Transparent Content Negotiation in HTTP
RFC 2295

Document Type RFC - Experimental (March 1998; No errata)
Last updated 2013-03-02
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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

Holtman & Mutz                Experimental                      [Page 1]
RFC 2295            Transparent Content Negotiation           March 1998

    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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