IETF RFC 2186: Internet Cache Protocol (ICP), version 2
This document describes version 2 of the Internet Cache Protocol (ICPv2) as currently implemented in two World-Wide Web proxy cache packages. ICP is a lightweight message format used for communicating among Web caches. ICP is used to exchange hints about the existence of URLs in neighbor caches. Caches exchange ICP queries and replies to gather information to use in selecting the most appropriate location from which to retrieve an object.
This document describes only the format and fields of ICP messages. A companion document (RFC2187) describes the application of ICP to Web caches. Several independent caching implementations now use ICP, and we consider it important to codify the existing practical uses of ICP for those trying to implement, deploy, and extend its use for their own purposes.