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Nottingham 3 Internet-Draft November 4, 2015 4 Obsoletes: 5988 (if approved) 5 Intended status: Standards Track 6 Expires: May 7, 2016 8 Web Linking 9 draft-nottingham-rfc5988bis-00 11 Abstract 13 This specification defines a way to indicate the relationships 14 between resources on the Web ("links") and the type of those 15 relationships ("link relation types"). 17 It also defines the use of such links in HTTP headers with the Link 18 header field. 20 Note to Readers 22 This is a work-in-progress to revise RFC5988. 24 The issues list can be found at https://github.com/mnot/I-D/labels/ 25 rfc5988bis . 27 The most recent (often, unpublished) draft is at 28 https://mnot.github.io/I-D/rfc5988bis/ . 30 Recent changes are listed at https://github.com/mnot/I-D/commits/gh- 31 pages/rfc5988bis . 33 Status of This Memo 35 This Internet-Draft is submitted in full conformance with the 36 provisions of BCP 78 and BCP 79. 38 Internet-Drafts are working documents of the Internet Engineering 39 Task Force (IETF). Note that other groups may also distribute 40 working documents as Internet-Drafts. The list of current Internet- 41 Drafts is at http://datatracker.ietf.org/drafts/current/. 43 Internet-Drafts are draft documents valid for a maximum of six months 44 and may be updated, replaced, or obsoleted by other documents at any 45 time. It is inappropriate to use Internet-Drafts as reference 46 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on May 7, 2016. 50 Copyright Notice 52 Copyright (c) 2015 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (http://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 This document may contain material from IETF Documents or IETF 66 Contributions published or made publicly available before November 67 10, 2008. The person(s) controlling the copyright in some of this 68 material may not have granted the IETF Trust the right to allow 69 modifications of such material outside the IETF Standards Process. 70 Without obtaining an adequate license from the person(s) controlling 71 the copyright in such materials, this document may not be modified 72 outside the IETF Standards Process, and derivative works of it may 73 not be created outside the IETF Standards Process, except to format 74 it for publication as an RFC or to translate it into languages other 75 than English. 77 Table of Contents 79 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 80 2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3 81 3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 82 4. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 4 83 4.1. Registered Relation Types . . . . . . . . . . . . . . . . 5 84 4.1.1. Registering Link Relation Types . . . . . . . . . . . 5 85 4.2. Extension Relation Types . . . . . . . . . . . . . . . . 6 86 5. The Link Header Field . . . . . . . . . . . . . . . . . . . . 7 87 5.1. Link Target . . . . . . . . . . . . . . . . . . . . . . . 7 88 5.2. Link Context . . . . . . . . . . . . . . . . . . . . . . 8 89 5.3. Relation Type . . . . . . . . . . . . . . . . . . . . . . 8 90 5.4. Target Attributes . . . . . . . . . . . . . . . . . . . . 8 91 5.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 9 92 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 93 6.1. Link HTTP Header Registration . . . . . . . . . . . . . . 10 94 6.2. Link Relation Type Registry . . . . . . . . . . . . . . . 11 95 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 96 8. Internationalisation Considerations . . . . . . . . . . . . . 12 97 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 98 9.1. Normative References . . . . . . . . . . . . . . . . . . 12 99 9.2. Informative References . . . . . . . . . . . . . . . . . 13 100 Appendix A. Using the Link Header with the HTML Format . . . . . 15 101 Appendix B. Using the Link Header with the Atom Format . . . . . 15 102 Appendix C. Changes from RFC5988 . . . . . . . . . . . . . . . . 16 103 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17 105 1. Introduction 107 This specification defines a way to indicate the relationships 108 between resources on the Web ("links") and the type of those 109 relationships ("link relation types"). 111 HTML [W3C.REC-html5-20141028] and Atom [RFC4287] both have well- 112 defined concepts of linking; this specification generalises this into 113 a framework that encompasses linking in these formats and 114 (potentially) elsewhere. 