RPM: Reverse Path Congestion Marking on P4 Programmable Switches

Transport layer congestion control relies on feedback signals that travel from the congested link to the receiver and back to the sender. This forward congestion control loop, first, requires at least one Round-Trip Time (RTT) to react to congestion and secondly, it depends on the downstream path after the bottleneck. The former property leads to a reaction time in the order of RTT+ bottleneck queue delay, while the second may amplify the unfairness due to heterogeneous RTT. In this paper, we present Reverse Path Congestion Marking (RPM) to accelerate the reaction to network congestion events without changing the end-host stack. RPM decouples the congestion signal from the downstream path after the bottleneck while maintaining the stability of the congestion control loop. We show that RPM improves throughput fairness for RTT-heterogeneous TCP flows as well as the flow completion time, especially for small Data Center TCP (DCTCP) flows. Finally, we show RPM evaluation results in a testbed built around P4 programmable ASIC switches.