Internet DRAFT - draft-wilde-profile-link
draft-wilde-profile-link
Network Working Group E. Wilde
Internet-Draft EMC Corporation
Intended status: Informational October 17, 2012
Expires: April 20, 2013
The 'profile' Link Relation Type
draft-wilde-profile-link-04
Abstract
This specification defines the 'profile' link relation type that
allows resource representations to indicate that they are following
one or more profiles. A profile is defined to not alter the
semantics of the resource representation itself, but to allow clients
to learn about additional semantics (constraints, conventions,
extensions) that are associated with the resource representation, in
addition to those defined by the media type and possibly other
mechanisms.
Editorial Note (to be removed by RFC Editor)
Please discuss this draft on the apps-discuss@ietf.org mailing list.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 20, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Profiles and Media Types . . . . . . . . . . . . . . . . . 5
3.2. Profile Context . . . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. hCard . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Dublin Core . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Podcasts . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. From -03 to -04 . . . . . . . . . . . . . . . . . . . . . . 8
7.2. From -02 to -03 . . . . . . . . . . . . . . . . . . . . . . 8
7.3. From -01 to -02 . . . . . . . . . . . . . . . . . . . . . . 8
7.4. From -00 to -01 . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
One of the foundations of the Internet and Web Architecture is the
fact that resource representations communicated through protocols
such as SMTP or HTTP are labeled with a 'media type', which allows a
client to understand at run time what 'type' of resource
representation it is handling. Sometimes, it would be useful for
servers and clients to include additional information about the
nature of the resource, so that a client understanding this
additional information could react in a way specific to that
specialization of the resource, where the specialization can be about
constraints, conventions, extensions, or any other aspects that do
not alter the basic media type semantics. HTML 4 [HTML401] has such
a mechanism built into the language, which is the 'profile' attribute
of the 'head' element. This mechanism, however, is specific to HTML
alone, and at the time of writing it seems as if HTML 5 will drop
support for this mechanism entirely.
RFC 5988 [RFC5988] "defines a framework for typed links that is not
specific to a particular serialization or application. It does so by
redefining the link relation registry established by Atom to have a
broader domain, and adding to it the relations that are defined by
HTML."
This specification registers a 'profile' link relation type according
to the rules of RFC 5988 [RFC5988]. This link relation type is
independent of the context in which it is used (however, the
representation must support typed links for this mechanism to work)
and does not constrain in any way the target of the linked URI. In
fact, for the purpose of this specification, the target URI does not
necessarily have to identify a dereferencable resource (or even use a
dereferencable URI scheme), and clients can treat the occurrence of a
specific URI in the same way as an XML namespace URI and invoke
specific behavior based on the assumption that a specific profile
target URI signals that a resource representation follows a specific
profile. Note that at the same time, it is possible for profile
target URIs to use dereferencable URIs and use a representation
(which is outside the scope of this specification) which represents
the information about the profile in a human- or machine-readable
way.
As one example, consider the case of podcasts, a specific kind of
feed using additional fields for media-related metadata. Using a
'profile' link, it would be easily possible for clients to understand
that a specific feed is supposed to be a podcast feed, and that it
may contain entries using podcast-specific fields. This may allow a
client to behave differently when handling such a feed (such as
rendering a podcast-specific UI), even when the current set of
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entries in the feed may not contain any podcast entries.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Profiles
The concept of a profile has no strict definition on the Internet or
on the Web. For the purpose of this specification, a profile can be
described as additional semantics that can be used to process a
resource representation, such as constraints, conventions,
extensions, or any other aspects that do not alter the basic media
type semantics. A profile MUST NOT change the semantics of the
resource representation when processed without profile knowledge, so
that clients both with and without knowledge of a profiled resource
can safely use the same representation. While this specification
associates profiles with resource representations, creators and users
of profiles MAY define and manage them in a way that they can be used
across media types and thus could be associated with a resource,
independent of its representations (i.e., using the same profile URI
for different media types). However, such a design is outside of the
scope of this specification, and clients SHOULD treat profiles as
being associated with a resource representation.