116 Furthermore, this specification formalises an HTTP header field for 117 conveying such links, having been originally defined in 118 Section 19.6.2.4 of [RFC2068], but removed from [RFC2616]. 120 2. Notational Conventions 122 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 123 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 124 document are to be interpreted as described in BCP 14, [RFC2119], as 125 scoped to those conformance targets. 127 This document uses the Augmented Backus-Naur Form (ABNF) notation of 128 [RFC7230], including the #rule, and explicitly includes the following 129 rules from it: quoted-string, token, SP (space), LOALPHA, DIGIT. 131 Additionally, the following rules are included from [RFC3986]: URI 132 and URI-Reference; from [RFC6838]: type-name and subtype-name; from 133 [W3C.CR-css3-mediaqueries-20090915]: media_query_list; from 134 [RFC5646]: Language-Tag; and from [I-D.ietf-httpbis-rfc5987bis], ext- 135 value and parmname. 137 3. Links 139 In this specification, a link is a typed connection between two 140 resources, and is comprised of: 142 o A link context, 144 o a link relation type (Section 4), 145 o a link target, and 147 o optionally, target attributes. 149 A link can be viewed as a statement of the form "{link context} has a 150 {link relation type} resource at {link target}, which has {target 151 attributes}". 153 Link contexts and link targets are both IRIs [RFC3987]. However, in 154 the common case, the link context will also be a URI [RFC3986], 155 because many protocols (such as HTTP) do not support dereferencing 156 IRIs. Likewise, the link target will be sometimes be converted to a 157 URI (see [RFC3987], Section 3.1) in places that do not support IRIs 158 (such as the Link header field defined in Section 5). 160 This specification does not place restrictions on the cardinality of 161 links; there can be multiple links to and from a particular target, 162 and multiple links of the same or different types between a given 163 context and target. Likewise, the relative ordering of links in any 164 particular serialisation, or between serialisations (e.g., the Link 165 header and in-content links) is not specified or significant in this 166 specification; applications that wish to consider ordering 167 significant can do so. 169 Target attributes are a set of key/value pairs that describe the link 170 or its target; for example, a media type hint. This specification 171 does not attempt to coordinate their names or use, but does provide 172 common target attributes for use in the Link HTTP header. 174 Finally, this specification does not define a general syntax for 175 expressing links, nor does it mandate a specific context for any 176 given link; it is expected that serialisations of links will specify 177 both aspects. One such serialisation is communication of links 178 through HTTP headers, specified in Section 5. 180 4. Link Relation Types 182 In the simplest case, a link relation type identifies the semantics 183 of a link. For example, a link with the relation type "copyright" 184 indicates that the resource identified by the link target is a 185 statement of the copyright terms applying to the current link 186 context. 188 Link relation types can also be used to indicate that the target 189 resource has particular attributes, or exhibits particular 190 behaviours; for example, a "service" link implies that the identified 191 resource is part of a defined protocol (in this case, a service 192 description). 194 Relation types are not to be confused with media types [RFC6838]; 195 they do not identify the format of the representation that results 196 when the link is dereferenced. Rather, they only describe how the 197 current context is related to another resource. 199 Relation types SHOULD NOT infer any additional semantics based upon 200 the presence or absence of another link relation type, or its own 201 cardinality of occurrence. An exception to this is the combination 202 of the "alternate" and "stylesheet" registered relation types, which 203 has special meaning in HTML for historical reasons. 205 There are two kinds of relation types: registered and extension. 207 4.1. Registered Relation Types 209 Well-defined relation types can be registered as tokens for 210 convenience and/or to promote reuse by other applications, using the 211 procedure in Section 4.1.1. 