Re: [iccrg] Fairness mechanism of BBRv2

Hi Mario,

Yes, that's right. The basic idea is:

(a) In a bottleneck that is providing loss or ECN signals, the BBRv2
flows will be doing a MD (multiplicative decrease) in the volume of
in-flight data (cwnd). As with the MD in Reno or CUBIC, this leaves an
amount of unused headroom (in the buffer and/or bottleneck link) that
is larger for high-rate flows, and smaller for low-rate flows. So the
convergence dynamics are somewhat analogous to CUBIC, which also uses
an MD and then super-linear increase process.

(b) In a bottleneck that has a deep buffer and is not providing loss
or ECN signals, the BBRv2 flows will be converging toward approximate
fairness using the same MI/MD  rate-based scheme used in BBRv1. The
basic reason that the flows pull toward fairness is that in each round
of probing, the MI probing from small flows produces a larger
proportional increase in their bandwidth estimate than does the MI
probing from large flows. Here the dynamics are a bit analogous to a
market with a fixed-rate set of sales transactions; if all the players
are trying to increase their market share (sales rate) by stocking
shelves with an MI relative to their sales in the last interval, the
small start-up producers will find it much easier to grow
multiplicatively for a while, while the bigger players will end up
noticing their market share is shrinking multiplicatively due to
losing customers to the smaller players.

I believe the (a) part is fairly closely analogous to the essence of
the well-studied CUBIC dynamics. For the (b) part, we have an
algebraic analysis and simulation scripts/graphs, if there is
interest. I might be able to post those after the IETF if there is
interest.

best,
neal

On Thu, Mar 28, 2019 at 5:30 PM Mario Hock <mario.hock@kit.edu> wrote:
>
> Neal,
>
> as just discussed during the ICCRG session (jabber question), can you
> give some details about the "convergence to fairness" mechanism of BBRv2
> (intra-protocol)?
>
> Did I get you right that either MD (multiplicative decrease) or MI
> (multiplicative increase) leads to fairness, depending on buffer size?
>
> Also, did you mean increase/decrease of CWnd or of rate?
>
> Best, Mario
>
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