Internet-Draft Koen Holtman/TUE Expires: February 14, 1997 August 14, 1996 Transparent Content Negotiation in HTTP draft-holtman-http-negotiation-02.txt STATUS OF THIS MEMO This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Distribution of this document is unlimited. Please send comments to the HTTP working group at . Discussions of the working group are archived at . General discussions about HTTP and the applications which use HTTP should take place on the mailing list. ABSTRACT HTTP allows one to put multiple versions of the same information under a single URL. Transparent content negotiation is a 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. Design goals for transparent content negotiation include low overhead on the request message size, downwards compatibility, extensibility, enabling the rapid introduction of new areas of negotiation, scalability, low cost of minimal support, end user control, and good cachability. TABLE OF CONTENTS 1 Introduction 1.1 Background 1.2 Note for readers 2 Terminology 2.1 Terms from HTTP/1.1 2.2 New terms 3 Notation 4 Overview 4.1 Content negotiation 4.2 HTTP/1.0 style negotiation scheme 4.3 Transparent content negotiation scheme 4.4 Optimizing the negotiation process 4.5 Retrieving a variant by hand 4.6 Dimensions of negotiation 4.7 Feature negotiation 5 Variant descriptions 5.1 Syntax 5.2 URI 5.3 Source-quality 5.4 Type, language, length, and charset. 5.5 Features 5.6 Description 5.7 Extension-attribute 6 Feature negotiation 6.1 Feature tags 6.2 Accept-Features header 6.3 Feature predicates 6.4 Features attribute 7 Feature negotiation examples 7.1 Use of feature tags 7.2 Use of numeric feature tags 8 Feature tag registration 8.1 Evolution of feature tags 8.2 Core set of feature tags 8.3 Feature tag design 9 Content negotiation response codes and headers 9.1 506 Variant Also Negotiates 9.2 Accept-Features 9.3 Content-Features 9.4 Alternates 9.5 Alternates-Older 9.6 Negotiate 9.7 Variant-Vary 10 Content negotiation responses 10.1 List response 10.2 Choice response 10.2.1 Construction by origin servers 10.2.2 Construction by proxies 10.3 Reusing the Alternates header 10.4 Extracting a normal response from a choice response 10.5 Elaborate Vary headers 10.5.1 Construction of an elaborate Vary header 10.5.2 Caching of an elaborate Vary header 11 The network negotiation algorithm 11.1 Input 11.2 Output 11.2.1 Output for proxies 11.2.2 Output for origin servers 11.2.3 Output for user agents 11.3 Computing the overall quality values 11.4 Definite and speculative quality values 11.5 Determining the result 11.5.1 Result for proxies and origin servers 11.5.2 Result for user agents 11.6 Construction of short requests 12 User agent support for transparent negotiation 12.1 Handling of responses 12.2 Presentation of a transparently negotiated resource 13 Origin server support for transparent negotiation 13.1 Requirements 13.2 Negotiation on transactions other than GET and HEAD 14 Proxy support for transparent negotiation 15 Security and privacy considerations 15.1 Accept headers revealing information of a private nature 15.2 Spoofing of responses from variant resources 16 Acknowledgments 17 References 18 Author's address 19 Appendices 19.1 Adding an Expires header to ensure HTTP/1.0 compatibility 19.2 Origin server implementation considerations 19.2.1 Implementation with a CGI script 19.2.2 Direct support by the HTTP server 19.2.3 Negotiable content authoring 19.3 Open issues in transparent content negotiation 19.4 Other negotiation specifications 19.4.1 User-Agent Display Attributes Headers 19.4.2 PEP: An Extension Mechanism for HTTP/1.1 19.5 Related issues 19.5.1 Current negotiation practice 19.5.2 Bandwidth negotiation 19.5.3 Content transformation by proxies 1 Introduction HTTP allows one to put multiple versions of the same information under a single URI. Each of these versions is called a `variant'. Transparent content negotiation is a mechanism, layered on top of the HTTP/1.1 protocol [1], 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. Transparent negotiation is called `transparent' because it makes all variants inside the origin server visible to outside parties. 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, allowing graceful evolution of the web * elimination of the need to choose between a `state of the art multimedia homepage' and one which can be viewed by all web users * ability to offer good service to a wider range of browsing platforms (from low-end PDA's to high-end VR setups) * elimination of the need for error-prone and cache-unfriendly User-Agent based negotiation * ability to construct sites without `click here for the X version' links * internationalization, and the ability to offer multi-lingual content without a bias towards one language. Nevertheless, the development of content negotiation has been a slow process, maybe because the expected benefits are mostly long-term benefits. Also, all immediately obvious content negotiation solutions involve the sending of larger HTTP request messages. Larger request messages are unacceptable to many people: a small overhead on the request size is often cited as the main technical requirement for any scheme which offers the above benefits. Therefore, a low overhead on the request message size is one of the main design goals of this specification; a large fraction of the text below is devoted to the definition of mechanisms for meeting this design goal. The desire to provide for good cachability in all cases accounts for another considerable part of the size of this document. Other design goals include downwards compatibility, extensibility, enabling the rapid introduction of new areas of negotiation, scalability, low cost of minimal support, and end user control. End user control, the option to manually retrieve other variants if desired, is the one redeeming quality of the `click here for the X version' type of negotiation, and it is considered important that this option is preserved. 1.2 Note for readers Some sections in this document discuss requirements for proxy caches only. Implementers of origin servers and user agents (but not implementers of user agent caches) can skip these sections. The sections which can be skipped are: 9.5 Alternates-Older 9.7 Variant-Vary 10.2.2 Choice response construction by proxies 10.3 Reusing the Alternates header 10.4 Extracting a normal response from a choice response 10.5.2 Caching of an elaborate Vary header 14 Proxy support for transparent negotiation The skipping of these sections is also recommended on first reading. 2 Terminology 2.1 Terms from HTTP/1.1 This specification mostly uses the terminology of the HTTP/1.1 specification [1]. The definitions below were reproduced 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. variant A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations is termed a `variant.' Use of the term `variant' does not necessarily imply that the resource is subject to content negotiation. 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. first-hand A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more proxies. A response is also first-hand if its validity has just been checked directly with the origin server. 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 which supports transparent content negotiation. A transparently negotiable resource always has a variant list bound to it, usually represented as an Alternates header. variant list The list of variants bound to a transparently negotiable resource. variant description A machine-readable description of a variant, 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 simple GET request. list response A list response contains the variant list of the negotiable resource, but no variant data. It is generated if the sever cannot or does not want to choose a particular best variant for the request. List responses are defined in Section 10.1. choice response A choice response contains both the variant list of the negotiable resource, and a representation of the best variant for the request. Choice responses are defined in Section 10.2. normal response A HTTP response which is not a list response or a choice response. Normal responses are never generated by transparently negotiable resources, and are always generated by variant resources. Accept headers The request headers Accept, Accept-Charset, Accept-Language, and Accept-Features. network negotiation algorithm A standardized algorithm by which a party in the negotiation process can choose a best variant on behalf of another party. The algorithm computes if the Accept headers in the request contain sufficient information to allow a choice, and, if so, which variant must be chosen. The network negotiation algorithm is defined in Section 11. neighbor Two resources are called neighbors if the absolute URI of the first resource up to its last slash equals the absolute URI of the second resource up to its last slash. The neighboring relation is important because of security considerations, see Section 15.2. [##Historical note: In the context of old versions of the HTTP/1.1 specification like [2], list responses can be called `pre-emptive negotiation responses', and choice responses can be called `reactive negotiation responses'.##] [##Note: In the context of the HTTP/1.1 specification [1], both list and choice responses are the result of server-driven negotiation, though a list response is a server-driven negotiation response meant to initiate agent-driven negotiation.##] 3 Notation The notation [## ... ##] in this document encloses an editorial comment. Such a comment will be either removed or replaced by real text in the final version of this document. The version of BNF used in this document is taken from [1], and many of the nonterminals used are defined in [1]. 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 ) 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 one 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 will have multiple variants. 4.3 Transparent content negotiation scheme The transparent 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.html.en" 0.9 {type text/html} {language en}}, {"paper.html.fr" 0.7 {type text/html} {language fr}}, {"paper.ps.en" 1.0 {type application/postscript} {language en}} The syntax and semantics of the variant descriptions in this list are covered in depth 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.html.en | ---------------------------------- > [normal response] return of html.en 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, the information about capabilities and preferences is only used by the user agent itself. Therefore, the sending of 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 depth 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 19:39:48 GMT Alternates: {"paper.html.en" 0.9 {type text/html} {language en}}, {"paper.html.fr" 0.7 {type text/html} {language fr}}, {"paper.ps.en" 1.0 {type application/postscript} {language en}} Vary: * Content-Type: text/html Content-Length: 227

