    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.

    Copyright Notice

       Copyright (c) 2013 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
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.

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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

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       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.

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       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).

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       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.

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          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.

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    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.

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    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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