Linkset: Media Types and a Link Relation Type for Link Sets
draft-ietf-httpapi-linkset-03
Network Working Group E. Wilde
Internet-Draft Axway
Intended status: Informational H. Van de Sompel
Expires: 5 January 2022 Data Archiving and Networked Services
4 July 2021
Linkset: Media Types and a Link Relation Type for Link Sets
draft-ietf-httpapi-linkset-03
Abstract
This specification defines two document formats and respective media
types for representing sets of links as stand-alone resources. One
format is JSON-based, the other aligned with the format for
representing links in the HTTP "Link" header field. This
specification also introduces a link relation type to support
discovery of sets of links.
Note to Readers
Please discuss this draft on the "Building Blocks for HTTP APIs"
mailing list (https://www.ietf.org/mailman/listinfo/httpapi).
Online access to all versions and files is available on GitHub
(https://github.com/ietf-wg-httpapi/linkset).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 5 January 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Third-Party Links . . . . . . . . . . . . . . . . . . . . 4
3.2. Challenges Writing to HTTP Link Header Field . . . . . . 4
3.3. Large Number of Links . . . . . . . . . . . . . . . . . . 5
4. Document Formats for Sets of Links . . . . . . . . . . . . . 5
4.1. HTTP Link Document Format: application/linkset . . . . . 6
4.2. JSON Document Format: application/linkset+json . . . . . 6
4.2.1. Set of Links . . . . . . . . . . . . . . . . . . . . 7
4.2.2. Link Context Object . . . . . . . . . . . . . . . . . 7
4.2.3. Link Target Object . . . . . . . . . . . . . . . . . 8
4.2.4. Link Target Attributes . . . . . . . . . . . . . . . 10
4.2.5. JSON Extensibility . . . . . . . . . . . . . . . . . 14
5. The "profile" attribute for media types to Represent Sets of
Links . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6. The "linkset" Relation Type for Linking to a Set of Links . . 15
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Set of Links Provided as application/linkset . . . . . . 16
7.2. Set of Links Provided as application/linkset+json . . . . 17
7.3. Discovering a Link Set via the "linkset" Link Relation
Type . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8.1. Link Relation Type: linkset . . . . . . . . . . . . . . . 20
8.2. Media Type: application/linkset . . . . . . . . . . . . . 20
8.3. Media Type: application/linkset+json . . . . . . . . . . 21
9. Security Considerations . . . . . . . . . . . . . . . . . . . 22
10. Normative References . . . . . . . . . . . . . . . . . . . . 23
11. Informative References . . . . . . . . . . . . . . . . . . . 24
Appendix A. JSON-LD Context . . . . . . . . . . . . . . . . . . 25
Appendix B. Implementation Status . . . . . . . . . . . . . . . 30
B.1. GS1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
B.2. FAIR Signposting Profile . . . . . . . . . . . . . . . . 31
B.3. Open Journal Systems (OJS) . . . . . . . . . . . . . . . 31
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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1. Introduction
Resources on the Web often use typed Web Links [RFC8288], either
embedded in resource representations, for example using the <link>
element for HTML documents, or conveyed in the HTTP "Link" header
field for documents of any media type. In some cases, however,
providing links in this manner is impractical or impossible and
delivering a set of links as a stand-alone document is preferable.
Therefore, this specification defines two document formats and
associated media types to represent sets of links. It also defines
the "linkset" relation type that supports discovery of any resource
that conveys a set of links as a stand-alone document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This specification uses the terms "link context" and "link target" as
defined in [RFC8288].
In the examples provided in this document, links in the HTTP "Link"
header field are shown on separate lines in order to improve
readability. Note, however, that as per Section 5.5 of
[I-D.ietf-httpbis-semantics], line breaks are deprecated in values
for HTTP fields; only whitespaces and tabs are supported as
separators.
3. Scenarios
The following sections outline scenarios in which providing links by
means of a standalone document instead of in an HTTP "Link" header
field or as links embedded in the resource representation is
advantageous or necessary.
For all scenarios, links could be provided by means of a stand-alone
document that is formatted according to the JSON-based serialization,
the serialization aligned with the HTTP "Link" field format, or both.
The former serialization is motivated by the widespread use of JSON
and related tools, which suggests that handling sets of links
expressed as JSON documents should be attractive to developers. The
latter serialization is provided for compatibility with the existing
serialization used in the HTTP "Link" field and to allow reuse of
tools created to handle it.
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It is important to keep in mind that when providing links by means of
a standalone representation, other links can still be provided using
other approaches, i.e. it is possible combine various mechanisms to
convey links.
