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2 Network Working Group E. Wilde
3 Internet-Draft Axway
4 Intended status: Informational H. Van de Sompel
5 Expires: 5 January 2022 Data Archiving and Networked Services
6 4 July 2021
8 Linkset: Media Types and a Link Relation Type for Link Sets
9 draft-ietf-httpapi-linkset-03
11 Abstract
13 This specification defines two document formats and respective media
14 types for representing sets of links as stand-alone resources. One
15 format is JSON-based, the other aligned with the format for
16 representing links in the HTTP "Link" header field. This
17 specification also introduces a link relation type to support
18 discovery of sets of links.
20 Note to Readers
22 Please discuss this draft on the "Building Blocks for HTTP APIs"
23 mailing list (https://www.ietf.org/mailman/listinfo/httpapi).
25 Online access to all versions and files is available on GitHub
26 (https://github.com/ietf-wg-httpapi/linkset).
28 Status of This Memo
30 This Internet-Draft is submitted in full conformance with the
31 provisions of BCP 78 and BCP 79.
33 Internet-Drafts are working documents of the Internet Engineering
34 Task Force (IETF). Note that other groups may also distribute
35 working documents as Internet-Drafts. The list of current Internet-
36 Drafts is at https://datatracker.ietf.org/drafts/current/.
38 Internet-Drafts are draft documents valid for a maximum of six months
39 and may be updated, replaced, or obsoleted by other documents at any
40 time. It is inappropriate to use Internet-Drafts as reference
41 material or to cite them other than as "work in progress."
43 This Internet-Draft will expire on 5 January 2022.
45 Copyright Notice
47 Copyright (c) 2021 IETF Trust and the persons identified as the
48 document authors. All rights reserved.
50 This document is subject to BCP 78 and the IETF Trust's Legal
51 Provisions Relating to IETF Documents (https://trustee.ietf.org/
52 license-info) in effect on the date of publication of this document.
53 Please review these documents carefully, as they describe your rights
54 and restrictions with respect to this document. Code Components
55 extracted from this document must include Simplified BSD License text
56 as described in Section 4.e of the Trust Legal Provisions and are
57 provided without warranty as described in the Simplified BSD License.
59 Table of Contents
61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
63 3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 3
64 3.1. Third-Party Links . . . . . . . . . . . . . . . . . . . . 4
65 3.2. Challenges Writing to HTTP Link Header Field . . . . . . 4
66 3.3. Large Number of Links . . . . . . . . . . . . . . . . . . 5
67 4. Document Formats for Sets of Links . . . . . . . . . . . . . 5
68 4.1. HTTP Link Document Format: application/linkset . . . . . 6
69 4.2. JSON Document Format: application/linkset+json . . . . . 6
70 4.2.1. Set of Links . . . . . . . . . . . . . . . . . . . . 7
71 4.2.2. Link Context Object . . . . . . . . . . . . . . . . . 7
72 4.2.3. Link Target Object . . . . . . . . . . . . . . . . . 8
73 4.2.4. Link Target Attributes . . . . . . . . . . . . . . . 10
74 4.2.5. JSON Extensibility . . . . . . . . . . . . . . . . . 14
75 5. The "profile" attribute for media types to Represent Sets of
76 Links . . . . . . . . . . . . . . . . . . . . . . . . . . 15
77 6. The "linkset" Relation Type for Linking to a Set of Links . . 15
78 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
79 7.1. Set of Links Provided as application/linkset . . . . . . 16
80 7.2. Set of Links Provided as application/linkset+json . . . . 17
81 7.3. Discovering a Link Set via the "linkset" Link Relation
82 Type . . . . . . . . . . . . . . . . . . . . . . . . . . 19
83 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
84 8.1. Link Relation Type: linkset . . . . . . . . . . . . . . . 20
85 8.2. Media Type: application/linkset . . . . . . . . . . . . . 20
86 8.3. Media Type: application/linkset+json . . . . . . . . . . 21
87 9. Security Considerations . . . . . . . . . . . . . . . . . . . 22
88 10. Normative References . . . . . . . . . . . . . . . . . . . . 23
89 11. Informative References . . . . . . . . . . . . . . . . . . . 24
90 Appendix A. JSON-LD Context . . . . . . . . . . . . . . . . . . 25
91 Appendix B. Implementation Status . . . . . . . . . . . . . . . 30
92 B.1. GS1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
93 B.2. FAIR Signposting Profile . . . . . . . . . . . . . . . . 31
94 B.3. Open Journal Systems (OJS) . . . . . . . . . . . . . . . 31
95 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 31
96 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
98 1. Introduction
100 Resources on the Web often use typed Web Links [RFC8288], either
101 embedded in resource representations, for example using the
102 element for HTML documents, or conveyed in the HTTP "Link" header
103 field for documents of any media type. In some cases, however,
104 providing links in this manner is impractical or impossible and
105 delivering a set of links as a stand-alone document is preferable.
107 Therefore, this specification defines two document formats and
108 associated media types to represent sets of links. It also defines
109 the "linkset" relation type that supports discovery of any resource
110 that conveys a set of links as a stand-alone document.
112 2. Terminology
114 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
115 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
116 "OPTIONAL" in this document are to be interpreted as described in BCP
117 14 [RFC2119] [RFC8174] when, and only when, they appear in all
118 capitals, as shown here.
120 This specification uses the terms "link context" and "link target" as
121 defined in [RFC8288].
123 In the examples provided in this document, links in the HTTP "Link"
124 header field are shown on separate lines in order to improve
125 readability. Note, however, that as per Section 5.5 of
126 [I-D.ietf-httpbis-semantics], line breaks are deprecated in values
127 for HTTP fields; only whitespaces and tabs are supported as
128 separators.
