RFC2295 - Transparent Content Negotiation in HTTP

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
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
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
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)
* 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 RFC2119 [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.
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.
variant description
A machine-readable description of a variant resource, usually found
in a variant list. A variant description contains the variant
resource URI and various attributes which describe properties of
the variant. Variant descriptions are defined in section 5.
variant resource
A resource from which a variant of a negotiable resource can be
retrieved with a normal HTTP/1.x GET request, i.e. a GET request
which does not use transparent content negotiation.
neighboring variant
A variant resource is called a neighboring variant resource of some
transparently negotiable HTTP resource if the variant resource has
a HTTP URL, and if the absolute URL of the variant resource up to
its last slash equals the absolute URL of the negotiable resource
up to its last slash, where equality is determined with the URI
comparison rules in section 3.2.3 of [1]. The property of being a
neighboring variant is important because of security considerations
(section 14.2). Not all variants of a negotiable resource need to
be neighboring variants. However, access to neighboring variants
can be more highly optimized by the use of remote variant selection
algorithms (section 7) and choice responses (section 10.2).
remote variant selection algorithm
A standardized algorithm by which a server can sometimes choose a
best variant on behalf of a negotiating user agent. The algorithm
typically computes whether the Accept- headers in the request
contain sufficient information to allow a choice, and if so, which
variant is the best variant. The use of a remote algorithm can
speed up the negotiation process.
list response
A list response returns the variant list of the negotiable
resource, but no variant data. It can be generated when the server
does not want to, or is not allowed to, return a particular best
variant for the request. List responses are defined in section
10.1.
choice response
A choice response returns a representation of the best variant for
the request, and may also return the variant list of the negotiable
resource. It can be generated when the server has sufficient
information to be able to choose the best variant on behalf the
user agent, but may only be generated if this best variant is a
neighboring variant. Choice responses are defined in section 10.2.
adhoc response
An adhoc response can be sent by an origin server as an extreme
measure, to achieve compatibility with a non-negotiating or buggy
client if this compatibility cannot be achieved by sending a list
or choice response. There are very little requirements on the
contents of an adhoc response. Adhoc responses are defined in
section 10.3.
Accept- headers
The request headers: Accept, Accept-Charset, Accept-Language, and
Accept-Features.
supports transparent content negotiation
From the viewpoint of an origin server or proxy, a user agent
supports transparent content negotiation if and only if it sends a
Negotiate header (section 8.4) which indicates such support.
server-side override
If a request on a transparently negotiated resource is made by a
client which supports transparent content negotiation, an origin
server is said to perform a server-side override if the server
ignores the directives in the Negotiate request header, and instead
uses a custom algorithm to choose an appropriate response. A
server-side override can sometimes be used to work around known
client bugs. It could also be used by protocol extensions on top
of transparent content negotiation.
3 Notation
The version of BNF used in this document is taken from [1], and many
of the nonterminals used are defined in [1]. Note that the
underlying charset is US-ASCII.
One new BNF construct is added:
1%rule
stands for one or more instances of "rule", separated by whitespace:
1%rule = rule *( 1*LWS rule )
This specification also introduces
number = 1*DIGIT
short-float = 1*3DIGIT [ "." 0*3DIGIT ]
This specification uses the same conventions as in [1] (see section
1.2 of [1]) for defining the significance of each particular
requirement.
4 Overview
This section gives an overview of transparent content negotiation.
It starts with a more general discussion of negotiation as provided
by HTTP.
4.1 Content negotiation
HTTP/1.1 allows web site authors to put multiple versions of the same
information under a single resource URI. Each of these versions is
called a `variant". For example, a resource http://x.org/paper could
bind to three different variants of a paper:
1. Html, English
2. HTML, French
3. Postscript, English
Content negotiation is the process by which the best variant is
selected if the resource is accessed. The selection is done by
matching the properties of the available variants to the capabilities
of the user agent and the preferences of the user.
It has always been possible under HTTP to have multiple
representations available for one resource, and to return the most
appropriate representation for each subsequent request. However,
HTTP/1.1 is the first version of HTTP which has provisions for doing
this in a cache-friendly way. These provisions include the Vary
response header, entity tags, and the If-None-Match request header.
4.2 HTTP/1.0 style negotiation scheme
The HTTP/1.0 protocol elements allow for a negotiation scheme as
follows:
Server _____ proxy _____ proxy _____ user
x.org cache cache agent
< ----------------------------------
GET http://x.org/paper
Accept- headers
choose

