rfc2295.txt

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Network Working Group                                         K. Holtman
Request for Comments: 2295                                           TUE
Category: Experimental                                           A. Mutz
                                                         Hewlett-Packard
                                                              March 1998


                Transparent Content Negotiation in HTTP

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
   an extensible negotiation mechanism, layered on top of HTTP, for
   automatically selecting the best version when the URL is accessed.
   This enables the smooth deployment of new web data formats and markup
   tags.

TABLE OF CONTENTS

   1  Introduction................................................4
    1.1 Background................................................4

   2  Terminology.................................................5
    2.1 Terms from HTTP/1.1.......................................5
    2.2 New terms.................................................6

   3  Notation....................................................8

   4  Overview....................................................9
    4.1 Content negotiation.......................................9
    4.2 HTTP/1.0 style negotiation scheme.........................9
    4.3 Transparent content negotiation scheme...................10
    4.4 Optimizing the negotiation process.......................12
    4.5 Downwards compatibility with non-negotiating user agents.14
    4.6 Retrieving a variant by hand.............................15
    4.7 Dimensions of negotiation................................15



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    4.8 Feature negotiation......................................15
    4.9 Length of variant lists..................................16
    4.10 Relation with other negotiation schemes.................16

   5  Variant descriptions.......................................17
    5.1 Syntax...................................................17
    5.2 URI......................................................17
    5.3 Source-quality...........................................18
    5.4 Type, charset, language, and length......................19
    5.5 Features.................................................19
    5.6 Description..............................................19
    5.7 Extension-attribute......................................20

   6  Feature negotiation........................................20
    6.1 Feature tags.............................................20
    6.1.1 Feature tag values.....................................21
    6.2 Feature sets.............................................21
    6.3 Feature predicates.......................................22
    6.4 Features attribute.......................................24

   7  Remote variant selection algorithms........................25
    7.1 Version numbers..........................................25

   8  Content negotiation status codes and headers...............25
    8.1 506 Variant Also Negotiates..............................25
    8.2 Accept-Features..........................................26
    8.3 Alternates...............................................27
    8.4 Negotiate................................................28
    8.5 TCN......................................................30
    8.6 Variant-Vary.............................................30

   9  Cache validators...........................................31
    9.1 Variant list validators..................................31
    9.2 Structured entity tags...................................31
    9.3 Assigning entity tags to variants........................32

   10 Content negotiation responses..............................32
    10.1 List response...........................................33
    10.2 Choice response.........................................34
    10.3 Adhoc response..........................................37
    10.4 Reusing the Alternates header...........................38
    10.5 Extracting a normal response from a choice response.....39
    10.6 Elaborate Vary headers..................................39
    10.6.1 Construction of an elaborate Vary header..............40
    10.6.2 Caching of an elaborate Vary header...................41
    10.7 Adding an Expires header for HTTP/1.0 compatibility.....41
    10.8 Negotiation on content encoding.........................41




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   11 User agent support for transparent negotiation.............42
    11.1 Handling of responses...................................42
    11.2 Presentation of a transparently negotiated resource.....42

   12 Origin server support for transparent negotiation..........43
    12.1 Requirements............................................43
    12.2 Negotiation on transactions other than GET and HEAD.....45

   13 Proxy support for transparent negotiation..................45

   14 Security and privacy considerations........................46
    14.1 Accept- headers revealing personal information..........46
    14.2 Spoofing of responses from variant resources............47
    14.3 Security holes revealed by negotiation..................47

   15 Internationalization considerations........................47

   16 Acknowledgments............................................47

   17 References.................................................48

   18 Authors' Addresses.........................................48

   19 Appendix: Example of a local variant selection algorithm...49
    19.1 Computing overall quality values........................49
    19.2 Determining the result..................................51
    19.3 Ranking dimensions......................................51

   20 Appendix: feature negotiation examples.....................52
    20.1 Use of feature tags.....................................52
    20.2 Use of numeric feature tags.............................53
    20.3 Feature tag design......................................53

   21 Appendix: origin server implementation considerations......54
    21.1 Implementation with a CGI script........................54
    21.2 Direct support by HTTP servers..........................55
    21.3 Web publishing tools....................................55

   22 Appendix: Example of choice response construction..........55

   23 Full Copyright Statement...................................58










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1  Introduction

   HTTP allows web site authors to put multiple versions of the same
   information under a single URI.  Each of these versions is called a
   `variant'.  Transparent content negotiation is an extensible
   negotiation mechanism for automatically and efficiently retrieving
   the best variant when a GET or HEAD request is made.  This enables
   the smooth deployment of new web data formats and markup tags.