130 3. Scenarios
132 The following sections outline scenarios in which providing links by
133 means of a standalone document instead of in an HTTP "Link" header
134 field or as links embedded in the resource representation is
135 advantageous or necessary.
137 For all scenarios, links could be provided by means of a stand-alone
138 document that is formatted according to the JSON-based serialization,
139 the serialization aligned with the HTTP "Link" field format, or both.
140 The former serialization is motivated by the widespread use of JSON
141 and related tools, which suggests that handling sets of links
142 expressed as JSON documents should be attractive to developers. The
143 latter serialization is provided for compatibility with the existing
144 serialization used in the HTTP "Link" field and to allow reuse of
145 tools created to handle it.
147 It is important to keep in mind that when providing links by means of
148 a standalone representation, other links can still be provided using
149 other approaches, i.e. it is possible combine various mechanisms to
150 convey links.
152 3.1. Third-Party Links
154 In some cases it is useful that links pertaining to a resource are
155 provided by a server other than the one that hosts the resource. For
156 example, this allows:
158 * Providing links in which the resource is involved not just as link
159 context but also as link target.
161 * Providing links pertaining to the resource that the server hosting
162 that resource is not aware of.
164 * External management of links pertaining to the resource in a
165 special-purpose link management service.
167 In such cases, links pertaining to a resource can be provided by
168 another, specific resource. That specific resource may be managed by
169 the same or by another custodian as the resource to which the links
170 pertain. For clients intent on consuming links provided in that
171 manner, it would be beneficial if the following conditions were met:
173 * Links are provided in a document that uses a well-defined media
174 type.
176 * The resource to which the provided links pertain is able to link
177 to the resource that provides these links using a well-known link
178 relation type.
180 These requirements are addressed in this specification through the
181 definition of two media types and a link relation type, respectively.
183 3.2. Challenges Writing to HTTP Link Header Field
185 In some cases, it is not straightforward to write links to the HTTP
186 "Link" header field from an application. This can, for example, be
187 the case because not all required link information is available to
188 the application or because the application does not have the
189 capability to directly write HTTP fields. In such cases, providing
190 links by means of a standalone document can be a solution. Making
191 the resource that provides these links discoverable can be achieved
192 by means of a typed link.
194 3.3. Large Number of Links
196 When conveying links in an HTTP "Link" header field, it is possible
197 for the size of the HTTP response fields to become unpredictable.
198 This can be the case when links are determined dynamically dependent
199 on a range of contextual factors. It is possible to statically
200 configure a web server to correctly handle large HTTP response fields
201 by specifying an upper bound for their size. But when the number of
202 links is unpredictable, estimating a reliable upper bound is
203 challenging.
205 Section 15 of HTTP [I-D.ietf-httpbis-semantics] defines error codes
206 related to excess communication by the user agent ("413 Request
207 Entity Too Large" and "414 Request-URI Too Long"), but no specific
208 error codes are defined to indicate that response field content
209 exceeds the upper bound that can be handled by the server, and thus
210 it has been truncated. As a result, applications take counter
211 measures aimed at controlling the size of the HTTP "Link" header
212 field, for example by limiting the links they provide to those with
213 select relation types, thereby limiting the value of the HTTP "Link"
214 header field to clients. Providing links by means of a standalone
215 document overcomes challenges related to the unpredictable nature of
216 the size of HTTP "Link" header fields.
218 4. Document Formats for Sets of Links
220 This section specifies two document formats to convey a set of links.
221 Both are based on the abstract model specified in Section 2 of Web
222 Linking [RFC8288] that defines a link as consisting of a "link
223 context", a "link relation type", a "link target", and optional
224 "target attributes":
226 * The format defined in Section 4.1 is identical to the payload of
227 the HTTP "Link" header field as specified in Web Linking Section 3
228 of [RFC8288].
230 * The format defined in Section 4.2 is based on JSON [RFC8259].
232 Note that Section 3.3 of [RFC8288] deprecates the "rev" construct
233 that was provided by [RFC5988] as a means to express links with a
234 directionality that is the inverse of direct links that use the "rel"
235 construct. In both serializations for link sets defined here,
236 inverse links SHOULD be represented as direct links using the "rel"
237 construct and by switching the position of the resources involved in
238 the link.
240 4.1. HTTP Link Document Format: application/linkset
242 This document format is identical to the payload of the HTTP "Link"
243 header field as defined in Section 3 of [RFC8288], more specifically
244 by its ABNF production rule for "Link" and subsequent ones. The use
245 of non-ASCII characters in the payload of the HTTP "Link" Header
246 field is not interoperable.
248 The assigned media type for this format is "application/linkset".
250 In order to support use cases where "application/linkset" documents
251 are re-used outside the context of an HTTP interaction, it is
252 RECOMMENDED to make them self-contained by adhering to the following
253 guidelines:
255 * For every link provided in the set of links, explicitly provide
256 the link context using the "anchor" attribute.
258 * For link context ("anchor" attribute) and link target ("href"
259 attribute), use URI References that are not relative references
260 (as defined in Section 4.1 of [RFC3986]).
262 If these recommendations are not followed, interpretation of links in
263 "application/linkset" documents will depend on which URI is used as
264 context.
266 It should be noted that the "application/linkset" format specified
267 here is different than the "application/link-format" format specified
268 in [RFC6690] in that the former fully matches the payload of the HTTP
269 "Link" header field as defined in Section 3 of [RFC8288], whereas the
270 latter introduces constraints on that definition to meet requirements
271 for Constrained RESTful Environments.
273 4.2. JSON Document Format: application/linkset+json
275 This document format uses JSON [RFC8259] as the syntax to represent a
276 set of links. The set of links follows the abstract model defined by
277 Web Linking Section 2 of [RFC8288].
279 The assigned media type for this format is "application/
280 linkset+json".
