In summary, RTT variance as a congestion signal is actually quite bad. It's normal to have variation even without congestion, so utilization would be worse if you backed off every time RTT increased.
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.
http://root.org/ip-development/e2e/end19940314-diagram.ps
In summary, RTT variance as a congestion signal is actually quite bad. It's normal to have variation even without congestion, so utilization would be worse if you backed off every time RTT increased.