Multiple Choices:

The HTML entity included in the response ensures compatibility with (existing) user agents which do not support transparent content negotiation. Such user agents will ignore the computer-readable list in the Alternates header and just display the HTML entity, allowing the user to select the best variant by hand. 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 complete responses. 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.html.en | has the variant in cache | -------------------------- > [normal response] return of html.en Second, the user agent can send small Accept headers which could allow a server to determine the choice the user agent would make on receiving the list. If the Accept headers contain enough information, the origin server can send back the variant 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 html.en and list This choosing based on small accept headers can be done with the network negotiation algorithm (Section 11). This algorithm takes the variant list and the Accept headers as input. It computes if the Accept headers contain sufficient information to allow a choice on behalf of the user agent, and, if so, which variant must be chosen. The response in the above diagram is called a choice response: it transmits both the chosen variant and the list of all variants bound to the negotiable resource. Choice responses are covered in depth in Section 10.2. As an example, the choice response in the above picture could be: HTTP/1.1 200 OK Date: Tue, 11 Jun 1996 19:39:54 GMT Content-Type: text/html Content-Length: 5327 Content-Location: paper.html.en Alternates: {"paper.html.en" 0.9 {type text/html} {language en}}, {"paper.html.fr" 0.7 {type text/html} {language fr}}, {"paper.ps.en" 1.0 {type application/postscript} {language en}} Vary: * 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.html.en | ---------- > [normal response] html.en | ---------------- > [choice response] html.en and list Here, the proxy cache nearest to the user agent was able to use a cached variant list to choose on behalf of the user agent. 4.5 Retrieving a variant by hand If a transparently negotiated resource is accessed, the user agent will always at some point receive the list of available variants. The user agent can use this list to make available a menu (in HTML or not) which lists all variants and their characteristics to the user. Such a menu will allow the user to randomly browse other variants, and will also make it possible to manually correct any sub-optimal choice made by the automatic negotiation process. 4.6 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. Transparent negotiation does not prohibit proxies to encode or decode a relayed or cached response on the fly: the response still contains the same variant as far as transparent content negotiation is concerned. 4.7 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 is the main 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'. 5 Variant descriptions 5.1 Syntax A variant can be described in a machine-readable way with a variant description. variant-descr = "{" <"> URI <"> source-quality [ "{" "type" media-type "}" ] [ "{" "charset" charset "}" ] [ "{" "language" 1#language-tag "}" ] [ "{" "length" 1*DIGIT "}" ] [ "{" "features" feature-list "}" ] [ "{" "description" quoted-string "}" ] [ extension-attribute ] "}" source-quality = qvalue 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 "}"> Examples are {"paper.html.fr" 0.7 {type text/html} {language fr}} {"paper.html.tables" 0.9 {type text/html} {features tables}} The various attributes which can be present in a variant description are covered in the subsections below. 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. It is important that content providers do not assign very low source quality values without good reason, as this would limit the ability of users to influence the negotiation process with their own preference settings. The following table should be used as a guide when assigning source quality values: 1.000 no degradation 0.999-0.900 no noticeable degradation 0.899-0.700 noticeable, but acceptable degradation 0.699-0.500 barely acceptable degradation 0.499-0.000 unacceptable degradation [##Issue to be resolved: can we come up with a word other than `degradation' that covers better the case of variants not converted from one source?##] When assigning source-quality values, content providers must not account for the size of the variant and its impact on transmission and rendering delays. Source-quality values are assigned assuming instantaneous transmission and rendering. This rule ensures that a future mechanism for bandwidth negotiation (see Appendix 19.5.2) can be cleanly added. 5.4 Type, language, length, and charset. The type, language, and length attributes of a variant description can carry the same information as their Content-* response header counterparts as defined in [1]. The counterpart of the charset attribute is the charset parameter which can be present in the Content-Type response header, for example Content-Type: text/html; charset=ISO-8859-4 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. The length attribute, if present, must reflect the length of the variant only, not the length of the variant plus the length of any objects inlined or embedded by the variant. Note: A server need not necessarily maintain a one-to-one correspondence between attributes in the description of a variant and the Content-* headers in responses containing that variant. For example, if the variant description contains a language attribute, the response does not necessarily have to contain a Content-Language header, and 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 features 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 is meant to provide a textual description of the variant, to be displayed by a user agent when showing a menu of available variants (Section 12.2). This attribute can be included if the URI and normal attributes of a variant are considered too opaque to allow interpretation by the user. 5.7 Extension-attribute The extension-attribute allows future specifications to incrementally define new dimensions of negotiation, and eases content negotiation experiments under HTTP/1.1. 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 the network negotiation algorithm if an unknown extension attribute is present in the variant list. 6 Feature negotiation Feature negotiation has been introduced in Section 4.7. This section defines the feature negotiation mechanism, Section 7 contains examples of its use. 6.1 Feature tags A feature tag (ftag) identifies a capability of a user agent or a preference of a user. A feature is said to be `present' in a user agent if the corresponding capability is implemented, or if the user has expressed corresponding preference. ftag = 1*<any CHAR except CTLs or tspecials or "!"> tspecials = "(" | ")" | "<" | ">" | "@" | "," | ";" | ":" | "\" | <"> | "/" | "[" | "]" | "?" | "=" | "{" | "}" | SP | HT (tspecials definition reproduced from [1]) Examples are ns_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 ns_tables ns_frames}} Feature-tags are case-insensitive. The definition of a feature tag may state that a feature tag, if present, must have associated with it a numeric value which reflects a particular capability or preference. For example, a feature tag `screenwidth' could be present with a value of 640. Note that it sometimes depends on context whether a feature tag expresses a capability or a preference. For a text-only browser, the `textonly' tag is naturally present, but the user of a graphical browser could set the tag to be present if text-only content is preferred to graphical content. As feature registration will 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 with tags unknown to it as absent. 