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2 Network Working Group M. Nottingham
3 Internet-Draft July 3, 2008
4 Updates: 4287 (if approved)
5 Intended status: Standards Track
6 Expires: January 4, 2009
8 HTTP Header Linking
9 draft-nottingham-http-link-header-02
11 Status of this Memo
13 By submitting this Internet-Draft, each author represents that any
14 applicable patent or other IPR claims of which he or she is aware
15 have been or will be disclosed, and any of which he or she becomes
16 aware will be disclosed, in accordance with Section 6 of BCP 79.
18 Internet-Drafts are working documents of the Internet Engineering
19 Task Force (IETF), its areas, and its working groups. Note that
20 other groups may also distribute working documents as Internet-
21 Drafts.
23 Internet-Drafts are draft documents valid for a maximum of six months
24 and may be updated, replaced, or obsoleted by other documents at any
25 time. It is inappropriate to use Internet-Drafts as reference
26 material or to cite them other than as "work in progress."
28 The list of current Internet-Drafts can be accessed at
29 http://www.ietf.org/ietf/1id-abstracts.txt.
31 The list of Internet-Draft Shadow Directories can be accessed at
32 http://www.ietf.org/shadow.html.
34 This Internet-Draft will expire on January 4, 2009.
36 Abstract
38 This document clarifies the status of the Link HTTP header and
39 attempts to consolidate link relations in a single registry.
41 Table of Contents
43 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
44 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
45 3. The Link Header Field . . . . . . . . . . . . . . . . . . . . 4
46 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
47 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
48 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
49 6.1. Normative References . . . . . . . . . . . . . . . . . . . 9
50 6.2. Informative References . . . . . . . . . . . . . . . . . . 9
51 Appendix A. Notes on HTML Links . . . . . . . . . . . . . . . . . 10
52 Appendix B. Notes on Atom Links . . . . . . . . . . . . . . . . . 11
53 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 11
54 Appendix D. Document history . . . . . . . . . . . . . . . . . . 11
55 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
56 Intellectual Property and Copyright Statements . . . . . . . . . . 13
58 1. Introduction
60 A means of indicating the relationships between documents on the Web
61 has been available for some time in HTML [W3C.REC-html401-19991224],
62 and was considered as a HTTP header in [RFC2068], but removed from
63 [RFC2616], due to a lack of implementation experience.
65 There have since surfaced many cases where a means of including this
66 information in HTTP headers has proved useful. However, because it
67 was removed, the status of the Link header is unclear, leading some
68 to consider minting new application-specific HTTP headers instead of
69 reusing it.
71 This document seeks to address these shortcomings.
73 Additionally, formats other than HTML -- namely, Atom [RFC4287] --
74 have also defined generic linking mechanisms that are similar to
75 those in HTML, but not identical. This document aims to reconcile
76 these differences when such links are expressed as headers.
78 [[ NOTE: This is a straw-man draft that is intended to give a ROUGH
79 idea of what it would take to align and consolidate the HTML and Atom
80 link relations into a single registry with reasonable extensibility
81 rules. In particular; a) it changes the registry for Atom link
82 relations, and the process for registration; b) it assigns more
83 generic semantics to several existing link relations, both Atom and
84 HTML; c) it changes the syntax of the Link header (in the case where
85 extensions are present). Feedback is welcome on the
86 ietf-http-wg@w3.org mailing list, although this is NOT a work item of
87 the HTTPBIS WG. ]]
89 2. Notational Conventions
91 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
92 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
93 document are to be interpreted as described in BCP 14, [RFC2119], as
94 scoped to those conformance targets.
96 This document uses the Augmented Backus-Naur Form (ABNF) notation of
97 [RFC2616], and explicitly includes the following rules from it:
98 quoted-string, token, SP (space). Additionally, the following rules
99 are included from [RFC3986]: URI-Reference, and from [RFC4288]: type-
100 name.
102 3. The Link Header Field
104 The Link entity-header field provides a means for describing a
105 relationship between two resources, generally between that of the
106 entity associated with the header and some other resource. An entity
107 MAY include multiple Link values.
109 The Link header field is semantically equivalent to the
110 element in HTML, as well as the atom:link element in Atom [RFC4287].
