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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5988 (ref. '1') (Obsoleted by RFC 8288) Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group E. Wilde 3 Internet-Draft EMC Corporation 4 Intended status: Standards Track April 15, 2012 5 Expires: October 17, 2012 7 The 'profile' Link Relation Type 8 draft-wilde-profile-link-01 10 Abstract 12 This specification defines the 'profile' link relation type that 13 allows resource representations to indicate that they are following 14 one or more profiles. A profile is defined to not alter the 15 semantics of the resource representation itself, but to allow clients 16 to learn about additional semantics (constraints, conventions, 17 extensions) that are associated with the resource representation, in 18 addition to those defined by the media type and possibly other 19 mechanisms. 21 Status of this Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on October 17, 2012. 38 Copyright Notice 40 Copyright (c) 2012 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4 57 3. Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 59 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 60 5.1. hCard . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 61 5.2. Dublin Core . . . . . . . . . . . . . . . . . . . . . . . . 5 62 5.3. Podcasts . . . . . . . . . . . . . . . . . . . . . . . . . 6 63 5.4. Atom Publishing Protocol (AtomPub) . . . . . . . . . . . . 6 64 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 65 7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 7 66 7.1. From -00 to -01 . . . . . . . . . . . . . . . . . . . . . . 7 67 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 68 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 69 8.2. Informative References . . . . . . . . . . . . . . . . . . 7 70 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 7 71 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 73 1. Introduction 75 One of the foundations of the Internet and Web Architecture is the 76 fact that resource representations communicated through protocols 77 such as SMTP or HTTP are labeled with a 'media type', which allows a 78 client to understand at run time what 'type' of resource 79 representation it is handling. Sometimes, it would be useful for 80 servers and clients to include additional information about the 81 nature of the resource, so that a client understanding this 82 additional information could react in a way specific to that 83 specialization of the resource, where the specialization can be about 84 constraints, conventions, extensions, or any other aspects that do 85 not alter the basic media type semantics. HTML 4 [3] has such a 86 mechanism built into the language, which is the 'profile' attribute 87 of the 'head' element. This mechanism, however, is specific to HTML 88 alone, and at the time of writing it seems as if HTML 5 will drop 89 support for this mechanism entirely. 91 RFC 5988 [1] "defines a framework for typed links that is not 92 specific to a particular serialization or application. It does so by 93 redefining the link relation registry established by Atom to have a 94 broader domain, and adding to it the relations that are defined by 95 HTML." 97 This specification registers a 'profile' link relation type according 98 to the rules of RFC 5988 [1]. This link relation type is independent 99 of the context in which it is used (however, the representation must 100 support typed links for this mechanism to work) and does not 101 constrain in any way the target of the linked URI. In fact, for the 102 purpose of this specification, the target URI does not necessarily 103 have to identify a dereferencable resource (or even use a 104 dereferencable URI scheme), and clients can treat the occurrence of a 105 specific URI in the same way as an XML namespace URI and invoke 106 specific behavior based on the assumption that a specific profile 107 target URI signals that a resource representation follows a specific 108 profile. Note that at the same time, it is possible for profile 109 target URIs to use referencable URIs and use a media type (which is 110 outside the scope of this specification) which represents the 111 information about the profile in a human- or machine-readable way. 113 As one example, consider the case of podcasts, a specific kind of 114 feed using additional fields for media-related metadata. Using a 115 'profile' link, it would be easily possible for clients to understand 116 that a specific feed is supposed to be a podcast feed, and that it 117 may contain entries using podcast-specific fields. This may allow a 118 client to behave differently when handling such a feed (such as 119 rendering a podcast-specific UI), even when the current set of 120 entries in the feed may not contain any podcast entries. 122 2. Terminology 124 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 125 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 126 document are to be interpreted as described in RFC 2119 [2]. 128 3. Profiles 130 The concept of a profile has no strict definition on the Internet or 131 on the Web. For the purpose of this specification, a profile can be 132 described as additional semantics that can be used to process a 133 resource representation, such as constraints, conventions, 134 extensions, or any other aspects that do not alter the basic media 135 type semantics. A profile MUST NOT change the semantics of the 136 resource representation when processed without profile knowledge, so 137 that clients both with and without knowledge of a profiled resource 138 can safely use the same representation. While this specification 139 associates profiles with resource representations, creators of 140 profiles MAY define and manage them in a way that they can be used 141 across media types and thus could be associated with a resource, 142 independent of its representations. However, such a design is 143 outside of the scope of this specification, and clients profiles 144 SHOULD treat them as being associated with a representation. 146 Profiles can be combined, meaning that a single resource 147 representation can conform to zero or any number of profiles. 148 Depending on the profile support of clients, it is possible that the 149 same resource representation, when linked to a number of profiles, 150 can be processed with different sets of processing rules, based on 151 the profile support of the clients. 