6.2 Accept-Features header The Accept-Features request header can be used by a client to give information about the presence or absence of certain features. Accept-Features = "Accept-Features" : #( ftag | "!" ftag | ftag "=" number | "*" ) number = 1*DIGIT An example is: Accept-Features: blex, !blebber, colordepth=5, * A feature tag must never occur twice in an Accept-Features header. In the Accept-Features header, an ftag without a "!" indicates that the corresponding feature is present, but that is has no numeric value associated with it. An ftag with a "!" indicates that the corresponding feature is absent. An ftag with a number indicates that the corresponding feature is present with the given numeric value. The special construct "*", if present, makes all true feature predicates (Section 6.3) on feature tags not present in the header. Absence of the Accept-Features header in a request is equivalent to the inclusion of Accept-Features: * 6.3 Feature predicates Feature predicates are used in the features attribute of a variant description. fpred = ( ftag ) | ( "!" ftag ) | ( ftag ">=" number ) | ( ftag "<" number ) Examples feature predicates are blebber, !blebber, colordepth>=5, blex<54 The network negotiation algorithm can compute the truth value of a feature predicate by using the contents of the Accept-Features header of the current request. If the ftag in a feature predicate appears in the Accept-Features header, the truth value of the predicate is defined as follows, depending on its form: ftag true if the feature is present according to the Accept-Features header, false otherwise. !ftag true if the feature is absent according to the Accept-Features header, false otherwise. ftag>=N true if the feature is present with a value greater than or equal to N according to the Accept-Features header, false otherwise. ftag<N true if the feature is present with a value less than N according to the Accept-Features header, false otherwise. [##Issues to be resolved: Add ftag<=N and ftag>N for symmetry? Add ftag=N? Add ftag=N as a synonym for ftag>=N?##] If the ftag in a feature predicate does not appear in the Accept-Features header, the value of the predicate is true if there is a "*" in the Accept-Features header, false otherwise. As an example, the header Accept-Features: blex, !blebber, colordepth=5, !screenwidth, * makes the following predicates true: blex, colordepth>=4, colordepth<6, colordepth, !screenwidth, frtnbf, !frtnbf, frtnbf>=4, frtnbf<4 and makes the following predicates false: !blex, blex>=0, blebber, colordepth>=6, !colordepth, screenwidth, screenwidth>=640, screenwidth<640 6.4 Features attribute The features attribute "{" "features" feature-list "}" is used in a variant description to specify how the presence or absence of particular features 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 = 1*3DIGIT [ "." 0*3DIGIT ] false-degradation = 1*3DIGIT [ "." 0*3DIGIT ] 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. The network negotiation algorithm can 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. [##Issue to be resolved: It is unknown yet whether this features attribute definition makes the right tradeoff between complexity and (ease of) expressive power. The attribute grammar above is designed to be parsable with simple non-recursive parsers. The true-improvement construct does not add expressive power in a theoretical sense, but does make the (automatic) construction of variant lists more straightforward in many cases.##] [##Aside for mathematicians: note the similarity between the feature-list [f1 !f2] [!f1 f3] and the conjunctive normal form of a boolean expression. This similarity implies good things about the expressive power of the features attribute. I have not yet explored how far this power extends into the non-boolean domain.##] 7 Feature negotiation examples This section contains examples of the use of feature tags in variant descriptions. [##Note: A future version of this document will probably revise and extend this section.##] 7.1 Use of feature tags Feature tags can be used in variant lists to express the quality degradation associated with the presence or absence of certain features. One example is {"index.html.plain" 0.7 }, {"index.html" 1.0 {features ns_tables ns_frames}} Here, the "{features ns_tables ns_frames}" part expresses that index.html uses the features tagged as ns_tables and ns_frames. If these features are absent, the overall quality of index.html degrades to 0. Another example is {"home.graphics" 1.0 {features !textonly}}, {"home.textonly" 0.7 } where the "{features !textonly}" part expresses that home.graphics requires the absence of the textonly feature. If the feature is present, the overall quality of home.graphics degrades to 0. The absence of a feature need not always degrade the overall quality to 0. In the example {"x.html.1" 1.0 {features fonts/0.7}} the absence of the fonts feature degrades the quality with a factor of 0.7. "fonts/0.7" can be pronounced as "fonts, or a degradation of 0.7". In the example {"x.html.2" 0.5 {features fonts:1.5}} the presence of the fonts feature improves the overall quality with a factor of 1.5. If the fonts feature is absent, the overall quality is not affected. "fonts:1.5" can be pronounced as "fonts improves with 1.5". Finally, in the example {"y.html" 1.0 {features [blebber wolx] }} The "[blebber wolx]" expresses that y.html requires the presence of the blebber feature or the wolx feature. This construct can be used in a number of cases: 1. blebber and wolx actually tag the same feature, but they were registered by different people, and some browsers say they support blebber while others say they support wolx. 2. blebber and wolx are HTML tags of different vendors which implement the same functionality, and which were used together in y.html without interference. 3. blebber and wolx are HTML tags of different vendors which implement the same functionality, and y.html uses conditional HTML to provide versions using both tags [##Note: conditional HTML does not yet exist, but it is something people are thinking about.##] 4. blebber is a complicated HTML tag with only a sketchy definition, implemented by one browser vendor, and wolx indicates implementation of a well-defined subset of the blebber tag by some other vendor(s). y.html uses only this well-defined subset. 7.2 Use of numeric feature tags As an example of negotiation in a numeric area, the following variant list describes four variants with title graphics designed for increasing screen widths: {"home.pda" 1.0 {features screenwidth<200}}, {"home.narrow" 1.0 {features screenwidth>=200 screenwidth<600}}, {"home.normal" 1.0 {features screenwidth>=600 screenwidth<1000}}, {"home.wide" 1.0 {features screenwidth>=1000}} The above variant list has a serious problem, however: a user agent not supporting screenwidth negotiation is given no guidance on which variant to select. For such a user agent, the absence of the screenwidth feature would degrade the overall quality of each variant to 0. A variant list which solves this problem is {"home.pda" 1.0 {features screenwidth<200}}, {"home.narrow" 1.0 {features screenwidth>=200 screenwidth<600}}, {"home.normal" 0.95 }, {"home.wide" 1.0 {features screenwidth>=1000}} With this list, a user agent which does not support the screenwidth feature will always select the home.normal variant. A user agent which does support the screenwidth feature will only select the home.normal variant if its screen width is in the range 600..999. 8 Feature tag registration [##A specification of the feature tag registration process has not yet been completed. This issue has yet to be discussed by the http-wg.##] [##According to current plans, everybody will be allowed to register feature tags: registration only requires that the tag follows the syntax rules, and that a definition of the meaning of the tag is supplied. In particular, registration will not require actual implementation of a feature, and there will be no test on whether the feature definition overlaps with another feature definition.##] [##It is unclear yet whether it is desirable for feature tags to have some hierarchical structure, which would make it easier, for example, to trace who registered the feature. All that is known now is that it is desirable to have short feature tags. A naming scheme like that for SGML-DTDs would probably yield feature tags which are too long. The main advantage of having a registration process at all is that it allows for short tags without running the risk of `tag collision'.##] 8.1 Evolution of feature tags The feature negotiation mechanism is designed not to break if 1000+ features, partially overlapping, are registered. This allows for feature tag creation to be an evolutionary process, in which many tags are created while only a few `survive' to become generally used. As the development of new web content formats is currently in an evolutionary phase, rather than a standardization phase, it is thought that this evolutionary approach to feature tag creation has the best chances of keeping up with new developments. It is expected that after the introduction of feature negotiation, an explosion in feature tag registration will occur, and that only some of these tags will end up being actively used to label variants. Web indexing robots could, while traversing the web, gather statistics about actual use of feature tags. These statistics could be used by individuals to compile lists, intended for content authors, of useful feature tags for particular areas of negotiation. Note that this indexing activity is orthogonal to the feature registration process. It is expected that, once an area of negotiation is well-understood, this process will converge on a commonly-used and commonly-recognized set of feature tags for that area. 8.2 Core set of feature tags [##In order to jump-start feature negotiation, it seems useful to define a `core set' of feature tags in a separate document. These feature tags would cover the areas of negotiation which are currently well understood, like negotiation on currently stable HTML extensions. The transparent content negotiation specification could then require (or strongly encourage) that user agents implementing transparent content negotiation must recognize (not support!) all tags in the core set. The core set could be defined in a non-standards track document which would be completed along with this document.