HTTP Working Group                                     Koen Holtman, TUE
Internet-Draft                              Andrew Mutz, Hewlett-Packard
Expires: January 7, 1998                                    July 7, 1997


                          Feature Tag Scenarios

                 draft-ietf-http-feature-scenarios-00.txt


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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 Dimensions of negotiation
   2.2 Feature tags

  3 Extensible negotiation mechanisms
   3.1 Non-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 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


 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.


 Basic concepts and definitions

2.1 Dimensions of negotiation

   Negotiation can be done on many things, and sometimes negotiation
   must be done on multiple things at the same time.  For example, in
   a World Wide Web application, a negotiation process might need to
   simultaneously select an appropriate media type and an appropriate
   language for the page which is to be transmitted.

   Such negotiation on multiple things can be seen as searching
   process, in which a permitted or optimal point has to be found in a
   multi-dimensional solution space.

   Dimensions can be simple, usually reflecting a simple yes/no
   choice, or more complex, reflecting a choice among many potential
   alternatives, for example the choice of a particular media type.


2.2 Feature tags

   A feature tag identifies a single dimension of negotiation, i.e.
   something which can be negotiated on.  Examples of things which can
   be negotiated on 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.

   It is expected that the majority of feature tags will label
   features in protocols or applications, the use of which needs to be
   negotiated on.  This explains the name `feature tag'.

   It is recognized that there is continuous growth in the number of
   things for which some form of negotiation is desirable, in
   particular because there is continuous growth in the feature set of
   Internet software.  By using the feature tag vocabulary, rather
   than an internal set of hard-coded values, to identify dimensions,
   negotiation mechanisms can keep up with this growth.

   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 things which may be identified by a feature tag,
   other than that it must be conceivable to negotiate on them in some
   Internet application.


 Extensible negotiation mechanisms

   We call a negotiation mechanism extensible if the set of dimensions
   on which the mechanism can negotiate is extensible, in stead of
   hard-coded inside the mechanism.

3.1 Non-extensible negotiation mechanisms: the case of the Web

   An example of a non-extensible negotiation mechanism is HTTP
   content negotiation mechanism which is based on the four Accept
   headers defined in HTTP/1.1 [2].  This mechanism is referred to as
   `HTTP/1.0 style negotiation' in [3], and it has the following
   dimensions hard-coded:

     1. MIME media type
     2. Charset
     3. Language
     4. Encoding

   Experience with the Web has shown that this mechanism is
   insufficient for meeting the negotiation needs.  Among other
   things, the Web has a great need to negotiate on things which do
   not fit the four dimensions above.  The existence of this need is
   shown by a large number of cases in which people have creatively
   abused HTTP protocol elements, which were not intended for that
   purpose, to extend the dimensions of negotiation.  Examples of this
   are the encoding of negotiation related information in `dynamic
   URLs' or cookies.  As an other example the HTTP User-Agent header
   has been widely abused by web sites to detect the presence of
   certain features in browsers, and by browsers to trigger certain
   features in web sites.


3.2 Extensible negotiation mechanisms for the Web

   In the IETF HTTP working group, it has been recognized that the
   number of needed dimensions 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 dimensions.  In stead, a number of extensible
   content negotiation mechanisms have been proposed.  All of these
   mechanisms share the need for an external vocabulary to identify
   dimensions, 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 central
   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.  Then, individual negotiation
   mechanisms could access the central database through some API.  The
   architecture is shown in the following picture.

     +------------------+   +----------+  +----------+   +-----------+
     | Native browser   |   | Plugin 1 |  | Plugin 2 |   |User       |
     | rendering engine |   +----------+  +----------+   |preferences|
     +------------------+     |               |          +-----------+
                  |           |               |             |
                  V           V               V             V
           +------------------------------------------------------+
           | Central database for negotiation related information |
           +------------------------------------------------------+
                  |         |         |                |
                 API       API       API              API
                  |         |         |                |
                  V         V         V                V
            +---------+ +---------+             +-------------+
            | Java    | | JScript |             | transparent |
            | based   | | based   | ..etc       | content     |
            | active  | | active  |             | negotiation |
            | content | | content |             | engine      |
            +---------+ +---------+             +-------------+

   In this architecture, the central database needs a vocabulary,
   independent from any particular negotiation mechanism, to identify
   individual dimensions of negotiation.  The proposed feature tag
   registry would offer this vocabulary.


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.

 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 Parties who would want to register feature tags

   Feature registration allows for the quick introduction of new
   dimensions of negotiation in extensible negotiation mechanisms.  In
   a Web context, the following parties might want to introduce new
   dimensions of negotiation:

     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.


 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.


 Acknowledgments

   The idea of creating a vocabulary of dimensions 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 for
   contributing to early discussions of feature tag registration.


 References

   [] N. Freed, J. Klensin, J. Postel, Multipurpose Internet Mail
       Extensions (MIME) Part Four: Registration Procedures.  RFC
       2048, BCP 13, Network Working Group, November 1996

   [] 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.

   [] K. Holtman, A. Mutz.  Transparent Content Negotiation in HTTP.
       Internet-Draft draft-ietf-http-negotiation-02.txt, HTTP Working
       Group.


 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 7, 1998