I'd like to add to this by saying that the new congestion control algorithms ought to be "friendly" to TCP to avoid congestion collapse. Backing off by a factor smaller than 2, or increasing additively by a delta more than 1 MSS per RTT are all tricks that would cause harm to normal TCP flows.

Granted that TCP isn't really "fair" between flows that have different RTTs, one could really justify tuning their TCP behaviour. But the challenge is to do this _automatically_, which is what TCP has so successfully done for the past few decades.

EDIT: DCCP decouples congestion control from reliable delivery and its congestion control algorithms are TCP friendly.