##] 8.3 Feature tag design When designing a new feature tag, it is important to take into account that existing user agents, which will not recognize the new tag, will treat the feature as absent. In general, a new feature tag needs to be designed in such a way that absence of the tag is the default case which reflects current practice. If this design principle is ignored, the resulting feature tag will generally be unusable. As an example, one could try to support negotiation between monochrome and color content by introducing a `color' feature tag, the presence of which would indicate the capability to display color graphics. However, if this new tag is used in a variant list, for example {"rainbow.gif" 1.0 {features color} } {"rainbow.mono.gif" 0.6 {features !color}} then existing user agents, which would not recognize the color tag, would all display the monochrome rainbow. The color tag is therefore unusable in situations where optimal results for existing user agents are desired. To provide for negotiation in this area, one needs to introduce a `monochrome' feature tag, the presence of which indicates that the user agent can only render monochrome graphics. 9 Content negotiation response codes and headers This specification adds one new response code to those defined in HTTP/1.1 [1]. This is a 5xx (Server Error) class response. It also adds six new headers. 9.1 506 Variant Also Negotiates 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. 9.2 Accept-Features This request header was defined in Section 6.2. 9.3 Content-Features The Content-Features response header can be used by a server to indicate how the presence or absence of particular features in the user agent affects the overall quality of the response. Content-Features = "Content-Features" ":" feature-list 9.4 Alternates The Alternates response header is used to convey the list of variants bound to a negotiable resource in a response, and can also contain other directives for the content negotiation process. Alternates = "Alternates" ":" 1#( variant-descr | alternates-directive ) alternates-directive = ( "max-age" "=" delta-seconds ) | ( "min-q" "=" short-float ) | "forward" | extension-alternates-directive delta-seconds = 1*DIGIT short-float = 1*3DIGIT [ "." 0*3DIGIT ] extension-alternates-directive = token [ "=" ( token | quoted-string ) ] An example is Alternates: {"paper.html.en" 0.9 {type text/html} {language en}}, {"paper.html.fr" 0.7 {type text/html} {language fr}}, {"paper.ps.en" 1.0 {type application/postscript} {language en}}, min-q=0.4 Any relative URI specified in a variant-descr field is relative to the request-URI. The max-age directive, if present, specifies the freshness lifetime of the information in the Alternates header. The min-q and forward directives, if present, can influence decisions made by the network negotiation algorithm on behalf of the origin server. Clients should ignore all extension-alternates-directives they do not understand. [##Question to be resolved: should this header be called `Variants'? Probably not, the 1.1 spec already announced that it would be called `Alternates'.##] 9.5 Alternates-Older The Alternates-Older response header conveys an estimate of the age difference between the Alternates header contained in a choice response and the variant data contained in the response. The age is measured as the time since generation or revalidation at the origin server, as in the HTTP/1.1 specification [1]. Alternates-Older = "Alternates-Older" ":" signed-delta-seconds signed-delta-seconds = [ "-" ] 1*DIGIT A positive value means that the Alternates header is older, a negative value means that the Alternates header is less old. If the Alternates-Older header is absent, it is known or estimated that there is no age difference between the Alternates header and the variant data. An example is Alternates-Older: 5433 This header is important for proxies which negotiate using cached variant lists, see Section 10.3. Only negotiating proxies need to be able to construct Alternates-Older headers, see Section 10.2.2. [##Note: The age of the Alternates header cannot be conveyed in an Alternates-Age header, because plain HTTP/1.1 caching proxies would only update the Age header, not the Alternates-Age header, when serving a choice response from cache memory.##] 9.6 Negotiate The Negotiate request header can contain directives for any content negotiation process initiated by the request. Presence of this header indicates that the user agent supports transparent content negotiation for the current request. Negotiate = "Negotiate" ":" #negotiate-directive negotiate-directive = token [ "=" ( token | quoted-string ) ] An example is Negotiate: This specification does not define any negotiate-directives, this is left to future extensions. Servers should ignore all negotiate-directives they do not understand. The presence or absence of a Negotiate header in a request is significant in calculations performed by the network negotiation algorithm. [#Question to be resolved: is it safer to put a keyword in the negotiate header to signal transparent negotiation capability? Some cache implementations might treat an empty Negotiate header the same as an absent Negotiate header under Vary: negotiate. Is the 1.1 spec clear enough on this?##] 9.7 Variant-Vary The Variant-Vary response header can be used in a list response to record any vary information which applies to the variant data 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 10.2.1. 10 Content negotiation responses A request on a transparently negotiated resource can yield two types of non-error responses: list responses and choice responses. A list response contains only the list of variants bound to the negotiable resource, and is generated if the sever cannot or does not want to choose a particular best variant for the request. A choice response contains both the list and a representation of the best variant. Normal responses are non-error responses generated by requests on resources which are not transparently negotiated. A normal response never contains an Alternates header, a list or choice response always contains an Alternates header. A list response always has the 300 (Multiple Choices) status code, a choice response never has the 300 status code. 10.1 List response A list response has the 300 response status code, and must contain, besides the normal headers required by HTTP, 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 a list response is: HTTP/1.1 300 Multiple Choices Date: Tue, 11 Jun 1996 20:02:21 GMT Alternates: {"paper.html.en" 0.9 {type text/html} {language en}}, {"paper.html.fr" 0.7 {type text/html} {language fr}}, {"paper.ps.en" 1.0 {type application/postscript} {language en}}, max-age=86400 Vary: negotiate, accept, accept-language Etag: "gonkyyyy" Cache-control: max-age=86400 Content-Type: text/html Content-Length: 227 <h2>Multiple Choices:</h2> <ul> <li><a href=paper.html.en>HTML, English version</a> <li><a href=paper.html.fr>HTML, French version</a> <li><a href=paper.ps.en>Postscript, English version</a> </ul> Note that the same max-age value appears both in the Alternates header and in the Cache-Control header. The Vary header in the response should ensure correct handling by HTTP/1.1 caching proxies not capable of transparent content negotiation. This header can either be Vary: * or a more elaborate header, see Section 10.5.1. Only the origin server may construct list responses. List responses are cachable unless indicated otherwise. A proxy may reuse a list response as a whole if the response is fresh in the sense of the HTTP/1.1 specification [1], even if the Alternates header in the response is no longer fresh by itself. According to the HTTP/1.1 specification [1], a user agent not capable of transparent content negotiation will, when receiving a list response, display the entity body included in the response. If the response contains a Location header, however, the user agent may automatically redirect to this location. The handling of list responses by clients supporting transparent content negotiation is described in Sections 12.1 and 14. [##Issue to be resolved: Some existing HTTP/1.0 user agents crash when getting a 300 response without a Location header. This problem may be big enough to warrant the allocation of an extra list response code outside of the 3xx class for possible use in responses to HTTP/1.0 clients.##] 10.2 Choice response A choice response merges a normal HTTP response from the chosen variant with a Content-Location header giving the location of the variant, and the Alternates and Vary headers bound to the negotiable resource. The cachability of a choice response as a whole is defined in the HTTP/1.1 specification [1]. Depending on the response code, a choice response as a whole is either cachable unless indicated otherwise, or uncachable. 10.2.1 Construction by origin servers Origin servers must construct choice responses with the following algorithm, or any other algorithm which gives equal results. 1. Construct a HTTP request message on the best variant resource by rewriting the request-URI and Host header (if appropriate) of the received request message on the negotiable resource. 2. Generate a valid HTTP response message for the request message constructed in step 1. 