The paper proposes the Explicit Control Protocol (XCP). It aims to address issues that TCP is not good at. For example, TCP is not very efficient in filling up the available bandwidth (not increasing fast enough) in high bandwidth environment. TCP has bias against high latency links because its throughput is inversely proportional to RTT.
XCP is a end-point AND gateway solution. Explicit congestion control information is exchanged between the end hosts and routers. The fairness controller (FC) and an efficiency controller (EC) are purposely separated for simplicity and modularity (separation of concerns). However, EC augments the feedback responses produced by FC. There is potentially a fixed-point issue here (i.e. Should EC's response affect FC's?). Also, allocation in terms of window size seems problematic. What if their windows are larger than they need?
The simulation results for XCP were unrealistically good. This is significant in that it shows how much we can improve from having more control information. It shows how much off TCP is from the "upper bound". A big problem however is the question of trust. It has to trust the end-points to cooperate by sending proper header info.