Reducing the Impact of RTT Unfairness using P4-Programmable Data Planes

This paper presents a system that mitigates the Round-trip Time (RTT) unfairness issue in non-programmable networks using P4-programmable data planes. In traditional loss-based congestion control algorithms (CCAs), RTT unfairness occurs when the flows with shorter RTTs obtain higher bandwidth shares with respect to the flows with longer RTTs. This behavior occurs due to the faster recovery period that flows with shorter RTTs experience after a loss event. On the other hand, more recent CCAs, such as the Bottleneck Bandwidth and Round-trip Time (BBR), present the opposite behavior, where the flows with longer RTTs achieve higher throughput than the ones with shorter RTTs. In this paper, the proposed system employs a P4-programmable data plane to monitor the RTT of flows traversing a non-programmable router at line rate using passive taps. The P4-programmable data plane analyzes the RTT of each flow, sub-sequently segregating them into different queues. This separation is aimed at minimizing the interaction between flows with varying RTTs. Results show that implementing flow separation improves the fairness of long flows, reduces the RTT of individual flows allocated in different queues, and improves the Flow Completion Times (FCTs) of short flows. P4, RTT unfairness, Transmission Control Protocol (TCP), Congestion Control Algorithm (CCA), Bottleneck Bandwidth and Round-trip Time (BBR).