idnits 2.17.1 draft-nottingham-http-link-header-04.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** The document seems to lack a License Notice according IETF Trust Provisions of 28 Dec 2009, Section 6.b.ii or Provisions of 12 Sep 2009 Section 6.b -- however, there's a paragraph with a matching beginning. Boilerplate error? (You're using the IETF Trust Provisions' Section 6.b License Notice from 12 Feb 2009 rather than one of the newer Notices. See https://trustee.ietf.org/license-info/.) Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- -- The draft header indicates that this document updates RFC4287, but the abstract doesn't seem to mention this, which it should. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year (Using the creation date from RFC4287, updated by this document, for RFC5378 checks: 2004-07-09) -- The document seems to contain a disclaimer for pre-RFC5378 work, and may have content which was first submitted before 10 November 2008. The disclaimer is necessary when there are original authors that you have been unable to contact, or if some do not wish to grant the BCP78 rights to the IETF Trust. If you are able to get all authors (current and original) to grant those rights, you can and should remove the disclaimer; otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (February 25, 2009) is 5538 days in the past. Is this intentional? 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 2616 (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) ** Obsolete normative reference: RFC 4288 (Obsoleted by RFC 6838) ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) -- Obsolete informational reference (is this intentional?): RFC 2068 (Obsoleted by RFC 2616) Summary: 4 errors (**), 0 flaws (~~), 1 warning (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Nottingham 3 Internet-Draft February 25, 2009 4 Updates: 4287 (if approved) 5 Intended status: Standards Track 6 Expires: August 29, 2009 8 Link Relations and HTTP Header Linking 9 draft-nottingham-http-link-header-04 11 Status of this Memo 13 This Internet-Draft is submitted to IETF in full conformance with the 14 provisions of BCP 78 and BCP 79. This document may contain material 15 from IETF Documents or IETF Contributions published or made publicly 16 available before November 10, 2008. The person(s) controlling the 17 copyright in some of this material may not have granted the IETF 18 Trust the right to allow modifications of such material outside the 19 IETF Standards Process. Without obtaining an adequate license from 20 the person(s) controlling the copyright in such materials, this 21 document may not be modified outside the IETF Standards Process, and 22 derivative works of it may not be created outside the IETF Standards 23 Process, except to format it for publication as an RFC or to 24 translate it into languages other than English. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF), its areas, and its working groups. Note that 28 other groups may also distribute working documents as Internet- 29 Drafts. 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 The list of current Internet-Drafts can be accessed at 37 http://www.ietf.org/ietf/1id-abstracts.txt. 39 The list of Internet-Draft Shadow Directories can be accessed at 40 http://www.ietf.org/shadow.html. 42 This Internet-Draft will expire on August 29, 2009. 44 Copyright Notice 46 Copyright (c) 2009 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents in effect on the date of 51 publication of this document (http://trustee.ietf.org/license-info). 52 Please review these documents carefully, as they describe your rights 53 and restrictions with respect to this document. 55 Abstract 57 This document specifies relation types for Web links, and defines a 58 registry for them. It also defines how to send such links in HTTP 59 headers with the Link header-field. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 64 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3 65 3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 66 4. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 4 67 5. The Link Header Field . . . . . . . . . . . . . . . . . . . . 5 68 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 69 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 70 8. Internationalisation Considerations . . . . . . . . . . . . . 11 71 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 72 9.1. Normative References . . . . . . . . . . . . . . . . . . . 12 73 9.2. Informative References . . . . . . . . . . . . . . . . . . 12 74 Appendix A. Notes on Using the Link Header with HTML4 . . . . . . 13 75 Appendix B. Notes on Using the Link Header with Atom . . . . . . 14 76 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 15 77 Appendix D. Document history . . . . . . . . . . . . . . . . . . 