3.1. Third-Party Links
In some cases it is useful that links pertaining to a resource are
provided by a server other than the one that hosts the resource. For
example, this allows:
* Providing links in which the resource is involved not just as link
context but also as link target.
* Providing links pertaining to the resource that the server hosting
that resource is not aware of.
* External management of links pertaining to the resource in a
special-purpose link management service.
In such cases, links pertaining to a resource can be provided by
another, specific resource. That specific resource may be managed by
the same or by another custodian as the resource to which the links
pertain. For clients intent on consuming links provided in that
manner, it would be beneficial if the following conditions were met:
* Links are provided in a document that uses a well-defined media
type.
* The resource to which the provided links pertain is able to link
to the resource that provides these links using a well-known link
relation type.
These requirements are addressed in this specification through the
definition of two media types and a link relation type, respectively.
3.2. Challenges Writing to HTTP Link Header Field
In some cases, it is not straightforward to write links to the HTTP
"Link" header field from an application. This can, for example, be
the case because not all required link information is available to
the application or because the application does not have the
capability to directly write HTTP fields. In such cases, providing
links by means of a standalone document can be a solution. Making
the resource that provides these links discoverable can be achieved
by means of a typed link.
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3.3. Large Number of Links
When conveying links in an HTTP "Link" header field, it is possible
for the size of the HTTP response fields to become unpredictable.
This can be the case when links are determined dynamically dependent
on a range of contextual factors. It is possible to statically
configure a web server to correctly handle large HTTP response fields
by specifying an upper bound for their size. But when the number of
links is unpredictable, estimating a reliable upper bound is
challenging.
Section 15 of HTTP [I-D.ietf-httpbis-semantics] defines error codes
related to excess communication by the user agent ("413 Request
Entity Too Large" and "414 Request-URI Too Long"), but no specific
error codes are defined to indicate that response field content
exceeds the upper bound that can be handled by the server, and thus
it has been truncated. As a result, applications take counter
measures aimed at controlling the size of the HTTP "Link" header
field, for example by limiting the links they provide to those with
select relation types, thereby limiting the value of the HTTP "Link"
header field to clients. Providing links by means of a standalone
document overcomes challenges related to the unpredictable nature of
the size of HTTP "Link" header fields.
4. Document Formats for Sets of Links
This section specifies two document formats to convey a set of links.
Both are based on the abstract model specified in Section 2 of Web
Linking [RFC8288] that defines a link as consisting of a "link
context", a "link relation type", a "link target", and optional
"target attributes":
* The format defined in Section 4.1 is identical to the payload of
the HTTP "Link" header field as specified in Web Linking Section 3
of [RFC8288].
* The format defined in Section 4.2 is based on JSON [RFC8259].
Note that Section 3.3 of [RFC8288] deprecates the "rev" construct
that was provided by [RFC5988] as a means to express links with a
directionality that is the inverse of direct links that use the "rel"
construct. In both serializations for link sets defined here,
inverse links SHOULD be represented as direct links using the "rel"
construct and by switching the position of the resources involved in
the link.
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4.1. HTTP Link Document Format: application/linkset
This document format is identical to the payload of the HTTP "Link"
header field as defined in Section 3 of [RFC8288], more specifically
by its ABNF production rule for "Link" and subsequent ones. The use
of non-ASCII characters in the payload of the HTTP "Link" Header
field is not interoperable.
The assigned media type for this format is "application/linkset".
In order to support use cases where "application/linkset" documents
are re-used outside the context of an HTTP interaction, it is
RECOMMENDED to make them self-contained by adhering to the following
guidelines:
* For every link provided in the set of links, explicitly provide
the link context using the "anchor" attribute.
* For link context ("anchor" attribute) and link target ("href"
attribute), use URI References that are not relative references
(as defined in Section 4.1 of [RFC3986]).
If these recommendations are not followed, interpretation of links in
"application/linkset" documents will depend on which URI is used as
context.
It should be noted that the "application/linkset" format specified
here is different than the "application/link-format" format specified
in [RFC6690] in that the former fully matches the payload of the HTTP
"Link" header field as defined in Section 3 of [RFC8288], whereas the
latter introduces constraints on that definition to meet requirements
for Constrained RESTful Environments.
4.2. JSON Document Format: application/linkset+json
This document format uses JSON [RFC8259] as the syntax to represent a
set of links. The set of links follows the abstract model defined by
Web Linking Section 2 of [RFC8288].
