Low Earth Orbit and the TCP congestion control problem

In this episode of PING, APNIC’s Chief Scientist Geoff Huston discusses the rise of Low Earth Orbiting (LEO) Satellite based Internet, and the consequences for end-to-end congestion control in TCP and related protocols. Modern TCP has mostly been tuned for constant delay, low loss paths and performs very well at balancing bandwidth amongst the cooperating users of such a link, achieving maximum use of the resource. But a consequence of the new LEO internet is a high degree of variability in delay, loss and consequently an unstable bandwidth, which means TCP congestion control methods aren’t working quite as well in this kind of Internet. A problem is, that with the emergence of TCP bandwidth estimation models such as BBR, and the rise of new transports like QUIC (which continue to use the classic TCP model for congestion control), we have a fundamental mismatch in how competing flows try to share the link. Geoff has been exploring this space with some tests from starlink home routers, and models of satellite visibility. His Labs starlink page shows a visualisation of behaviour of the starlink system, and a movie of views of the satellites in orbit. Read more about TCP, QUIC, LEO and Geoff’s measurements on the APNIC Blog and APNIC Labs