RFC 2296                     HTTP RVSA/1.0                    March 1998


   If the user agent discovers, though the receipt of a list or choice
   response, that a particular origin server contains transparently
   negotiated resources, it could dynamically lengthen future requests
   to this server, for example to

      GET /paper/chapter1 HTTP/1.1
      Host: x.org
      User-Agent: WuxtaWeb/2.4
      Negotiate: 1.0
      Accept: text/html, application/postscript;q=0.8, */*
      Accept-Language: en, fr;q=0.5, *;q=0.9
      Accept-Features: tables, *

   This will increase the chance that the remote variant selection
   algorithm will have sufficient information to choose on behalf of the
   user agent, thereby optimizing the negotiation process.  A good
   strategy for dynamic extension would be to extend the headers with
   those media types, languages, charsets, and feature tags mentioned in
   the variant lists of past responses from the server.

4.3 Differences between the local and the remote algorithm

   A user agent can only optimize content negotiation though the use of
   a remote algorithm if its local algorithm will generally make the
   same choice.  If a user agent receives a choice response containing a
   variant X selected by the remote algorithm, while the local algorithm
   would have selected Y, the user agent has two options:

     1. Retrieve Y in a subsequent request. This is sub-optimal
        because it takes time.

     2. Display X anyway.  This is sub-optimal because it makes the
        end result of the negotiation process dependent on factors that
        can randomly change.  For the next request on the same resource,
        and intermediate proxy cache could return a list response, which
        would cause the local algorithm to choose and retrieve Y instead
        of X.  Compared to a stable representation, a representation
        which randomly switches between X and Y (say, the version with
        and without frames) has a very low subjective quality for most
        users.

   As both alternatives above are unattractive, a user agent should try
   to avoid the above situation altogether.  The sections below discuss
   how this can be done.








 
RFC 2296                     HTTP RVSA/1.0                    March 1998


4.3.1 Avoiding major differences

   If the user agent enables the remote algorithm in this specification,
   it should generally use a local algorithm which closely resembles the
   remote algorithm.  The algorithm should for example also use
   multiplication to combine quality factors.  If the user agent
   combines quality factors by addition, it would be more advantageous
   to define a new remote variant selection algorithm, with a new major
   version number, for use by this agent.

4.3.2 Working around minor differences

   Even if a local algorithm uses multiplication to combine quality
   factors, it could use an extended quality formulae like

      Q = round5( qs * qt * qc * ql * qf ) * q_adjust

   in order to account for special interdependencies between dimensions,
   which are due to limitations of the user agent.  For example, if the
   user agent, for some reason, cannot handle the iso-8859-7 charset
   when rendering text/plain documents, the q_adjust factor would be 0
   when the text/plain - iso-8859-7 combination is present in the
   variant description, and 1 otherwise.

   By selectively withholding information from the remote variant
   selection algorithm, the user agent can ensure that the remote
   algorithm will never make a choice if the local q_adjust is less than
   1.  For example, to prevent the remote algorithm from ever returning
   a text/plain - iso-8859-7 choice response, the user agent should take
   care to never produce a request which exactly specifies the quality
   factors of both text/plain and iso-8859-7.  The omission of either
   factor from a request will cause the overall quality value of any
   text/plain - iso-8859-7 variant to be speculative, and variants with
   speculative quality values can never be returned in a choice
   response.

   In general, if the local q_adjust does not equal 1 for a particular