Profiles can be combined, meaning that a single resource
representation can conform to zero or any number of profiles.
Depending on the profile support of clients, it is possible that the
same resource representation, when linked to a number of profiles,
can be processed with different sets of processing rules, based on
the profile support of the clients.
Profiles are identified by URI, but as with for example XML namespace
URIs, the URI in this case only serves as an identifier, meaning that
the presence of a specific URI has to be sufficient for a client to
assert that a resource representation conforms to a profile. Clients
thus SHOULD treat profile URIs as identifiers and not as links, but
profiles MAY be defined in a way that the URIs do identify
retrievable profile description and thus can be accessed by clients
by dereferencing the profile URI. For profiles intended for use in
environments where clients may encounter unknown profile URIs,
profile maintainers SHOULD consider to make the profile URI
dereferencable and provide useful documentation at that URI. The
design and representation of such profile descriptions, however, is
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outside the scope of this specification.
3.1. Profiles and Media Types
A media type defines both the semantics and the serialization of a
specific type of content. In many cases, media types have some
extensibility or openness built-in, so that specific instances of the
media type can layer additional semantics on top of the media type's
foundations. In this case, a profile is the appropriate mechanism to
signal that the original semantics and processing model of the media
type still applies, but that an additional processing model can be
used to extract additional semantics. This is in contrast to a new
media type, which instead of just adding processing rules and
semantics, in most cases defines a complete set of processing rules
and semantics. As an example, XHTML is not a profile of XML but a
new media type because it introduces a complete new perspective of
the underlying XML structures, and from the XHTML point of view,
exposing the raw XML is not all that useful for clients. However,
hCard (see Section 5.1) is a profile of (X)HTML because it adds
processing rules that allow a client to extract additional semantics
from a representation, without changing any of the processing rules
and semantics of (X)HTML itself. While the line between a media type
and a profile might not always be easy to draw, the intention of
profiles is not to replace media types, but to add a more lightweight
and runtime-capable mechanism that allows servers and clients to be
more explicit in how a specific instance of a media type represents
concepts that are not defined by the media type itself, but by
additional conventions (the profile processing rules and semantics).
The idea of profiles is that they allow instances to clearly identify
what kind of mechanism they are using for expressing additional
semantics, should they follow a well-defined framework for doing so
(see Section 5 for examples). While this allows servers and clients
to represent the use of profiles, it does not make the profile
information visible outside of the representation itself, if the
representation is using embedded typed links. For newly defined
media types that may be used with profiles, it is therefore
recommended that they SHOULD define a media type parameter called
"profile", and specify that this media type parameter follows the
semantics of a profile as laid out in this document. This way,
clients can use this media type parameter to request a certain
profile when interacting, for example, with an HTTP server and
setting the Accept header. Representations using a "profile" media
type parameter still SHOULD include that value in the representation
using the "profile" link relation, since the media type label of a
representation can easily get lost when it is taken out of its
conversational context.
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Since a representation can link to more than one profile, the same
has to be possible for the corresponding media type parameter (if a
media type defines such a parameter). Media types defining a
"profile" parameter SHOULD define it as a whitespace-separated list
of profile URIs.
3.2. Profile Context
Profile links convey information about the use of profiles for a
media type. If they are used within a media type, they apply to the
context specified by that media type, which means that for example
profile links in the head element of an HTML document apply to the
document as a whole. The context of a profile extends to the scope
of where it is being used, which means that profiles used in profile
media type parameters (as described in Section 3.1) or used in HTTP
Link headers extend to the scope of the protocol in which they are
being used.
4. IANA Considerations
The link relation type below will be registered by IANA per Section
6.2.1 of RFC 5988 [RFC5988]:
Relation Name: profile
Description: Identifying that a resource representation conforms
to a certain profile, without affecting the non-profile semantics
of the resource representation.
Reference: [[ This document ]]
Notes: Profile URIs are primarily intended to be used as
identifiers, and thus clients SHOULD NOT indiscriminately access
profile URIs.
