[Bloat] BBR high RTT unfairness: Fifty Shades of Congestion Control: A Performance and Interactions Evaluation

[Roland Bless]

Hi Dave,

On 29.05.19 at 17:05 Dave Taht wrote:
> I have been trying to work through this paper:
> https://arxiv.org/pdf/1903.03852.pdf
> which is enormous and well worth reading.
> 
> I have a theory, though, about TABLE XII, which contrasts four BBR
> flows at different RTTs, in that BBRv1's probe phase makes a 200ms
> assumption, thus
> not seeing the real rtt at ong rtts, and thus the longest RTT flow
> gets the most bandwidth on this test, and the second (testable) theory
> is that were these rtts not exactly on the 100ms boundaries, we  would
> see more throughput fairness.

Nope, the main reason for RTT unfairness in BBRv1 is its
CWnd cap at 2*(RTT_min*est_bw) (2*estimated bottleneck BDP share).
As we showed in http://doc.tm.kit.edu/2017-kit-icnp-bbr-authors-copy.pdf
Section III: multiple BBR flows will always increase their CWnd up
to this point (except when the buffer capacity is smaller than a BDP).
Neal's explanation is in line with our findings.
Consequently, each flow will converge towards a share of RTT_min*est_bw
at the bottleneck queue, providing a larger bandwidth share for flows
with a larger RTT_min. See also Section V.F of our paper that also
evaluated RTT unfairness (moreover, the outcome depends also on the
bottleneck buffer size).

Unfortunately, they didn't test TCP-LoLa in this context, since it is
actually able to provide fairness among flows with different RTTs
(while still limiting the overall queuing delay). Moreover, Mario
and Felix improved the convergence speed by introducing FFBquick, see:
http://doc.tm.kit.edu/Poster/2019-FFBquick_Networking.pdf
for a quick glance on the challenges and the solution.
This was published as poster paper at Networking 2019:
M. Hock, R. Bless, F. Neumeister, M. Zitterbart: FFBquick: Fast
Convergence to Fairness for Delay-bounded Congestion Controls,
Networking 2019, Warsaw, Poland, May 20-22.

Regards
 Roland