15 78 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17 80 1. Introduction 82 A means of indicating the relationships between resources on the Web, 83 as well as indicating the type of those relationships, has been 84 available for some time in HTML [W3C.REC-html401-19991224], and more 85 recently in Atom [RFC4287]. These mechanisms, although conceptually 86 similar, are separately specified. However, links between resources 87 need not be format-specific; it can be useful to have typed links 88 that are independent of the format, especially when a resource has 89 representations in multiple formats. 91 To this end, this document defines a framework for typed links that 92 isn't specific to a particular serialisation or context of use. It 93 does so by re-defining the link relation registry established by Atom 94 to have a broader scope, and adding to it the relations that are 95 defined by HTML. 97 Furthermore, an HTTP header-field for conveying typed links was 98 defined in [RFC2068], but removed from [RFC2616], due to a lack of 99 implementation experience. Since then, it has been implemented in 100 some User-Agents (e.g., for stylesheets), and several additional use 101 cases have surfaced. Because it was removed, the status of the Link 102 header is unclear, leading some to consider minting new application- 103 specific HTTP headers instead of reusing it. This document addresses 104 this by re-specifying the Link header with updated but backwards- 105 compatible syntax. 107 [[ Feedback is welcome on the ietf-http-wg@w3.org mailing list, 108 although this is NOT a work item of the HTTPBIS WG. ]] 110 2. Notational Conventions 112 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 113 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 114 document are to be interpreted as described in BCP 14, [RFC2119], as 115 scoped to those conformance targets. 117 This document uses the Augmented Backus-Naur Form (ABNF) notation of 118 [RFC2616], and explicitly includes the following rules from it: 119 quoted-string, token, SP (space). Additionally, the following rules 120 are included from [RFC3986]: URI and URI-Reference, and from 121 [RFC4288]: type-name. 123 3. Links 125 In this specification, a link is a typed connection between two 126 resources that are identified by IRIs [RFC3987], and is comprised of: 127 o A context IRI, and 128 o A link relation type (Section 4), and 129 o A target IRI. 131 A link can be viewed as a statement of the form "(context IRI) has a 132 (relation type) resource at (target IRI)." 134 Note that in the common case, the context IRI will also be a URI 135 [RFC3986], because common protocols (such as HTTP) do not support 136 dereferencing IRIs. Likewise, the target IRI will be converted to a 137 URI in serialisations that do not support IRIs (e.g., the Link 138 header). 140 This specification does not place restrictions on the cardinality of 141 links; there can be multiple links from and to a particular IRI, and 142 multiple links of different types between two given IRIs. 144 Additionally, this specification does not define a general syntax for 145 expressing links, nor mandate a specific context for any given link; 146 it is expected that applications of links will specify both aspects. 147 One such application is communication of links through HTTP headers, 148 specified in Section 5. 150 Such applications may further constrain or extend links (e.g., 151 associating a media type hint). 153 4. Link Relation Types 155 A link relation type identifies the semantics of a link. For 156 example, a link with the relation type "copyright" indicates that the 157 resource identified by the target IRI is a statement of the copyright 158 terms applying to the current context IRI. 160 Relation types are not to be confused with media types [RFC4288]; 161 they do not identify the format of the representation that results 162 when the link is dereferenced. Rather, they only describe how the 163 current context is related to another resource. 165 As such, relation types are not format-specific, and MUST NOT specify 166 a particular format or media type that they are to be used with. 167 Likewise, the context IRI for a given link is usually determined by 168 the serialisation of the link (e.g., the Link header, a HTML 169 document, etc.); a relation type SHOULD NOT specify the context IRI. 171 Consuming implementations SHOULD ignore relation types that they do 172 not understand or have no need to process. 174 There are two kinds of relation types; registered and extension. 176 4.1. Registered Relation Types 178 Commonly-used relation types with a clear meaning that are shared 179 across applications can be registered as tokens for convenience and 180 to promote reuse. For example, "self" and "alternate" are registered 181 relation types, because they are broadly useful. 183 This draft establishes an IANA registry of such relation types; see 184 Section 6.2. 186 Registered relation types MUST conform to the token rule, and SHOULD 187 conform to the sgml-name rule for compatibility with deployed 188 implementations; 190 sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" ) 192 Names that differ only in case from existing entries (e.g., "Foo" and 193 "foo") MUST NOT be registered. 