Our study of the transport layer leads me to firmly believe that the end to end principle works so much better when the ends are informed about what is going on in the middle.
It seems that we spent almost all of our time discussing TCP congestion. Congestion is something that happens in the middle. The response to congestion is for the ends to do something about it. Many, many approaches have been proposed, yet all follow a common pattern, figure out what is going on in the middle and change your sending rate accordingly.
The advent of XCP and VCP shows to me that the most effective way to figure out what is going on in the middle, is for the middle to give explicit feedback to the ends. If this is true, then this may have implications which extend beyond just the transport layer. Explicit feedback from the middle to the ends may have broad applicability.