3. Check for an internal server configuration error. If the HTTP response message generated in step 2 contains an Alternates header, a Content-Location header, or has the 300 status code, then the best variant resource is not a proper end point in the negotiation process, and a 506 (Variant Also Negotiates) error response message should be generated instead of going to step 4. 4. Add a number of headers to the HTTP response message generated in step 2. a. Add a Content-Location header giving the location of the chosen variant. Note: According to the HTTP/1.1 specification [1], if the Content-Location header contains a relative URI, this URI is relative to the URI in the Content-Base header, if present. b. If any Vary headers are present in the response message from step 2, add, for every Vary header, a Variant-Vary header with a copy of the contents of this Vary header. c. Add the Alternates header bound to the negotiable resource. d. Add a Vary header to ensure correct handling by HTTP/1.1 caching proxies not capable of transparent content negotiation. This header can either be Vary: * or a more elaborate header, see Section 10.5.1. e. To ensure compatibility with HTTP/1.0 caching proxies which do not recognize the Vary header, an Expires header with a date in the past may be added. See Appendix 19.1 for more information. An example of a choice response generated by an origin server is: HTTP/1.1 200 OK Date: Tue, 11 Jun 1996 20:05:31 GMT Content-Type: text/html Last-Modified: Mon, 10 Jun 1996 10:01:14 GMT Etag: "497542" Cache-control: max-age=604800 Content-Length: 5327 Content-Location: paper.html.en Alternates: {"paper.html.en" 0.9 {type text/html} {language en}}, {"paper.html.fr" 0.7 {type text/html} {language fr}}, {"paper.ps.en" 1.0 {type application/postscript} {language en}}, max-age=86400 Vary: negotiate, accept, accept-language Expires: Thu, 01 Jan 1980 00:00:00 GMT <title>A paper about .... 10.2.2 Construction by proxies Proxies must construct choice responses with the following algorithm, or any other algorithm which gives equal results. 1. Same as in Section 10.2.1. 2. Obtain a fresh or first-hand HTTP response message for the request message constructed in step 1. The message can be obtained from cache memory, or by passing the constructed HTTP request message towards the origin server. 3. Same as in Section 10.2.1. 4. Add a number of headers to the HTTP response message generated in step 2. a. Same as in Section 10.2.1. b. Same as in Section 10.2.1. c. Add a fresh or first-hand Alternates header bound to the negotiable resource. Section 10.3 specifies how this header can be obtained. d. Add a Vary header to ensure correct handling by HTTP/1.1 caching proxies not capable of transparent content negotiation. This header can either be Vary: * or a more elaborate fresh or first-hand header, Section 10.5.2 specifies how this header can be obtained. e. Same as in Section 10.2.1. f. Compute the age difference between the Alternates header in the response in the following way. - Let variant_age be the age of the response obtained in step 2, calculated according to the rules in the HTTP/1.1 specification [1]. If the response is known to be first-hand, variant_age can be 0. - Let alternates_age be the age of the Alternates header added in step c, calculated according to the rules in Section 10.3, or 0 if the Alternates header is known to be first-hand. - Compute the age difference: alternates_older = alternates_age - variant_age If the computed alternates_older value is greater than 0, an Alternates-Older header with this value must be added to the response. If the value is 0, an Alternates-Older header with this value need not be added. If the value is less than 0, an Alternates-Older header with this value should be added. An example of a choice response which could be sent by a caching proxy is: HTTP/1.1 200 OK Date: Tue, 11 Jun 1996 20:05:31 GMT Content-Type: text/html Last-Modified: Mon, 10 Jun 1996 10:01:14 GMT Etag: "497542" Cache-control: max-age=604800 Content-Length: 5327 Age: 432000 Content-Location: paper.html.en Alternates: {"paper.html.en" 0.9 {type text/html} {language en}}, {"paper.html.fr" 0.7 {type text/html} {language fr}}, {"paper.ps.en" 1.0 {type application/postscript} {language en}}, max-age=86400 Vary: negotiate, accept, accept-language Alternates-Older: -410400 Expires: Thu, 01 Jan 1980 00:00:00 GMT <title>A paper about .... Here, the age of the variant data is 432000 seconds (5 days), while the age of the Alternates header is 432000 + -410400 = 21600 seconds (6 hours). 10.3 Reusing the Alternates header Proxy caches may extract and cache the Alternates header in any cachable list or choice response. If the Alternates header is fresh or first-hand, it may be reused to negotiate on behalf of the user agent or origin server (Section 14) and to construct choice responses (Section 10.2.2). The freshness of an Alternates header is determined in the following way. - Let response_age be the current age of the response from which the header was extracted, calculated according to the rules in the HTTP/1.1 specification [1]. - Let alternates_older be the value in the Alternates-Older header of this response, or 0 if no Alternates-Older header is present. - Let max_age be the value of the max-age directive in the Alternates header, or positive infinity if a max-age directive is absent. The age of the alternates header is alternates_age = response_age + alternates_older . The Alternates header is fresh if alternates_age < max_age . An Alternates header is fist-hand if it is used, without unnecessary delay, after being extracted from a first-hand response. 10.4 Extracting a normal response from a choice response If a proxy receives a choice response, it may extract and cache the normal HTTP response contained therein. The normal response can be extracted by taking a copy of the choice response and then deleting the Content-Location, Alternates, Vary, and Alternates-Older header, and renaming any Variant-Vary headers to Vary headers. [##Question to be resolved: A method to get rid of the Expires header which could have been added in a step 4.e of Section 10.2.1 and 10.2.2 could be invented. Should such a method be added to this specification? Probably not.##] This normal response may then be cached (as a HTTP response to the variant request as constructed in step 1. of Section 10.2.1) and reused to answer future direct requests on the variant resource according to the rules in the HTTP/1.1 specification [1]. This caching of extracted responses can increase overall efficiency with up to a factor 2. For security reasons (see Section 15.2), an extracted normal response may only be cached if the negotiable resource and the variant resource are neighbors. If they are not neighbors, the proxy should reject the choice response as a probable spoofing attempt and pass on a 502 (bad gateway) error response instead. 10.5 Elaborate Vary headers The Vary header added to a list or choice response should ensure correct handling by HTTP/1.1 caching proxies not capable of transparent content negotiation. The most simple Vary header which satisfies this requirement is Vary: * A more elaborate Vary header can be used to allow for certain optimizations which could be performed by some HTTP/1.1 caches which are not capable of transparent content negotiation, but which do cache multiple variants of one resource based on Vary header contents. 10.5.1 Construction of an elaborate Vary header Origin servers can construct a more elaborate Vary header in the following way. First, start with the header Vary: negotiate Then, if any of the following attributes is present in any variant description in the Alternates header, add the corresponding header name to the Vary header attribute | header name to add -----------+--------------------- type | accept charset | accept-charset language | accept-language features | accept-features The Vary header constructed in this way specifies the response variation which can be caused by the use of the network negotiation algorithm in proxies. If the origin server uses a custom negotiation algorithm which takes additional headers (for example User-Agent) into account, these should also be added to the Vary header. 10.5.2 Caching of an elaborate Vary header Proxy caches cannot construct elaborate Vary headers by themselves, because they may not assume that the origin server uses the standard network negotiation algorithm to vary its responses. However, when extracting an Alternates header from a response (Section 10.3) caches may also extract the Vary header in the response, and reuse it along with the Alternates header. A clean Vary header can however only be extracted if the variant does not vary itself, i.e. if a Variant-Vary header is absent. 11 The network negotiation algorithm The network negotiation algorithm is a standardized algorithm by which a party in the negotiation process can make a choice on behalf of another party. For example, a proxy could choose a particular variant on behalf of the user agent in order to speed up the negotiation process by cutting corners (see Section 4.4). General principles underlying the network negotiation algorithm are as follows. - When running on an origin server or proxy, the algorithm will usually only have partial user agent capabilities and preferences information as input. Therefore, instead of a choice for a best variant, the algorithm can also yield an `insufficient data, cannot choose' result. - Decisions are always made on either on behalf of the user agent or on behalf of the origin server. They are made on behalf of the user agent if the user agent indicates that it is capable of transparent content negotiation, and on behalf of the origin server otherwise. - By including an Alternates header in the response, the origin server gives upstream proxies the permission to perform actions prescribed by the network negotiation algorithm instead of actions prescribed by the plain HTTP specification. - It is expected that user agents will use the network negotiation algorithm by default, locally supplying their complete capabilities and preferences to the algorithm, but this is not required. User agents may use customized negotiation algorithms to automatically choose a variant. Such algorithms could for example account for cross-dependencies between dimensions of negotiation. Note that in this specification, `use of the network negotiation algorithm' means use of any algorithm which yields equal results. 