---------------------------------- >
Best variant
When the resource is accessed, the user agent sends (along with its
request) various Accept- headers which express the user agent
capabilities and the user preferences. Then the origin server uses
these Accept- headers to choose the best variant, which is returned
in the response.
The biggest problem with this scheme is that it does not scale well.
For all but the most minimal user agents, Accept- headers expressing
all capabilities and preferences would be very large, and sending
them in every request would be hugely inefficient, in particular
because only a small fraction of the resources on the web have
multiple variants.
4.3 Transparent content negotiation scheme
The transparent content negotiation scheme eliminates the need to
send huge Accept- headers, and nevertheless allows for a selection
process that always yields either the best variant, or an error
message indicating that user agent is not capable of displaying any
of the available variants.
Under the transparent content negotiation scheme, the server sends a
list with the available variants and their properties to the user
agent. An example of a list with three variants is
{"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript} {language en}}
The syntax and semantics of the variant descriptions in this list are
covered in section 5. When the list is received, the user agent can
choose the best variant and retrieve it. Graphically, the
communication can be represented as follows:
Server _____ proxy _____ proxy _____ user
x.org cache cache agent
< ----------------------------------
GET http://x.org/paper

----------------------------------- > [list response]
return of list
choose

< ----------------------------------
GET http://x.org/paper.1

---------------------------------- > [normal response]
return of paper.1
The first response returning the list of variants is called a `list
response". The second response is a normal HTTP response: it does
not contain special content negotiation related information. Only
the user agent needs to know that the second request actually
retrieves a variant. For the other parties in the communication, the
second transaction is indistinguishable from a normal HTTP
transaction.
With this scheme, information about capabilities and preferences is
only used by the user agent itself. Therefore, sending such
information in large Accept- headers is unnecessary. Accept- headers
do have a limited use in transparent content negotiation however; the
sending of small Accept- headers can often speed up the negotiation
process. This is covered in section 4.4.
List responses are covered in section 10.1. As an example, the list
response in the above picture could be:
HTTP/1.1 300 Multiple Choices
Date: Tue, 11 Jun 1996 20:02:21 GMT
TCN: list
Alternates: {"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript}
{language en}}
Vary: negotiate, accept, accept-language
ETag: "blah;1234"
Cache-control: max-age=86400
Content-Type: text/html
Content-Length: 227
<h2>Multiple Choices:</h2>
<ul>
<li><a href=paper.1>HTML, English version</a>
<li><a href=paper.2>HTML, French version</a>
<li><a href=paper.3>Postscript, English version</a>
</ul>
The Alternates header in the response contains the variant list. The
Vary header is included to ensure correct caching by plain HTTP/1.1
caches (see section 10.6). The ETag header allows the response to be
revalidated by caches, the Cache-Control header controls this
revalidation. The HTML entity included in the response allows the
user to select the best variant by hand if desired.
4.4 Optimizing the negotiation process
The basic transparent negotiation scheme involves two HTTP
transactions: one to retrieve the list, and a second one to retrieve
the chosen variant. There are however several ways to `cut corners"
in the data flow path of the basic scheme.
First, caching proxies can cache both variant lists and variants.
Such caching can reduce the communication overhead, as shown in the
following example:
Server _____ proxy _____ proxy __________ user
x.org cache cache agent
< --------------
GET ../paper

has the list
in cache

------------- > [list response]
list

choose

< --------------------------
GET ../paper.1

has the variant
in cache

-------------------------- > [normal response]
return of paper.1
Second, the user agent can send small Accept- headers, which may
contain enough information to allow the server to choose the best
variant and return it directly.
Server _____ proxy _____ proxy _____ user
x.org cache cache agent
< ----------------------------------
GET http://x.org/paper
small Accept- headers

able to choose on
behalf of user agent

---------------------------------- > [choice response]
return of paper.1 and list
This choosing based on small Accept- headers is done with a `remote
variant selection algorithm". Such an algorithm takes the variant
list and the Accept- headers as input. It then computes whether the
Accept- headers contain sufficient information to choose on behalf of
the user agent, and if so, which variant is the best variant. If the
best variant is a neighboring variant, it may be returned, together
with the variant list, in a choice response.
A server may only choose on behalf of a user agent supporting
transparent content negotiation if the user agent explicitly allows
the use of a particular remote variant selection algorithm in the
Negotiate request header. User agents with sophisticated internal
variant selection algorithms may want to disallow a remote choice, or
may want to allow it only when retrieving inline images. If the
local algorithm of the user agent is superior in only some difficult
areas of negotiation, it is possible to enable the remote algorithm
for the easy areas only. More information about the use of a remote
variant selection algorithm can be found in [3].
Choice responses are covered in section 10.2. For example, the
choice response in the above picture could be:
HTTP/1.1 200 OK
Date: Tue, 11 Jun 1996 20:05:31 GMT
TCN: choice
Content-Type: text/html
Last-Modified: Mon, 10 Jun 1996 10:01:14 GMT
Content-Length: 5327
Cache-control: max-age=604800
Content-Location: paper.1
Alternates: {"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript}
{language en}}
Etag: "gonkyyyy;1234"
Vary: negotiate, accept, accept-language
Expires: Thu, 01 Jan 1980 00:00:00 GMT
<title>A paper about ....
Finally, the above two kinds of optimization can be combined; a
caching proxy which has the list will sometimes be able to choose on
behalf of the user agent. This could lead to the following
communication pattern:
Server _____ proxy _____ proxy __________ user
x.org cache cache agent
< ---------------
GET ../paper
small Accept

