Improving TCP Fairness in Non-Programmable Networks Using P4-Programmable Data Planes

Round-trip Time (RTT) unfairness arises in traditional TCP loss-based Congestion Control Algorithms (CCAs) such as CUBIC when flows with shorter RTTs are allocated more bandwidth than those with longer RTTs. Conversely, newer CCAs such as the Bottleneck Bandwidth and Round-trip Time (BBR) exhibit the opposite behavior, where flows with longer RTTs are allocated more bandwidth than those with shorter RTTs. This paper presents a system that reduces RTT unfairness by rebalancing the router's queue according to the RTT of TCP flows. The proposed system uses a P4-programmable Data Plane (PDP) as a measurement tool to process a copy of the traffic on the network, between non-programmable (traditional) routers. The PDP generates per-flow fine-grained measurements of the RTT and the throughput of TCP flows, and creates control rules which are applied to a non-programmable router. These rules distribute the TCP flows into separate queues, according to a classification algorithm that mitigates RTT unfairness. Results show that the system improves fairness among competing flows and reduces the Flow Completion Time (FCT) of long flows, regardless of the CCA. Moreover, the system is capable of measuring the throughput of individual flows and rebalancing the bandwidth allocated to each queue. This approach ensures that the available bandwidth is effectively used.