112 Link = "Link" ":" #("<" URI-Reference ">"
113 *( ";" link-param ) )
115 link-param = ( ( "rel" "=" relationship )
116 | ( "type" "=" type-name )
117 | ( "title" "=" quoted-string )
118 | ( link-extension ) )
120 link-extension = token [ "=" ( token | quoted-string ) ]
122 relationship = URI-Reference |
123 <"> URI-Reference *( SP URI-Reference) <"> )
125 Relationship values that include a semicolon (";") or comma (",")
126 MUST be quoted.
128 The title parameter MAY be used to label the destination of a link
129 such that it can be used as identification within a human-readable
130 menu.
132 For example:
134 Link: ; rel="previous";
135 title="previous chapter"
137 This indicates that chapter2 is previous to this resource in a
138 logical navigation path.
140 3.1. Link Relationships
142 Relationship values are URIs that identify the type of link. If the
143 relationship is a relative URI, its base URI MUST be considered to be
144 "http://www.iana.org/assignments/relation/", and the value MUST be
145 present in the link relation registry.
147 4. IANA Considerations
149 4.1. Link Header Registration
151 This specification requires that the Message Header Registry entry
152 for "Link" in HTTP [RFC3864] be updated to refer to this document,
153 and located at "http://www.iana.org/assignments/relation/".
155 Header field: Link
156 Applicable protocol: http
157 Status: standard
158 Author/change controller:
159 IETF (iesg@ietf.org)
160 Internet Engineering Task Force
161 Specification document(s):
162 [ this document ]
164 4.2. Link Relation Registry
166 This specification is intended to update Atom to become the reference
167 for the Link Relation registry, and clarifies its nature and use.
169 A Link relation is a way of indicating the semantics of a link. Link
170 relations are not format-specific, and MUST NOT specify a particular
171 format or media type that they are to be used with.
173 The security considerations of following a particular link are not
174 determined by the link's relation type; they are determined by the
175 specific context of the use and the media type of the response.
177 Likewise, a link relation SHOULD NOT specify what the context of its
178 use is, although the media type of the dereferenced link may
179 constrain how it is applied.
181 Link relations that differ only in case (e.g., "Foo" and "foo") MUST
182 NOT be registered.
184 New relations MAY be registered, subject to IESG Approval, as
185 outlined in [RFC2434]. Requests should be made by email to IANA,
186 which will then forward the request to the IESG, requesting approval.
187 The request should use the following template:
189 o Relation Name:
190 o Description:
191 o Reference:
193 The Link Relation registry replaces the Atom Link Relation registry,
194 using the same address with the following contents:
196 o Relation Name: alternate
197 o Description: Designates a substitute for the link's context.
198 o Reference: [W3C.REC-html401-19991224]
200 o Relation Name: appendix
201 o Description: Refers to an appendix.
202 o Reference: [W3C.REC-html401-19991224]
204 o Relation Name: bookmark
205 o Description: Refers to a bookmark or entry point.
206 o Reference: [W3C.REC-html401-19991224]
208 o Relation Name: chapter
209 o Description: Refers to a chapter in a collection of resources.
210 o Reference: [W3C.REC-html401-19991224]
212 o Relation Name: contents
213 o Description: Refers to a table of contents.
214 o Reference: [W3C.REC-html401-19991224]
216 o Relation Name: copyright
217 o Description: Refers to a copyright statement.
218 o Reference: [W3C.REC-html401-19991224]
220 o Relation Name: current
221 o Description: Refers to a resource containing the most recent
222 item(s) in a collection of resources.
223 o Reference: [RFC5005]
225 o Relation Name: edit
226 o Description: Refers to a resource that can be used to edit the
227 link's context.
228 o Reference: [RFC5023]
230 o Relation Name: edit-media
231 o Description: Refers to a resource that can be used to edit media
232 associated with the link's context.
233 o Reference: [RFC5023]
235 o Relation Name: enclosure
236 o Description: Identifies a related resource that is potentially
237 large and might require special handling.
