Reproducing and Solving a Fallback Issue in TCP Prague.

TCP Prague is a proposed congestion control for L4S, a new Internet architecture. L4S promises very low queuing delay on the Internet. It achieves this by a fundamental change to the meaning of Explicit Congestion Notification (ECN) marks; Active Queuing Management (AQM) should mark packets with ECN a lot earlier and more frequently to better communicate the congestion state to congestion control algorithms. These algorithms use this information to fine tune their sending rate. TCP Prague does so by reducing its congestion window marginally on each marked packet, like Data Center TCP. It would, however, starve classic flows if they were to compete on a non-L4S ECN AQM, as this behavior conflicts with the classic interpretation of ECN marks. To avoid this, Prague tries to detect these cases and fall back to classic behavior to ensure fair flow rate sharing at such bottlenecks. A recent study by Sarpkaya et al. has shown this fallback to significantly hurt the Prague flow in L4S environments. We reproduce the issue and analyze it in different scenarios to highlight the severity of the issue. We then use eBPF to analyze the fallback implementation and find a bug that causes Prague always to fall back to classic behavior, even on an L4S AQM. We then also visualize the sharing characteristics of Prague in different scenarios, with the fallback corrected, and show that Prague now behaves as expected.