When less is more: BBR versus LEDBAT++

Abstract

We present a study of the interaction between two recent congestion control algorithms, LEDBAT++ and BBR. LEDBAT++ is a less-than-best-effort congestion control mechanism and BBR is model-based congestion control algorithm that aims to implement a best-effort traffic class. Naturally, the expectation is that LEDBAT++ yields in front of BBR. While we observe that this is the case in networks with moderate size buffer and when the base RTT is larger than 60 ms, we find that in other common conditions (base RTT smaller than 60 ms and moderate size network buffers), the interaction between LEDBAT++ and BBR is not as expected, as LEDBAT++ fails to yield in front of BBR. The observed behavior is different for the two existent versions of BBR. Specifically, LEDBAT++ competes in equal grounds against BBRv1, and BBRv2 yields in front of LEDBAT++. The BBRv2/LEDBAT++ interaction is exactly the opposite to the desired outcome. We present a number of experiments in diverse conditions that support our conclusions.