Fairness Improvement of BBR Congestion Control Algorithm for Different RTT Flows

BBR is a new congestion-based congestion control algorithm for improving performance of traditional TCP by proposed Google recently. Unlike other loss-based congestion control algorithms used so far, BBR determines the sending rate by measuring bottleneck bandwidth and round-trip propagation time. It was designed to operate without creating persistent queues in the bottleneck. However, it has been studied that in many situations, a persistent queue is formed which causes unintended problems. Most significant problem is the fairness among flows with different round-trip times (RTTs). Contrary to the typical behaviors in a traditional TCP, short RTT flow is always overwhelmed by the long RTT flow in BBR. Surprisingly, the short RTT flow's share is affected even with a minor RTT difference between flows. This paper proposes a bottleneck queue buildup suppression method to enhance the fairness between different RTT flows. We showed our proposed method could enhance significantly fairness, compared with the original BBR.