ISI This document contains an initial specification for the Explicit Control Protocol (XCP), an experimental congestion control protocol. XCP is designed to deliver the highest possible end-to-end throughput over a broad range of network infrastructure, including links with very large bandwidth-delay products, which are not well served by the current control algorithms. XCP is potentially applicable to any transport protocol, although initial testing has applied it to TCP in particular. XCP routers are required to perform a small calculation on congestion state carried in each data packet. XCP routers also periodically recalculate the local parameters required to provide fairness. On the other hand, there is no per-flow congestion state in XCP routers. XCP is currently not ready for wide-scale deployment on the public Internet and the intent of this document is to provide a starting point for future experimentation and development as well as to record the authors' implementation experiences and caveats.