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

automatic selection, though it also does not prevent any suchmechanism from being developed as an extension and used withinHTTP/1.1.Fielding, et al. Standards Track [Page 73]RFC 2616 HTTP/1.1 June 1999HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)status codes for enabling agent-driven negotiation when the server isunwilling or unable to provide a varying response using server-drivennegotiation.12.3 Transparent NegotiationTransparent negotiation is a combination of both server-driven andagent-driven negotiation. When a cache is supplied with a form of thelist of available representations of the response (as in agent-drivennegotiation) and the dimensions of variance are completely understoodby the cache, then the cache becomes capable of performing serverdrivennegotiation on behalf of the origin server for subsequentrequests on that resource.Transparent negotiation has the advantage of distributing thenegotiation work that would otherwise be required of the originserver and also removing the second request delay of agent-drivennegotiation when the cache is able to correctly guess the rightresponse.This specification does not define any mechanism for transparentnegotiation, though it also does not prevent any such mechanism frombeing developed as an extension that could be used within HTTP/1.1.13 Caching in HTTPHTTP is typically used for distributed information systems, whereperformance can be improved by the use of response caches. TheHTTP/1.1 protocol includes a number of elements intended to makecaching work as well as possible. Because these elements areinextricable from other aspects of the protocol, and because theyinteract with each other, it is useful to describe the basic cachingdesign of HTTP separately from the detailed descriptions of methods,headers, response codes, etc.