TCP Congestion Control Performance over Starlink

We examine the performance of 14 different Linux TCP congestion control (CC) variants over Starlink connectivity. We then focus on the two most commonly used CCs, BBR and Cubic, and the best performing other CC, Illinois. Multiple measurement campaigns were conducted to evaluate throughput performance for single and multiple flows, examine how the performance of the CCs relate to the length of the TCP flow, and investigate how the CCs share resources over Starlink. The results show that BBR (both v1 and v3) outperform Cubic in the current Starlink environment. Cubic, which is the Linux default CC, performs considerably worse than BBR, particularly for single flows. Cubic with HyStart also has additional slowstart performance issues, which can be mitigated by disabling HyStart. The Illinois CC is a promising alternative to BBR and although being less performant for single flows, it has better fairness properties. Leveraging earlier similar measurements for a longitudinal study, we further show that changes made to the Starlink infrastructure during 2023/2024 are likely to have resulted in significantly reduced Cubic throughput.