The assigned media type for this format is "application/
linkset+json".
In order to support use cases where "application/linkset+json"
documents are re-used outside the context of an HTTP interaction, it
is RECOMMENDED to make them self-contained by adhering to the
following guidelines:
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* For every link provided in the set of links, explicitly provide
the link context using the "anchor" member.
* For link context ("anchor" member) and link target ("href"
member), use URI References that are not relative references (as
defined in Section 4.1 of [RFC3986]).
If these recommendations are not followed, interpretation of
"application/linkset+json" will depend on which URI is used as
context URI.
The "application/linkset+json" serialization is designed such that it
can directly be used as the content of a JSON-LD serialization by
adding an appropriate context. Appendix A shows an example of a
possible context that, when added to a JSON serialization, allows it
to be interpreted as RDF.
4.2.1. Set of Links
In the JSON representation of a set of links:
* A set of links MUST be represented as a JSON object which MUST
have "linkset" as its sole member.
* The "linkset" member is an array in which a distinct JSON object -
the "link context object" (see Section 4.2.2) - MUST be used to
represent links that have the same link context.
* Even if there is only one link context object, it MUST be wrapped
in an array. Members other than link context objects MUST NOT be
included in this array.
4.2.2. Link Context Object
In the JSON representation one or more links that have the same link
context are represented by a JSON object, the link context object. A
link context object adheres to the following rules:
* Each link context object MAY have an "anchor" member with a value
that represents the link context. If present, this value MUST be
a URI Reference and SHOULD NOT be a relative reference as per
Section 4.1 of [RFC3986].
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* For each distinct relation type that the link context has with
link targets, a link context object MUST have an additional
member. This member is an array in which a distinct JSON object -
the "link target object" (see Section 4.2.3) - MUST be used for
each link target for which the relationship with the link context
(value of the encompassing anchor member) applies. The name of
this member expresses the relation type of the link as follows:
- For registered relation types (Section 2.1.1 of [RFC8288]), the
name of this member is the registered name of the relation
type.
- For extension relation types (Section 2.1.2 of [RFC8288]), the
name of this member is the URI that uniquely represents the
relation type.
* Even if there is only one link target object it MUST be wrapped in
an array. Members other than link target objects MUST NOT be
included in this array.
4.2.3. Link Target Object
In the JSON representation a link target is represented by a JSON
object, the link target object. A link target object adheres to the
following rules:
* Each link target object MUST have an "href" member with a value
that represents the link target. This value MUST be a URI
Reference and SHOULD NOT be a relative reference as per
Section 4.1 of [RFC3986]. Cases where the href member is present,
but no value is provided for it (i.e. the resource providing the
set of links is the target of the link in the link target object)
MUST be handled by providing an "href" member with an empty string
("href": "").
* In many cases, a link target is further qualified by target
attributes. Various types of attributes exist and they are
conveyed as additional members of the link target object as
detailed in Section 4.2.4.
The following example of a JSON-serialized set of links represents
one link with its core components: link context, link relation type,
and link target.
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{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo"}
]
}
]
}
Figure 1
The following example of a JSON-serialized set of links represents
two links that share link context and relation type but have
different link targets.
{
"linkset":
[
{ "anchor": "http://example.net/bar",
"item": [
{"href": "http://example.com/foo1"},
{"href": "http://example.com/foo2"}
]
}
]
}
Figure 2
The following example shows a set of links that represents two links,
each with a different link context, link target, and relation type.
One relation type is registered, the other is an extension relation
type.
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{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo1"}
]
},
{ "anchor": "http://example.net/boo",
"http://example.com/relations/baz" : [
{"href": "http://example.com/foo2"}
]
}
]
}
Figure 3
4.2.4. Link Target Attributes
A link may be further qualified by target attributes. Three types of
attributes exist:
* Attributes defined in Section 3.4.1 of Web Linking [RFC8288].
* Extension attributes defined and used by communities as allowed by
Section 3.4.2 of [RFC8288].
* Internationalized versions of the "title" attribute defined by
[RFC8288] and of extension attributes allowed by Section 3.4 of
[RFC8288].
The handling of these different types of attributes is described in
the sections below.
4.2.4.1. Target Attributes Defined by Web Linking
Section 3.4.1 of [RFC8288] defines the following target attributes
that may be used to annotate links: "hreflang", "media", "title",
"title*", and "type"; these target attributes follow different
occurrence and value patterns. In the JSON representation, these
attributes MUST be conveyed as additional members of the link target
object as follows:
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* "hreflang": The optional and repeatable "hreflang" target
attribute MUST be represented by an array (even if there only is
one value to be represented), and each value in that array MUST be
a string - representing one value of the "hreflang" target
attribute for a link - which follows the same model as in the
[RFC8288] syntax.
