So the sysctl is host ECN, and the altq part is router ECN? If yes, what prevents us from enabling host ECN by default (what are the drawbacks), either by switching the sysctl or via a rc.conf setting? I am aware that no rc.conf support exists yet, but IMO this would be easy to write.
What is the difference between host ECN and router ECN? For me (with just the Wikipedia-knowledge about ECN) it looks like in both cases a system should look if any buffer involved passed a specific fill-level and then send a ECN message. I can understand that different AQM algorithms may want to modify the ECN trigger level/behavior, so it is maybe a little bit more complicated, but from a big picture view I assume this is what is happening and I do not see a difference between host and router behavior.
Can someome please shed some light on it. An URL would be sufficient.