Small Packets, Big Difference: L4S Congestion Control at Low Bandwidth

Abstract

TCP Prague is a congestion control algorithm (CCA) for the low latency, low loss and scalable throughput (L4S) architecture, which targets very low queuing delay by leveraging fine-grained explicit congestion notification (ECN) signals. Most prior evaluations of Prague focus on high-rate scenarios and its TCP-friendliness. Prague can also, however, cope with low-bandwidth bottlenecks or compete with elastic and inelastic flows—leaving only a small fair share per flow. In such cases, Prague reduces its segment size to maintain approximately two packets per virtual round trip time (RTT). The current design derives the segment size from both the pacing rate and RTT, making it sensitive to noisy RTT conditions and potentially degrading performance. We redesign segment sizing to depend solely on the pacing_rate using three schemes—linear, exponential, and logarithmic. We implement these schemes in a modified Prague kernel and evaluate them in our testbed against both the default Prague segment-sizing algorithm and a fixed-segment-size baseline. On throttled links, our schemes reduce retransmissions by more than a factor of 20 and increase feedback granularity. Under both elastic (inter- and intra-CCA) and inelastic competition, they improve throughput stability and fairness.