greenbytes The Hypertext Transfer Protocol (HTTP) specifies a state identifier, called "Entity Tag", to be returned in the "ETag" response header. However, the description of this header for write operations such as PUT is incomplete, and has caused confusion among developers and protocol designers, and potentially interoperability problems. This document explains the problem in detail and suggests both a clarification for a revision to the HTTP/1.1 specification (RFC2616) and a new header for use in responses, making HTTP entity tags more useful for user agents that want to avoid round-trips to the server after modifying a resource.