* "media": The optional and not repeatable "media" target attribute
MUST be represented by a "media" member in the link target object,
and its value MUST be a string that follows the same model as in
the [RFC8288] syntax.
* "type": The optional and not repeatable "type" target attribute
MUST be represented by a "type" member in the link target object,
and its value MUST be a string that follows the same model as in
the [RFC8288] syntax.
* "title": The optional and not repeatable "title" target attribute
MUST be represented by a "title" member in the link target object,
and its value MUST be a string that follows the same model as in
the [RFC8288] syntax.
* "title*": The optional and not repeatable "title*" target
attribute is motivated by character encoding and language issues
and follows the model defined in [RFC8187]. The details of the
JSON representation that applies to title* are described in
Section 4.2.4.2.
The following example illustrates how the repeatable "hreflang" and
the not repeatable "type" target attributes are represented in a link
target object.
{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo",
"type": "text/html",
"hreflang": [ "en" , "de" ]
}
]
}
]
}
Figure 4
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4.2.4.2. Internationalized Target Attributes
In addition to the target attributes described in Section 4.2.4.1,
Section 3.4 of [RFC8288] also supports attributes that follow the
content model of [RFC8187]. In [RFC8288], these target attributes
are recognizable by the use of a trailing asterisk in the attribute
name, such as "title*". The content model of [RFC8187] uses a
string-based microsyntax that represents the character encoding, an
optional language tag, and the escaped attribute value encoded
according to the specified character encoding.
The JSON serialization for these target attributes MUST be as
follows:
* An internationalized target attribute is represented as a member
of the link context object with the same name (including the *) of
the attribute.
* The character encoding information as prescribed by [RFC8187] is
not preserved; instead, the content of the internationalized
attribute is represented in the character encoding used for the
JSON set of links.
* The value of the internationalized target attribute is an array
that contains one or more JSON objects. The name of one member of
such JSON object is "value" and its value is the actual content
(in its unescaped version) of the internationalized target
attribute, i.e. the value of the attribute from which the encoding
and language information are removed. The name of another,
optional, member of such JSON object is "language" and its value
is the language tag [RFC5646] for the language in which the
attribute content is conveyed.
The following example illustrates how the "title*" target attribute
defined by Section 3.4.1 of [RFC8288] is represented in a link target
object.
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{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo",
"type": "text/html",
"hreflang": [ "en" , "de" ],
"title": "Next chapter",
"title*": [ { "value": "nächstes Kapitel" ,
"language" : "de" } ]
}
]
}
]
}
Figure 5
The above example assumes that the German title contains an umlaut
character (in the native syntax it would be encoded as title*=UTF-
8'de'n%c3%a4chstes%20Kapitel), which gets encoded in its unescaped
form in the JSON representation. Implementations MUST properly
decode/encode internationalized target attributes that follow the
model of [RFC8187] when transcoding between the "application/linkset"
and the "application/linkset+json" formats.
4.2.4.3. Extension Target Attributes
Extension target attributes are attributes that are not defined by
Section 3.4.1 of [RFC8288] (as listed in Section 4.2.4.1), but are
nevertheless used to qualify links. They can be defined by
communities in any way deemed necessary, and it is up to them to make
sure their usage is understood by target applications. However,
lacking standardization, there is no interoperable understanding of
these extension attributes. One important consequence is that their
cardinality is unknown to generic applications. Therefore, in the
JSON serialization, all extension target attributes are treated as
repeatable.
The JSON serialization for these target attributes MUST be as
follows:
* An extension target attribute is represented as a member of the
link context object with the same name of the attribute, including
the * if applicable.
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* The value of an extension attribute MUST be represented by an
array, even if there only is one value to be represented.
* If the extension target attribute does not have a name with a
trailing asterisk, then each value in that array MUST be a string
that represents one value of the attribute.
* If the extension attribute has a name with a trailing asterisk (it
follows the content model of [RFC8187]), then each value in that
array MUST be a JSON object. The value of each such JSON object
MUST be structured as described in Section 4.2.4.2.
The example shows a link target object with three extension target
attributes. The value for each extension target attribute is an
array. The two first are regular extension target attributes, with
the first one ("foo") having only one value and the second one
("bar") having two. The last extension target attribute ("baz*")
follows the naming rule of [RFC8187] and therefore is encoded
according to the serialization described in Section 4.2.4.2.
