With few exceptions, the path to deployment for any Internet technology requires that there be some benefit to unilateral adoption of the new technology. In an Internet where the technology is not fully deployed, is an individual better off sticking to the status quo, or adopting the new technology? This question is especially relevant in the context of the Low Latency, Low Loss, Scalable Throughput (L4S) architecture, where the full benefit is realized only when compatible protocols (scalable congestion control, accurate ECN, and flow isolation at queues) are adopted at both endpoints of a connection and also at the bottleneck router. In this paper, we consider the perspective of the sender of an L4S flow using scalable congestion control, without knowing whether the bottleneck router uses an L4S queue, or whether other flows sharing the bottleneck queue are also using scalable congestion control. We show that whether the sender uses TCP Prague or BBRv2 as the scalable congestion control, it cannot be assured that it will not harm or be harmed by another flow sharing the bottleneck link. We further show that the harm is not necessarily mitigated when a scalable flow shares a bottleneck with multiple classic flows. Finally, we evaluate the approach of BBRv3, where scalable congestion control is used only when the path delay is small, with ECN feedback ignored otherwise, and show that it does not solve the coexistence problem.

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