The 'profile' Link Relation Type

Independent Submission                                          E. Wilde
Request for Comments: 6906                               EMC Corporation
Category: Informational                                       March 2013
ISSN: 2070-1721


                    The 'profile' Link Relation Type

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 not to 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.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6906.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Profiles ........................................................3
      3.1. Profiles and Media Types ...................................4
      3.2. Profile Context ............................................5
   4. IANA Considerations .............................................5
   5. Examples ........................................................6
      5.1. hCard ......................................................6
      5.2. Dublin Core ................................................6
      5.3. Podcasts ...................................................7
   6. Security Considerations .........................................7
   7. Acknowledgements ................................................7
   8. References ......................................................7
      8.1. Normative References .......................................7
      8.2. Informative References .....................................7

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.  This would allow a client understanding this
   additional information to 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.  However, this mechanism is specific to HTML
   alone; 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 isn't
   specific to a particular serialisation 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].  Links with this relation type
   can be used in representations that support typed links as well as in
   HTTP Link headers.  The profile link relation type is independent of
   the context in which it is used 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).
   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 to use a representation (which is outside the scope of this
   specification) that represents the information about the profile in a
   human- or machine-readable way.

   As one example, consider the case of podcasts, where a specific kind
   of feed uses 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.  (Section
   5.3 gives more details for this example.)

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 allows them to
   be used across media types; thus, they could be associated with a
   resource, independent of their 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.  However, as is the case 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.  Thus, clients 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.

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
   built-in extensibility or openness, so that specific instances of the
   media type can layer additional semantics on top of the media type's
   foundation.  In this case, a profile is the appropriate mechanism to
   signal that the original semantics and processing model of the media
   type still apply, but that an additional processing model can be used
   to extract additional semantics.  This is in contrast to a new media
   type that, instead of just adding processing rules and semantics
   defines a complete set of processing rules and semantics in most
   cases.  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 objective 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
   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.  This means, for example, that
   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 has been 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: [RFC6906]

      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 are already defined
   (and thus could be readily used with a 'profile' link) and of some
   potential additional profiles.  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 one
   hypothetical example is a non-HTML example that is based on feeds.

5.1.  hCard

   The hCard profile uses http://microformats.org/profile/hcard as its
   defining URI and is essentially a mechanism on 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 is then
   represented in the media type (in this case, using microformats).
   Alternatively, it would be possible to represent vCard information
   through the mechanisms of RDF in Attributes (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 HTML page.  In contrast to hCard,
   which uses microformats as its foundation, the Dublin Core profile
   defines its own way of embedding metadata into HTML, and it does so
   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 only 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.

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.  Acknowledgements

   Thanks for comments and suggestions provided by Erlend Hamnaberg,
   Markus Lanthaler, Simon Mayer, Mark Nottingham, Julian Reschke, James
   Snell, Herbert Van de Sompel, and Tim Williams.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

8.2.  Informative References

   [HTML401]  Le 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.

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/