153 Profiles are identified by URI, but as with for example XML namespace 154 URIs, the URI in this case only serves as an identifier, meaning that 155 the presence of a specific URI has to be sufficient for a client to 156 assert that a resource representation conforms to a profile. Clients 157 thus SHOULD treat profile URIs as identifiers and not as links, but 158 profiles MAY be defined in a way that the URIs do identify 159 retrievable profile description and thus can be accessed by clients 160 by dereferencing the profile URI. For profiles intended for use in 161 environments where clients may encounter unknown profile URIs, 162 profile maintainers SHOULD consider to make the profile URI 163 dereferencable and provide useful documentation at that URI. The 164 design of such profile descriptions, however, is outside the scope of 165 this specification. 167 4. IANA Considerations 169 The link relation type below has been registered by IANA per Section 170 6.2.1 of RFC 5988 [1]: 172 Relation Name: profile 174 Description: Identifying that a resource representation conforms 175 to a certain profile, without affecting the non-profile semantics 176 of the resource representation. 178 Reference: [[ This document ]] 180 Notes: Profile URIs are primarily intended to be used as 181 identifiers, and thus clients SHOULD NOT indiscriminately access 182 profile URIs. 184 5. Examples 186 This section lists some examples of profiles that already are defined 187 today (and thus could be readily used with a 'profile' link), and of 188 some potential additional examples. Since so far, profiles have been 189 mostly limited to HTML (because of the support of profiles in HTML), 190 the two examples of existing profiles are HTML profiles, and the two 191 hypothetical examples are non-HTML examples. 193 5.1. hCard 195 The hCard profile uses http://microformats.org/profile/hcard as its 196 defining URI and is essentially a mechanism how vCard [4] information 197 can be embedded in an HTML page using the mechanisms provided by 198 microformats. It is thus a good example for how profiles might on 199 the one hand define a model-based extension of the original media 200 type (in this case adding vCard fields), and how they also have to 201 define specific ways of how that model extension then is represented 202 in the media type (in this case, using microformats). Alternatively, 203 it would be possible to represent vCard information through the 204 mechanisms of RDFa or microdata, but since these would be different 205 conventions that a client would need to follow to extract the vCard 206 data, they would be identified by different profiles. 208 5.2. Dublin Core 210 Dublin Core metadata identified by the profile 211 http://dublincore.org/documents/2008/08/04/dc-html/ can be used to 212 embed Dublin Core metadata in an HRML page. In contrast to hCard, 213 which is using microformats as its foundation, the Dublin Core 214 profile defines its own way of embedding metadata into HTML, and does 215 so by using HTML elements. The interesting difference to 216 hCard is that Dublin Core not only defines metadata to be embedded in 217 HTML, it also allows links to be added as metadata, in which case the 218 profile not just describes additional data to be found within the 219 representation, but also allows the representation to be linked to 220 additional resources. 222 5.3. Podcasts 224 Podcasts are an extension of feed formats, and define a substantial 225 set of additional attributes to reflect the fact that the resources 226 in podcast feeds are time-based media formats such as audio and 227 video. While there is no profile URI for podcasts, the current 228 definition (maintained by Apple) at 229 http://www.apple.com/itunes/podcasts/specs.html could serve as such a 230 URI, or it could by updated to include such a URI. Podcasts are 231 feeds with special behavior, and while it is possible to follow a 232 podcast feed using a generic feed reader, a podcast-aware feed reader 233 will be able to extract additional information from the feed, and 234 thus can implement more sophisticated services or present a more 235 sophisticated UI for podcast feeds. The Apple page referenced above 236 describes the implementation of one such specialized podcast feed 237 reader, Apple iTunes. 239 5.4. Atom Publishing Protocol (AtomPub) 241 The Atom Publishing Protocol (AtomPub) [5] has no mechanism for 242 signalling to clients that a feed supports AtomPub, this is only 243 discoverable for clients if they know that a feed's URI appears in a 244 service document (because they have found that service document 245 through some means outside of the AtomPub specification). By adding 246 a profile link to a feed supporting AtomPub (using Atom's generic 247 link element), an AtomPub feed could be self-describing in the sense 248 that clients could discover a feed's support for AtomPub just by 249 looking at the feed itself. While this approach would require an 250 update of the AtomPub specification, future specifications could 251 easily include such a profile URI as part of the specification 252 itself, and profile links then could serve as the generic discovery 253 mechanism for these extensions of a feed's capabilities. 255 6. Security Considerations 257 The 'profile' relation type is not known to introduce any new 258 security issues not already discussed in RFC 5988 [1] for generic use 259 of Web linking mechanisms. 261 7. Change Log 263 Note to RFC Editor: Please remove this section before publication. 265 7.1. From -00 to -01 267 o Updated security considerations. 269 o Minor typographical changes. 271 o Added section with examples. 273 o Made it clear that profiles are about resource representations, 274 and not about resources. 276 o Added structured examples section with fours examples (Dublin 277 Core, HCard, AtomPub, and Podcasts) 279 8. References 281 8.1. Normative References 283 [1] Nottingham, M., "Web Linking", RFC 5988, October 2010. 285 [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement 286 Levels", RFC 2119, March 1997. 288 8.2. Informative References 290 [3] Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01 Specification", 291 World Wide Web Consortium Recommendation REC-html401-19991224, 292 December 1999, . 294 [4] Perreault, S., "vCard Format Specification", RFC 6350, 295 August 2011. 297 [5] Gregorio, J. and B. Hora, "The Atom Publishing Protocol", 298 RFC 5023, October 2010. 300 Appendix A. Acknowledgements 302 Thanks for comments and suggestions provided by Erlend Hamnaberg, 303 Markus Lanthaler, Simon Mayer, Mark Nottingham, and Tim Williams. 305 Author's Address 307 Erik Wilde 308 EMC Corporation 309 6801 Koll Center Parkway 310 Pleasanton, CA 94566 311 U.S.A. 313 Phone: +1-925-6006244 314 Email: erik.wilde@emc.com 315 URI: http://dret.net/netdret/