm, say we get a "all is fine" delay probe at time X, at X+1 the
> capacity drops to 50% and we incur a drop, will the most recent delay data
> actually be informative for the near future?
>

Usually it takes an ACK (a dupack or ACK carrying a SACK block) ACKing data
that transited the network path *after* the loss to infer the loss
(consistent with the RACK <https://www.rfc-editor.org/rfc/rfc8985>
philosophy), and that ACK will usually provide a delay sample. So when
there is loss usually there will be a delay signal that is at least as
fresh as the loss signal, providing a hint about the state of the
bottleneck queue after the loss. So even with loss I'd imagine that using
that most recent delay data should usually be informative about the near
future.

best regards,
neal

> Regards
>         Sebastian
>
>
> > Now that BBRv2 reacts to packet loss, are you making any of these
> considerations too?
> >
> > This is not something I plan to present in iccrg tomorrow, just
> something I was curious about :)
> >
> > Warmest regards,
> > Ayush
> >
> > On Fri, Aug 26, 2022 at 9:36 PM 'Neal Cardwell' via BBR Development <
> bbr-dev at googlegroups.com> wrote:
> > Yes, I agree the assumptions are key here. One key aspect of this paper
> is that it focuses on the steady-state behavior of bulk flows.
> >
> > Once you allow for short flows (like web pages, RPCs, etc) to
> dynamically enter and leave a bottleneck, the considerations become
> different. As is well-known, Reno/CUBIC will starve themselves if new flows
> enter and cause loss too frequently. For CUBIC, for a somewhat typical 30ms
> broadband path with a flow fair share of 25 Mbit/sec, if new flows enter
> and cause loss more frequently than roughly every 2 seconds then CUBIC will
> not be able to utilize its fair share. For a high-speed WAN path, with
> 100ms RTT and fair share of 10 Gbit/sec,  if new flows enter and cause loss
> more frequently than roughly every 40 seconds then CUBIC will not be able
> to utilize its fair share. Basically, loss-based CC can starve itself in
> some very typical kinds of dynamic scenarios that happen in the real world.
> >
> > BBR is not trying to maintain a higher throughput than CUBIC in these
> kinds of scenarios with steady-state bulk flows. BBR is trying to be robust
> to the kinds of random packet loss that happen in the real world when there
> are flows dynamically entering/leaving a bottleneck.
> >
> > cheers,
> > neal
> >
> >
> >
> >
> > On Thu, Aug 25, 2022 at 8:01 PM Dave Taht via Bloat <
> bloat at lists.bufferbloat.net> wrote:
> > I rather enjoyed this one. I can't help but wonder what would happen
> > if we plugged some different assumptions into their model.
> >
> > https://www.comp.nus.edu.sg/~bleong/publications/imc2022-nash.pdf
> >
> > --
> > FQ World Domination pending:
> https://blog.cerowrt.org/post/state_of_fq_codel/
> > Dave Täht CEO, TekLibre, LLC
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> >
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