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rfc:rfc2296

Network Working Group K. Holtman Request for Comments: 2296 TUE Category: Experimental A. Mutz

                                                       Hewlett-Packard
                                                            March 1998
        HTTP Remote Variant Selection Algorithm -- RVSA/1.0

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

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

ABSTRACT

 HTTP allows web site authors to put multiple versions of the same
 information under a single URL.  Transparent content negotiation is a
 mechanism for automatically selecting the best version when the URL
 is accessed.  A remote variant selection algorithm can be used to
 speed up the transparent negotiation process. This document defines
 the remote variant selection algorithm with the version number 1.0.

TABLE OF CONTENTS

 1  Introduction...............................................2
 2  Terminology and notation...................................2
 3  The remote variant selection algorithm.....................2
  3.1 Input....................................................2
  3.2 Output...................................................3
  3.3 Computing overall quality values.........................3
  3.4 Definite and speculative quality values..................5
  3.5 Determining the result...................................6
 4  Use of the algorithm.......................................7
  4.1 Using quality factors to rank preferences................7
  4.2 Construction of short requests...........................8
  4.2.1 Collapsing Accept- header elements.....................8
  4.2.2 Omitting Accept- headers...............................9
  4.2.3 Dynamically lengthening requests.......................9
  4.3 Differences between the local and the remote algorithm..10
  4.3.1 Avoiding major differences............................11
  4.3.2 Working around minor differences......................11

Holtman & Mutz Experimental [Page 1] RFC 2296 HTTP RVSA/1.0 March 1998

 5  Security and privacy considerations.......................11
 6  Acknowledgments...........................................12
 7  References................................................12
 8  Authors' Addresses........................................12
 9  Full Copyright Statement..................................13

1 Introduction

 HTTP allows web site authors to put multiple versions (variants) of
 the same information under a single URL.  Transparent content
 negotiation [2] is a mechanism for automatically selecting the best
 variant when the URL is accessed.  A remote variant selection
 algorithm can be used by a HTTP server to choose a best variant on
 behalf of a negotiating user agent.  The use of a remote algorithm
 can speed up the transparent negotiation process by eliminating a
 request-response round trip.
 This document defines the remote variant selection algorithm with the
 version number 1.0.  The algorithm computes whether the Accept-
 headers in the request contain sufficient information to allow a
 choice, and if so, which variant must be chosen.

2 Terminology and notation

 This specification uses the terminology and notation of the HTTP
 transparent content negotiation specification [2].

3 The remote variant selection algorithm

 This section defines the remote variant selection algorithm with the
 version number 1.0.  To implement this definition, a server MAY run
 any algorithm which gives equal results.
   Note: According to [2], servers are always free to return a list
   response instead of running a remote algorithm.  Therefore,
   whenever a server may run a remote algorithm, it may also run a
   partial implementation of the algorithm, provided that the partial
   implementation always returns List_response when it cannot compute
   the real result.

3.1 Input

   The algorithm is always run for a particular request on a
   particular transparently negotiable resource.  It takes the
   following information as input.
  1. The variant list of the resource, as present in the Alternates
     header of the resource.

Holtman & Mutz Experimental [Page 2] RFC 2296 HTTP RVSA/1.0 March 1998

  2. (Partial) Information about capabilities and preferences of the
     user agent for this particular request, as given in the Accept-
     headers of the request.
 If a fallback variant description
     {"fallback.html"}
 is present in the Alternates header, the algorithm MUST interpret it
 as the variant description
     {"fallback.html" 0.000001}
 The extremely low source quality value ensures that the fallback
 variant only gets chosen if all other options are exhausted.

3.2 Output

 As its output, the remote variant selection algorithm and will yield
 the appropriate action to be performed.  There are two possibilities:
    Choice_response
      The Accept- headers contain sufficient information to make a
      choice on behalf of the user agent possible, and the best
      variant MAY be returned in a choice response.
    List_response
      The Accept- headers do not contain sufficient information to
      make a choice on behalf of the user agent possible.  A list
      response MUST be returned, allowing the user agent to make the
      choice itself.

3.3 Computing overall quality values

 As a first step in the remote variant selection algorithm, the
 overall qualities of the individual variants in the list are
 computed.
 The overall quality Q of a variant is the value
    Q = round5( qs * qt * qc * ql * qf )
 where round5 is a function which rounds a floating point value to 5
 decimal places after the point, and where the factors qs, qt, qc, ql,
 and qf are determined as follows.

Holtman & Mutz Experimental [Page 3] RFC 2296 HTTP RVSA/1.0 March 1998

    qs Is the source quality factor in the variant description.
    qt The media type quality factor is 1 if there is no type
       attribute in the variant description, or if there is no Accept
       header in the request.  Otherwise, it is the quality assigned
       by the Accept header to the media type in the type attribute.
         Note: If a type is matched by none of the elements of an
         Accept header, the Accept header assigns the quality factor 0
         to that type.
    qc The charset quality factor is 1 if there is no charset
       attribute in the variant description, or if there is no
       Accept-Charset header in the request.  Otherwise, the charset
       quality factor is the quality assigned by the Accept-Charset
       header to the charset in the charset attribute.
    ql The language quality factor is 1 if there is no language
       attribute in the variant description, or if there is no
       Accept-Language header in the request.  Otherwise, the language
       quality factor is the highest quality factor assigned by the
       Accept-Language header to any one of the languages listed in
       the language attribute.
    qf The features quality factor is 1 if there is no features
       attribute in the variant description, or if there is no
       Accept-Features header in the request.  Otherwise, it is the
       quality degradation factor for the features attribute, see
       section 6.4 of [2].
 As an example, if a variant list contains the variant description
   {"paper.html.en" 0.7 {type text/html} {language fr}}
 and if the request contains the Accept- headers
   Accept: text/html:q=1.0, */*:q=0.8
   Accept-Language: en;q=1.0, fr;q=0.5
 the remote variant selection algorithm will compute an overall
 quality for the variant as follows:
   {"paper.html.fr" 0.7 {type text/html} {language fr}}
                     |           |                 |
                     |           |                 |
                     V           V                 V
           round5 ( 0.7   *     1.0        *      0.5 ) = 0.35000

