Towards a Deeper Understanding of TCP BBR Congestion Control

In 2016, Google published the bottleneck bandwidth and round-trip time (BBR) congestion control algorithm. Unlike established loss- or delay-based algorithms like CUBIC or Vegas, BBR claims to operate without creating packet loss or filling buffers. Because of these prospects and promising initial performance results, BBR has gained wide-spread attention. As such it has been subject to behavior and performance analysis, which confirmed the results, but also revealed critical flaws.Because BBR is still work in progress, measurement results have limited validity for the future. In this paper we present our publicly available framework for reproducible TCP measurements based on network emulation. In a case study, we analyze the TCP BBR algorithm, reproduce and confirm weaknesses of the current BBR implementation, and provide further insights. We also contribute an analysis of BBR's inter-flow synchronization behavior, showing that it reaches fairness equilibrium for long lived flows.