5. Examples
This section lists some examples of profiles that already are defined
today (and thus could be readily used with a 'profile' link), and of
some potential additional examples. Since so far, profiles have been
mostly limited to HTML (because of the support of profiles in HTML),
the two examples of existing profiles are HTML profiles, and the two
hypothetical examples are non-HTML examples.
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5.1. hCard
The hCard profile uses http://microformats.org/profile/hcard as its
defining URI and is essentially a mechanism how vCard [RFC6350]
information can be embedded in an HTML page using the mechanisms
provided by microformats. It is thus a good example for how profiles
might on the one hand define a model-based extension of the original
media type (in this case adding vCard fields), and how they also have
to define specific ways of how that model extension then is
represented in the media type (in this case, using microformats).
Alternatively, it would be possible to represent vCard information
through the mechanisms of RDFa or microdata, but since these would be
different conventions that a client would need to follow to extract
the vCard data, they would be identified by different profiles.
5.2. Dublin Core
Dublin Core metadata identified by the profile
http://dublincore.org/documents/2008/08/04/dc-html/ can be used to
embed Dublin Core metadata in an HRML page. In contrast to hCard,
which is using microformats as its foundation, the Dublin Core
profile defines its own way of embedding metadata into HTML, and does
so by using HTML <link> elements. The interesting difference to
hCard is that Dublin Core not only defines metadata to be embedded in
HTML, it also allows links to be added as metadata, in which case the
profile not just describes additional data to be found within the
representation, but also allows the representation to be linked to
additional resources.
5.3. Podcasts
Podcasts are an extension of feed formats, and define a substantial
set of additional attributes to reflect the fact that the resources
in podcast feeds are time-based media formats such as audio and
video. While there is no profile URI for podcasts, the current
definition (maintained by Apple) at
http://www.apple.com/itunes/podcasts/specs.html could serve as such a
URI, or it could by updated to include such a URI. Podcasts are
feeds with special behavior, and while it is possible to follow a
podcast feed using a generic feed reader, a podcast-aware feed reader
will be able to extract additional information from the feed, and
thus can implement more sophisticated services or present a more
sophisticated UI for podcast feeds. The Apple page referenced above
describes the implementation of one such specialized podcast feed
reader, Apple iTunes.
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6. Security Considerations
The 'profile' relation type is not known to introduce any new
security issues not already discussed in RFC 5988 [RFC5988] for
generic use of Web linking mechanisms.
7. Change Log
Note to RFC Editor: Please remove this section before publication.
7.1. From -03 to -04
o Changed category from "Standard" to "Informational".
o Minor textual changes.
7.2. From -02 to -03
o Removed the AtomPub example, which seemed to cause more confusion
than clarification.
o Minor textual changes.
7.3. From -01 to -02
o Feedback directed to apps-discuss@ietf.org
o Improved explanation of difference between media types and
profiles in "Profiles and Media Types" (Section 3.1).
o Added section on "Profiles and Media Types" (Section 3.1).
o Added section on "Profile Context" (Section 3.2).
7.4. From -00 to -01
o Updated security considerations.
o Made it clear that profiles are about resource representations,
and not about resources.
o Added examples section (Section 3.1) with four examples (Dublin
Core, HCard, AtomPub, and Podcasts).
o Minor textual changes.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
8.2. Informative References
[HTML401] Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
[RFC6350] Perreault, S., "vCard Format Specification", RFC 6350,
August 2011.
Appendix A. Acknowledgements
Thanks for comments and suggestions provided by Erlend Hamnaberg,
Markus Lanthaler, Simon Mayer, Mark Nottingham, Julian Reschke, James
Snell, and Tim Williams.
Author's Address
Erik Wilde
EMC Corporation
6801 Koll Center Parkway
Pleasanton, CA 94566
U.S.A.
Phone: +1-925-6006244
Email: erik.wilde@emc.com
URI: http://dret.net/netdret/
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