  The Low Latency, Low Loss, Scalable Throughput (L4S) architecture has the
potential to reduce queuing delay when it is deployed at endpoints and routers
throughout the Internet. However, it is not clear how TCP Prague, a prototype
scalable congestion control for L4S, behaves when L4S is not yet universally
deployed. Specifically, we consider the question: in a partial L4S deployment,
will a user benefit by unilaterally switching from the status quo TCP to TCP
Prague? To address this question, we evaluate the performance of a TCP Prague
flow when sharing an L4S or non-L4S bottleneck queue with a non-L4S flow. Our
findings suggest that the L4S congestion control, TCP Prague, has less
favorable throughput or fairness properties than TCP Cubic or BBR in some
coexistence scenarios, which may hinder adoption.
