The 'profile' Link Relation Type
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This is an older version of an Internet-Draft that was ultimately published as RFC 6906.
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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 firstname.lastname@example.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- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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 Wilde Expires December 9, 2012 [Page 1] Internet-Draft "profile" Link Type June 2012 (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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as 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 Wilde Expires December 9, 2012 [Page 2] Internet-Draft "profile" Link Type June 2012 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. Wilde Expires December 9, 2012 [Page 3] Internet-Draft "profile" Link Type June 2012 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. Wilde Expires December 9, 2012 [Page 4] Internet-Draft "profile" Link Type June 2012 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 Wilde Expires December 9, 2012 [Page 5] Internet-Draft "profile" Link Type June 2012 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] Wilde Expires December 9, 2012 [Page 6] Internet-Draft "profile" Link Type June 2012 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 Wilde Expires December 9, 2012 [Page 7] Internet-Draft "profile" Link Type June 2012 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 email@example.com 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 Wilde Expires December 9, 2012 [Page 8] Internet-Draft "profile" Link Type June 2012 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: firstname.lastname@example.org URI: http://dret.net/netdret/ Wilde Expires December 9, 2012 [Page 9]