The Behavior and Performance of TCP Congestion Control Algorithms under Multi-path Routing

Multiple paths with equal-cost are commonly found on the Internet, which will trigger the multi-path routing to balance the traffic across those paths. On the other hand, TCP as the reliable transport protocol will treat those multiple paths of equal-cost similar to other single paths. This paper studies the behavior of several TCP Congestion Control Algorithms (e.g. Reno, CUBIC, BBR) and the impact of multi-path routing across multiple paths of equal-cost. The comparison analysis focuses on the link delay, loss rate, packet reordering. Based on the experiment results, we found TCP packet reordering occurs when multi-path routing is employed, but on the overall does not significantly degrade the measured throughput. On the link delay variations, the average RTT estimation on BBR is 20% lower than the average RTT estimation on Reno and CUBIC. While in loss rate variations from 1 to 10%, the average measured throughput on BBR is steadily achieve near the available bandwidth. On the contrary, Reno and CUBIC experience dropped of the average measured throughput up to 86% lower.