195 Registered relation types MUST be compared in a case-insensitive 196 fashion. 198 Although they are specified as tokens, applications wishing to 199 internally refer to an extension relation type using a URI MAY do so 200 by considering it relative to the base URI 201 "http://www.iana.org/assignments/relation/". However, the URI form 202 of a registered relation type SHOULD NOT be serialised when an 203 application specifies the use of a relation type, because a consuming 204 implementation may not recognise it. 206 4.2. Extension Relation Types 208 Applications that don't merit a registered relation type may use an 209 extension relation type. An extension relation type is a URI 210 [RFC3986] that, when dereferenced, SHOULD yield a document describing 211 that relation type. 213 Extension relation types MUST be compared in a case-sensitive 214 fashion, character-by-character. 216 5. The Link Header Field 218 The Link entity-header field provides a means for conveying one or 219 more links in HTTP headers. It is semantically equivalent to the 220 element in HTML, as well as the atom:link feed-level element 221 in Atom [RFC4287]. 223 Link = "Link" ":" #link-value 224 link-value = "<" URI-Reference ">" *( ";" link-param ) ) 225 link-param = ( ( "rel" "=" relation-types ) 226 | ( "rev" "=" relation-types ) 227 | ( "type" "=" type-name ) 228 | ( "title" "=" quoted-string ) 229 | ( "title*" "=" enc2231-string ) 230 | ( "anchor" "=" <"> URI-Reference <"> ) 231 | ( link-extension ) ) 232 link-extension = token [ "=" ( token | quoted-string ) ] 233 enc2231-string = , 234 Section 7> 235 relation-types = relation-type | 236 <"> relation-type *( SP relation-type ) <"> 237 relation-type = reg-relation-type | ext-relation-type 238 reg-relation-type = token 239 ext-relation-type = URI 241 For example: 243 Link: ; rel="previous"; 244 title="previous chapter" 246 indicates that chapter2 is previous to this resource in a logical 247 navigation path. 249 Each link-value conveys one target IRI as a URI-Reference (after 250 conversion, if necessary) inside angle brackets ("<>"). If the URI- 251 Reference is relative, it MUST be resolved as per [RFC3986]. Note 252 that any base IRI from the body's content is not applied. 254 By default, the context of a link conveyed in the Link header field 255 is the IRI associated with the representation it occurs in. When 256 present, the anchor parameter overrides this with another URI, such 257 as a fragment of this resource, or a third resource (i.e., when the 258 anchor value is an absolute URI). 260 Normally, the relation type of a link is conveyed in the "rel" 261 parameter's value. The "rev" parameter has also been used for this 262 purpose historically by some formats, and is included here for 263 compatibility with those uses, but its use is not encouraged nor 264 defined by this specification. 266 Note that extension relation types are REQUIRED to be absolute URIs 267 in Link headers, and MUST be quoted if they contain a semicolon (";") 268 or comma (","). 270 The title parameter is used to label the destination of a link such 271 that it can be used as a human-readable identifier (e.g. a menu 272 entry). The title* parameter MAY be used to instead to encode this 273 label in an alternate character set, and/or contain language 274 information as per [RFC2231]. When using the enc2231-string syntax, 275 producers MUST NOT use a charset value other than 'ISO-8859-1' or 276 'UTF-8'. 278 Note that link-values may convey multiple links between the same 279 target and context IRIs; for example 281 Link: ; rel=index; 282 rel="start http://example.net/relation/other" 284 Here, the link to "http://example.org/" has the registered relation 285 types "index" and "start", and the extension relation type 286 "http://example.net/relation/other". 288 6. IANA Considerations 290 6.1. Link Header Registration 292 This specification updates the Message Header Registry entry for 293 "Link" in HTTP [RFC3864] to refer to this document. 295 Header field: Link 296 Applicable protocol: http 297 Status: standard 298 Author/change controller: 299 IETF (iesg@ietf.org) 300 Internet Engineering Task Force 301 Specification document(s): 302 [ this document ] 304 6.2. Link Relation Type Registry 306 This specification establishes the Link Relation Type Registry, 307 located at , and updates 308 Atom [RFC4287] to refer to it in place of the "Registry of Link 309 Relations". 311 The requirements for registered relation types are described in 312 Section 4.1. 314 Relation types may be registered on the advice of a Designated Expert 315 (appointed by the IESG or their delegate), with a Specification 316 Required (using terminology from [RFC5226]). 318 Registration requests consist of the completed registration template 319 below, typically published in an RFC or Open Standard (in the sense 320 described by [RFC2026], section 7). However, to allow for the 321 allocation of values prior to publication, the Designated Expert may 322 approve registration once they are satisfied that an RFC (or other 323 Open Standard) will be published. 