This paper describes the eXtensible Control Protocol and aims to address TCP’s inherent instability (overestimating and resizing the window), and conservative nature (slow-start). Like in RED, XCP involves the router to provide an explicit feedback. Each data packet has a feedback field updated by the routers along the path, and eventually returned to the sender via ACKs. Since every router updates the feedback field, the sender can identify the “weakest link” and behave accordingly. I think XCP is a very powerful idea and evaluations show that it clearly performs better than TCP.
While this paper leaves itself open to criticism of nodes manipulating the feedback fields and hijacking the router resources, I think that is a larger problem and wouldn’t criticize XCP in particular. Most ideas that propose differentiated servicing (QoS), have to deal with the problem of admission control and dishonest hosts. I think Scott’s paper on the future of the Internet also mentions the point of admission control. A possible solution to that problem could be on the lines of deploying some sort of a middle-box that monitors the tampering of the feedback field (assuming symmetric routes) and incorporation of the feedback by the hosts. Solutions like XCP can be very useful for solving DDoS problems.
Overall, I found the paper very interesting and suggest we keep it in the list. Also, if this stays, RED can be replaced by a different paper.