Congestion Control for High Bandwidth-Delay Product Networks

This paper presents a new transport layer protocol called the Explicit Control Protocol (XPC). XPC addresses the problems of TCP where the the bandwidth is underutilized when TCP conservatively controls congestions through slow start and fairness control through the gateways. The proposal for XPC is to use control theory to model the protocol, and decouple the fairness and efficiency control mechanisms used to maintain flexibility and control. However, unlike the previous paper we read where the routers drop packets to control congestion, XPC instead appends a congestion header to all its data packets being sent around. The routers fill out congestion information in the congestion header for those packets and as the packets get routed from destination to destination, the most congested router would be the last to update the packet header, which then the end nodes use to control their sending window.

The XPC router splits efficiency and fairness into two separate algorithms, where as TCP uses the drop packet mechanism to maintain both. This allows the XPC protocol to efficiently use the underused bandwidth when recovering from a congestion slow down. And because it is decoupled from the fairness control, it doesn't have to worry about nodes not getting bandwidth, it just needs to keep the total utilized bandwidth at a high efficiency. The Fairness controller is the one that maintains the bandwidth fairness of the router.

The results of this paper seems to me that is was almost unrealistically good. There were almost no dropped packets, and the performance of the protocol is good when network is un-congested, and also good when the network is congested. Intuitively it seems that because this protocol and gateway doesn't drop packets (intentionally), the quality should be better, and it was built to overcome the flaws of TCP regarding recovery of congestion control mechanisms, thus, its performance can only be better than TCP, while the overhead includes an added packet header for each packet with different control algorithms. It's hard to believe just based on those it can have such a promising results.