213 Registered relation type names MUST conform to the reg-rel-type rule, 214 and MUST be compared character-by-character in a case-insensitive 215 fashion. They SHOULD be appropriate to the specificity of the 216 relation type; i.e., if the semantics are highly specific to a 217 particular application, the name should reflect that, so that more 218 general names are available for less specific use. 220 Registered relation types MUST NOT constrain the media type of the 221 link context, and MUST NOT constrain the available representation 222 media types of the link target. However, they can specify the 223 behaviours and properties of the target resource (e.g., allowable 224 HTTP methods, request and response media types that must be 225 supported). 227 Applications that wish to refer to a registered relation type with a 228 URI [RFC3986] MAY do so by prepending 229 "http://www.iana.org/assignments/relation/" to its name. Note that 230 the resulting strings are not considered equivalent to the registered 231 relation types by many processors, and SHOULD NOT be serialised 232 unless the application using link relations specifically allows them. 234 4.1.1. Registering Link Relation Types 236 Relation types are registered on the advice of a Designated Expert 237 (appointed by the IESG or their delegate), with a Specification 238 Required (using terminology from [RFC5226]). 240 The Expert(s) will establish procedures for requesting registrations, 241 and make them available from the registry page. 243 Registration requests consist of at least the following information: 245 o Relation Name: 247 o Description: 249 o Reference: 251 The Expert(s) MAY define additional fields to be collected in the 252 registry. 254 General requirements for registered relation types are described in 255 Section 4.1. 257 See the registry for examples of the description field; generally, it 258 SHOULD identify the semantics in terms of the link's context and 259 target. 261 Registrations MUST reference a freely available, stable 262 specification. 264 Note that relation types can be registered by third parties, if the 265 Expert(s) determine that an unregistered relation type is widely 266 deployed and not likely to be registered in a timely manner. 268 Decisions (or lack thereof) made by the Expert(s) can be first 269 appealed to Application Area Directors (contactable using app- 270 ads@tools.ietf.org email address or directly by looking up their 271 email addresses on http://www.iesg.org/ website) and, if the 272 appellant is not satisfied with the response, to the full IESG (using 273 the iesg@iesg.org mailing list). 275 4.2. Extension Relation Types 277 Applications that don't wish to register a relation type can use an 278 extension relation type, which is a URI [RFC3986] that uniquely 279 identifies the relation type. Although the URI can point to a 280 resource that contains a definition of the semantics of the relation 281 type, clients SHOULD NOT automatically access that resource to avoid 282 overburdening its server. 284 When extension relation types are compared, they MUST be compared as 285 strings (after converting to URIs if serialised in a different 286 format, such as a XML QNames [W3C.REC-xml-names-20091208]) in a case- 287 insensitive fashion, character-by-character. Because of this, all- 288 lowercase URIs SHOULD be used for extension relations. 290 Note that while extension relation types are required to be URIs, a 291 serialisation of links can specify that they are expressed in another 292 form, as long as they can be converted to URIs. 294 5. The Link Header Field 296 The Link entity-header field provides a means for serialising one or 297 more links in HTTP headers. 299 Link = "Link" ":" #link-value 300 link-value = "<" URI-Reference ">" *( ";" link-param ) 301 link-param = ( ( "rel" "=" relation-types ) 302 | ( "anchor" "=" <"> URI-Reference <"> ) 303 | ( "rev" "=" relation-types ) 304 | ( "hreflang" "=" Language-Tag ) 305 | ( "media" "=" 306 ( media_query_list | ( <"> media_query_list <"> ) ) 307 ) 308 | ( "title" "=" quoted-string ) 309 | ( "title*" "=" ext-value ) 310 | ( "type" "=" ( media-type | quoted-mt ) ) 311 | ( link-extension ) ) 312 link-extension = ( parmname [ "=" ( ptoken | quoted-string ) ] ) 313 | ( ext-name-star "=" ext-value ) 314 ext-name-star = parmname "*" ; reserved for RFC5987-profiled 315 ; extensions. Whitespace NOT 316 ; allowed in between. 