Alternates = "Alternates" ":" variant-list

       variant-list = 1#( variant-description
                        | fallback-variant
                        | list-directive )

       fallback-variant = "{" <"> URI <"> "}"

       list-directive = ( "proxy-rvsa" "=" <"> 0#rvsa-version <"> )




 
RFC 2295            Transparent Content Negotiation           March 1998


                        | extension-list-directive

       extension-list-directive =
                        token [ "=" ( token | quoted-string ) ]

   An example is

     Alternates: {"paper.1" 0.9 {type text/html} {language en}},
                 {"paper.2" 0.7 {type text/html} {language fr}},
                 {"paper.3" 1.0 {type application/postscript}
                     {language en}},
                 proxy-rvsa="1.0, 2.5"

   Any relative URI specified in a variant-description or fallback-
   variant field is relative to the request-URI.  Only one fallback-
   variant field may be present.  If the variant selection algorithm of
   the user agent finds that all described variants are unacceptable,
   then it SHOULD choose the fallback variant, if present, as the best
   variant.  If the user agent computes the overall quality values of
   the described variants, and finds that several variants share the
   highest value, then the first variant with this value in the list
   SHOULD be chosen as the best variant.

   The proxy-rvsa directive restricts the use of remote variant
   selection algorithms by proxies. If present, a proxy MUST ONLY use
   algorithms which have one of the version numbers listed, or have the
   same major version number and a higher minor version number as one of
   the versions listed.  Any restrictions set by proxy-rvsa come on top
   of the restrictions set by the user agent in the Negotiate request
   header.  The directive proxy-rvsa="" will disable variant selection
   by proxies entirely.  Clients SHOULD ignore all extension-list-
   directives they do not understand.

   A variant list may contain multiple differing descriptions of the
   same variant.  This can be convenient if the variant uses conditional
   rendering constructs, or if the variant resource returns multiple
   representations using a multipart media type.

8.4 Negotiate

   The Negotiate request header can contain directives for any content
   negotiation process initiated by the request.

      Negotiate = "Negotiate" ":" 1#negotiate-directive

      negotiate-directive = "trans"
                          | "vlist"
                          | "guess-small"




 
RFC 2295            Transparent Content Negotiation           March 1998


                          | rvsa-version
                          | "*"
                          | negotiate-extension

      negotiate-extension = token [ "=" token ]

   Examples are

      Negotiate: 1.0, 2.5
      Negotiate: *

   The negotiate directives have the following meaning

      "trans"
        The user agent supports transparent content negotiation for
        the current request.

      "vlist"
        The user agent requests that any transparently negotiated
        response for the current request includes an Alternates
        header with the variant list bound to the negotiable resource.
        Implies "trans".

      "guess-small"
        The user agent allows origin servers to run a custom algorithm
        which guesses the best variant for the request, and to return
        this variant in a choice response, if the resulting choice