ICAP, the Internet Content Adaptation Protocol, is part of an evolving architecture that facilitates better distribution and caching for the web. ICAP is, in essence, a lightweight protocol for executing a 'remote procedure call' on HTTP messages. That is, it allows ICAP clients to pass HTTP messages to ICAP servers for 'adaptation' -- some sort of transformation or other processing. The server executes its transformation service on messages and sends back responses to the client, usually with modified messages. The adapted messages may be either HTTP requests or HTTP responses.