317 ptoken = 1*ptokenchar 318 ptokenchar = "!" | "#" | "$" | "%" | "&" | "'" | "(" 319 | ")" | "*" | "+" | "-" | "." | "/" | DIGIT 320 | ":" | "<" | "=" | ">" | "?" | "@" | ALPHA 321 | "[" | "]" | "^" | "_" | "`" | "{" | "|" 322 | "}" | "~" 323 media-type = type-name "/" subtype-name 324 quoted-mt = <"> media-type <"> 325 relation-types = relation-type 326 | <"> relation-type *( 1*SP relation-type ) <"> 327 relation-type = reg-rel-type | ext-rel-type 328 reg-rel-type = LOALPHA *( LOALPHA | DIGIT | "." | "-" ) 329 ext-rel-type = URI 331 5.1. Link Target 333 Each link-value conveys one target IRI as a URI-Reference (after 334 conversion to one, if necessary; see [RFC3987], Section 3.1) inside 335 angle brackets ("<>"). If the URI-Reference is relative, parsers 336 MUST resolve it as per [RFC3986], Section 5. Note that any base IRI 337 from the message's content is not applied. 339 5.2. Link Context 341 By default, the context of a link conveyed in the Link header field 342 is the IRI of the requested resource. 344 When present, the anchor parameter overrides this with another URI, 345 such as a fragment of this resource, or a third resource (i.e., when 346 the anchor value is an absolute URI). If the anchor parameter's 347 value is a relative URI, parsers MUST resolve it as per [RFC3986], 348 Section 5. Note that any base URI from the body's content is not 349 applied. 351 Consuming implementations can choose to ignore links with an anchor 352 parameter. For example, the application in use might not allow the 353 link context to be assigned to a different resource. In such cases, 354 the entire link is to be ignored; consuming implementations MUST NOT 355 process the link without applying the anchor. 357 Note that depending on HTTP status code and response headers, the 358 link context might be "anonymous" (i.e., no link context is 359 available). For instance, this is the case on a 404 response to a 360 GET request. 362 5.3. Relation Type 364 The relation type of a link is conveyed in the "rel" parameter's 365 value. The "rel" parameter MUST NOT appear more than once in a given 366 link-value; occurrences after the first MUST be ignored by parsers. 368 The "rev" parameter has been used in the past to indicate that the 369 semantics of the relationship are in the reverse direction. That is, 370 a link from A to B with REL="X" expresses the same relationship as a 371 link from B to A with REV="X". "rev" is deprecated by this 372 specification because it often confuses authors and readers; in most 373 cases, using a separate relation type is preferable. 375 Note that extension relation types are REQUIRED to be absolute URIs 376 in Link headers, and MUST be quoted if they contain a semicolon (";") 377 or comma (",") (as these characters are used as delimiters in the 378 header itself). 380 5.4. Target Attributes 382 The "hreflang", "media", "title", "title*", "type", and any link- 383 extension link-params are considered to be target attributes for the 384 link. 386 The "hreflang" parameter, when present, is a hint indicating what the 387 language of the result of dereferencing the link should be. Note 388 that this is only a hint; for example, it does not override the 389 Content-Language header of a HTTP response obtained by actually 390 following the link. Multiple "hreflang" parameters on a single link- 391 value indicate that multiple languages are available from the 392 indicated resource. 394 The "media" parameter, when present, is used to indicate intended 395 destination medium or media for style information (see 396 [W3C.REC-html5-20141028], Section 4.2.4). Its value MUST be quoted 397 if it contains a semicolon (";") or comma (","), and there MUST NOT 398 be more than one "media" parameter in a link-value. 400 The "title" parameter, when present, is used to label the destination 401 of a link such that it can be used as a human-readable identifier 402 (e.g., a menu entry) in the language indicated by the Content- 403 Language header (if present). The "title" parameter MUST NOT appear 404 more than once in a given link-value; occurrences after the first 405 MUST be ignored by parsers. 