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RFC 2296 - HTTP Remote Variant Selection Algorithm -- RVSA/1.0


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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

   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.

    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.

      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

   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, *

   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

     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.

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

   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'.

   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.

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.

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

9  Full Copyright Statement

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

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   The limited permissions granted above are perpetual and will not be
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   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
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