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

used.Multiple warnings MAY be attached to a response (either by the originserver or by a cache), including multiple warnings with the same codenumber. For example, a server might provide the same warning withtexts in both English and Basque.When multiple warnings are attached to a response, it might not bepractical or reasonable to display all of them to the user. Thisversion of HTTP does not specify strict priority rules for decidingwhich warnings to display and in what order, but does suggest someheuristics.13.1.3 Cache-control MechanismsThe basic cache mechanisms in HTTP/1.1 (server-specified expirationtimes and validators) are implicit directives to caches. In somecases, a server or client might need to provide explicit directivesto the HTTP caches. We use the Cache-Control header for this purpose.The Cache-Control header allows a client or server to transmit avariety of directives in either requests or responses. Thesedirectives typically override the default caching algorithms. As ageneral rule, if there is any apparent conflict between headervalues, the most restrictive interpretation is applied (that is, theone that is most likely to preserve semantic transparency). However,Fielding, et al. Standards Track [Page 77]RFC 2616 HTTP/1.1 June 1999in some cases, cache-control directives are explicitly specified asweakening the approximation of semantic transparency (for example,"max-stale" or "public").The cache-control directives are described in detail in section 14.9.13.1.4 Explicit User Agent WarningsMany user agents make it possible for users to override the basiccaching mechanisms. For example, the user agent might allow the userto specify that cached entities (even explicitly stale ones) arenever validated. Or the user agent might habitually add "Cache-Control: max-stale=3600" to every request. The user agent SHOULD NOTdefault to either non-transparent behavior, or behavior that resultsin abnormally ineffective caching, but MAY be explicitly configuredto do so by an explicit action of the user.