{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{ "href": "http://example.com/foo",
"type": "text/html",
"foo": [ "foovalue" ],
"bar": [ "barone", "bartwo" ],
"baz*": [ { "value": "bazvalue" ,
"language" : "en" } ]
}
]
}
]
}
Figure 6
4.2.5. JSON Extensibility
The extensibility of the JSON document format for representing a set
of links is restricted to the extensibility provided by [RFC8288].
The Web linking model provides for the use of extension target
attributes as discussed in Section 4.2.4.3. Extensions based on the
JSON syntax MUST NOT be used, and MUST be ignored when found in a
JSON linkset document.
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This limitation of the JSON format allows to unambiguously round trip
between links provided in the HTTP "Link" header field, sets of links
serialized according to the "application/linkset" format, and sets of
links serialized according to the "application/linkset+json" format.
5. The "profile" attribute for media types to Represent Sets of Links
As a means to convey specific constraints or conventions (as per
[RFC6906]) that apply to a link set document, the "profile" attribute
MAY be used in conjunction with the media types "application/linkset"
and "application/linkset+json" detailed in Section 4.1 and
Section 4.2, respectively. For example, the attribute could be used
to indicate that a link set uses a specific, limited set of link
relation types.
The value of the "profile" attribute MUST be a non-empty list of
space-separated URIs, each of which identifies specific constraints
or conventions that apply to the link set document. Profile URIs MAY
be registered in the IANA Profile URI Registry in the manner
specified by [RFC7284].
The presence of a "profile" attribute in conjunction with the
"application/linkset" and "application/linkset+json" media types does
not change the semantics of a link set. As such, clients with and
without knowledge of profile URIs can use the same representation.
6. The "linkset" Relation Type for Linking to a Set of Links
The target of a link with the "linkset" relation type provides a set
of links, including links in which the resource that is the link
context participates.
A link with the "linkset" relation type MAY be provided in the header
field and/or the body of a resource's representation. It may also be
discovered by other means, such as through client-side information.
A resource MAY provide more than one link with a "linkset" relation
type. Multiple such links can refer to the same set of links
expressed using different media types, or to different sets of links,
potentially provided by different third-party services.
A user agent that follows a "linkset" link MUST be aware that the set
of links provided by the resource that is the target of the link can
contain links in which the resource that is the context of the link
does not participate; it MAY decide to ignore those links.
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A user agent that follows a "linkset" link and obtains links for
which anchors and targets are expressed as relative references (as
per Section 4.1 of [RFC3986]) MUST determine what the context is for
these links; it SHOULD ignore links for which it is unable to
unambiguously make that determination.
7. Examples
Section 7.1 and Section 7.2 show examples whereby a set of links is
provided as "application/linkset" and "application/linkset+json"
documents, respectively. Section 7.3 illustrates the use of the
"linkset" link relation type to support discovery of sets of links.
7.1. Set of Links Provided as application/linkset
Figure 7 shows a client issuing an HTTP GET request against resource
<https://example.org/links/resource1>.
GET /links/resource1 HTTP/1.1
Host: example.org
Figure 7: Client HTTP GET request
Figure 8 shows the response to the GET request of Figure 7. The
response contains a Content-Type header field specifying that the
media type of the response is "application/linkset". A set of links,
revealing authorship and versioning related to resource
<https://example.org/resource1>, is provided in the response body.
The HTTP "Link" header field indicates the availability of an
alternate representation of the set of links using media type
"application/linkset+json".
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HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:35:51 GMT
Server: Apache-Coyote/1.1
Content-Length: 1023
Content-Type: application/linkset
Link: <https://example.org/links/resource1>
; rel="alternate"
; type="application/linkset+json"
<https://authors.example.net/johndoe>
; rel="author"
; type="application/rdf+xml"
; anchor="https://example.org/resource1",
<https://example.org/resource1?version=3>
; rel="latest-version"
; type="text/html"
; anchor="https://example.org/resource1",
<https://example.org/resource1?version=2>
; rel="predecessor-version"
; type="text/html"
; anchor="https://example.org/resource1?version=3",
<https://example.org/resource1?version=1>
; rel="predecessor-version"
; type="text/html"
; anchor="https://example.org/resource1?version=2",
<https://example.org/resource1?version=1>
; rel="memento"
; type="text/html"
; datetime="Thu, 13 Jun 2019 09:34:33 GMT"
; anchor="https://example.org/resource1",
<https://example.org/resource1?version=2>
; rel="memento"
; type="text/html"
; datetime="Sun, 21 Jul 2019 12:22:04 GMT"
; anchor="https://example.org/resource1",
<https://authors.example.net/alice>
; rel="author"
; anchor="https://example.org/resource1#comment=1"
Figure 8: Response to HTTP GET includes a set of links
7.2. Set of Links Provided as application/linkset+json
Figure 9 shows the client issuing an HTTP GET request against
<https://example.org/links/resource1>. In the request, the client
uses an "Accept" header field to indicate it prefers a response in
the "application/linkset+json" format.