11.1 Input The algorithm is always executed for a particular request on a particular transparently negotiable resource. It takes the following information as input. 1. The list of variants of the resource, as present in the Alternates header of the resource. 2. Any "min-q" and "forward" directives pertaining to the negotiation process, as specified by the Alternates header of the resource. 3. (Partial) Information about capabilities and preferences of the user agent for this particular request, as given in the Accept headers of the request, or, if the algorithm runs on a user agent, as given by the internal capabilities and preferences database. 4. A boolean indicating whether or not the user agent is capable of transparent content negotiation for this request. (True if the request includes a Negotiate header, false otherwise). 11.2 Output As its output, the network negotiation algorithm and will yield the appropriate action to be performed. Actions have labels `ACT_UA' for actions performed on behalf of the user agent, and `ACT_OS' for actions performed on behalf of the origin server. 11.2.1 Output for proxies If the algorithm runs on a proxy, there are five different possible results. The first two results can be generated if the user agent is capable of transparent content negotiation: Choice_UA Choice on behalf of the user agent. The best variant may be retrieved and returned in a choice response. List_UA No choice possible, return the list. The Accept headers do not contain sufficient information to make a choice on behalf of the user agent possible. A list response should be returned, allowing the user agent to make the choice itself. The following results can be generated if the user agent is not capable of transparent content negotiation: Choice_OS Choice on behalf of the origin server. The best variant may be retrieved and returned in a choice response. List_OS No choice possible, return the list. The Accept headers do not contain sufficient information to make a choice on behalf of the origin server possible. A list response should be returned, allowing the user to manually select the best variant. Forward_OS No choice possible, forward request towards the origin server. The Accept headers do not contain sufficient information to make a choice on behalf of the origin server possible. The request should be forwarded towards the origin server so that it can choose the most appropriate action with a custom negotiation algorithm. 11.2.2 Output for origin servers If the algorithm runs on an origin server, all results above, except Forward_OS, are possible, with the same interpretations. 11.2.3 Output for user agents If the algorithm runs on a user agent, only the results List_UA and Choice_UA are possible, with the following interpretations. Choice_UA Automatic choice. A particular variant X must be retrieved and displayed. List_UA No automatic choice possible, none of of the variants can be rendered according to the preferences and capabilities database. An appropriate action like showing an error message with the list of variants to the user must be performed. 11.3 Computing the overall quality values As a first step in the network negotiation algorithm, the overall qualities of the individual variants in the list are computed. The overall quality Q of a variant is the value Q = qs * qt * qc * ql * qf where the factors qs, qt, qc, ql, and qf are determined as follows. qs Is the source quality factor in the variant description. qt The media type quality factor is 1 if there is no type attribute in the variant description, or if there is no Accept header in the request. Otherwise, it is the quality assigned by the Accept header to the media type in the type attribute. Note: If a type is matched by none of the elements of an Accept header, the Accept header assigns the quality factor 0 to that type. qc The charset quality factor is 1 if there is no charset attribute in the variant description, or if there is no Accept-Charset header in the request. Otherwise, the charset quality factor is the quality assigned by the Accept-Charset header to charset in the charset attribute. Note: If a charset does not appear in an Accept-Charset header, the Accept-Charset header assigns the quality factor 0 to that charset. ql The language quality factor is 1 if there is no language attribute in the variant description, or if there is no Accept-Language header in the request. Otherwise, the language quality factor is the highest quality factor assigned by the Accept-Language header to any one of the languages listed in the language attribute. qf The features quality factor is 1 if there is no features attribute in the variant description, or if there is no Accept-Features header in the request. Otherwise, it is the feature negotiation quality degradation value for the features attribute, see Section 6.4. As an example, if a variant list contains the variant description {"paper.html.en" 0.7 {type text/html} {language fr}} and if the request contains the Accept headers Accept: text/html:q=1.0, */*:q=0.8 Accept-Language: en;q=1.0, fr;q=0.5 the network negotiation algorithm will compute an overall quality for the variant as follows: {"paper.html.en" 0.7 {type text/html} {language fr}} | | | | | | V V V 0.7 * 1.0 * 0.5 = 0.35 With the above Accept headers, the complete variant list {"paper.html.en" 0.9 {type text/html} {language en}}, {"paper.html.fr" 0.7 {type text/html} {language fr}}, {"paper.ps.en" 1.0 {type application/postscript} {language en}} would yield the following computations: qs * qt * qc * ql * qf = Q --- --- --- --- --- ---- paper.html.en: 0.9 * 1.0 * 1.0 * 1.0 * 1.0 = 0.9 paper.html.fr: 0.7 * 1.0 * 1.0 * 0.5 * 1.0 = 0.35 paper.ps.en: 1.0 * 0.8 * 1.0 * 1.0 * 1.0 = 0.8 11.4 Definite and speculative quality values An overall quality value computed by an origin server or proxy can be either definite or speculative. An overall quality value for a variant is definite if it was computed without using any wildcard characters '*' in the Accept headers, and without the need to use the absence of a particular Accept header. An overall quality value is speculative otherwise. As an example, in the previous section, the quality values of paper.html.en and paper.html.fr are definite, and the quality value of paper.ps.en is speculative because the type application/postscript was matched to the range */*. Definiteness can be defined more formally as follows. An overall quality value Q is definite if the same quality value Q could be computed after the request message is changed in the following way: 1. If an Accept, Accept-Charset, Accept-Language, or Accept-Features, header is missing from the request, add this header with an empty field. 2. Delete any media ranges containing a wildcard character '*' from the Accept header. Delete the language-range '*', if present, from the Accept-Language header. Delete the wildcard '*', if present, from the Accept-Features header. As another example, the overall quality factor for the variant {"blah.html" 1 {language en-gb} {features blebber [x y]}} is 1 and definite with the Accept headers Accept-Language: en-gb, fr Accept-Features: blebber, x, !y, * and Accept-Language: en, fr Accept-Features: blebber, x, * The overall quality factor is still 1, but speculative, with the Accept headers Accept-language: en-gb, fr Accept-Features: blebber, !y, * and Accept-Language: fr, * Accept-Features: blebber, x, !y, * 11.5 Determining the result The best variant, as determined by the network negotiation algorithm, is the one variant with the highest overall quality value, or, if there are multiple variants which share the highest overall quality, the first variant in the list with this value. 11.5.1 Result for proxies and origin servers When running on a proxy or origin server, the result of the network negotiation algorithm is determined as follows. 1. If the negotiable resource and the best variant resource are not neighbors, then the result is List_UA if the user agent is capable of transparent content negotiation, List_OS otherwise. This rule exists for security reasons: it prevents some forms of spoofing, see Section 15.2. 2. If the above rule does not apply, then a. If the user agent is capable of transparent content negotiation, the result is Choice_UA if the best variant has a definite overall quality value greater than 0, and List_UA otherwise. b. If the user agent is not capable of transparent content negotiation, the result is Choose_OS if the best variant has a (definite or speculative) overall quality value which is greater than 0, or at least the value of the "min-q" directive if this directive is present in the Alternates header. If the result is not Choose_OS, it is Forward_OS if the Alternates header contains the "forward" directive and the algorithm is running on a proxy, and List_OS otherwise. 