We describe the structure and functionality of the Internet Cache Protocol (ICP) and its implementation in the Squid Web Caching software. ICP is a lightweight message format used for communication among Web caches. Caches exchange ICP queries and replies to gather information to use in selecting the most appropriate location from which to retrieve an object.

We present background on the history of ICP, and discuss issues in ICP deployment, efficiency, security, and interaction with other aspects of Web trac behavior. We catalog successes, failures, and lessons learned from using ICP to deploy a global Web cache hierarchy.