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GET links/resource1 HTTP/1.1
Host: example.org
Accept: application/linkset+json
Figure 9: Client HTTP GET request expressing preference for
"application/ linkset+json" response
Figure 10 shows the response to the HTTP GET request of Figure 9.
The set of links is serialized according to the media type
"application/linkset+json".
HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:46:22 GMT
Server: Apache-Coyote/1.1
Content-Type: application/linkset+json
Link: <https://example.org/links/resource1>
; rel="alternate"
; type="application/linkset"
Content-Length: 1349
{
"linkset": [
{
"anchor": "https://example.org/resource1",
"author": [
{
"href": "https://authors.example.net/johndoe",
"type": "application/rdf+xml"
}
],
"memento": [
{
"href": "https://example.org/resource1?version=1",
"type": "text/html",
"datetime": "Thu, 13 Jun 2019 09:34:33 GMT"
},
{
"href": "https://example.org/resource1?version=2",
"type": "text/html",
"datetime": "Sun, 21 Jul 2019 12:22:04 GMT"
}
],
"latest-version": [
{
"href": "https://example.org/resource1?version=3",
"type": "text/html"
}
]
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},
{
"anchor": "https://example.org/resource1?version=3",
"predecessor-version": [
{
"href": "https://example.org/resource1?version=2",
"type": "text/html"
}
]
},
{
"anchor": "https://example.org/resource1?version=2",
"predecessor-version": [
{
"href": "https://example.org/resource1?version=1",
"type": "text/html"
}
]
},
{
"anchor": "https://example.org/resource1#comment=1",
"author": [
{
"href": "https://authors.example.net/alice"
}
]
}
]
}
Figure 10: Response to the client's request for the set of links
7.3. Discovering a Link Set via the "linkset" Link Relation Type
Figure 11 shows a client issuing an HTTP HEAD request against
resource <https://example.org/resource1>.
HEAD resource1 HTTP/1.1
Host: example.org
Figure 11: Client HTTP HEAD request
Figure 12 shows the response to the HEAD request of Figure 11. The
response contains an HTTP "Link" header field with a link that has
the "linkset" relation type. It indicates that a set of links is
provided by resource <https://example.org/links/resource1>, which
provides a representation with media type "application/linkset+json".
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HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:45:54 GMT
Server: Apache-Coyote/1.1
Link: <https://example.org/links/resource1>
; rel="linkset"
; type="application/linkset+json"
Content-Length: 236
Content-Type: text/html;charset=utf-8
Figure 12: Response to HTTP HEAD request
Section 7.2 shows a client obtaining a set of links by issuing an
HTTP GET on the target of the link with the "linkset" relation type,
<https://example.org/links/resource1>.
8. IANA Considerations
8.1. Link Relation Type: linkset
The link relation type below should be registered by IANA per
Section 6.2.1 of Web Linking [RFC8288]:
Relation Name: linkset
Description: The link target of a link with the "linkset" relation
type provides a set of links, including links in which the link
context of the link participates.
Reference: [[ This document ]]
8.2. Media Type: application/linkset
The Internet media type [RFC6838] for a natively encoded linkset is
application/linkset.
Type name: application
Subtype name: linkset
Required parameters: none
Optional parameters: profile
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Encoding considerations: Linksets are encoded according to the
definition of [RFC8288]. The encoding of [RFC8288] is based on
the general encoding rules of [I-D.ietf-httpbis-semantics], with
the addition of allowing indicating character encoding and
language for specific parameters as defined by [RFC8187].
Security considerations: The security considerations of [[ This
document ]] apply.
Interoperability considerations: N/A
Published specification: [[ This document ]]
Applications that use this media type: This media type is not
specific to any application, as it can be used by any application
that wants to interchange web links.
Additional information:
Magic number(s): N/A
File extension(s): This media type does not propose a specific
extension.