282 In order to support use cases where "application/linkset+json"
283 documents are re-used outside the context of an HTTP interaction, it
284 is RECOMMENDED to make them self-contained by adhering to the
285 following guidelines:
287 * For every link provided in the set of links, explicitly provide
288 the link context using the "anchor" member.
290 * For link context ("anchor" member) and link target ("href"
291 member), use URI References that are not relative references (as
292 defined in Section 4.1 of [RFC3986]).
294 If these recommendations are not followed, interpretation of
295 "application/linkset+json" will depend on which URI is used as
296 context URI.
298 The "application/linkset+json" serialization is designed such that it
299 can directly be used as the content of a JSON-LD serialization by
300 adding an appropriate context. Appendix A shows an example of a
301 possible context that, when added to a JSON serialization, allows it
302 to be interpreted as RDF.
304 4.2.1. Set of Links
306 In the JSON representation of a set of links:
308 * A set of links MUST be represented as a JSON object which MUST
309 have "linkset" as its sole member.
311 * The "linkset" member is an array in which a distinct JSON object -
312 the "link context object" (see Section 4.2.2) - MUST be used to
313 represent links that have the same link context.
315 * Even if there is only one link context object, it MUST be wrapped
316 in an array. Members other than link context objects MUST NOT be
317 included in this array.
319 4.2.2. Link Context Object
321 In the JSON representation one or more links that have the same link
322 context are represented by a JSON object, the link context object. A
323 link context object adheres to the following rules:
325 * Each link context object MAY have an "anchor" member with a value
326 that represents the link context. If present, this value MUST be
327 a URI Reference and SHOULD NOT be a relative reference as per
328 Section 4.1 of [RFC3986].
330 * For each distinct relation type that the link context has with
331 link targets, a link context object MUST have an additional
332 member. This member is an array in which a distinct JSON object -
333 the "link target object" (see Section 4.2.3) - MUST be used for
334 each link target for which the relationship with the link context
335 (value of the encompassing anchor member) applies. The name of
336 this member expresses the relation type of the link as follows:
338 - For registered relation types (Section 2.1.1 of [RFC8288]), the
339 name of this member is the registered name of the relation
340 type.
342 - For extension relation types (Section 2.1.2 of [RFC8288]), the
343 name of this member is the URI that uniquely represents the
344 relation type.
346 * Even if there is only one link target object it MUST be wrapped in
347 an array. Members other than link target objects MUST NOT be
348 included in this array.
350 4.2.3. Link Target Object
352 In the JSON representation a link target is represented by a JSON
353 object, the link target object. A link target object adheres to the
354 following rules:
356 * Each link target object MUST have an "href" member with a value
357 that represents the link target. This value MUST be a URI
358 Reference and SHOULD NOT be a relative reference as per
359 Section 4.1 of [RFC3986]. Cases where the href member is present,
360 but no value is provided for it (i.e. the resource providing the
361 set of links is the target of the link in the link target object)
362 MUST be handled by providing an "href" member with an empty string
363 ("href": "").
365 * In many cases, a link target is further qualified by target
366 attributes. Various types of attributes exist and they are
367 conveyed as additional members of the link target object as
368 detailed in Section 4.2.4.
370 The following example of a JSON-serialized set of links represents
371 one link with its core components: link context, link relation type,
372 and link target.
374 {
375 "linkset":
376 [
377 { "anchor": "http://example.net/bar",
378 "next": [
379 {"href": "http://example.com/foo"}
380 ]
381 }
382 ]
383 }
385 Figure 1
387 The following example of a JSON-serialized set of links represents
388 two links that share link context and relation type but have
389 different link targets.
391 {
392 "linkset":
393 [
394 { "anchor": "http://example.net/bar",
395 "item": [
396 {"href": "http://example.com/foo1"},
397 {"href": "http://example.com/foo2"}
398 ]
399 }
400 ]
401 }
403 Figure 2
405 The following example shows a set of links that represents two links,
406 each with a different link context, link target, and relation type.
407 One relation type is registered, the other is an extension relation
408 type.
410 {
411 "linkset":
412 [
413 { "anchor": "http://example.net/bar",
414 "next": [
415 {"href": "http://example.com/foo1"}
416 ]
417 },
418 { "anchor": "http://example.net/boo",
419 "http://example.com/relations/baz" : [
420 {"href": "http://example.com/foo2"}
421 ]
422 }
423 ]
424 }
426 Figure 3
428 4.2.4. Link Target Attributes
430 A link may be further qualified by target attributes. Three types of
431 attributes exist:
433 * Attributes defined in Section 3.4.1 of Web Linking [RFC8288].
435 * Extension attributes defined and used by communities as allowed by
436 Section 3.4.2 of [RFC8288].
438 * Internationalized versions of the "title" attribute defined by
439 [RFC8288] and of extension attributes allowed by Section 3.4 of
440 [RFC8288].
442 The handling of these different types of attributes is described in
443 the sections below.
445 4.2.4.1. Target Attributes Defined by Web Linking
447 Section 3.4.1 of [RFC8288] defines the following target attributes
448 that may be used to annotate links: "hreflang", "media", "title",
449 "title*", and "type"; these target attributes follow different
450 occurrence and value patterns. In the JSON representation, these
451 attributes MUST be conveyed as additional members of the link target
452 object as follows:
454 * "hreflang": The optional and repeatable "hreflang" target
455 attribute MUST be represented by an array (even if there only is
456 one value to be represented), and each value in that array MUST be
457 a string - representing one value of the "hreflang" target
458 attribute for a link - which follows the same model as in the
459 [RFC8288] syntax.
461 * "media": The optional and not repeatable "media" target attribute
462 MUST be represented by a "media" member in the link target object,
463 and its value MUST be a string that follows the same model as in
464 the [RFC8288] syntax.
