RFC 2295
Transparent Content Negotiation in HTTP
Pages: 58
Experimental
Experimental
Part 1 of 2 – Pages 1 to 24
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 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.
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=titlebanner.gif width=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.
(next page on part 2)