407 The "title*" parameter can be used to encode this label in a 408 different character set, and/or contain language information as per 409 [I-D.ietf-httpbis-rfc5987bis]. The "title*" parameter MUST NOT 410 appear more than once in a given link-value; occurrences after the 411 first MUST be ignored by parsers. If the parameter does not contain 412 language information, its language is indicated by the Content- 413 Language header (when present). 415 If both the "title" and "title*" parameters appear in a link-value, 416 processors SHOULD use the "title*" parameter's value. 418 The "type" parameter, when present, is a hint indicating what the 419 media type of the result of dereferencing the link should be. Note 420 that this is only a hint; for example, it does not override the 421 Content-Type header of a HTTP response obtained by actually following 422 the link. There MUST NOT be more than one type parameter in a link- 423 value. 425 5.5. Examples 427 For example: 429 Link: ; rel="previous"; 430 title="previous chapter" 432 indicates that "chapter2" is previous to this resource in a logical 433 navigation path. 435 Similarly, 437 Link: ; rel="http://example.net/foo" 439 indicates that the root resource ("/") is related to this resource 440 with the extension relation type "http://example.net/foo". 442 The example below shows an instance of the Link header encoding 443 multiple links, and also the use of RFC 5987 encoding to encode both 444 non-ASCII characters and language information. 446 Link: ; 447 rel="previous"; title*=UTF-8'de'letztes%20Kapitel, 448 ; 449 rel="next"; title*=UTF-8'de'n%c3%a4chstes%20Kapitel 451 Here, both links have titles encoded in UTF-8, use the German 452 language ("de"), and the second link contains the Unicode code point 453 U+00E4 ("LATIN SMALL LETTER A WITH DIAERESIS"). 455 Note that link-values can convey multiple links between the same link 456 target and link context; for example: 458 Link: ; 459 rel="start http://example.net/relation/other" 461 Here, the link to "http://example.org/" has the registered relation 462 type "start" and the extension relation type 463 "http://example.net/relation/other". 465 6. IANA Considerations 467 In addition to the actions below, IANA should terminate the Link 468 Relation Application Data Registry, as it has not been used, and 469 future use is not anticipated. 471 6.1. Link HTTP Header Registration 473 This specification updates the Message Header registry entry for 474 "Link" in HTTP [RFC3864] to refer to this document. 476 Header field: Link 477 Applicable protocol: http 478 Status: standard 479 Author/change controller: 480 IETF (iesg@ietf.org) 481 Internet Engineering Task Force 482 Specification document(s): 483 [RFC&rfc.number;] 485 6.2. Link Relation Type Registry 487 This specification updates the registration procedures for the Link 488 Relation Type registry; see Section 4.1.1. The Expert(s) and IANA 489 will interact as outlined below. 491 IANA will direct any incoming requests regarding the registry to the 492 processes established by the Expert(s); typically, this will mean 493 referring them to the registry HTML page. 495 The Expert(s) will provide registry data to IANA in an agreed form 496 (e.g. a specific XML format). IANA will publish: * The raw registry 497 data * The registry data, transformed into HTML * The registry data 498 in any alternative formats provided by the Expert(s) 500 Each published document will be at a URL agreed to by IANA and the 501 Expert(s), and IANA will set HTTP response headers on them as 502 (reasonably) requested by the Expert(s). 504 Additionally, the HTML generated by IANA will: * Take directions from 505 the Expert(s) as to the content of the HTML page's introductory text 506 and markup * Include a stable HTML fragment identifier for each 507 registered link relation 509 All registry data documents MUST include Simplified BSD License text 510 as described in Section 4.e of the Trust Legal Provisions 511 (). 513 7. Security Considerations 515 The content of the Link header field is not secure, private or 516 integrity-guaranteed, and due caution should be exercised when using 517 it. Use of Transport Layer Security (TLS) with HTTP ([RFC2818] and 518 [RFC2817]) is currently the only end-to-end way to provide such 519 protection. 