Session

Linux TCP Cubic defaults to using HyStart to exit slow start and transition to congestion avoidance, but HyStart can perform poorly over networks with delay variation, exiting slow start prematurely. HyStart++ was designed to improve HyStart by adding Conservative Slow Start, an intermediate phase between slow start and congestion avoidance that reduces the exponential growth until either confirming a transition to congestion avoidance or going back to slow start.  Our work validates an implementation of HyStart++ for Linux, confirming adherence of the code base to the Request for Comments (RFC 9406) and demonstrating functionality through case studies under known conditions.  Then, we evaluate HyStart++ over several network paths, focusing on networks with high delay variation, including satellite links. Our results show that HyStart++ improves upon HyStart by continuing to grow after HyStart would have exited slow start, leading to a higher congestion window and better early goodput in some cases. However, HyStart++’s reaction to delay variation restricts utilization,  motivating further adjustments to HyStart++ and other slow start mechanisms to improve slow start for wireless access networks.