466 * "type": The optional and not repeatable "type" target attribute
467 MUST be represented by a "type" member in the link target object,
468 and its value MUST be a string that follows the same model as in
469 the [RFC8288] syntax.
471 * "title": The optional and not repeatable "title" target attribute
472 MUST be represented by a "title" member in the link target object,
473 and its value MUST be a string that follows the same model as in
474 the [RFC8288] syntax.
476 * "title*": The optional and not repeatable "title*" target
477 attribute is motivated by character encoding and language issues
478 and follows the model defined in [RFC8187]. The details of the
479 JSON representation that applies to title* are described in
480 Section 4.2.4.2.
482 The following example illustrates how the repeatable "hreflang" and
483 the not repeatable "type" target attributes are represented in a link
484 target object.
486 {
487 "linkset":
488 [
489 { "anchor": "http://example.net/bar",
490 "next": [
491 {"href": "http://example.com/foo",
492 "type": "text/html",
493 "hreflang": [ "en" , "de" ]
494 }
495 ]
496 }
497 ]
498 }
500 Figure 4
502 4.2.4.2. Internationalized Target Attributes
504 In addition to the target attributes described in Section 4.2.4.1,
505 Section 3.4 of [RFC8288] also supports attributes that follow the
506 content model of [RFC8187]. In [RFC8288], these target attributes
507 are recognizable by the use of a trailing asterisk in the attribute
508 name, such as "title*". The content model of [RFC8187] uses a
509 string-based microsyntax that represents the character encoding, an
510 optional language tag, and the escaped attribute value encoded
511 according to the specified character encoding.
513 The JSON serialization for these target attributes MUST be as
514 follows:
516 * An internationalized target attribute is represented as a member
517 of the link context object with the same name (including the *) of
518 the attribute.
520 * The character encoding information as prescribed by [RFC8187] is
521 not preserved; instead, the content of the internationalized
522 attribute is represented in the character encoding used for the
523 JSON set of links.
525 * The value of the internationalized target attribute is an array
526 that contains one or more JSON objects. The name of one member of
527 such JSON object is "value" and its value is the actual content
528 (in its unescaped version) of the internationalized target
529 attribute, i.e. the value of the attribute from which the encoding
530 and language information are removed. The name of another,
531 optional, member of such JSON object is "language" and its value
532 is the language tag [RFC5646] for the language in which the
533 attribute content is conveyed.
535 The following example illustrates how the "title*" target attribute
536 defined by Section 3.4.1 of [RFC8288] is represented in a link target
537 object.
539 {
540 "linkset":
541 [
542 { "anchor": "http://example.net/bar",
543 "next": [
544 {"href": "http://example.com/foo",
545 "type": "text/html",
546 "hreflang": [ "en" , "de" ],
547 "title": "Next chapter",
548 "title*": [ { "value": "nächstes Kapitel" ,
549 "language" : "de" } ]
550 }
551 ]
552 }
553 ]
554 }
556 Figure 5
558 The above example assumes that the German title contains an umlaut
559 character (in the native syntax it would be encoded as title*=UTF-
560 8'de'n%c3%a4chstes%20Kapitel), which gets encoded in its unescaped
561 form in the JSON representation. Implementations MUST properly
562 decode/encode internationalized target attributes that follow the
563 model of [RFC8187] when transcoding between the "application/linkset"
564 and the "application/linkset+json" formats.
566 4.2.4.3. Extension Target Attributes
568 Extension target attributes are attributes that are not defined by
569 Section 3.4.1 of [RFC8288] (as listed in Section 4.2.4.1), but are
570 nevertheless used to qualify links. They can be defined by
571 communities in any way deemed necessary, and it is up to them to make
572 sure their usage is understood by target applications. However,
573 lacking standardization, there is no interoperable understanding of
574 these extension attributes. One important consequence is that their
575 cardinality is unknown to generic applications. Therefore, in the
576 JSON serialization, all extension target attributes are treated as
577 repeatable.
579 The JSON serialization for these target attributes MUST be as
580 follows:
582 * An extension target attribute is represented as a member of the
583 link context object with the same name of the attribute, including
584 the * if applicable.
586 * The value of an extension attribute MUST be represented by an
587 array, even if there only is one value to be represented.
589 * If the extension target attribute does not have a name with a
590 trailing asterisk, then each value in that array MUST be a string
591 that represents one value of the attribute.
593 * If the extension attribute has a name with a trailing asterisk (it
594 follows the content model of [RFC8187]), then each value in that
595 array MUST be a JSON object. The value of each such JSON object
596 MUST be structured as described in Section 4.2.4.2.
598 The example shows a link target object with three extension target
599 attributes. The value for each extension target attribute is an
600 array. The two first are regular extension target attributes, with
601 the first one ("foo") having only one value and the second one
602 ("bar") having two. The last extension target attribute ("baz*")
603 follows the naming rule of [RFC8187] and therefore is encoded
604 according to the serialization described in Section 4.2.4.2.
606 {
607 "linkset":
608 [
609 { "anchor": "http://example.net/bar",
610 "next": [
611 { "href": "http://example.com/foo",
612 "type": "text/html",
613 "foo": [ "foovalue" ],
614 "bar": [ "barone", "bartwo" ],
615 "baz*": [ { "value": "bazvalue" ,
616 "language" : "en" } ]
617 }
618 ]
619 }
620 ]
621 }
623 Figure 6
625 4.2.5. JSON Extensibility
627 The extensibility of the JSON document format for representing a set
628 of links is restricted to the extensibility provided by [RFC8288].
629 The Web linking model provides for the use of extension target
630 attributes as discussed in Section 4.2.4.3. Extensions based on the
631 JSON syntax MUST NOT be used, and MUST be ignored when found in a
632 JSON linkset document.
634 This limitation of the JSON format allows to unambiguously round trip
635 between links provided in the HTTP "Link" header field, sets of links
636 serialized according to the "application/linkset" format, and sets of
637 links serialized according to the "application/linkset+json" format.