521 Applications that take advantage of typed links should consider the 522 attack vectors opened by automatically following, trusting, or 523 otherwise using links gathered from HTTP headers. In particular, 524 Link headers that use the "anchor" parameter to associate a link's 525 context with another resource should be treated with due caution. 527 The Link entity-header field makes extensive use of IRIs and URIs. 528 See [RFC3987] for security considerations relating to IRIs. See 529 [RFC3986] for security considerations relating to URIs. See 530 [RFC7230] for security considerations relating to HTTP headers. 532 8. Internationalisation Considerations 534 Link targets may need to be converted to URIs in order to express 535 them in serialisations that do not support IRIs. This includes the 536 Link HTTP header. 538 Similarly, the anchor parameter of the Link header does not support 539 IRIs, and therefore IRIs must be converted to URIs before inclusion 540 there. 542 Relation types are defined as URIs, not IRIs, to aid in their 543 comparison. It is not expected that they will be displayed to end 544 users. 546 Note that registered Relation Names are required to be lower-case 547 ASCII letters. 549 9. References 551 9.1. Normative References 553 [I-D.ietf-httpbis-rfc5987bis] 554 Reschke, J., "Indicating Character Encoding and Language 555 for HTTP Header Field Parameters", draft-ietf-httpbis- 556 rfc5987bis-00 (work in progress), October 2015. 558 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 559 3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996, 560 . 562 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 563 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 564 RFC2119, March 1997, 565 . 567 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 568 Procedures for Message Header Fields", BCP 90, RFC 3864, 569 DOI 10.17487/RFC3864, September 2004, 570 . 572 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 573 Resource Identifier (URI): Generic Syntax", STD 66, RFC 574 3986, DOI 10.17487/RFC3986, January 2005, 575 . 577 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource 578 Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987, 579 January 2005, . 581 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 582 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 583 DOI 10.17487/RFC5226, May 2008, 584 . 586 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying 587 Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, 588 September 2009, . 590 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 591 Specifications and Registration Procedures", BCP 13, RFC 592 6838, DOI 10.17487/RFC6838, January 2013, 593 . 595 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 596 Protocol (HTTP/1.1): Message Syntax and Routing", RFC 597 7230, DOI 10.17487/RFC7230, June 2014, 598 . 600 [W3C.CR-css3-mediaqueries-20090915] 601 Lie, H., Celik, T., Glazman, D., and A. Kesteren, "Media 602 Queries", World Wide Web Consortium CR CR-css3- 603 mediaqueries-20090915, September 2009, 604 . 606 9.2. Informative References 608 [RFC2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T. 609 Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", 610 RFC 2068, DOI 10.17487/RFC2068, January 1997, 611 . 613 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 614 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 615 Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/ 616 RFC2616, June 1999, 617 . 619 [RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within 620 HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000, 621 . 623 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/ 624 RFC2818, May 2000, 625 . 627 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom 628 Syndication Format", RFC 4287, DOI 10.17487/RFC4287, 629 December 2005, . 631 [W3C.REC-html-rdfa-20150317] 632 Sporny, M., "HTML+RDFa 1.1 - Second Edition", World Wide 633 Web Consortium Recommendation REC-html-rdfa-20150317, 634 March 2015, 635 . 637 [W3C.REC-html5-20141028] 638 Hickson, I., Berjon, R., Faulkner, S., Leithead, T., 639 Navara, E., O'Connor, E., and S. Pfeiffer, "HTML5", 640 World Wide Web Consortium Recommendation REC- 641 html5-20141028, October 2014, 642 . 644 [W3C.REC-xml-names-20091208] 645 Bray, T., Hollander, D., Layman, A., Tobin, R., and H. 646 Thompson, "Namespaces in XML 1.0 (Third Edition)", World 647 Wide Web Consortium Recommendation REC-xml-names-20091208, 648 December 2009, 649 . 