able to choose
on behalf

< ----------
GET ../paper.1

---------- > [normal response]
paper.1
---------------- > [choice response]
paper.1 and list
Note that this cutting of corners not only saves bandwidth, it also
eliminates delays due to packet round trip times, and reduces the
load on the origin server.
4.5 Downwards compatibility with non-negotiating user agents
To handle requests from user agents which do not support transparent
content negotiation, this specification allows the origin server to
revert to a HTTP/1.0 style negotiation scheme. The specification of
heuristics for such schemes is beyond the scope of this document.
4.6 Retrieving a variant by hand
It is always possible for a user agent to retrieve the variant list
which is bound to a negotiable resource. The user agent can use this
list to make available a menu of all variants and their
characteristics to the user. Such a menu allows the user to randomly
browse other variants, and makes it possible to manually correct any
sub-optimal choice made by the automatic negotiation process.
4.7 Dimensions of negotiation
Transparent content negotiation defines four dimensions of
negotiation:
1. Media type (MIME type)
2. Charset
3. Language
4. Features
The first three dimensions have traditionally been present in HTTP.
The fourth dimension is added by this specification. Additional
dimensions, beyond the four mentioned above, could be added by future
specifications.
Negotiation on the content encoding of a response (gzipped,
compressed, etc.) is left outside of the realm of transparent
negotiation. See section 10.8 for more information.
4.8 Feature negotiation
Feature negotiation intends to provide for all areas of negotiation
not covered by the type, charset, and language dimensions. Examples
are negotiation on
* HTML extensions
* Extensions of other media types
* Color capabilities of the user agent
* Screen size
* Output medium (screen, paper, ...)
* Preference for speed vs. preference for graphical detail
The feature negotiation framework (section 6) is the principal means
by which transparent negotiation offers extensibility; a new
dimension of negotiation (really a sub-dimension of the feature
dimension) can be added without the need for a new standards effort
by the simple registration of a `feature tag".
4.9 Length of variant lists
As a general rule, variant lists should be short: it is expected that
a typical transparently negotiable resource will have 2 to 10
variants, depending on its purpose. Variant lists should be short
for a number of reasons:
1. The user must be able to pick a variant by hand to correct a
bad automatic choice, and this is more difficult with a long
variant list.
2. A large number of variants will decrease the efficiency of
internet proxy caches.
3. Long variant lists will make some transparently negotiated
responses longer.
In general, it is not desirable to create a transparently negotiable
resource with hundreds of variants in order to fine-tune the
graphical presentation of a resource. Any graphical fine-tuning
should be done, as much as possible, by using constructs which act at
the user agent side, for example
<center><img src=http://www.QQread.com/net-protocol/titlebanner.gifwidth=100%
alt="MegaBozo Corp"></center>
In order to promote user agent side fine tuning, which is more
scalable than fine tuning over the network, user agents which
implement a scripting language for content rendering are encouraged
to make the availability of this language visible for transparent
content negotiation, and to allow rendering scripts to access the
capabilities and preferences data used for content negotiation, as
far as privacy considerations permit this.
4.10 Relation with other negotiation schemes
The HTTP/1.x protocol suite allows for many different negotiation
mechanisms. Transparent content negotiation specializes in scalable,
interoperable negotiation of content representations at the HTTP
level. It is intended that transparent negotiation can co-exist with
other negotiation schemes, both open and proprietary, which cover
different application domains or work at different points in the
author-to-user chain. Ultimately, it will be up to the resource
author to decide which negotiation mechanism, or combination of
negotiation mechanisms, is most appropriate for the task at hand.
5 Variant descriptions
5.1 Syntax
A variant can be described in a machine-readable way with a variant
description.
variant-description =
"{" <"> URI <"> source-quality *variant-attribute"}"
source-quality = qvalue
variant-attribute = "{" "type" media-type "}"
"{" "charset" charset "}"
"{" "language" 1#language-tag "}"
"{" "length" 1*DIGIT "}"
"{" "features" feature-list "}"
"{" "description"
quoted-string [ language-tag ] "}"
extension-attribute
extension-attribute = "{" extension-name extension-value "}"
extension-name = token
extension-value = *( token quoted-string LWS
extension-specials )
extension-specials =
<any element of tspecials except <"> and "}">
The feature-list syntax is defined in section 6.4.
Examples are
{"paper.2" 0.7 {type text/html} {language fr}}
{"paper.5" 0.9 {type text/html} {features tables}}
{"paper.1" 0.001}
The various attributes which can be present in a variant description
are covered in the subsections below. Each attribute may appear only
once in a variant description.
5.2 URI
The URI attribute gives the URI of the resource from which the
variant can be retrieved with a GET request. It can be absolute or
relative to the Request-URI. The variant resource may vary (on the
Cookie request header, for example), but MUST NOT engage in
transparent content negotiation itself.
5.3 Source-quality
