rfc2296.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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



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   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.



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    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.




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      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




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   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, *




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   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



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     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}}