238 o Reference: [RFC4287]
240 o Relation Name: first
241 o Description: A URI that refers to the furthest preceding resource
242 in a series of resources.
244 o Reference:
246 o Relation Name: glossary
247 o Description: Refers to a glossary of terms.
248 o Reference: [W3C.REC-html401-19991224]
250 o Relation Name: help
251 o Description: Refers to a resource offering help (more information,
252 links to other sources information, etc.)
253 o Reference: [W3C.REC-html401-19991224]
255 o Relation Name: index
256 o Description: Refers to an index.
257 o Reference: [W3C.REC-html401-19991224]
259 o Relation Name: last
260 o Description: A URI that refers to the furthest following resource
261 in a series of resources.
262 o Reference:
264 o Relation Name: license
265 o Description: Refers to a license associated with the link's
266 context.
267 o Reference: [RFC4946]
269 o Relation Name: next
270 o Description: Refers to the next resource in a ordered series of
271 resources.
272 o Reference: [W3C.REC-html401-19991224]
274 o Relation Name: next-archive
275 o Description: Refers to the immediately following archive resource.
276 o Reference: [RFC5005]
278 o Relation Name: payment
279 o Description: indicates a resource where payment is accepted.
280 o Reference:
281
283 o Relation Name: prev
284 o Description: Refers to the previous resource in an ordered series
285 of resources. Synonym for "previous".
286 o Reference: [W3C.REC-html401-19991224]
288 o Relation Name: previous
289 o Description: Refers to the previous resource in an ordered series
290 of resources. Synonym for "prev".
292 o Reference: [W3C.REC-html401-19991224]
294 o Relation Name: prev-archive
295 o Description: Refers to the immediately preceding archive resource.
296 o Reference: [RFC5005]
298 o Relation Name: related
299 o Description: Identifies a related resource.
300 o Reference: [RFC4287]
302 o Relation Name: replies
303 o Description: Identifies a resource that is a reply to the context
304 of the link.
305 o Reference: [RFC4685]
307 o Relation Name: section
308 o Description: Refers to a section in a collection of resources.
309 o Reference: [W3C.REC-html401-19991224]
311 o Relation Name: self
312 o Description: Conveys an identifier for the link's context.
313 o Reference: [RFC4287]
315 o Relation Name: start
316 o Description: Refers to the first resource in a collection of
317 resources.
318 o Reference: [W3C.REC-html401-19991224]
320 o Relation Name: stylesheet
321 o Description: Refers to an external style sheet.
322 o Reference: [W3C.REC-html401-19991224]
324 o Relation Name: subsection
325 o Description: Refers to a resource serving as a subsection in a
326 collection of resources.
327 o Reference: [W3C.REC-html401-19991224]
329 o Relation Name: via
330 o Description: Identifies a resource that is the source of the
331 information in the link's context.
332 o Reference: [RFC4287]
334 5. Security Considerations
336 The content the Link headers is not secure, private or integrity-
337 guaranteed, and due caution should be excercised when using it.
339 Applications that take advantage of these mechanisms should consider
340 the attack vectors opened by automatically following, trusting, or
341 otherwise using links gathered from HTTP headers.
343 6. References
345 6.1. Normative References
347 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
348 Requirement Levels", BCP 14, RFC 2119, March 1997.
350 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
351 IANA Considerations Section in RFCs", BCP 26, RFC 2434,
352 October 1998.
354 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
355 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
356 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
358 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
359 Procedures for Message Header Fields", BCP 90, RFC 3864,
360 September 2004.
362 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
363 Resource Identifier (URI): Generic Syntax", STD 66,
364 RFC 3986, January 2005.
366 [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
367 Registration Procedures", BCP 13, RFC 4288, December 2005.
369 6.2. Informative References
371 [RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
372 Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
373 RFC 2068, January 1997.
375 [RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication
376 Format", RFC 4287, December 2005.
378 [RFC4685] Snell, J., "Atom Threading Extensions", RFC 4685,
379 September 2006.
381 [RFC4946] Snell, J., "Atom License Extension", RFC 4946, July 2007.