11.5.2 Result for user agents When running on a user agent, the result of the network negotiation algorithm is Choice_UA if the best variant has an overall quality value greater than 0, and List_UA otherwise. 11.6 Construction of short requests Under normal HTTP semantics, the interpretation of the request header `Accept: */*' is `I accept all media types with a quality of 1.0'. One of the most important properties of the network negotiation algorithm is that this interpretation is different if the header is sent by a user agent which supports transparent content negotiation. In this case, the interpretation of the header is `I accept several media types, and assign quality factors from 0.0 up to 1.0 to them. If this information is insufficient to make a choice on my behalf, do not make a choice but send the list of variants'. The complete absence of an Accept header from the request would be interpreted in the same way. This means that a user agent which supports transparent content negotiation can send a minimal request, without any Accept headers but with a Negotiate header, by default: GET /paper HTTP/1.1 Host: x.org User-Agent: WuxtaWeb/2.4 Negotiate: The sending of this minimal request will never have an adverse effect on the quality of the transparent negotiation process: if the requested resource happens to be transparently negotiated, the user agent will always get a list response, so that it can choose the best variant itself. Only the speed of the negotiation process can be affected, because servers will no longer be able to cut corners by choosing on behalf of the user agent. If it is found that the requests to a particular origin server often return a less efficient list response, the user agent can dynamically introduce short Accept headers in its future requests to that server, for example GET /paper HTTP/1.1 Host: x.org User-Agent: WuxtaWeb/2.4 Negotiate: Accept: text/html, application/postscript:q=0.8, */* Accept-language: en, fr;q=0.5 This will increase the chance that the network negotiation algorithm will have sufficient information to choose on behalf of the user agent, thereby optimizing the negotiation process. A good strategy for dynamic extension would be to extend the request with with those media types, languages, charsets, and feature tags mentioned in the variant lists of past list responses from the server. The Accept header in the extended request, if present, will generally contain "*/*" to indicate that only partial information is given in this header. The Accept-Features header, if present, will generally contain a "*" for the same reason. 12 User agent support for transparent negotiation This section specifies the requirements a user agent must satisfy in order to support transparent negotiation. If the user agent contains an internal cache, this cache should satisfy the requirements for proxy caches in Section 14. 12.1 Handling of responses If a list response is received when a resource is accessed, the user agent must be able to automatically choose, retrieve, and display the best variant, or display an error message if it is not capable of displaying any of the available variants. The negotiation algorithm which chooses the best variant (or chooses to display an error message) need not be the network negotiation algorithm. If a choice response is received when a resource is accessed, the usual action is to automatically display the enclosed variant. A user agent may however choose to apply its (custom) negotiation algorithm to the received variant list, and to automatically retrieve and display another variant if the algorithm indicates that this other variant has a better quality. When receiving a choice response, a user agent should check if the if the negotiable resource and the chosen variant resource are neighbors. If this is not the case, the user agent should reject the choice response as a probable spoofing attempt and display an error message, for example by internally replacing the choice response with a 502 (bad gateway) response. 12.2 Presentation of a transparently negotiated resource If the user agent is displaying, in a main window (i.e. not as an embedded or inlined object), a variant as the end result of a request on a transparently negotiated resource, the following requirements must be met. 1. The user agent should make available though its user interface some indication that the resource being displayed is a negotiated resource. 2. The user agent should allow the user to review a list of all variants bound to the negotiable resource, and to manually retrieve another variant if desired. There are two general ways of providing such a list. First, the information in the Alternates header of the negotiable resource could be used to make an annotated menu of variants. Second, the entity included in a list response of the negotiable resource could be displayed. Note that a list response can always be obtained by doing a GET request on the negotiable resource with a Negotiate header but without Accept headers. If only this second option is provided, the user agent should also provide the user with a means to review the real variant list as used by the negotiation algorithm (see Section 15.1). 3. If the user agent shows the URI of the displayed information to the user, it should be the negotiable resource URI, not the variant URI. 4. Similarly, if the user agent stores a reference to the displayed information for future use, for example in a hotlist, it should store the negotiable resource URI, not the variant URI. It is encouraged, but not required, that some of the above functionality is also made available for inlined or embedded objects, and when a variant which was selected manually is being displayed. 13 Origin server support for transparent negotiation This section covers origin server support from a HTTP viewpoint. see Appendix 19.2 for implementation considerations. 13.1 Requirements To implement transparent negotiation on a resource, the origin server must be able to send a list response when getting a GET request on the resource. It should also be able to send appropriate list responses for HEAD requests. The origin server must always send a list response, never a choice response, for a request with a Negotiate header but without Accept headers. The origin server may also return choice responses, except in the special case above. The negotiation algorithm used need not be the network negotiation algorithm. Negotiability is a binary property: a resource is either transparently negotiated, or it is not. Origin servers should not vary the negotiability of a resource, or the variant list bound to that resource, based on the request headers which are received. The variant list and the property of being negotiated may however change through time. The Cache-Control header and the max-age directive of the Alternates header can be used to control the propagation of such time-dependent changes through caches. It is the responsibility of the author of the negotiable resource to ensure that all resources in the variant list serve the intended content, and that the variant resources do not engage in transparent content negotiation themselves. 13.2 Negotiation on transactions other than GET and HEAD If a resource is transparently negotiable, this only has an impact on the GET and HEAD transactions on the resource. Other transactions are not affected. If it is desired that transparent content negotiation is done on, for example, the end result of a POST request, then a 303 (See Other) POST response which redirects to a negotiable resource could be generated. See the HTTP/1.1 specification [1] for details. 14 Proxy support for transparent negotiation Transparent content negotiation is designed to work through proxies which only implement the HTTP/1.1 specification [1]. Thus, in a sense, all HTTP/1.1 proxies support transparent content negotiation. Plain HTTP/1.1 allows proxies to cache list and choice responses, and to efficiently revalidate them by using the If-None-Match header. This specification defines additional optimization mechanisms. First, a proxy may cache and reuse an Alternates header bound to a negotiable resource (Section 10.3). If it is fresh or first-hand, the Alternates header may be reused to negotiate on behalf of the user agent or origin server and to construct choice responses (Section 10.2.2). Proxies must not use custom negotiation algorithms, they may only use the network negotiation algorithm. Second, if a proxy receives a choice response, it may extract and cache the normal response embedded therein, as described in Section 10.4. 15 Security and privacy considerations 15.1 Accept headers revealing information of a private nature Accept headers, in particular Accept-Language headers, may reveal information which the user would rather keep private unless it will directly improve the quality of service. For example, a user may not want to send language preferences to sites which do not offer multi-lingual content. The transparent content negotiation mechanism allows user agents to omit sending of the Accept-Language header by default, without this affecting the outcome of the negotiation process if transparently negotiated multi-lingual content is accessed. However, even if Accept headers are never sent, the automatic selection and retrieval of a variant by a user agent will reveal a preference for this variant to the server. A malicious service author could provide a page with `fake' negotiability on (ethnicity-correlated) languages, with all variants actually being the same English document, as a means of obtaining privacy-sensitive information. Such a plot would however be visible to an alert victim if the list of available variants and their properties is reviewed. Some additional privacy considerations connected to Accept headers are discussed in [1]. 