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The Internet Cache Protocol (ICP) is a lightweight way for web caches to ask neighboring caches whether they have a requested object. It helps a cache decide where to fetch an object; it does not normally carry the object itself. HTTP handles the content transfer.
What ICP does in a web cache
When a cache does not have a requested object locally, it can send ICP queries to nearby caches and receive replies such as HIT or MISS. Those replies are hints about availability, which the querying cache can use to choose a source. The object is then retrieved through the appropriate web-transfer path, typically HTTP. RFC 2186 describes ICP as communication between web caches, while RFC 2187 explains how it can be applied in cache hierarchies.
In the hierarchy described by RFC 2187, a parent cache sits higher in the hierarchy and a sibling cache is at the same level. A neighbor or peer is one hop away. A cache may query these peers after a local miss. RFC 2186 says the query-and-reply exchange typically needs to complete within one or two seconds before the cache starts retrieving elsewhere; that is the RFC’s design context, not a present-day performance guarantee.
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How ICP differs from HTTP
| Protocol | Role in this process |
|---|---|
| ICP | Exchanges cache-availability hints between caches. |
| HTTP | Transfers the requested object from the selected source. |
A HIT reply therefore does not mean that the ICP message itself contains the object. The ordinary ICP role is to report availability; HTTP remains responsible for transferring the content and applying its normal processing.
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What ICPv2 messages contain
RFC 2186 documents ICP version 2 (ICPv2). Each message has a fixed 20-octet header followed by a variable payload. The header includes an opcode, version, message length, request number, options, option data and sender host address. Values use network byte order, and the message length cannot exceed 16,384 octets. Depending on the opcode, the payload most often contains a null-terminated URL string. RFC 2186 specifies the message format.
Common replies include HIT and MISS, alongside error-related opcodes. These are status signals used in cache coordination, not a substitute for a successful HTTP object transfer.
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Why ICP commonly uses UDP—and what that means
RFC 2186 describes UDP as common practice, but does not require ICP to use UDP exclusively. The RFC’s rationale is that cache queries need quick replies and UDP avoids TCP setup overhead. A missing reply can also indicate a failed or congested path. These are design considerations in the 1997 memo, not evidence that UDP guarantees faster results in every current deployment.
Because UDP is connectionless, reply loss and security need consideration in deployment. RFC 2187 discusses these concerns alongside the effect of cache configuration on ICP’s usefulness. The protocol’s value depends on the particular cache setup: RFC 2187 notes that ICP may be a net benefit in some situations and a net cost in others.
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Why ICP_OP_HIT_OBJ is discouraged
ICP_OP_HIT_OBJ is an exceptional opcode intended to include object data in an ICP reply. RFC 2186 does not recommend this use. Embedding content this way can bypass ordinary HTTP processing, including authorization and age validation. It can also make UDP messages large enough to fragment. For these reasons, a HIT_OBJ response should not be treated as equivalent to retrieving an object through the normal HTTP path. RFC 2186 documents the warning.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What ICPv2’s standard status means
RFC 2186 is an informational memo documenting ICPv2 as implemented in proxy-cache packages; it explicitly does not specify an Internet standard. RFC 3040 later included ICP in a broader taxonomy of web replication and caching protocols, but that classification does not make ICP a universal requirement or establish how widely it is used today. RFC 3040 provides the broader taxonomy.
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The cited RFCs describe the protocol and its application, but do not establish current adoption across cache products. For compatibility or configuration decisions, check the documentation for the specific cache implementation in use.
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