383 [RFC5005] Nottingham, M., "Feed Paging and Archiving", RFC 5005,
384 September 2007.
386 [RFC5023] Gregorio, J. and B. de hOra, "The Atom Publishing
387 Protocol", RFC 5023, October 2007.
389 [W3C.REC-html401-19991224]
390 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
391 Specification", W3C REC REC-html401-19991224,
392 December 1999.
394 Appendix A. Notes on HTML Links
396 HTML motivated the original syntax of the Link header, and many of
397 the design decisions in this document are driven by a desire to stay
398 compatible with these uses.
400 All of the link relations defined by HTML4 have been included in the
401 link relation registry, so they can be used without modification.
402 However, extension link relations work differently in HTML4 and the
403 Link header; the former uses a document-wide "profile" URI to scope
404 the relations, while the latter allows the use of full URIs on
405 individual relations.
407 Therefore, when using the profile mechanism in HTML4, it is necessary
408 to map the profiled link relations to URIs when expressed in Link
409 headers. For example, in HTML:
411
412
413
414
415 [...]
417 could be represented as a header like this;
419 Link: ; rel="http://example.com/profile1/foo"
421 Profile authors should note this when creating profile URIs; it may
422 be desirable to use URIs that end in a delimiter (e.g., "/" or "#"),
423 to make extracting the specific relation in use easier.
425 HTML defines link relation values as case-insensitive, while the Link
426 header's syntax does not. Therefore, it is important to case-
427 normalise relation values in HTML before comparing or converting them
428 to Link headers.
430 HTML also defines several attributes on links that are not explicitly
431 defined by the Link header. Although most of these are believed to
432 be defunct, they can be used as link-extensions.
434 Appendix B. Notes on Atom Links
436 Atom conveys links in the atom:link element. When serialising an
437 atom:link into a Link header, it is necessary to convert IRIs (if
438 used) to URIs.
440 Additionally, since the base URI for link relations in Link headers
441 is fixed, extension links (i.e,. those not in the registry) MUST be
442 represented as absolute URIs.
444 Note also that while the Link header allows multiple relations to be
445 associated with a single link, atom:link does not. In this case, a
446 single Link header may map to several atom:link elements.
448 As with HTML, atom:link defines some attributes that are not
449 explicitly mirrored in the Link header syntax, but they may also be
450 used as link-extensions.
452 Appendix C. Acknowledgements
454 This specification lifts the definition of the Link header from
455 RFC2068; credit for it belongs entirely to the authors of and
456 contributors to that document. The link relation registrations
457 themselves are sourced from several documents; see the applicable
458 references.
460 The author would like to thank the many people who commented upon,
461 encouraged and gave feedback to this draft, especially including
462 Frank Ellermann and Julian Reschke.
464 Appendix D. Document history
466 -02
468 o Dropped XLink language.
469 o Removed 'made' example.
470 o Removed 'rev'. Can still be used as an extension.
471 o Added HTML reference to introduction.
472 o Required relationship values that have a ; or , to be quoted.
473 o Changed base URI for relation values.
475 o Noted registry location.
476 o Added advisory text about HTML profile URIs.
477 o Disallowed registration of relations that only differ in case.
478 o Clarified language about IRIs in Atom.
479 o Added descriptions for 'first', 'last', and 'payment', referring
480 to current IANA registry entries, as these were sourced from
481 e-mail. Will this cause self-referential implosion?
482 o Explicitly updates RFC4287.
483 o Added 'type' parameter.
484 o Removed unnecessary advice about non-HTML relations in HTML
485 section.
487 -01
489 o Changed syntax of link-relation to one or more URI; dropped
490 Profile.
491 o Dropped anchor parameter; can still be an extension.
492 o Removed Link-Template header; can be specified by templates spec
493 or elsewhere.
494 o Straw-man for link relation registry.
496 -00
498 o Initial draft; normative text lifted from RFC2068.
500 Author's Address
502 Mark Nottingham
504 Email: mnot@mnot.net
505 URI: http://www.mnot.net/
507 Full Copyright Statement
509 Copyright (C) The IETF Trust (2008).
511 This document is subject to the rights, licenses and restrictions
512 contained in BCP 78, and except as set forth therein, the authors
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515 This document and the information contained herein are provided on an
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