Are we heading towards a BBR-dominant Internet?

Since its introduction in 2016, BBR has grown in popularity rapidly and likely already accounts for more than 40% of the Internet's downstream traffic. In this paper, we investigate the following question: given BBR's performance benefits and rapid adoption, is BBR likely to completely replace CUBIC just like how CUBIC replaced New Reno? We present a mathematical model that allows us to estimate BBR's throughput to within a 5% error when competing with CUBIC flows. Using this model, we show that even though BBR currently has a throughput advantage over CUBIC, this advantage will be diminished as the proportion of BBR flows increases. Therefore, if throughput is a key consideration, it is likely that the Internet will reach a stable mixed distribution of CUBIC and BBR flows. This mixed distribution will be a Nash Equilibrium where none of the flows will have the performance incentive to switch between CUBIC and BBR. Our methodology is also applicable to other recently proposed congestion control algorithms, like BBRv2 and PCC Vivace. We make a bold prediction that BBR is unlikely to completely replace the CUBIC on the Internet in the near future.