Coupled Slow-Start: Improving the Efficiency and Friendliness of MPTCP's Slow-Start

Abstract

Multipath TCP (MPTCP) is designed to offer higher throughput than single-path TCP, and meanwhile MPTCP flow is fair to concurrent TCP flows at the bottleneck. Although the coupled congestion control in current MPTCP can achieve the goals by coupling different subflows, it only focuses on Congestion Avoidance but each subflow still behaves like an independent TCP flow in Slow-Start. However, during Slow-Start, MPTCP is unfair to concurrent TCP flows as it uses more network resources at the shared bottleneck than single-path TCP. Worse still, since the exponential growth of multiple concurrent subflows' congestion windows often results in serious buffer overflow and packet loss at the shared bottleneck, the latency of short flows using MPTCP is often not as good as using TCP. This leads to the fact that MPTCP cannot satisfy the above design goals when handling short flows. To address this issue, we present a Coupled Slow-Start (CSS) Algorithm in this paper. CSS links the exponential growth of subflows' congestion windows to ensure the fairness and reduce the burstiness of Slow- Start. To reduce the packet loss, CSS resets the Slow-Start Threshold (ssthresh) of different subflows for MPTCP to safely move to Congestion Avoidance when it achieves its expected throughput. Our simulation shows that CSS can lower short flows' latency of up to 45% and significantly reduce the packet loss in two typical network environments, meanwhile CSS is TCP-friendly at the shared bottleneck. Simulation results also indicate that CSS can perform at least as well as original MPTCP for the bulk data transfer in common network environments.