The source-quality attribute gives the quality of the variant, as a
representation of the negotiable resource, when this variant is
rendered with a perfect rendering engine on the best possible output
medium.
If the source-quality is less than 1, it often expresses a quality
degradation caused by a lossy conversion to a particular data format.
For example, a picture originally in JPEG form would have a lower
source quality when translated to the XBM format, and a much lower
source quality when translated to an ASCII-art variant. Note
however, that degradation is a function of the source; an original
piece of ASCII-art may degrade in quality if it is captured in JPEG
form.
The source-quality could also represent a level of quality caused by
skill of language translation, or ability of the used media type to
capture the intended artistic expression.
Servers should use the following table a guide when assigning source
quality values:
1.000 perfect representation
0.900 threshold of noticeable loss of quality
0.800 noticeable, but acceptable quality reduction
0.500 barely acceptable quality
0.300 severely degraded quality
0.000 completely degraded quality
The same table can be used by local variant selection algorithms (see
appendix 19) when assigning degradation factors for different content
rendering mechanisms. Note that most meaningful values in this table
are close to 1. This is due to the fact that quality factors are
generally combined by multiplying them, not by adding them.
When assigning source-quality values, servers should not account for
the size of the variant and its impact on transmission and rendering
delays; the size of the variant should be stated in the length
attribute and any size-dependent calculations should be done by the
variant selection algorithm. Any constant rendering delay for a
particular media type (for example due to the startup time of a
helper application) should be accounted for by the user agent, when
assigning a quality factor to that media type.
5.4 Type, charset, language, and length
The type attribute of a variant description carries the same
information as its Content-Type response header counterpart defined
in [1], except for any charset information, which MUST be carried in
the charset attribute. For, example, the header
Content-Type: text/html; charset=ISO-8859-4
has the counterpart attributes
{type text/html} {charset ISO-8859-4}
The language and length attributes carry the same information as
their Content-* response header counterparts in [1]. The length
attribute, if present, MUST thus reflect the length of the variant
alone, and not the total size of the variant and any objects inlined
or embedded by the variant.
Though all of these attributes are optional, it is often desirable to
include as many attributes as possible, as this will increase the
quality of the negotiation process.
Note: A server is not required to maintain a one-to-one
correspondence between the attributes in the variant description
and the Content-* headers in the variant response. For example,
if the variant description contains a language attribute, the
response does not necessarily have to contain a Content-Language
header. If a Content-Language header is present, it does not have
to contain an exact copy of the information in the language
attribute.
5.5 Features
The features attribute specifies how the presence or absence of
particular feature tags in the user agent affects the overall quality
of the variant. This attribute is covered in section 6.4.
5.6 Description
The description attribute gives a textual description of the variant.
It can be included if the URI and normal attributes of a variant are
considered too opaque to allow interpretation by the user. If a user
agent is showing a menu of available variants compiled from a variant
list, and if a variant has a description attribute, the user agent
SHOULD show the description attribute of the variant instead of
showing the normal attributes of the variant. The description field
uses the UTF-8 character encoding scheme [5], which is a superset of
US-ASCII, with ""%" HEX HEX" encoding. The optional language tag MAY
be used to specify the language used in the description text.
5.7 Extension-attribute
The extension-attribute allows future specifications to incrementally
define dimensions of negotiation which cannot be created by using the
feature negotiation framework, and eases content negotiation
experiments. In experimental situations, servers MUST ONLY generate
extension-attributes whose names start with "x-". User agents SHOULD
ignore all extension attributes they do not recognize. Proxies MUST
NOT run a remote variant selection algorithm if an unknown extension
attribute is present in the variant list.
6 Feature negotiation
This section defines the feature negotiation mechanism. Feature
negotiation has been introduced in section 4.8. Appendix 19 contains
examples of feature negotiation.
6.1 Feature tags
A feature tag (ftag) identifies something which can be negotiated on,
for example a property (feature) of a representation, a capability
(feature) of a user agent, or the preference of a user for a
particular type of representation. The use of feature tags need not
be limited to transparent content negotiation, and not every feature
tag needs to be usable in the HTTP transparent content negotiation
framework.
ftag = token quoted-string
Note: A protocol-independent system for feature tag registration
is currently being developed in the IETF. This specification does
not define any feature tags. In experimental situations, the use
of tags which start with "x." is encouraged.
Feature tags are used in feature sets (section 6.2) and in feature
