[Podcast] Measuring Starlink TCP performance

By on 2 May 2024

Category: Tech matters

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In this episode of PING, APNIC’s Chief Scientist Geoff Huston discusses Starlink, examining how modern TCP flow control algorithms manage the considerable variability in loss and delay evident across the satellite network. His recent investigations involve conducting measurements with Starlink terminals in both Australia and the USA, at various times of the day, to understand the system’s behaviour more comprehensively.

Starlink has broken new ground in Low Earth Orbit (LEO) Internet services. Unlike Geostationary Earth Orbit (GEO) satellite services, which have a long delay but constant visibility of the satellite in stationary orbit above, Starlink requires the consumer’s equipment to continuously reselect a new satellite as they move overhead in orbit. In fact, Starlink LEOs require a new satellite to be picked every 15 seconds. This means there’s a high degree of variability in the behaviour of the link, both between signal quality to each satellite, and in the brief interval of loss occurring at each satellite reselection window.

It’s a miracle TCP can survive, and even excel with protocols like BBR, under the right conditions. This is largely due to the transition from the traditional slow start, fast backoff model found in Cubic and Reno protocols to a more assertive approach in bandwidth estimation. With BBR, TCP continuously explores available bandwidth, pushing its throughput to remarkable levels when conditions allow.

Read more about satellites, TCP, and flow control algorithms on the APNIC Blog and the IETF website.

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