rfc2296.txt

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   and suppose that the user prefers "el" over "en", while the user
   agent can render "ISO-8859-1" with a higher quality than "ISO-8859-
   7".  If the Accept- headers are

     Accept-Language: gr, en;q=0.8
     Accept-Charset: ISO-8859-1, ISO-8859-7;q=0.6, *

   then the remote variant selection algorithm would choose the English
   variant, because this variant has the least overall quality
   degradation.  But if the Accept- headers are

     Accept-Language: gr, en;q=0.8
     Accept-Charset: ISO-8859-1, ISO-8859-7;q=0.95, *

   then the algorithm would choose the Greek variant.  In general, the
   Accept- header with the biggest spread between its quality factors
   gets the highest precedence.  If a user agent allows the user to set
   the quality factors for some headers, while other factors are hard-
   coded, it should use a low spread on the hard-coded factors and a
   high spread on the user-supplied factors, so that the user settings
   take precedence over the built-in settings.





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4.2 Construction of short requests

   In a request on a transparently negotiated resource, a user agent
   need not send a very long Accept- header, which lists all of its
   capabilities, to get optimal results.  For example, instead of
   sending

     Accept: image/gif;q=0.9, image/jpeg;q=0.8, image/png;q=1.0,
             image/tiff;q=0.5, image/ief;q=0.5, image/x-xbitmap;q=0.8,
             application/plugin1;q=1.0, application/plugin2;q=0.9

   the user agent can send

     Accept: image/gif;q=0.9, */*;q=1.0

   It can send this short header without running the risk of getting a
   choice response with, say, an inferior image/tiff variant.  For
   example, with the variant list

     {"x.gif" 1.0 {type image/gif}}, {"x.tiff" 1.0 {type image/tiff}},

   the remote algorithm will compute a definite overall quality of 0.9
   for x.gif and a speculative overall quality value of 1.0 for x.tiff.
   As the best variant has a speculative quality value, the algorithm
   will not choose x.tiff, but return a list response, after which the
   selection algorithm of the user agent will correctly choose x.gif.
   The end result is the same as if the long Accept- header above had
   been sent.

   Thus, user agents can vary the length of the Accept- headers to get
   an optimal tradeoff between the speed with which the first request is
   transmitted, and the chance that the remote algorithm has enough
   information to eliminate a second request.

4.2.1 Collapsing Accept- header elements

   This section discusses how a long Accept- header which lists all
   capabilities and preferences can be safely made shorter.  The remote
   variant selection algorithm is designed in such a way that it is
   always safe to shorten an Accept or Accept-Charset header by two
   taking two header elements `A;q=f' and `B;q=g' and replacing them by
   a single element `P;q=m' where P is a wildcard pattern that matches
   both A and B, and m is the maximum of f and g.  Some examples are

      text/html;q=1.0, text/plain;q=0.8       -->    text/*;q=1.0
      image/*;q=0.8, application/*;q=0.7      -->    */*;q=0.8

      iso-8859-5;q=1.0, unicode-1-1;q=0.8     -->    *;q=1.0



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   Note that every `;q=1.0' above is optional, and can be omitted:

      iso-8859-7;q=0.6, *                     -->    *

   For Accept-Language, it is safe to collapse all language ranges
   with the same primary tag into a wildcard:

      en-us;q=0.9, en-gb;q=0.7, en;q=0.8, da  -->    *;q=0.9, da

   It is also safe to collapse a language range into a wildcard, or to
   replace it by a wildcard, if its primary tag appears only once:

      *;q=0.9, da                             -->    *

   Finally, in the Accept-Features header, every feature expression
   can be collapsed into a wildcard, or replaced by a wildcard:

      colordepth!=5, *                        -->    *

4.2.2 Omitting Accept- headers


   According to the HTTP/1.1 specification [1], the complete absence of
   an Accept header from the request is equivalent to the presence of
   `Accept: */*'.  Thus, if the Accept header is collapsed to `Accept:
   */*', a user agent may omit it entirely.  An Accept-Charset, Accept-
   Language, or Accept-Features header which only contains `*' may also
   be omitted.

4.2.3 Dynamically lengthening requests

   In general, a user agent capable of transparent content negotiation
   can send short requests by default.  Some short Accept- headers could
   be included for the benefit of existing servers which use HTTP/1.0
   style negotiation (see section 4.2 of [2]).  An example is

      GET /paper HTTP/1.1
      Host: x.org
      User-Agent: WuxtaWeb/2.4
      Negotiate: 1.0
      Accept-Language: en, *;q=0.9

   If the Accept- headers included in such a default request are not
   suitable as input to the remote variant selection algorithm, the user
   agent can disable the algorithm by sending `Negotiate: trans' instead
   of `Negotiate: 1.0'.





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   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.







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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
   combination X - Y - Z, then a remote choice can be prevented by
   always omitting at least one of the elements of the combination from
   the Accept- headers, and adding a suitable wildcard pattern to match
   the omitted element, if such a pattern is not already present.

5  Security and privacy considerations

   This specification introduces no security and privacy considerations
   not already covered in [2].  See [2] for a discussion of privacy
   risks connected to the sending of Accept- headers.




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6  Acknowledgments

   Work on HTTP content negotiation has been done since at least 1993.
   The authors are unable to trace the origin of many of the ideas
   incorporated in this document.  Many members of the HTTP working
   group have contributed to the negotiation model in this
   specification.  The authors wish to thank the individuals who have
   commented on earlier versions of this document, including Brian
   Behlendorf, Daniel DuBois, Ted Hardie, Larry Masinter, and Roy T.
   Fielding.

7  References

   [1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and
       T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
       2068, January 1997.

   [2] Holtman, K., and A. Mutz, "Transparent Content Negotiation in
       HTTP", RFC 2295, March 1998.

8  Authors' Addresses

   Koen Holtman
   Technische Universiteit Eindhoven
   Postbus 513
   Kamer HG 6.57
   5600 MB Eindhoven (The Netherlands)

   EMail: [e-mail]


   Andrew H. Mutz
   Hewlett-Packard Company
   1501 Page Mill Road 3U-3
   Palo Alto CA 94304, USA

   Fax:   [phone]
   EMail: [e-mail]













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9  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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