325 The registration template is: 327 o Relation Name: 328 o Description: 329 o Reference: 331 Upon receiving a registration request (usually via IANA), the 332 Designated Expert should request review and comment from the 333 apps-discuss@ietf.org mailing list (or a successor designated by the 334 APPS Area Directors). Before a period of 30 days has passed, the 335 Designated Expert will either approve or deny the registration 336 request, communicating this decision both to the review list and to 337 IANA. Denials should include an explanation and, if applicable, 338 suggestions as to how to make the request successful. 340 The Link Relation Type registry's initial contents are: 342 o Relation Name: alternate 343 o Description: Designates a substitute for the link's context. 344 o Reference: [W3C.REC-html401-19991224] 346 o Relation Name: appendix 347 o Description: Refers to an appendix. 348 o Reference: [W3C.REC-html401-19991224] 350 o Relation Name: bookmark 351 o Description: Refers to a bookmark or entry point. 352 o Reference: [W3C.REC-html401-19991224] 354 o Relation Name: chapter 355 o Description: Refers to a chapter in a collection of resources. 356 o Reference: [W3C.REC-html401-19991224] 358 o Relation Name: contents 359 o Description: Refers to a table of contents. 360 o Reference: [W3C.REC-html401-19991224] 362 o Relation Name: copyright 363 o Description: Refers to a copyright statement that applies to the 364 link's context. 366 o Reference: [W3C.REC-html401-19991224] 368 o Relation Name: current 369 o Description: Refers to a resource containing the most recent 370 item(s) in a collection of resources. 371 o Reference: [RFC5005] 373 o Relation Name: describedby 374 o Description: Refers to a resource providing information about the 375 link's context. 376 o Documentation: 378 o Relation Name: edit 379 o Description: Refers to a resource that can be used to edit the 380 link's context. 381 o Reference: [RFC5023] 383 o Relation Name: edit-media 384 o Description: Refers to a resource that can be used to edit media 385 associated with the link's context. 386 o Reference: [RFC5023] 388 o Relation Name: enclosure 389 o Description: Identifies a related resource that is potentially 390 large and might require special handling. 391 o Reference: [RFC4287] 393 o Relation Name: first 394 o Description: An IRI that refers to the furthest preceding resource 395 in a series of resources. 396 o Reference: 398 o Relation Name: glossary 399 o Description: Refers to a glossary of terms. 400 o Reference: [W3C.REC-html401-19991224] 402 o Relation Name: help 403 o Description: Refers to a resource offering help (more information, 404 links to other sources information, etc.) 405 o Reference: [W3C.REC-html401-19991224] 407 o Relation Name: index 408 o Description: Refers to an index. 409 o Reference: [W3C.REC-html401-19991224] 411 o Relation Name: last 412 o Description: An IRI that refers to the furthest following resource 413 in a series of resources. 414 o Reference: 416 o Relation Name: license 417 o Description: Refers to a license associated with the link's 418 context. 419 o Reference: [RFC4946] 421 o Relation Name: next 422 o Description: Refers to the next resource in a ordered series of 423 resources. 424 o Reference: [W3C.REC-html401-19991224] 426 o Relation Name: next-archive 427 o Description: Refers to the immediately following archive resource. 428 o Reference: [RFC5005] 430 o Relation Name: payment 431 o Description: indicates a resource where payment is accepted. 432 o Reference: 433 435 o Relation Name: prev 436 o Description: Refers to the previous resource in an ordered series 437 of resources. Synonym for "previous". 438 o Reference: [W3C.REC-html401-19991224] 440 o Relation Name: previous 441 o Description: Refers to the previous resource in an ordered series 442 of resources. Synonym for "prev". 443 o Reference: [W3C.REC-html401-19991224] 445 o Relation Name: prev-archive 446 o Description: Refers to the immediately preceding archive resource. 447 o Reference: [RFC5005] 449 o Relation Name: related 450 o Description: Identifies a related resource. 451 o Reference: [RFC4287] 453 o Relation Name: replies 454 o Description: Identifies a resource that is a reply to the context 455 of the link. 456 o Reference: [RFC4685] 457 o Relation Name: section 458 o Description: Refers to a section in a collection of resources. 459 o Reference: [W3C.REC-html401-19991224] 461 o Relation Name: self 462 o Description: Conveys an identifier for the link's context. 463 o Reference: [RFC4287] 465 o Relation Name: start 466 o Description: Refers to the first resource in a collection of 467 resources. 468 o Reference: [W3C.REC-html401-19991224] 470 o Relation Name: stylesheet 471 o Description: Refers to an external style sheet. 472 o Reference: [W3C.REC-html401-19991224] 474 o Relation Name: subsection 475 o Description: Refers to a resource serving as a subsection in a 476 collection of resources. 