However, TCP is not IP, which is the point of my presentation at Vienna.
We can hardly disagree. But TCP is not SCTP, UDP, DCCP or ICMP either. The systems level argument for a layer 3.5 solution is that it can cover all cases, including ones we have not invented yet.
To deny such a sweeping argument of Brian, a counter example is just enough.
The flip side of that argument is that different transport layers have different ways of coping with multiple paths between two nodes. Currently, TCP does not; SCTP has an integrated support for multi-path; UDP is a pass-through layer and leaves multi-path handling to the application;