IETag

IETag is not a widely recognized standalone standard term. In most technical contexts, what is meant is ETag, the entity tag used in HTTP for cache validation and conditional requests. The acronym “IETag” is sometimes encountered as a misnomer or shorthand that blends the IETF (Internet Engineering Task Force) and ETag, but there is no separate IETF specification specifically for “IETag.” The official standardization of ETag is through IETF publications, notably RFCs that define how entity tags function in HTTP.

An ETag is a validator that identifies a specific version of a resource. When a server responds to a request for a resource, it may include an ETag header with a quoted value. Clients store this value and can reuse it in subsequent requests using conditional headers such as If-None-Match to ask whether the resource has changed. If the ETag matches the current version, the server can respond with a 304 Not Modified status, saving bandwidth. Conversely, If-Match can be used by clients to ensure a write operation (like PUT) only succeeds if the resource has not changed since it was last retrieved.

ETags can be strong or weak. A strong ETag requires byte-for-byte equivalence, while a weak ETag (prefixed with W/) signals semantic equivalence for cached content that may have undergone non-substantive changes. ETag values are generated by the server and may be hashes, version numbers, or other identifiers.

See also: ETag, HTTP caching, If-None-Match, If-Match, RFC 7232.