The paper is about delay-based congestion control algorithms such as BBR, Vegas, etc. From their conclusion: "We offer three key conclusions for CCA designers

The paper is about delay-based congestion control algorithms such as BBR, Vegas, etc. From their conclusion: "We offer three key conclusions for CCA designers, who should model (or better estimate) non-congestive delays explicitly in delay-convergent CCAs. First, to utilize the link efficiently, a CCA must maintain a queue that is larger than the non-congestive delay on the path; second, this alone is not enough to avoid starvation, but in addition the variation in the queueing delay in steady state must also be greater than one-half of the delay jitter; and third, if we have a prior upper bound on sending rate, we may be able to avoid starvation while also reducing the queueing delay variation." So what non-congestive delays are the problem? A key one is the way modern WiFi agregates multiple packets into AMPDUs for transmission to reduce the number of medium acquisitions it needs to perform. When WiFi is under heavy load this gives good throughput, at the expense of per-packet jitter. If I understand correctly, their conclusions mean delay-based congestion control loses the signal it needs to converge when the target queue size is similar to the non-congestive jitter. Many of us working on congestion control have wondered about this in the context of BBR, and it's great that this paper formalizes the issue. The implication is that such delay-based schemes need to add perhaps a few tends of milliseconds to their queue delay targets to avoid potential starvation. Doesn't seem like a real roadblock, as per the article title, but the desire to reduce latency further does perhaps increase the incentive for more ECN/AQM research. It's perfectly possible to return ECN congestion marking before a queue starts to build, so this latency cost isn't fundamental.