HTTP: The Definitive Guide

Internet Cache Protocol

The Internet Cache Protocol (ICP) allows caches to look for content hits in sibling caches. If a cache does not have the content requested in an HTTP message, it can find out if the content is in a nearby sibling cache and, if so, retrieve the content from there, hopefully avoiding a more costly query to an origin server. ICP can be thought of as a cache clustering protocol. It is a redirection protocol in the sense that the final destination of an HTTP request message can be determined by a series of ICP queries.

ICP is an object discovery protocol. It asks nearby caches, all at the same time, if any of them have a particular URL in their caches. The nearby caches send back a short message saying “HIT” if they have that URL or “MISS” if they don’t. The cache is then free to open an HTTP connection to a neighbor cache that has the object.

ICP is simple and lightweight. ICP messages are 32-bit packed structures in network byte order, making them easy to parse. They are carried in UDP datagrams for efficiency. UDP is an unreliable Internet protocol, which means that the data can get destroyed in transit, so programs that speak ICP need to have timeouts to detect lost datagrams.

Here is a brief description of the parts of an ICP message:

Opcode

The opcode is an 8-bit value that describes the meaning of the ICP message. Basic opcodes are ICP_OP_QUERY request messages and ICP_OP_HIT and ICP_OP_MISS response messages.

Version

The 8-bit version number describes the ...