One document matched: draft-ietf-http-feature-scenarios-01.txt
Differences from draft-ietf-http-feature-scenarios-00.txt
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
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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 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
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