HTTP Working Group Koen Holtman, TUE Internet-Draft Andrew Mutz, Hewlett-Packard Expires: January 28, 1998 July 28, 1997 Feature Tag Scenarios draft-ietf-http-feature-scenarios-01.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. A HTML version of this document can be found at . ABSTRACT Recent Internet applications, such as the World Wide Web, tie together a great diversity in data formats, client and server platforms, and communities. This has created a need for various kinds of negotiation mechanisms, which tailor the data which is exchanged, or the exchange process itself, to the capabilities and preferences of the parties involved. Extensible negotiation mechanisms need a vocabulary to identify various things which can be negotiated on. To promote interoperability, a registration process is needed to ensure that that this vocabulary, which can be shared between negotiation mechanisms, is developed in an orderly, well-specified, and public manner. This document discusses requirements and scenarios the registration of this vocabulary, which is the vocabulary of feature tags. Feature tag registration is foreseen as an ongoing, open process which will keep pace with the introduction of new features by software vendors, and other parties such as standards bodies. TABLE OF CONTENTS 1 Introduction 2 Basic concepts and definitions 2.1 Areas of negotiation and feature tags 2.2 Complexity of negotiation 2.3 The result in an area of negotiation 3 Extensible negotiation mechanisms 3.1 The need for extensible negotiation mechanisms: the case of the Web 3.2 Extensible negotiation mechanisms for the Web 3.3 Extensible negotiation mechanisms for other Internet protocols 4 Feature tag registration 4.1 IANA registration procedures 4.2 Examples of parties who would want to register feature tags 4.3 Feature tag registration scenario 4.4 Volume considerations 4.5 Danger of excessive registration 5 Security considerations 6 Acknowledgments 7 References 8 Authors' addresses 1 Introduction Recent Internet applications, such as the World Wide Web, tie together a great diversity in data formats, client and server platforms, and communities. This has created a need for various kinds of negotiation mechanisms, which tailor the data which is exchanged, or the exchange process itself, to the capabilities and preferences of the parties involved. Extensible negotiation mechanisms need a vocabulary to identify various things which can be negotiated on. To promote interoperability, a registration process is needed to ensure that that this vocabulary, which can be shared between negotiation mechanisms, is developed in an orderly, well-specified, and public manner. This document discusses requirements and scenarios the registration of this vocabulary, which is the vocabulary of feature tags. Feature tag registration is foreseen as an ongoing, open process which will keep pace with the introduction of new features by software vendors, and other parties such as standards bodies. 2 Basic concepts and definitions 2.1 Areas of negotiation and feature tags Something which can be negotiated on is called an `area of negotiation' in this document. Examples of areas of negotiation are: * the MIME media type of the data which is transmitted * the language of the text document which is transmitted * the color depth of the screen on which something is to be displayed * whether the recipient supports the `floating 5 dimensional tables' feature * the fonts which are available to the recipient * whether a Web user prefers speed over graphical detail * whether the recipient is capable of displaying graphical content * whether the user prefers a blue background with purple dots over a green background with pictures of small furry animals, except on Fridays. A feature tag identifies a single area of negotiation. It is expected that the majority of feature tags will identify new areas of negotiation, in which the object of negotiation is to decide on the presence or use of some new feature in a software product. This explains the name `feature tag'. It is recognized that there is continuous growth in the number of areas in which some form of negotiation is desirable. To keep up with this growth, extensible negotiation mechanisms are needed, which refer to the feature tag vocabulary to identify new areas of negotiation, rather than relying on hard-coded knowledge about a few areas. To avoid the duplication of work, and to promote the interoperable naming of areas of negotiation across protocols and applications, the feature tag namespace should not be bound to a particular protocol or negotiation mechanism. Also, there should be no prior restriction on the areas of negotiation which may be identified by a feature tag, other than that it must be conceivable to negotiate in these areas in the context of some Internet application. 2.2 Complexity of negotiation Negotiation processes can often be complex. Two frequent sources of complexity are: 1. An area of negotiation may be inherently complex. For example, negotiating on the use of a particular media type is inherently more complex than negotiating on the presence of a single feature, because there are more possible outcomes. 2. There may be complex of interdependencies between the choices in different areas of negotiation. For example, if the following versions of a document are available on a Web server: * text/html, English * text/plain, French * audio/x-wav, German then the content negotiation mechanism cannot treat the areas of `MIME media type negotiation' and `language negotiation' as separate. It is recognized that extensible negotiation mechanisms will often differ in the type and amount of complexity they can handle. Thus, though negotiation mechanisms share the feature tag namespace, it will not be the case that every tag is usable in every negotiation mechanism, or that every negotiation mechanism will be able to handle all possible interdependencies. 