predicates (section 6.3). Feature predicates are in turn used in
features attributes (section 6.4), which are used in variant
descriptions (section 5). Variant descriptions can be transmitted in
Alternates headers (section 8.3).
The US-ASCII charset is used for feature tags. Feature tag
comparison is case-insensitive. A token tag XYZ is equal to a
quoted-string tag "XYZ". Examples are
tables, fonts, blebber, wolx, screenwidth, colordepth
An example of the use of feature tags in a variant description is:
{"index.html" 1.0 {type text/html} {features tables frames}}
This specification follows general computing practice in that it
places no restrictions on what may be called a feature. At the
protocol level, this specification does not distinguish between
different uses of feature tags: a tag will be processed in the same
way, no matter whether it identifies a property, capability, or
preference. For some tags, it may be fluid whether the tag
represents a property, preference, or capability. For example, in
content negotiation on web pages, a "textonly" tag would identify a
capability of a text-only user agent, but the user of a graphical
user agent may use this tag to specify that text-only content is
preferred over graphical content.
6.1.1 Feature tag values
The definition of a feature tag may state that a feature tag can have
zero, one, or more values associated with it. These values
specialize the meaning of the tag. For example, a feature tag
`paper" could be associated with the values `A4" and `A5".
tag-value = token quoted-string
The US-ASCII charset is used for feature tag values. Equality
comparison for tag values MUST be done with a case-sensitive, octet-
by-octet comparison, where any ""%" HEX HEX" encodings MUST be
processed as in [1]. A token value XYZ is equal to a quoted-string
value "XYZ".
6.2 Feature sets
The feature set of a user agent is a data structure which records the
capabilities of the user agent and the preferences of the user.
Feature sets are used by local variant selection algorithms (see
appendix 19 for an example). A user agent can use the Accept-
Features header (section 8.2) to make some of the contents of its
feature set known to remote variant selection algorithms.
Structurally, a feature set is a possibly empty set, containing
records of the form
( feature tag , set of feature tag values )
If a record with a feature tag is present in the set, this means that
the user agent implements the corresponding capability, or that the
user has expressed the corresponding preference.
Each record in a feature set has a, possibly empty, set of tag
values. For feature tags which cannot have values associated with
it, this set is always empty. For feature tags which can have zero,
one, or more values associated with it, this set contains those
values currently associated with the tag. If the set of a feature
tag T has the value V in it, it is said that `the tag T is present
with the value V".
This specification does not define a standard notation for feature
sets. An example of a very small feature set, in a mathematical
notation, is
{ ( "frames" , { } ) ,
( "paper" , { "A4" , "A5" } )
}
As feature registration is expected to be an ongoing process, it is
generally not possible for a user agent to know the meaning of all
feature tags it can possibly encounter in a variant description. A
user agent SHOULD treat all features tags unknown to it as absent
from its feature set.
A user agent may change the contents of its feature set depending on
the type of request, and may also update it to reflect changing
conditions, for example a change in the window size. Therefore, when
considering feature negotiation, one usually talks about `the feature
set of the current request".
6.3 Feature predicates
Feature predicates are predicates on the contents of feature sets.
They appear in the features attribute of a variant description.
fpred = [ "!" ] ftag
ftag ( "=" "!=" ) tag-value
ftag "=" "[" numeric-range "]"
numeric-range = [ number ] "-" [ number ]
Feature predicates are used in features attributes (section 6.4),
which are used in variant descriptions (section 5). Variant
descriptions can be transmitted in Alternates headers (section 8.3).
Examples of feature predicates are
blebber, !blebber, paper=a4, colordepth=5, blex!=54,
dpi=[300-599], colordepth=[24-]
Using the feature set of the current request, a user agent SHOULD
compute the truth value of the different feature predicates as
follows.
ftag true if the feature is present, false otherwise
!ftag true if the feature is absent, false otherwise
ftag=V true if the feature is present with the value V,
false otherwise,
ftag!=V true if the feature is not present with the value V,
false otherwise,
ftag=[N-M] true if the feature is present with at least one
numeric value, while the highest value with which it
is present in the range N-M, false otherwise. If N
is missing, the lower bound is 0. If M is missing,
the upper bound is infinity.
As an example, with the feature set
{ ( "blex" , { } ),
( "colordepth" , { "5" } ),
( "UA-media" , { "stationary" } ),
( "paper" , { "A4", "A3" } ) ,
( "x-version" , { "104", "200" } )
}
the following predicates are true:
blex, colordepth=[4-], colordepth!=6, colordepth, !screenwidth, UA-
media=stationary, UA-media!=screen, paper=A4, paper =!A0,
colordepth=[ 4 - 6 ], x-version=[100-300], x-version=[200-300]
and the following predicates are false:
!blex, blebber, colordepth=6, colordepth=foo, !colordepth,
screenwidth, screenwidth=640, screenwidth!=640, x-version=99, UA-
media=screen, paper=A0, paper=a4, x-version=[100-199], wuxta
6.4 Features attribute
The features attribute, for which section 5.1 defines the syntax