651 Appendix A. Using the Link Header with the HTML Format 653 HTML motivated the original syntax of the Link header, and many of 654 the design decisions in this document are driven by a desire to stay 655 compatible with it. 657 In HTML, the link element can be mapped to links as specified here by 658 using the "href" attribute for the target URI, and "rel" to convey 659 the relation type, as in the Link header. The context of the link is 660 the URI associated with the entire HTML document. 662 All of the link relation types defined by HTML have been included in 663 the Link Relation Type registry, so they can be used without 664 modification. However, there are several potential ways to serialise 665 extension relation types into HTML, including: 667 o As absolute URIs, 669 o using the RDFa [W3C.REC-html-rdfa-20150317] convention of mapping 670 token prefixes to URIs (in a manner similar to XML name spaces). 672 Individual applications of linking will therefore need to define how 673 their extension links should be serialised into HTML. 675 Surveys of existing HTML content have shown that unregistered link 676 relation types that are not URIs are (perhaps inevitably) common. 677 Consuming HTML implementations ought not consider such unregistered 678 short links to be errors, but rather relation types with a local 679 scope (i.e., their meaning is specific and perhaps private to that 680 document). 682 HTML also defines several attributes on links that can be see as 683 target attributes, including "media", "hreflang", "type" and "sizes". 685 Finally, the HTML specification gives a special meaning when the 686 "alternate" and "stylesheet" relation types coincide in the same 687 link. Such links ought to be serialised in the Link header using a 688 single list of relation-types (e.g., rel="alternate stylesheet") to 689 preserve this relationship. 691 Appendix B. Using the Link Header with the Atom Format 693 Atom conveys links in the atom:link element, with the "href" 694 attribute indicating the link target and the "rel" attribute 695 containing the relation type. The context of the link is either a 696 feed locator or an entry ID, depending on where it appears; 697 generally, feed-level links are obvious candidates for transmission 698 as a Link header. 700 When serialising an atom:link into a Link header, it is necessary to 701 convert link targets (if used) to URIs. 703 Atom defines extension relation types in terms of IRIs. This 704 specification re-defines them as URIs, to simplify and reduce errors 705 in their comparison. 707 Atom allows registered link relation types to be serialised as 708 absolute URIs. Such relation types SHOULD be converted to the 709 appropriate registered form (e.g., 710 "http://www.iana.org/assignments/relation/self" to "self") so that 711 they are not mistaken for extension relation types. 713 Furthermore, Atom link relation types are always compared in a case- 714 sensitive fashion; therefore, registered link relation types SHOULD 715 be converted to their registered form (usually, lowercase) when 716 serialised in an Atom document. 718 Note also that while the Link header allows multiple relations to be 719 serialised in a single link, atom:link does not. In this case, a 720 single link-value may map to several atom:link elements. 722 As with HTML, atom:link defines some attributes that are not 723 explicitly mirrored in the Link header syntax, but they can also be 724 used as link-extensions to maintain fidelity. 726 Appendix C. Changes from RFC5988 728 This specification has the following differences from its 729 predecessor, RFC5988: 731 o The initial relation type registrations were removed, since 732 they've already been registered by 5988. 734 o The introduction has been shortened. 736 o The Link Relation Application Data Registry has been removed. 738 o Incorporated errata. 740 o Updated references. 742 o Link cardinality was clarified. 744 o Terminology was changed from "target IRI" and "context IRI" to 745 "link target" and "link context" respectively. 747 o A convention for assigning a URI to registered relation types was 748 defined. 750 o Removed misleading statement that the link header field is 751 semantically equivalent to HTML and Atom links. 753 o More carefully defined how the Experts and IANA should interact. 755 Author's Address 757 Mark Nottingham 759 EMail: mnot@mnot.net 760 URI: http://www.mnot.net/