2.3 The result in an area of negotiation During negotiation, negotiation mechanisms will often need to transmit (canonical representations of) the possible results in various areas of negotiation over the wire. Also, at the end of a negotiation process, the mechanism may need to return (a canonical representation of) the result to the application which invoked it. In many areas of negotiation, there will be a natural, canonical representation of the result. For example, in the area * whether the recipient supports the `floating 5 dimensional tables' feature the canonical representation of the result is a boolean value (yes, the feature is supported, or no, the feature is not supported). In the area * the MIME media type of the data which is transmitted the canonical representation of the result will be a MIME media type identifier like text/html or application/postscript. In some areas of negotiation, the result could be a compound value (e.g. a coordinate in a 3D space). To promote interoperability, the registration entry of a feature tag can include a definition of the canonical representations of the possible results in the corresponding area of negotiation. 3 Extensible negotiation mechanisms We call a negotiation mechanism extensible if the set of areas on which the mechanism can negotiate is extensible, instead of hard-coded inside the mechanism. 3.1 The need for extensible negotiation mechanisms: the case of the Web HTTP [2] has traditionally recognized four areas of negotiation: 1. MIME media type 2. Charset 3. Language 4. Encoding HTTP provides support for these areas of negotiation by defining identifiers (Accept headers) for these areas, and defining canonical representations of the possible results in these areas. Experience with the Web has shown there is a great need to negotiate on things which do not fit the four areas above. This need has shown itself in a number of ways: - Web servers routinely use (abuse) other headers than the Accept headers for negotiation purposes. In particular, the HTTP User-Agent header has been widely used by web sites to detect the presence of certain features in browsers, and by browsers to trigger certain features in web sites, even though such use is error-prone, and conflicts with the original purpose of the User-Agent header. - Web servers routinely use `dynamic URLs' and cookies to encode negotiation related information like user preferences. This can be cache-unfriendly, in particular in the case of `dynamic URLs'. - During the standardization of HTTP, several proposals for additional Accept headers, matching additional areas of negotiation, were made. These proposals have been rejected in favor of developing an extensible negotiation mechanism. - There has been pressure to extend the MIME media type parameter mechanism to allow for the naming of, and negotiation on, new features in media types already registered, something which is explicitly disallowed in the MIME type registration rules. It was recognized that this pressure would be better addressed by creating a new namespace independent from the MIME media type space. 3.2 Extensible negotiation mechanisms for the Web In the IETF HTTP working group, it has been recognized that the number of areas for Web content negotiation is growing so rapidly that the IETF would never be able to keep up with this growth by by continually revising a negotiation mechanism with a hard-coded set of areas. Instead, a number of extensible content negotiation mechanisms have been proposed. All of these mechanisms share the need for an external vocabulary to identify areas, a vocabulary which can be updated quickly enough to keep pace with the introduction of new features by Web software vendors. The proposed extensible content negotiation mechanisms are transparent content negotiation [2], and negotiation mechanisms based on various forms of "active content". In "active content" negotiation, the web server returns an executable program which is run by the browser. This program then accesses a database of negotiation related settings inside the browser, and chooses and renders the most appropriate content. Note that there are several existing and proposed forms of active content. To tie everything together, a browser architecture with a common internal database for negotiation related information has been proposed. The database would be filled to reflect the capabilities of all browser components, both native components and plugins, and the preference settings made by the user. Feature tags would serve as the keys under which the database entries for different areas of negotiation are stored. Individual negotiation mechanisms could access the central database through some API. The architecture is shown in the following picture. +-----------------------------------------------------------------+ | BROWSER | | | | +------------------+ +----------+ +----------+ +-----------+ | | | Native browser | | Plugin 1 | | Plugin 2 | |User | | | | rendering engine | +----------+ +----------+ |preferences| | | +------------------+ | | +-----------+ | | | | | | | | V V V V | | +------------------------------------------------------+ | | | Common internal database for negotiation information | | | +------------------------------------------------------+ | | | | | | | | API API API API | | | | | | | | V V V V | | +---------+ +---------+ +-------------+ | | | Java | | JScript | | transparent | | | | based | | based | ..etc | content | | | | active | | active | | negotiation | | | | content | | content | | engine | | | +---------+ +---------+ +-------------+ | | | +-----------------------------------------------------------------+ 3.3 Extensible negotiation mechanisms for other Internet protocols Extensible negotiation mechanisms for Internet printing and Internet fax are currently under investigation in the IETF. It has been proposed to make multipart/alternative negotiation in Internet mail more extensible, in particular if the mail client is part of a Web browser, by adapting some of the protocol elements developed for transparent content negotiation [2] to Internet mail. 4 Feature tag registration 4.1 IANA registration procedures Examples of IANA registration procedures can be found in [1]. There has been some confusion over what the IANA will register. Jon Postel told us that: The IANA is pretty good at keeping lists. It is not so good at evaluating the merits (or lack thereof) of the requests to add things to lists. [...] So, yes the IANA would keep the list of "feature tags" provided that that there is either a very simple way to decide if requests pass muster, or a designated someone else will be available to make that