477 o Reference: [W3C.REC-html401-19991224] 479 o Relation Name: via 480 o Description: Identifies a resource that is the source of the 481 information in the link's context. 482 o Reference: [RFC4287] 484 7. Security Considerations 486 The content of the Link header-field is not secure, private or 487 integrity-guaranteed, and due caution should be exercised when using 488 it. 490 Applications that take advantage of typed links should consider the 491 attack vectors opened by automatically following, trusting, or 492 otherwise using links gathered from HTTP headers. In particular, 493 Link headers that use the "anchor" parameter to associate a link's 494 context with another resource should be treated with due caution. 496 8. Internationalisation Considerations 498 Target IRIs may need to be converted to URIs in order to serialise 499 them in applications that do not support IRIs. This includes the 500 Link HTTP header. 502 Similarly, the anchor parameter of the Link header does not support 503 IRIs, and therefore IRIs must be converted to URIs before inclusion 504 there. 506 Relation types are defined as URIs, not IRIs, to aid in their 507 comparison. It is not expected that they will be displayed to end 508 users. 510 9. References 512 9.1. Normative References 514 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 515 3", BCP 9, RFC 2026, October 1996. 517 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 518 Requirement Levels", BCP 14, RFC 2119, March 1997. 520 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 521 Word Extensions: Character Sets, Languages, and 522 Continuations", RFC 2231, November 1997. 524 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 525 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 526 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 528 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 529 Procedures for Message Header Fields", BCP 90, RFC 3864, 530 September 2004. 532 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 533 Resource Identifier (URI): Generic Syntax", STD 66, 534 RFC 3986, January 2005. 536 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource 537 Identifiers (IRIs)", RFC 3987, January 2005. 539 [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and 540 Registration Procedures", BCP 13, RFC 4288, December 2005. 542 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 543 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 544 May 2008. 546 9.2. Informative References 548 [RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. 549 Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", 550 RFC 2068, January 1997. 552 [RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication 553 Format", RFC 4287, December 2005. 555 [RFC4685] Snell, J., "Atom Threading Extensions", RFC 4685, 556 September 2006. 558 [RFC4946] Snell, J., "Atom License Extension", RFC 4946, July 2007. 560 [RFC5005] Nottingham, M., "Feed Paging and Archiving", RFC 5005, 561 September 2007. 563 [RFC5023] Gregorio, J. and B. de hOra, "The Atom Publishing 564 Protocol", RFC 5023, October 2007. 566 [W3C.REC-html401-19991224] 567 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01 568 Specification", W3C REC REC-html401-19991224, 569 December 1999. 571 Appendix A. Notes on Using the Link Header with HTML4 573 HTML motivated the original syntax of the Link header, and many of 574 the design decisions in this document are driven by a desire to stay 575 compatible with these uses. 577 In HTML4, the link element can be mapped to links as specified here 578 by using the "href" attribute for the target URI, and "rel" to convey 579 both the relation type, as in the Link header. The context of the 580 link is the URI associated with the entire HTML document. 582 HTML4 also has a "rev" parameter for links that allows a link's 583 relation to be reversed. The Link header has a "rev" parameter to 584 allow the expression of these links in HTTP headers, but its use is 585 not encouraged, due to the confusion this mechanism causes as well as 586 conflicting interpretations among HTML versions. 588 All of the link relations defined by HTML4 have been included in the 589 link relation registry, so they can be used without modification. 590 However, extension link relations work differently in HTML4 and the 591 Link header; the former uses a document-wide "profile" URI to scope 592 the relations, while the latter allows the use of full URIs on 593 individual relations. 595 Therefore, when using the profile mechanism in HTML4, it is necessary 596 to map the profiled link relations to URIs when expressed in Link 597 headers. For example, in HTML: 599 600 601 602 603 [...] 605 could be represented as a header like this; 607 Link: ; rel="http://example.com/profile1/foo" 609 Profile authors should note this when creating profile URIs; it may 610 be desirable to use URIs that end in a delimiter (e.g., "/" or "#"), 611 to make extracting the specific relation in use easier. 613 Surveys of existing HTML content have shown that unregistered link 614 relation types that are not URIs are (perhaps inevitably) common. 