   This specification defines transparent content negotiation as an
   extension on top of the HTTP/1.1 protocol [1].  However, use of this
   extension does not require use of HTTP/1.1: transparent content
   negotiation can also be done if some or all of the parties are
   HTTP/1.0 [2] systems.

   Transparent content negotiation is called `transparent' because it
   makes all variants which exist inside the origin server visible to
   outside parties.

     Note: Some members of the IETF are currently undertaking a number
     of activities which are loosely related to this experimental
     protocol.  First, there is an effort to define a protocol-
     independent registry for feature tags.  The intention is that this
     experimental protocol will be one of the clients of the registry.
     Second, some research is being done on content negotiation systems
     for other transport protocols (like internet mail and internet fax)
     and on generalized negotiation systems for multiple transport
     protocols.  At the time of writing, it is unclear if or when this
     research will lead to results in the form of complete negotiation
     system specifications.  It is also unclear to which extent possible
     future specifications can or will re-use elements of this
     experimental protocol.

1.1 Background

   The addition of content negotiation to the web infrastructure has
   been considered important since the early days of the web.  Among the
   expected benefits of a sufficiently powerful system for content
   negotiation are

     * smooth deployment of new data formats and markup tags will
       allow graceful evolution of the web

     * eliminating the need to choose between a `state of the art
       multimedia homepage' and one which can be viewed by all web users

     * enabling good service to a wider range of browsing
       platforms (from low-end PDA's to high-end VR setups)



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     * eliminating error-prone and cache-unfriendly
       User-Agent based negotiation

     * enabling construction of sites without `click here for the X
       version' links

     * internationalization, and the ability to offer multi-lingual
       content without a bias towards one language.

2  Terminology

   The words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in
   this document are to be interpreted as described in RFC 2119 [4].

   This specification uses the term `header' as an abbreviation for for
   `header field in a request or response message'.

2.1 Terms from HTTP/1.1

   This specification mostly uses the terminology of the HTTP/1.1
   specification [1].  For the convenience of the reader, this section
   reproduces some key terminology definition from [1].

   request
     An HTTP request message.

   response
     An HTTP response message.

   resource
     A network data object or service that can be identified by a URI.
     Resources may be available in multiple representations (e.g.
     multiple languages, data formats, size, resolutions) or vary in
     other ways.

   content negotiation
     The mechanism for selecting the appropriate representation when
     servicing a request.

   client
     A program that establishes connections for the purpose of sending
     requests.

   user agent
     The client which initiates a request.  These are often browsers,
     editors, spiders (web-traversing robots), or other end user tools.





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   server
     An application program that accepts connections in order to service
     requests by sending back responses.  Any given program may be
     capable of being both a client and a server; our use of these terms
     refers only to the role being performed by the program for a
     particular connection, rather than to the program's capabilities in
     general.  Likewise, any server may act as an origin server, proxy,
     gateway, or tunnel, switching behavior based on the nature of each
     request.

   origin server
     The server on which a given resource resides or is to be created.

   proxy
     An intermediary program which acts as both a server and a client
     for the purpose of making requests on behalf of other clients.
     Requests are serviced internally or by passing them on, with
     possible translation, to other servers.  A proxy must implement
     both the client and server requirements of this specification.

   age
     The age of a response is the time since it was sent by, or
     successfully validated with, the origin server.

   fresh
     A response is fresh if its age has not yet exceeded its freshness
     lifetime.

2.2 New terms

   transparently negotiable resource
     A resource, identified by a single URI, which has multiple
     representations (variants) associated with it.  When servicing a
     request on its URI, it allows selection of the best representation
     using the transparent content negotiation mechanism.  A
     transparently negotiable resource always has a variant list bound
     to it, which can be represented as an Alternates header (defined in
     section 8.3).

   variant list
     A list containing variant descriptions, which can be bound to a
     transparently negotiable resource.









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   variant description