639 5. The "profile" attribute for media types to Represent Sets of Links
641 As a means to convey specific constraints or conventions (as per
642 [RFC6906]) that apply to a link set document, the "profile" attribute
643 MAY be used in conjunction with the media types "application/linkset"
644 and "application/linkset+json" detailed in Section 4.1 and
645 Section 4.2, respectively. For example, the attribute could be used
646 to indicate that a link set uses a specific, limited set of link
647 relation types.
649 The value of the "profile" attribute MUST be a non-empty list of
650 space-separated URIs, each of which identifies specific constraints
651 or conventions that apply to the link set document. Profile URIs MAY
652 be registered in the IANA Profile URI Registry in the manner
653 specified by [RFC7284].
655 The presence of a "profile" attribute in conjunction with the
656 "application/linkset" and "application/linkset+json" media types does
657 not change the semantics of a link set. As such, clients with and
658 without knowledge of profile URIs can use the same representation.
660 6. The "linkset" Relation Type for Linking to a Set of Links
662 The target of a link with the "linkset" relation type provides a set
663 of links, including links in which the resource that is the link
664 context participates.
666 A link with the "linkset" relation type MAY be provided in the header
667 field and/or the body of a resource's representation. It may also be
668 discovered by other means, such as through client-side information.
670 A resource MAY provide more than one link with a "linkset" relation
671 type. Multiple such links can refer to the same set of links
672 expressed using different media types, or to different sets of links,
673 potentially provided by different third-party services.
675 A user agent that follows a "linkset" link MUST be aware that the set
676 of links provided by the resource that is the target of the link can
677 contain links in which the resource that is the context of the link
678 does not participate; it MAY decide to ignore those links.
680 A user agent that follows a "linkset" link and obtains links for
681 which anchors and targets are expressed as relative references (as
682 per Section 4.1 of [RFC3986]) MUST determine what the context is for
683 these links; it SHOULD ignore links for which it is unable to
684 unambiguously make that determination.
686 7. Examples
688 Section 7.1 and Section 7.2 show examples whereby a set of links is
689 provided as "application/linkset" and "application/linkset+json"
690 documents, respectively. Section 7.3 illustrates the use of the
691 "linkset" link relation type to support discovery of sets of links.
693 7.1. Set of Links Provided as application/linkset
695 Figure 7 shows a client issuing an HTTP GET request against resource
696 .
698 GET /links/resource1 HTTP/1.1
699 Host: example.org
701 Figure 7: Client HTTP GET request
703 Figure 8 shows the response to the GET request of Figure 7. The
704 response contains a Content-Type header field specifying that the
705 media type of the response is "application/linkset". A set of links,
706 revealing authorship and versioning related to resource
707 , is provided in the response body.
708 The HTTP "Link" header field indicates the availability of an
709 alternate representation of the set of links using media type
710 "application/linkset+json".
712 HTTP/1.1 200 OK
713 Date: Mon, 12 Aug 2019 10:35:51 GMT
714 Server: Apache-Coyote/1.1
715 Content-Length: 1023
716 Content-Type: application/linkset
717 Link:
718 ; rel="alternate"
719 ; type="application/linkset+json"
721
722 ; rel="author"
723 ; type="application/rdf+xml"
724 ; anchor="https://example.org/resource1",
725
726 ; rel="latest-version"
727 ; type="text/html"
728 ; anchor="https://example.org/resource1",
729
730 ; rel="predecessor-version"
731 ; type="text/html"
732 ; anchor="https://example.org/resource1?version=3",
733
734 ; rel="predecessor-version"
735 ; type="text/html"
736 ; anchor="https://example.org/resource1?version=2",
737
738 ; rel="memento"
739 ; type="text/html"
740 ; datetime="Thu, 13 Jun 2019 09:34:33 GMT"
741 ; anchor="https://example.org/resource1",
742
743 ; rel="memento"
744 ; type="text/html"
745 ; datetime="Sun, 21 Jul 2019 12:22:04 GMT"
746 ; anchor="https://example.org/resource1",
747
748 ; rel="author"
749 ; anchor="https://example.org/resource1#comment=1"
751 Figure 8: Response to HTTP GET includes a set of links
753 7.2. Set of Links Provided as application/linkset+json
755 Figure 9 shows the client issuing an HTTP GET request against
756 . In the request, the client
757 uses an "Accept" header field to indicate it prefers a response in
758 the "application/linkset+json" format.
760 GET links/resource1 HTTP/1.1
761 Host: example.org
762 Accept: application/linkset+json
764 Figure 9: Client HTTP GET request expressing preference for
765 "application/ linkset+json" response
767 Figure 10 shows the response to the HTTP GET request of Figure 9.
768 The set of links is serialized according to the media type
769 "application/linkset+json".
