absence of particular feature tags in the user agent affects the
   overall quality of the variant.

       feature-list = 1%feature-list-element

       feature-list-element = ( fpred | fpred-bag )
                              [ ";" [ "+" true-improvement  ]
                                    [ "-" false-degradation ]
                              ]

       fpred-bag = "[" 1%fpred "]"

       true-improvement   =  short-float
       false-degradation  =  short-float

   Features attributes are used in variant descriptions (section 5).
   Variant descriptions can be transmitted in Alternates headers
   (section 8.3).

   Examples are:

       {features !textonly [blebber !wolx] colordepth=3;+0.7}

       {features !blink;-0.5 background;+1.5 [blebber !wolx];+1.4-0.8}

   The default value for the true-improvement is 1.  The default value
   for the false-degradation is 0, or 1 if a true-improvement value is
   given.

   A user agent SHOULD, and a remote variant selection algorithm MUST
   compute the quality degradation factor associated with the features
   attribute by multiplying all quality degradation factors of the
   elements of the feature-list.  Note that the result can be a factor
   greater than 1.

   A feature list element yields its true-improvement factor if the
   corresponding feature predicate is true, or if at least one element
   of the corresponding fpred-bag is true. The element yields its
   false-degradation factor otherwise.






 
RFC 2295            Transparent Content Negotiation           March 1998


7  Remote variant selection algorithms

   A remote variant selection algorithm is a standardized algorithm by
   which a server can choose a best variant on behalf of a negotiating
   user agent.  The use of a remote algorithm can speed up the
   negotiation process by eliminating a request-response round trip.

   A remote algorithm typically computes whether the Accept- headers in
   the request contain sufficient information to allow a choice, and if
   so, which variant is the best variant.  This specification does not
   define any remote algorithms, but does define a mechanism to
   negotiate on the use of such algorithms.

7.1 Version numbers

   A version numbering scheme is used to distinguish between different
   remote variant selection algorithms.

      rvsa-version = major "." minor

      major = 1*4DIGIT
      minor = 1*4DIGIT

   An algorithm with the version number X.Y, with Y>0, MUST be downwards
   compatible with all algorithms from X.0 up to X.Y.  Downwards
   compatibility means that, if supplied with the same information, the
   newer algorithm MUST make the same choice, or a better choice, as the
   old algorithm.  There are no compatibility requirements between
   algorithms with different major version numbers.

8  Content negotiation status codes and headers

   This specification adds one new HTTP status code, and introduces six
   new HTTP headers.  It also extends the semantics of an existing
   HTTP/1.1 header.

8.1 506 Variant Also Negotiates

   The 506 status code indicates that the server has an internal
   configuration error: the chosen variant resource is configured to
   engage in transparent content negotiation itself, and is therefore
   not a proper end point in the negotiation process.