Analysis of Careful Resumption of Internet Congestion Control from Retained Path State

This paper presents the first detailed evaluation of Careful Resume, a new Congestion Control mechanism for Internet transports designed to accelerate connection startup based on retained path state. Traditional Congestion Control relies on Slow Start, which introduces significant latency for paths with a high Bandwidth-Delay Product. Careful Resume employs safety mechanisms to prevent excessive congestion when end-to-end signals suggest that the path characteristics, such as bandwidth or round-trip time (RTT), may have changed. We evaluate this safety using a testbed, followed by experiments using real-world Internet paths, and finally present initial experiences from use in a content delivery node.
Results show that this new mechanism can significantly reduce the Time To Last Byte for a connection. In our tested scenarios, the reduction was by more than three RTTs. This finding aligns with results from testing Internet paths, where we observed reductions ranging from 2.8 to 3.4 RTTs on geostationary satellite and 4G/5G mobile paths. Our results also show that Careful Resume can be effectively used with HyStart++ to avoid further path capacity overshoot. This analysis demonstrates the benefit of using Careful Resume, and provides evidence that this mechanism does not unduly harm other connections that share the capacity of a network bottleneck.