771 HTTP/1.1 200 OK
772 Date: Mon, 12 Aug 2019 10:46:22 GMT
773 Server: Apache-Coyote/1.1
774 Content-Type: application/linkset+json
775 Link:
776 ; rel="alternate"
777 ; type="application/linkset"
778 Content-Length: 1349
780 {
781 "linkset": [
782 {
783 "anchor": "https://example.org/resource1",
784 "author": [
785 {
786 "href": "https://authors.example.net/johndoe",
787 "type": "application/rdf+xml"
788 }
789 ],
790 "memento": [
791 {
792 "href": "https://example.org/resource1?version=1",
793 "type": "text/html",
794 "datetime": "Thu, 13 Jun 2019 09:34:33 GMT"
795 },
796 {
797 "href": "https://example.org/resource1?version=2",
798 "type": "text/html",
799 "datetime": "Sun, 21 Jul 2019 12:22:04 GMT"
800 }
801 ],
802 "latest-version": [
803 {
804 "href": "https://example.org/resource1?version=3",
805 "type": "text/html"
806 }
807 ]
809 },
810 {
811 "anchor": "https://example.org/resource1?version=3",
812 "predecessor-version": [
813 {
814 "href": "https://example.org/resource1?version=2",
815 "type": "text/html"
816 }
817 ]
818 },
819 {
820 "anchor": "https://example.org/resource1?version=2",
821 "predecessor-version": [
822 {
823 "href": "https://example.org/resource1?version=1",
824 "type": "text/html"
825 }
826 ]
827 },
828 {
829 "anchor": "https://example.org/resource1#comment=1",
830 "author": [
831 {
832 "href": "https://authors.example.net/alice"
833 }
834 ]
835 }
836 ]
837 }
839 Figure 10: Response to the client's request for the set of links
841 7.3. Discovering a Link Set via the "linkset" Link Relation Type
843 Figure 11 shows a client issuing an HTTP HEAD request against
844 resource .
846 HEAD resource1 HTTP/1.1
847 Host: example.org
849 Figure 11: Client HTTP HEAD request
851 Figure 12 shows the response to the HEAD request of Figure 11. The
852 response contains an HTTP "Link" header field with a link that has
853 the "linkset" relation type. It indicates that a set of links is
854 provided by resource , which
855 provides a representation with media type "application/linkset+json".
857 HTTP/1.1 200 OK
858 Date: Mon, 12 Aug 2019 10:45:54 GMT
859 Server: Apache-Coyote/1.1
860 Link:
861 ; rel="linkset"
862 ; type="application/linkset+json"
863 Content-Length: 236
864 Content-Type: text/html;charset=utf-8
866 Figure 12: Response to HTTP HEAD request
868 Section 7.2 shows a client obtaining a set of links by issuing an
869 HTTP GET on the target of the link with the "linkset" relation type,
870 .
872 8. IANA Considerations
874 8.1. Link Relation Type: linkset
876 The link relation type below should be registered by IANA per
877 Section 6.2.1 of Web Linking [RFC8288]:
879 Relation Name: linkset
881 Description: The link target of a link with the "linkset" relation
882 type provides a set of links, including links in which the link
883 context of the link participates.
885 Reference: [[ This document ]]
887 8.2. Media Type: application/linkset
889 The Internet media type [RFC6838] for a natively encoded linkset is
890 application/linkset.
892 Type name: application
894 Subtype name: linkset
896 Required parameters: none
898 Optional parameters: profile
899 Encoding considerations: Linksets are encoded according to the
900 definition of [RFC8288]. The encoding of [RFC8288] is based on
901 the general encoding rules of [I-D.ietf-httpbis-semantics], with
902 the addition of allowing indicating character encoding and
903 language for specific parameters as defined by [RFC8187].
905 Security considerations: The security considerations of [[ This
906 document ]] apply.
908 Interoperability considerations: N/A
910 Published specification: [[ This document ]]
912 Applications that use this media type: This media type is not
913 specific to any application, as it can be used by any application
914 that wants to interchange web links.
916 Additional information:
918 Magic number(s): N/A
920 File extension(s): This media type does not propose a specific
921 extension.
923 Macintosh file type code(s): TEXT
925 Person & email address to contact for further information: Erik
926 Wilde
928 Intended usage: COMMON
930 Restrictions on usage: none
932 Author: Erik Wilde
934 Change controller: IETF
936 8.3. Media Type: application/linkset+json
938 The Internet media type [RFC6838] for a JSON-encoded linkset is
939 application/linkset+json.
941 Type name: application
943 Subtype name: linkset+json
945 Required parameters: none
946 Optional parameters: profile
948 Encoding considerations: The encoding considerations of [RFC8259]
949 apply
951 Security considerations: The security considerations of [[ This
952 document ]] apply.
954 Interoperability considerations: The interoperability
955 considerations of [RFC8259] apply.
957 Published specification: [[ This document ]]
959 Applications that use this media type: This media type is not
960 specific to any application, as it can be used by any application
961 that wants to interchange web links.
963 Additional information:
965 Magic number(s): N/A
967 File extension(s): JSON documents often use ".json" as the file
968 extension, and this media type does not propose a specific
969 extension other than this generic one.
971 Macintosh file type code(s): TEXT
973 Person & email address to contact for further information: Erik
974 Wilde
976 Intended usage: COMMON
978 Restrictions on usage: none
980 Author: Erik Wilde
982 Change controller: IETF
984 9. Security Considerations
986 The security considerations of Web Linking [RFC8288] apply, as long
987 as they are not specifically discussing the risks of exposing
988 information in HTTP header fields.
990 In general, links may cause information leakage when they expose
991 information (such as URIs) that can be sensitive or private. Links
992 may expose "hidden URIs" that are not supposed to be openly shared,
993 and may not be sufficiently protected. Ideally, none of the URIs
994 exposed in links should be supposed to be "hidden"; instead, if these
995 URIs are supposed to be limited to certain users, then technical
996 measures should be put in place so that accidentally exposing them
997 does not cause any harm.
999 For the specific mechanisms defined in this specification, two
1000 security considerations should be taken into account:
1002 * The Web Linking model always has an "implicit context", which is
1003 the resource of the HTTP interaction. This original context can
1004 be lost or can change when self-contained link representations are
1005 moved. Changing the context can change the interpretation of
1006 links when they have no explicit anchor, or when they use relative
1007 URIs. Applications may choose to ignore links that have no
1008 explicit anchor or that use relative URIs when these are exchanged
1009 in stand-alone resources.
1011 * The model introduced in this specification supports "3rd party
1012 links", where one party can provide links that have another
1013 party's resource as an anchor. Depending on the link semantics
1014 and the application context, it is important to verify that there
1015 is sufficient trust in that 3rd party to allow it to provide these
1016 links. Applications may choose to treat 3rd party links
1017 differently than cases where a resource and the links for that
1018 resource are provided by the same party.
