XCP

This paper details the eXtensible Control Protocol, which isn't particularly extensible. However, it attempts to improve upon TCP, particularly in instances of high delay/bandwidth product. This is done by breaking end-to-end and sticking a layer above IP in the routers that explicitly informs the protocol users about current congestion, and some complicated mathematics.
I found this interesting as I have worked on DTN, something else that works on TCP's irresponsible on high delay links. The systems seem unrelated, as this relies on ACKs for transfer of information, which DTN does not assume.
The most interesting aspect of the paper is the complete disregard of the end-to-end principle. I'm not sure how meaningful this is, tying the congestion control into the routers. TCP does a beautiful thing in assuming drops mean congestion. This is an assumption based on the lower layer, requiring no changes at the router. I feel as though a more interesting research agenda may be to reassess the assumptions the transport layer can make about the IP layer, now that the IP layer has RED and such. Perhaps backing off less on a drop, these things.