with the variant list, in a choice response.

   A server may only choose on behalf of a user agent supporting
   transparent content negotiation if the user agent explicitly allows
   the use of a particular remote variant selection algorithm in the
   Negotiate request header.  User agents with sophisticated internal
   variant selection algorithms may want to disallow a remote choice, or
   may want to allow it only when retrieving inline images.  If the
   local algorithm of the user agent is superior in only some difficult
   areas of negotiation, it is possible to enable the remote algorithm
   for the easy areas only.  More information about the use of a remote
   variant selection algorithm can be found in [3].

   Choice responses are covered in section 10.2.  For example, the
   choice response in the above picture could be:

     HTTP/1.1 200 OK
     Date: Tue, 11 Jun 1996 20:05:31 GMT
     TCN: choice
     Content-Type: text/html
     Last-Modified: Mon, 10 Jun 1996 10:01:14 GMT
     Content-Length: 5327
     Cache-control: max-age=604800
     Content-Location: paper.1
     Alternates: {"paper.1" 0.9 {type text/html} {language en}},




 
RFC 2295            Transparent Content Negotiation           March 1998


                 {"paper.2" 0.7 {type text/html} {language fr}},
                 {"paper.3" 1.0 {type application/postscript}
                     {language en}}
     Etag: "gonkyyyy;1234"
     Vary: negotiate, accept, accept-language
     Expires: Thu, 01 Jan 1980 00:00:00 GMT

     A paper about ....

   Finally, the above two kinds of optimization can be combined; a
   caching proxy which has the list will sometimes be able to choose on
   behalf of the user agent.  This could lead to the following
   communication pattern:

      Server _____ proxy _____ proxy __________ user
      x.org        cache       cache            agent

                                 < ---------------
                                 |  GET ../paper
                                 |  small Accept
                                 |
                              able to choose
                                on behalf
                                 |
                     < ----------
                     |  GET ../paper.1
                     |
                      ---------- >   [normal response]
                        paper.1  |
                                  ---------------- >  [choice response]
                                   paper.1 and list

   Note that this cutting of corners not only saves bandwidth, it also
   eliminates delays due to packet round trip times, and reduces the
   load on the origin server.

4.5 Downwards compatibility with non-negotiating user agents

   To handle requests from user agents which do not support transparent
   content negotiation, this specification allows the origin server to
   revert to a HTTP/1.0 style negotiation scheme.  The specification of
   heuristics for such schemes is beyond the scope of this document.










 
RFC 2295            Transparent Content Negotiation           March 1998


4.6 Retrieving a variant by hand

   It is always possible for a user agent to retrieve the variant list
   which is bound to a negotiable resource.  The user agent can use this
   list to make available a menu of all variants and their
   characteristics to the user.  Such a menu allows the user to randomly
   browse other variants, and makes it possible to manually correct any
   sub-optimal choice made by the automatic negotiation process.

4.7 Dimensions of negotiation