Holtman & Mutz Experimental [Page 4] RFC 2296 HTTP RVSA/1.0 March 1998

 With the above Accept- headers, the complete variant list
   {"paper.html.en" 0.9 {type text/html} {language en}},
   {"paper.html.fr" 0.7 {type text/html} {language fr}},
   {"paper.ps.en"   1.0 {type application/postscript} {language en}}
 would yield the following computations:
          round5 ( qs  * qt  * qc  * ql  * qf  ) =   Q
                   ---   ---   ---   ---   ---     -------
    paper.html.en: 0.9 * 1.0 * 1.0 * 1.0 * 1.0   = 0.90000
    paper.html.fr: 0.7 * 1.0 * 1.0 * 0.5 * 1.0   = 0.35000
    paper.ps.en:   1.0 * 0.8 * 1.0 * 1.0 * 1.0   = 0.80000

3.4 Definite and speculative quality values

 A computed overall quality value can be either definite or
 speculative.  An overall quality value is definite if it was computed
 without using any wildcard characters '*' in the Accept- headers, and
 without the need to use the absence of a particular Accept- header.
 An overall quality value is speculative otherwise.
 As an example, in the previous section, the quality values of
 paper.html.en and paper.html.fr are definite, and the quality value
 of paper.ps.en is speculative because the type application/postscript
 was matched to the range */*.
 Definiteness can be defined more formally as follows.  An overall
 quality value Q is definite if the same quality value Q can be
 computed after the request message is changed in the following way:
     1. If an Accept, Accept-Charset, Accept-Language, or
        Accept-Features header is missing from the request, add this
        header with an empty field.
     2. Delete any media ranges containing a wildcard character '*'
        from the Accept header.  Delete any wildcard '*' from the
        Accept-Charset, Accept-Language, and Accept-Features headers.
 As another example, the overall quality factor for the variant
   {"blah.html" 1 {language en-gb} {features blebber [x y]}}
 is 1 and definite with the Accept- headers
   Accept-Language: en-gb, fr
   Accept-Features: blebber, x, !y, *

Holtman & Mutz Experimental [Page 5] RFC 2296 HTTP RVSA/1.0 March 1998

 and
   Accept-Language: en, fr
   Accept-Features: blebber, x, *
 The overall quality factor is still 1, but speculative, with the
 Accept- headers
   Accept-language: en-gb, fr
   Accept-Features: blebber, !y, *
 and
   Accept-Language: fr, *
   Accept-Features: blebber, x, !y, *

3.5 Determining the result

 The best variant, as determined by the remote variant selection
 algorithm, is the one variant with the highest overall quality value,
 or, if there are multiple variants which share the highest overall
 quality, the first variant in the list with this value.
 The end result of the remote variant selection algorithm is
 Choice_response if all of the following conditions are met
    a. the overall quality value of the best variant is greater
       than 0
    b. the overall quality value of the best variant is a definite
       quality value
    c. the variant resource is a neighbor of the negotiable
       resource.  This last condition exists to ensure that a
       security-related restriction on the generation of choice
       responses is met, see sections 10.2 and 14.2 of [2].
 In all other cases, the end result is List_response.
 The requirement for definiteness above affects the interpretation of
 Accept- headers in a dramatic way.  For example, it causes the remote
 algorithm to interpret the header
   Accept: image/gif;q=0.9, */*;q=1.0
 as
   `I accept image/gif with a quality of 0.9, and assign quality

Holtman & Mutz Experimental [Page 6] RFC 2296 HTTP RVSA/1.0 March 1998

   factors up to 1.0 to other media types.  If this information is
   insufficient to make a choice on my behalf, do not make a choice
   but send the list of variants'.
 Without the requirement, the interpretation would have been
   `I accept image/gif with a quality of 0.9, and all other media
   types with a quality of 1.0'.

4 Use of the algorithm

 This section discusses how user agents can use the remote algorithm
 in an optimal way.  This section is not normative, it is included for
 informational purposes only.

4.1 Using quality factors to rank preferences

 Using quality factors, a user agent can not only rank the elements
 within a particular Accept- header, it can also express precedence
 relations between the different Accept- headers.  Consider for
 example the following variant list:
   {"paper.english" 1.0 {language en} {charset ISO-8859-1}},
   {"paper.greek"   1.0 {language el} {charset ISO-8859-7}}
 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.

Holtman & Mutz Experimental [Page 7] RFC 2296 HTTP RVSA/1.0 March 1998

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

Holtman & Mutz Experimental [Page 8] RFC 2296 HTTP RVSA/1.0 March 1998

 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'.

Holtman & Mutz Experimental [Page 9] 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.

Holtman & Mutz Experimental [Page 10] 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
 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.

Holtman & Mutz Experimental [Page 11] RFC 2296 HTTP RVSA/1.0 March 1998

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: koen@win.tue.nl
 Andrew H. Mutz
 Hewlett-Packard Company
 1501 Page Mill Road 3U-3
 Palo Alto CA 94304, USA
 Fax:   +1 415 857 4691
 EMail: mutz@hpl.hp.com

Holtman & Mutz Experimental [Page 12] RFC 2296 HTTP RVSA/1.0 March 1998

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.

Holtman & Mutz Experimental [Page 13]

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