decision. So two types of registration namespaces can be created: a) a space with feature tag review process performed by the IETF b) a space with very basic registration rules which do not take the merit of the feature tag into account. To quote [1], this type of registration "does not imply endorsement, approval, or recommendation by the IANA or IETF or even certification that the specification is adequate." If extensible negotiation mechanisms are to keep up with the speed of development in the Web, a type b) registration process for feature tags seems necessary, if only because the IETF does not have the manpower required for a review process which can keep up with the speed of Web development. It is proposed that feature tag registration closely mimics the new MIME media type registration rules in [1], providing both type a) and b) namespaces. This proposal is based on the observation that the rules in [1] seem to be working nicely. 4.2 Examples of parties who would want to register feature tags Feature registration allows for the quick introduction of new areas of negotiation in extensible negotiation mechanisms. In a Web context, examples of parties which might want to introduce new areas of negotiation are: 1. Browser and browser component vendors, when inventing and implementing new features or components. 2. The IETF or some other standards body, when creating a new standard for a content format, or when identifying a new type of user preference (for example a preference for representations without animated gifs). 3. Content authors, when identifying a new type of user preference and creating new content to match. A fast registration process is mainly needed for registration by group 1 and 3. For 2, a slower process would suffice. 4.3 Feature tag registration scenario Below is a scenario, in a Web context, for the registration of a feature tag which succeeds in being generally used. Time Action (months) t+0 Browser vendor A invents the new feature XY. t+1 Vendor A starts implementing XY, and completes a feature tag registration form for the `g.xy' tag. t+2 Vendor A submits the form and the IANA registers the `g.xy' feature tag. t+2.1 Vendor A releases a new browser version, which a) implements the feature XY b) has an entry under `g.xy' in its negotiation database, which tells extensible negotiation mechanisms that this feature is present t+2.5 `Early adopter' content authors start making content representations which use XY. t+3 Vendor B starts implementing XY in their own browser. t+3 The `g.xy' tag appears in lists of useful tags maintained by third parties. t+3.5 Vendor B releases a new browser version, which a) implements the feature XY b) has an entry under `g.xy' in its negotiation database, which tells extensible negotiation mechanisms that this feature is present t+3.5 Many content authors start making content representations which use XY. t+4 Vendor C starts implementing XY, and invents the extension XY_COLOR. t+4.5 Vendor C registers the `g.xy_color' feature tag. t+4.5 Vendor C releases a new browser version, which a) implements the features XY and XY_COLOR b) has appropriate entries under `g.xy' and `g.xy_color' in its database t+10 90% of all deployed browsers support XY. Content authors start using XY it without bothering to provide an alternate representation. 4.4 Volume considerations Feature tag registration which will have to keep pace with the introduction of new features by vendors. In particular in the Web domain, vendors are moving fast, and this will inevitably lead to a feature tag namespace which contains a lot of tags. Also, a lot of tags will be `dead' tags, tags related to features which failed to take off and gain widespread use. Compare this to the situation in the USENET newsgroup namespace. Like a list of all MIME media types, a list of all registered feature tags will generally be too long to be useful to any content author. Third parties could filter the feature tag namespace and compile short lists of useful tags. In the Web domain, Web indexing robots could, while traversing the web, gather statistics about actual use of feature tags; these statistics could help in compiling lists. 4.5 Danger of excessive registration One danger for the feature tag namespace is the emergence of excessive registration as seen in the internet domain name system (DNS) namespace. We speculate that the various forces which contributed to the DNS registration problems are absent for feature tags: feature tags will not be very visible to end users, and registration of a feature tag does not mean you get exclusive use. We therefore do not expect excessive registration to occur. We note it has not occured (so far) in the MIME media type namespace. Of course it is possible to update the registration procedure if excessive registration _does_ occur. A necessary precaution is to reserve a part of the feature tag namespace for future use. 5 Security considerations When used, negotiation mechanisms usually reveal some information about one party to other parties. This may raise privacy concerns, and may allow a malicious party to make more educated guesses about the presence of security holes in the other party. 6 Acknowledgments The idea of creating a vocabulary of areas of negotiation, which is maintained in a central open registry, is due to discussions on extensible negotiation mechanisms in the IETF HTTP working group. The authors wish to thank Larry Masinter and Graham Klyne for contributing to discussions about feature tag registration. 7 References [1] N. Freed, J. Klensin, J. Postel, Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures. RFC 2048, BCP 13, Network Working Group, November 1996 [2] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and T. Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1. RFC 2068, HTTP Working Group, January, 1997. [3] K. Holtman, A. Mutz. Transparent Content Negotiation in HTTP. Internet-Draft draft-ietf-http-negotiation-03.txt, HTTP Working Group. July 1997. 8 Authors' addresses Koen Holtman Technische Universiteit Eindhoven Postbus 513 Kamer HG 6.57 5600 MB Eindhoven (The Netherlands) Email: koen@win.tue.nl Andrew H. Mutz Hewlett-Packard Company 1501 Page Mill Road 3U-3 Palo Alto CA 94304, USA Fax +1 415 857 4691 Email: mutz@hpl.hp.com Expires: January 28, 1998