From my completely unscientific tinkering with Vegas idea few years ago I remember that there was an easily detectable pattern in RTT change during the congestion. I was testing using UDP between my home and a colo server and I saw RTT climb up to several times its original level in a matter of two round trips.

> It's normal to have variation even without congestion

Certainly, but the question is if congestion-based variation is substantially different from an ambient variation. I am guessing that it is. Reacting to just a RTT increase is pretty dumb. The congestion marker clearly needs to be a bit more advanced than that.