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Versions: 00 01 02 03 04 RFC 6906

Network Working Group                                           E. Wilde
Internet-Draft                                           EMC Corporation
Intended status: Standards Track                            June 7, 2012
Expires: December 9, 2012


                    The 'profile' Link Relation Type
                      draft-wilde-profile-link-02

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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on December 9, 2012.

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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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.


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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     5.2.  Dublin Core . . . . . . . . . . . . . . . . . . . . . . . . 7
     5.3.  Podcasts  . . . . . . . . . . . . . . . . . . . . . . . . . 7
     5.4.  Atom Publishing Protocol (AtomPub)  . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   7.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     7.1.  From -01 to -02 . . . . . . . . . . . . . . . . . . . . . . 8
     7.2.  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 referencable 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
   entries in the feed may not contain any podcast entries.



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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 in different mdeia 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
   outside the scope of this specification.





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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.

   Since a representation can link to more than one profile, the same
   has to be possible for the corresponding media type parameter (if a



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   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.

5.1.  hCard

   The hCard profile uses http://microformats.org/profile/hcard as its
   defining URI and is essentially a mechanism how vCard [RFC6350]



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   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.

5.4.  Atom Publishing Protocol (AtomPub)

   The Atom Publishing Protocol (AtomPub) [RFC5023] has no mechanism for
   signalling to clients that a feed supports AtomPub, this is only
   discoverable for clients if they know that a feed's URI appears in a
   service document (because they have found that service document



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   through some means outside of the AtomPub specification).  By adding
   a profile link to a feed supporting AtomPub (using Atom's generic
   link element), an AtomPub feed could be self-describing in the sense
   that clients could discover a feed's support for AtomPub just by
   looking at the feed itself.  While this approach would require an
   update of the AtomPub specification, future specifications could
   easily include such a profile URI as part of the specification
   itself, and profile links then could serve as the generic discovery
   mechanism for these extensions of a feed's capabilities.


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 -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).

7.2.  From -00 to -01

   o  Updated security considerations.

   o  Minor typographical changes.

   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).


8.  References




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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>.

   [RFC5023]  Gregorio, J. and B. Hora, "The Atom Publishing Protocol",
              RFC 5023, October 2010.

   [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, 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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