12.1 Server-driven Negotiation


If the selection of the best representation for a response is made by an algorithm located at the server, it is called server-driven negotiation. Selection is based on the available representations of the response (the dimensions over which it can vary; e.g. language, content-coding, etc.) and the contents of particular header fields in the request message or on other information pertaining to the request (such as the network address of the client).

Server-driven negotiation is advantageous when the algorithm for selecting from among the available representations is difficult to describe to the user agent, or when the server desires to send its "best guess" to the client along with the first response (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to improve the server's guess, the user agent MAY include request header fields (Accept, Accept-Language, Accept-Encoding, etc.) which describe its preferences for such a response.

Server-driven negotiation has disadvantages:
      1. It is impossible for the server to accurately determine what
         might be "best" for any given user, since that would require
         complete knowledge of both the capabilities of the user agent
         and the intended use for the response (e.g., does the user want
         to view it on screen or print it on paper?).
      2. Having the user agent describe its capabilities in every
         request can be both very inefficient (given that only a small
         percentage of responses have multiple representations) and a
         potential violation of the user's privacy.
      3. It complicates the implementation of an origin server and the
         algorithms for generating responses to a request.
      4. It may limit a public cache's ability to use the same response
         for multiple user's requests.


HTTP/1.1 includes the following request-header fields for enabling server-driven negotiation through description of user agent capabilities and user preferences: Accept (section 14.1), Accept- Charset (section 14.2), Accept-Encoding (section 14.3), Accept- Language (section 14.4), and User-Agent (section 14.43). However, an origin server is not limited to these dimensions and MAY vary the response based on any aspect of the request, including information outside the request-header fields or within extension header fields not defined by this specification.

The Vary header field can be used to express the parameters the server uses to select a representation that is subject to server- driven negotiation. See section 13.6 for use of the Vary header field by caches and section 14.44 for use of the Vary header field by servers.