1020 10. Normative References
1022 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1023 Requirement Levels", BCP 14, RFC 2119,
1024 DOI 10.17487/RFC2119, March 1997,
1025 .
1027 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
1028 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
1029 May 2017, .
1031 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
1032 Interchange Format", STD 90, RFC 8259,
1033 DOI 10.17487/RFC8259, December 2017,
1034 .
1036 [RFC8288] Nottingham, M., "Web Linking", RFC 8288,
1037 DOI 10.17487/RFC8288, October 2017,
1038 .
1040 [RFC8187] Reschke, J., "Indicating Character Encoding and Language
1041 for HTTP Header Field Parameters", RFC 8187,
1042 DOI 10.17487/RFC8187, September 2017,
1043 .
1045 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
1046 Resource Identifier (URI): Generic Syntax", STD 66,
1047 RFC 3986, DOI 10.17487/RFC3986, January 2005,
1048 .
1050 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
1051 Specifications and Registration Procedures", BCP 13,
1052 RFC 6838, DOI 10.17487/RFC6838, January 2013,
1053 .
1055 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
1056 Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
1057 September 2009, .
1059 [RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
1060 Code: The Implementation Status Section", RFC 6982,
1061 DOI 10.17487/RFC6982, July 2013,
1062 .
1064 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
1065 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
1066 .
1068 [RFC6906] Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
1069 DOI 10.17487/RFC6906, March 2013,
1070 .
1072 [RFC7284] Lanthaler, M., "The Profile URI Registry", RFC 7284,
1073 DOI 10.17487/RFC7284, June 2014,
1074 .
1076 [I-D.ietf-httpbis-semantics]
1077 Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
1078 Semantics", Work in Progress, Internet-Draft, draft-ietf-
1079 httpbis-semantics-15, 30 March 2021,
1080 .
1083 11. Informative References
1085 [W3C.REC-json-ld-20140116]
1086 Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0",
1087 World Wide Web Consortium Recommendation REC-json-ld-
1088 20140116, 16 January 2014,
1089 .
1091 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
1092 Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
1093 December 2005, .
1095 [RFC5988] Nottingham, M., "Web Linking", RFC 5988,
1096 DOI 10.17487/RFC5988, October 2010,
1097 .
1099 Appendix A. JSON-LD Context
1101 A set of links rendered according to the JSON serialization defined
1102 in Section 4.2 can be interpreted as RDF triples by adding a JSON-LD
1103 context [W3C.REC-json-ld-20140116] that maps the JSON keys to
1104 corresponding Linked Data terms. And, as per
1105 [W3C.REC-json-ld-20140116] section 6.8 (https://www.w3.org/TR/2014/
1106 REC-json-ld-20140116/#interpreting-json-as-json-ld), when delivering
1107 a link set that is rendered according to the "application/
1108 linkset+json" media type to a user agent, a server can convey the
1109 availability of such a JSON-LD context by using a link with the
1110 relation type "http://www.w3.org/ns/json-ld#context" in the HTTP
1111 "Link" header.
1113 Using the latter approach to support discovery of a JSON-LD Context,
1114 the response to the GET request of Figure 9 against the URI of a set
1115 of links would be as shown in Figure 13.
1117 HTTP/1.1 200 OK
1118 Date: Mon, 12 Aug 2019 10:48:22 GMT
1119 Server: Apache-Coyote/1.1
1120 Content-Type: application/linkset+json
1121 Link:
1122 ; rel="http://www.w3.org/ns/json-ld#context"
1123 ; type="application/ld+json"
1124 Content-Length: 1349
1126 {
1127 "linkset": [
1128 {
1129 "anchor": "https://example.org/resource1",
1130 "author": [
1131 {
1132 "href": "https://authors.example.net/johndoe",
1133 "type": "application/rdf+xml"
1134 }
1135 ],
1136 "memento": [
1137 {
1138 "href": "https://example.org/resource1?version=1",
1139 "type": "text/html",
1140 "datetime": "Thu, 13 Jun 2019 09:34:33 GMT"
1141 },
1142 {
1143 "href": "https://example.org/resource1?version=2",
1144 "type": "text/html",
1145 "datetime": "Sun, 21 Jul 2019 12:22:04 GMT"
1146 }
1147 ],
1148 "latest-version": [
1149 {
1150 "href": "https://example.org/resource1?version=3",
1151 "type": "text/html"
1152 }
1153 ]
1154 },
1155 {
1156 "anchor": "https://example.org/resource1?version=3",
1157 "predecessor-version": [
1158 {
1159 "href": "https://example.org/resource1?version=2",
1160 "type": "text/html"
1161 }
1162 ]
1163 },
1164 {
1165 "anchor": "https://example.org/resource1?version=2",
1166 "predecessor-version": [
1167 {
1168 "href": "https://example.org/resource1?version=1",
1169 "type": "text/html"
1170 }
1171 ]
1172 },
1173 {
1174 "anchor": "https://example.org/resource1#comment=1",
1175 "author": [
1176 {
1177 "href": "https://authors.example.net/alice"
1178 }
1179 ]
1180 }
1182 ]
1183 }
1185 Figure 13: Using a typed link to support discovery of a JSON-LD
1186 Context for a Set of Links
1188 In order to obtain the JSON-LD Context conveyed by the server, the
1189 user agent issues an HTTP GET against the link target of the link
1190 with the "http://www.w3.org/ns/json-ld#context" relation type. The
1191 response to this GET is shown in Figure 14. This particular JSON-LD
1192 context maps "application/linkset+json" representations of link sets
1193 to Dublin Core Terms. It also renders each link relation as a URI
1194 Reference, inspired by the same approach used for Atom [RFC4287]
1195 described in Appendix A.2 of [RFC8288].