615 Consuming HTML implementations should not consider such unregistered 616 short links to be errors, but rather relation types with a local 617 scope (i.e., their meaning is specific and perhaps private to that 618 document). 620 HTML4 also defines several attributes on links that are not 621 explicitly defined by the Link header. These attributes can be 622 serialised as link-extensions to maintain fidelity. 624 Appendix B. Notes on Using the Link Header with Atom 626 Atom conveys links in the atom:link element, with the "href" 627 attribute indicating the target IRI and the "rel" attribute 628 containing the relation type. The context of the link is either a 629 feed IRI or an entry ID, depending on where it appears; generally, 630 feed-level links are candidates for transmission as a Link header. 632 When serialising an atom:link into a Link header, it is necessary to 633 convert target IRIs (if used) to URIs. 635 Atom defines extension relation types in terms of IRIs. This 636 specification defines them as URIs, to aid in their comparison. 638 Atom allows registered link relation types to be serialised as 639 absolute URIs, because a base URI is defined for the registry. Such 640 relation types SHOULD be converted to the appropriate registered form 641 (e.g., "http://www.iana.org/assignments/relation/self" to "self") so 642 that they are not mistaken for extension relation types. 644 Note also that while the Link header allows multiple relations to be 645 associated with a single link, atom:link does not. In this case, a 646 single link-value may map to several atom:link elements. 648 As with HTML, atom:link defines some attributes that are not 649 explicitly mirrored in the Link header syntax, but they may also be 650 used as link-extensions. 652 Appendix C. Acknowledgements 654 This specification lifts the idea and definition for the Link header 655 from RFC2068; credit for it belongs entirely to the authors of and 656 contributors to that document. The link relation registrations 657 themselves are sourced from several documents; see the applicable 658 references. 660 The author would like to thank the many people who commented upon, 661 encouraged and gave feedback to this draft, especially including 662 Frank Ellermann, Roy Fielding and Julian Reschke. 664 Appendix D. Document history 666 [[ to be removed by the RFC editor before publication as an RFC. ]] 668 -04 670 o Defined context as a resource, rather than a representation. 671 o Removed concept of link directionality; relegated to a deprecated 672 Link header extension. 673 o Relation types split into registered (non-URI) and extension 674 (URI). 675 o Changed wording around finding URIs for registered relation types. 676 o Changed target and context URIs to IRIs (but not extension 677 relation types). 678 o Add RFC2231 encoding for title parameter, explicit BNF for title*. 679 o Add i18n considerations. 680 o Specify how to compare relation types. 681 o Changed registration procedure to Designated Expert. 682 o Softened language around presence of relations in the registry. 683 o Added describedby relation. 684 o Re-added 'anchor' parameter, along with security consideration for 685 third-party anchors. 687 o Softened language around HTML4 attributes that aren't directly 688 accommodated. 689 o Various tweaks to abstract, introduction and examples. 691 -03 693 o Inverted focus from Link headers to link relations. 694 o Specified was a link relation type is. 695 o Based on discussion, re-added 'rev'. 696 o Changed IESG Approval to IETF Consensus for relation registrations 697 (i.e., require a document). 698 o Updated RFC2434 reference to RFC5226. 699 o Registered relations SHOULD conform to sgml-name. 700 o Cautioned against confusing relation types with media types. 702 -02 704 o Dropped XLink language. 705 o Removed 'made' example. 706 o Removed 'rev'. Can still be used as an extension. 707 o Added HTML reference to introduction. 708 o Required relationship values that have a ; or , to be quoted. 709 o Changed base URI for relation values. 710 o Noted registry location. 711 o Added advisory text about HTML profile URIs. 712 o Disallowed registration of relations that only differ in case. 713 o Clarified language about IRIs in Atom. 714 o Added descriptions for 'first', 'last', and 'payment', referring 715 to current IANA registry entries, as these were sourced from 716 e-mail. Will this cause self-referential implosion? 717 o Explicitly updates RFC4287. 718 o Added 'type' parameter. 719 o Removed unnecessary advice about non-HTML relations in HTML 720 section. 722 -01 724 o Changed syntax of link-relation to one or more URI; dropped 725 Profile. 726 o Dropped anchor parameter; can still be an extension. 727 o Removed Link-Template header; can be specified by templates spec 728 or elsewhere. 729 o Straw-man for link relation registry. 731 -00 732 o Initial draft; normative text lifted from RFC2068. 734 Author's Address 736 Mark Nottingham 738 Email: mnot@mnot.net 739 URI: http://www.mnot.net/