"{" "features" feature-list "}"
is used in a variant description to specify how the presence or
absence of particular feature tags in the user agent affects the
overall quality of the variant.
feature-list = 1%feature-list-element
feature-list-element = ( fpred fpred-bag )
[ ";" [ "+" true-improvement ]
[ "-" false-degradation ]
]
fpred-bag = "[" 1%fpred "]"
true-improvement = short-float
false-degradation = short-float
Features attributes are used in variant descriptions (section 5).
Variant descriptions can be transmitted in Alternates headers
(section 8.3).
Examples are:
{features !textonly [blebber !wolx] colordepth=3;+0.7}
{features !blink;-0.5 background;+1.5 [blebber !wolx];+1.4-0.8}
The default value for the true-improvement is 1. The default value
for the false-degradation is 0, or 1 if a true-improvement value is
given.
A user agent SHOULD, and a remote variant selection algorithm MUST
compute the quality degradation factor associated with the features
attribute by multiplying all quality degradation factors of the
elements of the feature-list. Note that the result can be a factor
greater than 1.
A feature list element yields its true-improvement factor if the
corresponding feature predicate is true, or if at least one element
of the corresponding fpred-bag is true. The element yields its
false-degradation factor otherwise.
7 Remote variant selection algorithms
A remote variant selection algorithm is a standardized algorithm by
which a server can choose a best variant on behalf of a negotiating
user agent. The use of a remote algorithm can speed up the
negotiation process by eliminating a request-response round trip.
A remote algorithm typically computes whether the Accept- headers in
the request contain sufficient information to allow a choice, and if
so, which variant is the best variant. This specification does not
define any remote algorithms, but does define a mechanism to
negotiate on the use of such algorithms.
7.1 Version numbers
A version numbering scheme is used to distinguish between different
remote variant selection algorithms.
rvsa-version = major "." minor
major = 1*4DIGIT
minor = 1*4DIGIT
An algorithm with the version number X.Y, with Y>0, MUST be downwards
compatible with all algorithms from X.0 up to X.Y. Downwards
compatibility means that, if supplied with the same information, the
newer algorithm MUST make the same choice, or a better choice, as the
old algorithm. There are no compatibility requirements between
algorithms with different major version numbers.
8 Content negotiation status codes and headers
This specification adds one new HTTP status code, and introduces six
new HTTP headers. It also extends the semantics of an existing
HTTP/1.1 header.
8.1 506 Variant Also Negotiates
The 506 status code indicates that the server has an internal
configuration error: the chosen variant resource is configured to
engage in transparent content negotiation itself, and is therefore
not a proper end point in the negotiation process.
8.2 Accept-Features
The Accept-Features request header can be used by a user agent to
give information about the presence or absence of certain features in
the feature set of the current request. Servers can use this
information when running a remote variant selection algorithm.
Note: the name `Accept-Features" for this header was chosen
because of symmetry considerations with other Accept- headers,
even though the Accept-Features header will generally not contain
an exhaustive list of features which are somehow `accepted". A
more accurate name of this header would have been `Feature-Set-
Info".
Accept-Features = "Accept-Features" ":"
#( feature-expr *( ";" feature-extension ) )
feature-expr = [ "!" ] ftag
ftag ( "=" "!=" ) tag-value
ftag "=" "{" tag-value "}"
"*"
feature-extension = token [ "=" ( token quoted-string ) ]
No feature extensions are defined in this specification. An example
is:
Accept-Features: blex, !blebber, colordepth={5}, !screenwidth,
paper = A4, paper!="A2", x-version=104, *
The different feature expressions have the following meaning:
ftag ftag is present
!ftag ftag is absent
ftag=V ftag is present with the value V
ftag!=V ftag is present, but not with the value V
ftag={V} ftag is present with the value V, and not with any
other values
* the expressions in this header do not fully describe
the feature set: feature tags not mentioned in this
header may also be present, and, except for the case
ftag={V}, tags may be present with more values than
mentioned.
Absence of the Accept-Features header in a request is equivalent to
the inclusion of
Accept-Features: *
By using the Accept-Features header, a remote variant selection
algorithm can sometimes determine the truth value of a feature
predicate on behalf of the user agent. For example, with the header
Accept-Features: blex, !blebber, colordepth={5}, !screenwidth,
paper = A4, paper!="A2", x-version=104, *
the algorithm can determine that the following predicates are true:
blex, colordepth=[4-], colordepth!=6, colordepth, !screenwidth,
paper=A4, colordepth=[4-6]
and that the following predicates are false:
!blex, blebber, colordepth=6, colordepth=foo, !colordepth,
screenwidth, screenwidth=640, screenwidth!=640,
but the truth value of the following predicates cannot be
determined:
UA-media=stationary, UA-media!=screen, paper!=a0,
x-version=[100-300], x-version=[200-300], x-version=99,
UA-media=screen, paper=A0, paper=a4, x-version=[100-199], wuxta
8.3 Alternates
The Alternates response header is used to convey the list of variants
bound to a negotiable resource. This list can also include
directives for any content negotiation process. If a response from a
