  Low Earth orbit (LEO) satellite mega-constellations with hundreds or
thousands of satellites and inter-satellite links (ISLs) have the potential to
provide global end-to-end connectivity. Furthermore, if the physical distance
between source and destination is sufficiently long, end-to-end routing over
the LEO constellation can provide lower latency when compared to the
terrestrial infrastructure due to the faster propagation of electromagnetic
waves in space than in optic fiber. However, the frequent route changes due to
the movement of the satellites result in the out-of-order delivery of packets,
causing sudden changes to the Round-Trip Time (RTT) that can be misinterpreted
as congestion by congestion control algorithms. In this paper, the performance
of three widely used congestion control algorithms, Cubic, Reno, and BBR, is
evaluated in an emulated LEO satellite constellation with Free-Space Optical
(FSO) ISLs. Furthermore, we perform a sensitivity analysis for Cubic by
changing the satellite constellation parameters, length of the routes, and the
positions of the source and destination to identify problematic routing
scenarios. The results show that route changes can have profound transient
effects on the goodput of the connection, posing problems for typical broadband
applications.
