Round-trip times provide an upper bound on what should be considered a burst, since one RTT is enough time to provide feedback to throttle a burst. Of course, different traffic flows through a bottleneck can have drastically different RTTs, so this isn't too helpful.
In practice, CoDel works well for almost all current network technologies with its target parameter at the default of 5ms of allowable queueing delay. Given the structure of today's networks, an individual packet is unlikely to pass through more than a handful of congested bottlenecks, and a small multiple of 5ms of added delay is a tolerable worst-case comparable to the speed of light delays of long-distance connections.
It's definitely somewhat unsatisfying to not have formal derivations of optimal parameters. But reasonable defaults that have been tested in the real world are still a huge improvement over the old way of having network devices that don't even attempt to handle congestion intelligently.
In practice, CoDel works well for almost all current network technologies with its target parameter at the default of 5ms of allowable queueing delay. Given the structure of today's networks, an individual packet is unlikely to pass through more than a handful of congested bottlenecks, and a small multiple of 5ms of added delay is a tolerable worst-case comparable to the speed of light delays of long-distance connections.
It's definitely somewhat unsatisfying to not have formal derivations of optimal parameters. But reasonable defaults that have been tested in the real world are still a huge improvement over the old way of having network devices that don't even attempt to handle congestion intelligently.