Network Working Group R. Fielding Request for Comments: 2616 ...

RFC 2616 HTTP/1.1 June 199913.5 Constructing Responses From CachesThe purpose of an HTTP cache is to store information received inresponse to requests for use in responding to future requests. Inmany cases, a cache simply returns the appropriate parts of aresponse to the requester. However, if the cache holds a cache entrybased on a previous response, it might have to combine parts of a newresponse with what is held in the cache entry.13.5.1 End-to-end and Hop-by-hop HeadersFor the purpose of defining the behavior of caches and non-cachingproxies, we divide HTTP headers into two categories:- End-to-end headers, which are transmitted to the ultimaterecipient of a request or response. End-to-end headers inresponses MUST be stored as part of a cache entry and MUST betransmitted in any response formed from a cache entry.- Hop-by-hop headers, which are meaningful only for a singletransport-level connection, and are not stored by caches orforwarded by proxies.The following HTTP/1.1 headers are hop-by-hop headers:- Connection- Keep-Alive- Proxy-Authenticate- Proxy-Authorization- TE- Trailers- Transfer-Encoding- UpgradeAll other headers defined by HTTP/1.1 are end-to-end headers.Other hop-by-hop headers MUST be listed in a Connection header,(section 14.10) to be introduced into HTTP/1.1 (or later).13.5.2 Non-modifiable HeadersSome features of the HTTP/1.1 protocol, such as DigestAuthentication, depend on the value of certain end-to-end headers. Atransparent proxy SHOULD NOT modify an end-to-end header unless thedefinition of that header requires or specifically allows that.