transparently negotiable resource includes an Alternates header, this
header MUST contain the complete variant list bound to the negotiable
resource. Responses from resources which do not support transparent
content negotiation MAY also use Alternates headers.
Alternates = "Alternates" ":" variant-list
variant-list = 1#( variant-description
fallback-variant
list-directive )
fallback-variant = "{" <"> URI <"> "}"
list-directive = ( "proxy-rvsa" "=" <"> 0#rvsa-version <"> )
extension-list-directive
extension-list-directive =
token [ "=" ( token quoted-string ) ]
An example is
Alternates: {"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript}
{language en}},
proxy-rvsa="1.0, 2.5"
Any relative URI specified in a variant-description or fallback-
variant field is relative to the request-URI. Only one fallback-
variant field may be present. If the variant selection algorithm of
the user agent finds that all described variants are unacceptable,
then it SHOULD choose the fallback variant, if present, as the best
variant. If the user agent computes the overall quality values of
the described variants, and finds that several variants share the
highest value, then the first variant with this value in the list
SHOULD be chosen as the best variant.
The proxy-rvsa directive restricts the use of remote variant
selection algorithms by proxies. If present, a proxy MUST ONLY use
algorithms which have one of the version numbers listed, or have the
same major version number and a higher minor version number as one of
the versions listed. Any restrictions set by proxy-rvsa come on top
of the restrictions set by the user agent in the Negotiate request
header. The directive proxy-rvsa="" will disable variant selection
by proxies entirely. Clients SHOULD ignore all extension-list-
directives they do not understand.
A variant list may contain multiple differing descriptions of the
same variant. This can be convenient if the variant uses conditional
rendering constructs, or if the variant resource returns multiple
representations using a multipart media type.
8.4 Negotiate
The Negotiate request header can contain directives for any content
negotiation process initiated by the request.
Negotiate = "Negotiate" ":" 1#negotiate-directive
negotiate-directive = "trans"
"vlist"
"guess-small"
rvsa-version
"*"
negotiate-extension
negotiate-extension = token [ "=" token ]
Examples are
Negotiate: 1.0, 2.5
Negotiate: *
The negotiate directives have the following meaning
"trans"
The user agent supports transparent content negotiation for
the current request.
"vlist"
The user agent requests that any transparently negotiated
response for the current request includes an Alternates
header with the variant list bound to the negotiable resource.
Implies "trans".
"guess-small"
The user agent allows origin servers to run a custom algorithm
which guesses the best variant for the request, and to return
this variant in a choice response, if the resulting choice
response is smaller than or not much larger than a list
response. The definition of `not much larger" is left to
origin server heuristics. Implies "vlist" and "trans".
rvsa-version
The user agent allows origin servers and proxies to run the
remote variant selection algorithm with the indicated version
number, or with the same major version number and a higher
minor version number. If the algorithm has sufficient
information to choose a best, neighboring variant, the origin
server or proxy MAY return a choice response with this
variant. Implies "trans".
"*"
The user agent allows origin servers and proxies to run any
remote variant selection algorithm. The origin server may
even run algorithms which have not been standardized. If the
algorithm has sufficient information to choose a best,
neighboring variant, the origin server or proxy MAY return a
choice response with this variant. Implies "trans".
Servers SHOULD ignore all negotiate-directives they do not
understand. If the Negotiate header allows a choice between multiple
remote variant selection algorithms which are all supported by the
server, the server SHOULD use some internal precedence heuristics to
select the best algorithm.
8.5 TCN
The TCN response header is used by a server to signal that the
resource is transparently negotiated.
TCN = "TCN" ":" #( response-type
server-side-override-directive
tcn-extension )
response-type = "list" "choice" "adhoc"
server-side-override-directive = "re-choose" "keep"
tcn-extension = token [ "=" ( token quoted-string ) ]
If the resource is not transparently negotiated, a TCN header MUST
NOT be included in any response. If the resource is transparently
negotiated, a TCN header, which includes the response-type value of
the response, MUST be included in every response with a 2xx status
code or any 3xx status code, except 304, in which it MAY be included.
A TCN header MAY also be included, without a response-type value, in
other responses from transparently negotiated resources.
A server-side override directive MUST be included if the origin
server performed a server-side override when choosing the response.
If the directive is "re-choose", the server MUST include an
Alternates header with the variant bound to the negotiable resource
in the response, and user agent SHOULD use its internal variant
selection algorithm to choose, retrieve, and display the best variant
from this list. If the directive is "keep" the user agent SHOULD NOT
renegotiate on the response, but display it directly, or act on it
directly if it is a redirection response.
Clients SHOULD ignore all tcn-extensions they do not understand.
8.6 Variant-Vary
The Variant-Vary response header can be used in a choice response to
record any vary information which applies to the variant data (the