15.2 Spoofing of responses from variant resources The caching optimization in Section 10.4 gives the implementer of a negotiable resource control over the responses cached for all of its variant resources which are neighbors. This is a security problem if a neighboring variant resource belongs to another author. To provide security in this case, the HTTP server will have to filter the Content-Location headers in the choice responses generated by the negotiable resource implementation. 16 Acknowledgments Work on HTTP content negotiation has been done since at least 1993. The author is unable to trace the origin of many of the ideas incorporated in this document. This document builds on an earlier specification of content negotiation as recorded in [2]. Many members of the HTTP working group have contributed to the work reflected in this document. The author wishes to thank the individuals who have commented on earlier versions of this document, including Brian Behlendorf, Daniel DuBois, Larry Masinter, and Roy T. Fielding. 17 References [1] Fielding et al, Hypertext Transfer Protocol -- HTTP/1.1. Internet-Draft draft-ietf-http-v11-spec-06.txt, HTTP Working Group, July 4, 1996 [2] Roy T. Fielding, Henrik Frystyk Nielsen, and Tim Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1. Internet-Draft draft-ietf-http-v11-spec-01.txt, HTTP Working Group, January, 1996. 18 Author's address Koen Holtman Technische Universiteit Eindhoven Postbus 513 Kamer HG 6.57 5600 MB Eindhoven (Holland) e-mail: koen@win.tue.nl 19 Appendices 19.1 Adding an Expires header to ensure HTTP/1.0 compatibility To ensure compatibility with HTTP/1.0 caching proxies which do not recognize the Vary header, an Expires header with a date in the past can be be added to the response, for example Expires: Thu, 01 Jan 1980 00:00:00 GMT If this is done by an origin server, it should usually also include a Cache-Control header for the benefit of HTTP/1.1 caches, for example Cache-Control: max-age=604800 which overrides the freshness lifetime of zero seconds specified by the included Expires header. 19.2 Origin server implementation considerations 19.2.1 Implementation with a CGI script [##Has yet to be written. This section will contain a sample CGI shell script.##] 19.2.2 Direct support by the HTTP server [##Has yet to be written. Will discuss the interface which a transparent content negotiation server module could offer to content authors. Will provide references to existing implementations of content negotiation modules.##] 19.2.3 Negotiable content authoring [##Has yet to be written. Mainly intends to mention that authoring tools could automatically produce multiple variants together with an appropriate variant list.##] 19.3 Open issues in transparent content negotiation At the time of writing of this specification, the following issues are still open. - The feature tag registration process has yet to be defined. - A core set of feature tags (Section 8.2) has not yet been defined, though there has been some related work on display attributes, see Appendix 19.4.1. - Though it is expected that the feature negotiation framework will solve many current and future negotiation problems, it is also expected that there will remain current and future negotiation problems not solved by feature negotiation. Currently, there is little experience in this area. 19.4 Other negotiation specifications 19.4.1 User-Agent Display Attributes Headers (Comments based on the internet draft draft-mutz-http-attributes-00.txt and on messages from Andy Mutz.) User-Agent Display Attributes provide a means for an HTTP client to inform a server about its display capabilities. The draft mentioned above defines a number of such attributes, and presents them as part of a HTTP/1.0 style negotiation framework (See Section 4.2). However, the the author of the above draft has said that the draft does not intend to specify a negotiation framework, but instead intends to use the framework proposed for negotiating the existing header information. PEP (Section 19.4.2) and transparent content negotiation are mentioned as possible frameworks. In the opinion of the author of this document, the feature negotiation framework would be an appropriate mechanism for negotiation on display attributes. It seems possible and desirable to encapsulate display attributes in the feature negotiation mechanism, and to make them part of the core set of feature tags (Section 8.2). The work on feature negotiation has its historical origins in the desire to negotiate on display agent attributes with a minimum of overhead to the request size. The work started after a discussion of color content negotiation on the http-wg mailing list showed that many participants found the introduction of a header like Accept-Color unacceptable. Reasons cited were the request size overhead, and the potential explosion of special-case Accept-* headers, all of which would require special-case implementations, implied by the introduction of Accept-Color. 19.4.2 PEP: An Extension Mechanism for HTTP/1.1 (Comments based on the internet draft draft-khare-http-pep-01.txt.) PEP is a system for HTTP clients, servers, and proxies to reliably reason about custom extensions to HTTP. Its purpose places PEP one meta-level above transparent content negotiation, which is intended as a (standardized) extension of HTTP. Transparent content negotiation and HTTP/1.1 were both carefully designed to work together without requiring meta-negotiation. Therefore, transparent content negotiation does not require or rely on the use of PEP. Though PEP and transparent content negotiation both offer extensible negotiation frameworks, they operate in different areas of negotiation. PEP negotiates on the use of mutually understood protocol extensions, while transparent content negotiation is a method for efficiently retrieving the best representation of some information. Thus, deployment and use of PEP is orthogonal to deployment and use of transparent content negotiation. Though some elements of PEP resemble some elements of transparent content negotiation, it does not seem advantageous to attempt to generalize (parts of) both specifications in order to allow sharing of code by implementations. The two specifications seem to have more unrelated parts than similar parts. For example, the handling of incomplete information in Accept headers in transparent content negotiation has no counterpart in PEP. On the other hand, PEP's processing of `scope' has no counterpart in transparent content negotiation. 19.5 Related issues 19.5.1 Current negotiation practice In current practice, most user agents send short Accept headers in every request. These headers usually inadequately describe the user agent capabilities and user preferences, except maybe for inline image requests. Servers providing multiple representations of the same information usually do so under different URIs, and allow users to manually select a representation by clicking a particular link. Some HTTP servers implement extensive negotiation capabilities based on HTTP/1.0 headers. However, with current user agents, these capabilities can seldom be used effectively. Some origin servers use the contents of the User-Agent request header for negotiation purposes. These contents sometimes allow the server to infer information about capabilities which cannot be expressed with the existing Accept headers. Some sites use Netscape cookies (persistent client state http cookies) to implement a user preferences mechanism. 19.5.2 Bandwidth negotiation With a bandwidth negotiation mechanism, the time needed to retrieve a particular variant over the network can be taken into account during the negotiation process. Work on mechanisms for bandwidth negotiation has been done since at least 1993, but this work has not yet yielded a successful standard mechanism for bandwidth negotiation. Cascaded proxy caches introduce additional complexity in this area. This specification does not attempt to solve the problem of bandwidth negotiation. There is some hope that the feature negotiation framework will allow the introduction of an adequate solution for bandwidth negotiation. 19.5.3 Content transformation by proxies Recently, there has been some attention to content transformation by proxies. An example is the transformation of an image/gif response to a shorter image/jpeg response, with potential degraded quality, to save bandwidth. Another example is the transformation of an application/postscript response to a text/html response, with definite degraded quality, to allow viewing by a user agent which cannot handle postscript. The impact of such transformations on the quality of content on the web, and the impact on the current model for the allocation of trust to various parties in a HTTP transaction, has not yet been fully explored. Content transformation could be a useful optimization for transparent content negotiation. A response with a variant list {"map.gif" 1.0 {type image/gif} {features !monochrome}}, {"map.mono.gif" 0.8 {type image/gif} {features monochrome}} could include an additional response header Allow-Transform: "map.gif" -> "map.mono.gif" with "dither-2" to allow a proxy which has cached "map.gif" to create "map.mono.gif" on demand, by applying a particular standardized transformation to the "map.gif" data. Another interesting possibility would be Allow-Transform: "map.gif" -> "map.mono.gif" with "http://x.org/java/filters/my_dither" Expires: February 14, 1997