Accelerating End-host Congestion Response using P4 Programmable Switches

TL;DR

This paper introduces Reverse Path Congestion Marking (RPM), a method using P4 programmable switches to speed up congestion response times without modifying end-hosts, improving fairness and flow completion times.

Contribution

The paper presents RPM, a novel congestion signaling technique that decouples congestion feedback from downstream paths, implemented on P4 switches to enhance reaction speed.

Findings

RPM improves throughput fairness among heterogeneous RTT flows.

RPM reduces flow completion times, especially for small DCTCP flows.

Evaluation on P4 switches demonstrates effective acceleration of congestion response.

Abstract

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 rount-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.