entity body combined with some of the entity headers) contained in
the response, rather than to the response as a whole.
Variant-Vary = "Variant-Vary" ":" ( "*" 1#field-name )
Use of the Variant-Vary header is discussed in section 10.2.
9 Cache validators
To allow for correct and efficient caching and revalidation of
negotiated responses, this specification extends the caching model of
HTTP/1.1 [1] in various ways.
This specification does not introduce a `variant-list-max-age"
directive which explicitly bounds the freshness lifetime of a cached
variant list, like the `max-age" Cache-Control directive bounds the
freshness lifetime of a cached response. However, this specification
does ensure that a variant list which is sent at a time T by the
origin server will never be re-used without revalidation by
semantically transparent caches after the time T+M. This M is the
maximum of all freshness lifetimes assigned (using max-age directives
or Expires headers) by the origin server to
a. the responses from the negotiable resource itself, and
b. the responses from its neighboring variant resources
If no freshness lifetimes are assigned by the origin server, M is the
maximum of the freshness lifetimes which were heuristically assigned
by all caches which can re-use the variant list.
9.1 Variant list validators
A variant list validator is an opaque value which acts as the cache
validator of a variant list bound to a negotiable resource.
variant-list-validator = <quoted-string not containing any ";">
If two responses contain the same variant list validator, a cache can
treat the Alternates headers in these responses as equivalent (though
the headers themselves need not be identical).
9.2 Structured entity tags
A structured entity tag consists of a normal entity tag of which the
opaque string is extended with a semicolon followed by the text
(without the surrounding quotes) of a variant list validator:
normal variant list structured
entity tag validator entity tag
-------------+----------------+-----------------
"etag" "vlv" "etag;vlv"
W/"etag" "vlv" W/"etag;vlv"
Note that a structured entity tag is itself also an entity tag. The
structured nature of the tag allows caching proxies capable of
transparent content negotiation to perform some optimizations defined
in section 10. When not performing such optimizations, a structured
tag SHOULD be treated as a single opaque value, according to the
general rules in HTTP/1.1. Examples of structured entity tags are:
"xyzzy;1234" W/"xyzzy;1234" "gonkxxxx;1234" "a;b;c;;1234"
In the last example, the normal entity tag is "a;b;c;" and the
variant list validator is "1234".
If a transparently negotiated response includes an entity tag, it
MUST be a structured entity tag. The variant list validator in the
structured tag MUST act as a validator for the variant list contained
in the Alternates header. The normal entity tag in the structured
tag MUST act as a validator of the entity body in the response and of
all entity headers except Alternates.
9.3 Assigning entity tags to variants
To allow for correct revalidation of transparently negotiated
responses by clients, origin servers SHOULD generate all normal
entity tags for the neighboring variant resources of the negotiable
resource in such a way that
1. the same tag is never used by two different variants,
unless this tag labels exactly the same entity on all occasions,
2. if one normal tag "X" is a prefix of another normal tag "XY",
then "Y" must never be a semicolon followed by a variant list
validator.
10 Content negotiation responses
If a request on a transparently negotiated resource yields a response
with a 2xx status code or any 3xx status code except 304, this
response MUST always be either a list response, a choice response, or
an adhoc response. These responses MUST always include a TCN header
which specifies their type. Transparently negotiated responses with
other status codes MAY also include a TCN header.
The conditions under which the different content negotiation
responses may be sent are defined in section 12.1 for origin servers
and in section 13 for proxies.
After having constructed a list, choice, or adhoc response, a server
MAY process any If-No-Match or If-Range headers in the request
message and shorten the response to a 304 (Not Modified) or 206
(Partial Content) response, following the rules in the HTTP/1.1
specification [1]. In this case, the entity tag of the shortened
response will identify it indirectly as a list, choice, or adhoc
response.
10.1 List response
A list response returns the variant list of the negotiable resource,
but no variant data. It can be generated when the server does not
want to, or is not allowed to, return a particular best variant for
the request. If the user agent supports transparent content
negotiation, the list response will cause it to select a best variant
and retrieve it.
A list response MUST contain (besides the normal headers required by
HTTP) a TCN header which specifies the "list" response-type, the
Alternates header bound to the negotiable resource, a Vary header and
(unless it was a HEAD request) an entity body which allows the user
to manually select the best variant.
An example of a list response is
HTTP/1.1 300 Multiple Choices
Date: Tue, 11 Jun 1996 20:02:21 GMT
TCN: list
Alternates: {"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript}
{language en}}
Vary: negotiate, accept, accept-language
ETag: "blah;1234"
Cache-control: max-age=86400
Content-Type: text/html
Content-Length: 227
<h2>Multiple Choices:</h2>
<ul>
<li><a href=paper.1>HTML, English version</a>
<li><a href=paper.2>HTML, French version</a>
<li><a href=pape