1197 HTTP/1.1 200 OK
1198 Content-Type: application/ld+json
1199 Content-Length: 708
1201 {
1202 "@context": [
1203 {
1204 "@vocab": "http://www.iana.org/assignments/relation/",
1205 "anchor": "@id",
1206 "href": "@id",
1207 "linkset": "@graph",
1208 "_linkset": "@graph",
1209 "title": {
1210 "@id": "http://purl.org/dc/terms/title"
1211 },
1212 "title*": {
1213 "@id": "http://purl.org/dc/terms/title"
1214 },
1215 "type": {
1216 "@id": "http://purl.org/dc/terms/format"
1217 },
1218 "datetime": {
1219 "@id": "http://purl.org/dc/terms/date"
1220 }
1221 },
1222 {
1223 "language": "@language",
1224 "value": "@value",
1225 "hreflang": {
1226 "@id": "http://purl.org/dc/terms/language",
1227 "@container": "@set"
1228 }
1229 }
1230 ]
1231 }
1233 Figure 14: JSON-LD Context mapping to Dublin Core Terms and IANA
1234 assignments
1236 Applying the JSON-LD context of Figure 14 to the link set of
1237 Figure 13 allows transforming the "application/linkset+json" link set
1238 to an RDF link set. Figure 15 shows the latter represented by means
1239 of the "text/turtle" RDF serialization.
1241
1242
1243 "application/rdf+xml" .
1244
1245
1246 .
1247
1248
1249 .
1250
1251
1252 .
1253
1254
1255 .
1256
1257
1258 .
1259
1260
1261 "Thu, 13 Jun 2019 09:34:33 GMT" .
1262
1263
1264 "text/html" .
1265
1266
1267 "Sun, 21 Jul 2019 12:22:04 GMT" .
1268
1269
1270 "text/html" .
1271
1272
1273 .
1274
1275
1276 "text/html" .
1277
1278
1279 .
1281 Figure 15: RDF serialization of the link set resulting from
1282 applying the JSON-LD context
1284 Note that the JSON-LD context of Figure 14 does not handle (meta)link
1285 relations of type ""linkset"" as they are in conflict with the top-
1286 level JSON key. A workaround is to rename the top-level key to
1287 ""_linkset"" in the "application/linkset+json" before transforming a
1288 link set to JSON-LD.
1290 Appendix B. Implementation Status
1292 This section is to be removed before publishing as an RFC.
1294 This section records the status of known implementations of the
1295 protocol defined by this specification at the time of posting of this
1296 Internet-Draft, and is based on a proposal described in RFC 6982
1297 [RFC6982]. The description of implementations in this section is
1298 intended to assist the IETF in its decision processes in progressing
1299 drafts to RFCs. Please note that the listing of any individual
1300 implementation here does not imply endorsement by the IETF.
1301 Furthermore, no effort has been spent to verify the information
1302 presented here that was supplied by IETF contributors. This is not
1303 intended as, and must not be construed to be, a catalog of available
1304 implementations or their features. Readers are advised to note that
1305 other implementations may exist.
1307 According to RFC 6982, "this will allow reviewers and working groups
1308 to assign due consideration to documents that have the benefit of
1309 running code, which may serve as evidence of valuable experimentation
1310 and feedback that have made the implemented protocols more mature.
1311 It is up to the individual working groups to use this information as
1312 they see fit".
1314 B.1. GS1
1316 GS1 is a provider of barcodes (GS1 GTINs and EAN/UPC) for retail
1317 products and manages an ecology of services and standards to leverage
1318 them at a global scale. GS1 has indicated that it will implement
1319 this "linkset" specification as a means to allow requesting and
1320 representing links pertaining to products from various retailers.
1321 Currently, the GS1 Digital Link specification makes an informative
1322 reference to version 03 of the "linkset" I-D. GS1 expresses
1323 confidence that this will become a normative reference in the next
1324 iteration of that specification, likely to be ratified as a GS1
1325 standard around February 2021.
1327 B.2. FAIR Signposting Profile
1329 The FAIR Signposting Profile is a community specification aimed at
1330 improving machine navigation of scholarly objects on the web through
1331 the use of typed web links pointing at e.g. web resources that are
1332 part of a specific object, persistent identifiers for the object and
1333 its authors, license information pertaining to the object. The
1334 specification encourages the use of Linksets and initial
1335 implementations are ongoing, for example, for the open source
1336 Dataverse data repository platform that was initiated by Harvard
1337 University and is meanwhile used by research institutions, worldwide.
1339 B.3. Open Journal Systems (OJS)
1341 Open Journal Systems (OJS) is an open-source software for the
1342 management of peer-reviewed academic journals, and is created by the
1343 Public Knowledge Project (PKP), released under the GNU General Public
1344 License. Open Journal Systems (OJS) is a journal management and
1345 publishing system that has been developed by PKP through its
1346 federally funded efforts to expand and improve access to research.
1348 The OJS platform has implemented "linkset" support as an alternative
1349 way to provide links when there are more than a configured limit
1350 (they consider using about 10 as a good default, for testing purpose
1351 it is currently set to 8).
1353 Acknowledgements
1355 Thanks for comments and suggestions provided by Phil Archer,
1356 Dominique Guinard, Mark Nottingham, Julian Reschke, Stian Soiland-
1357 Reyes, and Sarven Capadisli.
1359 Authors' Addresses
1361 Erik Wilde
1362 Axway
1364 Email: erik.wilde@dret.net
1365 URI: http://dret.net/netdret/
1367 Herbert Van de Sompel
1368 Data Archiving and Networked Services
1370 Email: herbert.van.de.sompel@dans.knaw.nl
1371 URI: https://orcid.org/0000-0002-0715-6126