Characterising LEDBAT Performance Through Bottlenecks Using PIE, FQ-CoDel and FQ-PIE Active Queue Management

Low Extra Delay Background Transport (LEDBAT) is defined in RFC6817 as a congestion control algorithm for lower than best effort transport service that reacts to both delay and loss congestion signals. LEDBAT allows bulk transfer applications (such peer-to-peer file transfer and software updates) to utilize available capacity in the background while limiting additional forward queuing delays at network bottlenecks to 100ms. New Active Queue Management (AQM) schemes similarly aim for low latencies by dropping or marking packets when the bottleneck queuing delay exceeds thresholds much lower than 100ms. Due to renewed interest in deploying modern AQMs on home broadband services, we experimentally evaluate and characterize the impact of placing PIE, FQ-CoDel and FQ-PIE variants of AQM in the path of flows generated by libutp (a widely deployed UDP-based LEDBAT implementation). We uncover, and propose solutions to, some differences between libutp and RFC6817 that lead to poor utilization and incorrect inter-flow capacity sharing over AQM bottlenecks.