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

[Ayush Mishra <ayumishra.95@gmail.com>]

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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."

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.

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 (and deciding how to react to them) when packet loss is
coupled with the most recent delay measurement too. 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@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@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
>> _______________________________________________
>> Bloat mailing list
>> Bloat@lists.bufferbloat.net
>> https://lists.bufferbloat.net/listinfo/bloat
>>
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