[toast0]

You can measure jitter, but you don't know how much of the delay was due to congestion, and how much was due to other factors.

Something mentioned elsewhere in the thread is wifi physical layers may wait to send small packets until they get a few small packets to aggregate or a timeout. Other systems that aggregate packets may do something similar.

On time division multiplexed systems, it may take measurable time to wait for an assigned slot, even if the channel isn't congested. Some packets would get lucky and have a short wait and others would have a longer wait.

This would be challenging to signal as the delay is added at a hop-by-hop level, but whether the delay is significant enough to signal is unknowable at that level. Maybe you could ask all hops to add to a field indicating milliseconds of delay (or some other fixed increments), but I don't think there's room for that in existing fields and you'd have a heck of a time getting meaningful support. ECN took how long to be usable on the internet, because some middleboxes would drop connections with it enabled, to say nothing of devices actually flagging congestion.