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.