TCP slow start is designed to begin at a conservative bitrate, but quickly ramp up to the available bandwidth. To avoid overshooting, TCP slow start has a HyStart mode (on by default in Linux) that may exit slow start before packets are lost. Unfortunately, HyStart may also exit slow start prematurely, making it take longer for TCP to reach intended bitrates. This is especially problematic for links with high bandwidth and high latency, such as an satellite Internet connection. This paper evaluates TCP HyStart performance over a commercial satellite Internet link, first evaluating how sender and receiver buffer size settings might limit throughput and subsequently assessing TCP with HyStart on versus TCP with HyStart off for both isolated and simultaneous flows. Analysis shows HyStart on (the default) significantly degrades TCP start-up performance, with average throughputs about half that of HyStart off during starting performance.

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See also:

• Saahil Claypool, Jae Chung, and Mark Claypool. Comparison of TCP Congestion Control Performance over a Satellite Network, In Proceedings of the Passive and Active Measurement Conference (PAM), Virtual Conference, March 29-31, 2021. Online at: http://www.cs.wpi.edu/~claypool/papers/tcp-sat-pam-21/