Macintosh file type code(s): TEXT
Person & email address to contact for further information: Erik
Wilde <erik.wilde@dret.net>
Intended usage: COMMON
Restrictions on usage: none
Author: Erik Wilde <erik.wilde@dret.net>
Change controller: IETF
8.3. Media Type: application/linkset+json
The Internet media type [RFC6838] for a JSON-encoded linkset is
application/linkset+json.
Type name: application
Subtype name: linkset+json
Required parameters: none
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Optional parameters: profile
Encoding considerations: The encoding considerations of [RFC8259]
apply
Security considerations: The security considerations of [[ This
document ]] apply.
Interoperability considerations: The interoperability
considerations of [RFC8259] apply.
Published specification: [[ This document ]]
Applications that use this media type: This media type is not
specific to any application, as it can be used by any application
that wants to interchange web links.
Additional information:
Magic number(s): N/A
File extension(s): JSON documents often use ".json" as the file
extension, and this media type does not propose a specific
extension other than this generic one.
Macintosh file type code(s): TEXT
Person & email address to contact for further information: Erik
Wilde <erik.wilde@dret.net>
Intended usage: COMMON
Restrictions on usage: none
Author: Erik Wilde <erik.wilde@dret.net>
Change controller: IETF
9. Security Considerations
The security considerations of Web Linking [RFC8288] apply, as long
as they are not specifically discussing the risks of exposing
information in HTTP header fields.
In general, links may cause information leakage when they expose
information (such as URIs) that can be sensitive or private. Links
may expose "hidden URIs" that are not supposed to be openly shared,
and may not be sufficiently protected. Ideally, none of the URIs
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exposed in links should be supposed to be "hidden"; instead, if these
URIs are supposed to be limited to certain users, then technical
measures should be put in place so that accidentally exposing them
does not cause any harm.
For the specific mechanisms defined in this specification, two
security considerations should be taken into account:
* The Web Linking model always has an "implicit context", which is
the resource of the HTTP interaction. This original context can
be lost or can change when self-contained link representations are
moved. Changing the context can change the interpretation of
links when they have no explicit anchor, or when they use relative
URIs. Applications may choose to ignore links that have no
explicit anchor or that use relative URIs when these are exchanged
in stand-alone resources.
* The model introduced in this specification supports "3rd party
links", where one party can provide links that have another
party's resource as an anchor. Depending on the link semantics
and the application context, it is important to verify that there
is sufficient trust in that 3rd party to allow it to provide these
links. Applications may choose to treat 3rd party links
differently than cases where a resource and the links for that
resource are provided by the same party.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
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[RFC8187] Reschke, J., "Indicating Character Encoding and Language
for HTTP Header Field Parameters", RFC 8187,
DOI 10.17487/RFC8187, September 2017,
<https://www.rfc-editor.org/info/rfc8187>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013,
<https://www.rfc-editor.org/info/rfc6982>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6906] Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
DOI 10.17487/RFC6906, March 2013,
<https://www.rfc-editor.org/info/rfc6906>.
[RFC7284] Lanthaler, M., "The Profile URI Registry", RFC 7284,
DOI 10.17487/RFC7284, June 2014,
<https://www.rfc-editor.org/info/rfc7284>.
[I-D.ietf-httpbis-semantics]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-15, 30 March 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-15>.
11. Informative References
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[W3C.REC-json-ld-20140116]
Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0",
World Wide Web Consortium Recommendation REC-json-ld-
20140116, 16 January 2014,
<https://www.w3.org/TR/2014/REC-json-ld-20140116>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<https://www.rfc-editor.org/info/rfc5988>.
Appendix A. JSON-LD Context
A set of links rendered according to the JSON serialization defined
in Section 4.2 can be interpreted as RDF triples by adding a JSON-LD
context [W3C.REC-json-ld-20140116] that maps the JSON keys to
corresponding Linked Data terms. And, as per
[W3C.REC-json-ld-20140116] section 6.8 (https://www.w3.org/TR/2014/
REC-json-ld-20140116/#interpreting-json-as-json-ld), when delivering
a link set that is rendered according to the "application/
linkset+json" media type to a user agent, a server can convey the
availability of such a JSON-LD context by using a link with the
relation type "http://www.w3.org/ns/json-ld#context" in the HTTP
"Link" header.
Using the latter approach to support discovery of a JSON-LD Context,
the response to the GET request of Figure 9 against the URI of a set
of links would be as shown in Figure 13.
HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:48:22 GMT
Server: Apache-Coyote/1.1
Content-Type: application/linkset+json
Link: <https://example.org/contexts/linkset.jsonld>
; rel="http://www.w3.org/ns/json-ld#context"
; type="application/ld+json"
Content-Length: 1349
{
"linkset": [
{
"anchor": "https://example.org/resource1",
"author": [
{
"href": "https://authors.example.net/johndoe",
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"type": "application/rdf+xml"
}
],
"memento": [
{
"href": "https://example.org/resource1?version=1",
"type": "text/html",
"datetime": "Thu, 13 Jun 2019 09:34:33 GMT"
},
{
"href": "https://example.org/resource1?version=2",
"type": "text/html",
"datetime": "Sun, 21 Jul 2019 12:22:04 GMT"
}
],
"latest-version": [
{
"href": "https://example.org/resource1?version=3",
"type": "text/html"
}
]
},
{
"anchor": "https://example.org/resource1?version=3",
"predecessor-version": [
{
"href": "https://example.org/resource1?version=2",
"type": "text/html"
}
]
},
{
"anchor": "https://example.org/resource1?version=2",
"predecessor-version": [
{
"href": "https://example.org/resource1?version=1",
"type": "text/html"
}
]
},
{
"anchor": "https://example.org/resource1#comment=1",
"author": [
{
"href": "https://authors.example.net/alice"
}
]
}
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]
}
Figure 13: Using a typed link to support discovery of a JSON-LD
Context for a Set of Links
In order to obtain the JSON-LD Context conveyed by the server, the
user agent issues an HTTP GET against the link target of the link
with the "http://www.w3.org/ns/json-ld#context" relation type. The
response to this GET is shown in Figure 14. This particular JSON-LD
context maps "application/linkset+json" representations of link sets
to Dublin Core Terms. It also renders each link relation as a URI
Reference, inspired by the same approach used for Atom [RFC4287]
described in Appendix A.2 of [RFC8288].
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HTTP/1.1 200 OK
Content-Type: application/ld+json
Content-Length: 708
{
"@context": [
{
"@vocab": "http://www.iana.org/assignments/relation/",
"anchor": "@id",
"href": "@id",
"linkset": "@graph",
"_linkset": "@graph",
"title": {
"@id": "http://purl.org/dc/terms/title"
},
"title*": {
"@id": "http://purl.org/dc/terms/title"
},
"type": {
"@id": "http://purl.org/dc/terms/format"
},
"datetime": {
"@id": "http://purl.org/dc/terms/date"
}
},
{
"language": "@language",
"value": "@value",
"hreflang": {
"@id": "http://purl.org/dc/terms/language",
"@container": "@set"
}
}
]
}
Figure 14: JSON-LD Context mapping to Dublin Core Terms and IANA
assignments
Applying the JSON-LD context of Figure 14 to the link set of
Figure 13 allows transforming the "application/linkset+json" link set
to an RDF link set. Figure 15 shows the latter represented by means
of the "text/turtle" RDF serialization.
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<https://authors.example.net/johndoe>
<http://purl.org/dc/terms/format>
"application/rdf+xml" .
<https://example.org/resource1#comment=1>
<http://www.iana.org/assignments/relation/author>
<https://authors.example.net/alice> .
<https://example.org/resource1>
<http://www.iana.org/assignments/relation/author>
<https://authors.example.net/johndoe> .
<https://example.org/resource1>
<http://www.iana.org/assignments/relation/latest-version>
<https://example.org/resource1?version=3> .
<https://example.org/resource1>
<http://www.iana.org/assignments/relation/memento>
<https://example.org/resource1?version=1> .
<https://example.org/resource1>
<http://www.iana.org/assignments/relation/memento>
<https://example.org/resource1?version=2> .
<https://example.org/resource1?version=1>
<http://purl.org/dc/terms/date>
"Thu, 13 Jun 2019 09:34:33 GMT" .
<https://example.org/resource1?version=1>
<http://purl.org/dc/terms/format>
"text/html" .
<https://example.org/resource1?version=2>
<http://purl.org/dc/terms/date>
"Sun, 21 Jul 2019 12:22:04 GMT" .
<https://example.org/resource1?version=2>
<http://purl.org/dc/terms/format>
"text/html" .
<https://example.org/resource1?version=2>
<http://www.iana.org/assignments/relation/predecessor-version>
<https://example.org/resource1?version=1> .
<https://example.org/resource1?version=3>
<http://purl.org/dc/terms/format>
"text/html" .
<https://example.org/resource1?version=3>
<http://www.iana.org/assignments/relation/predecessor-version>
<https://example.org/resource1?version=2> .
Figure 15: RDF serialization of the link set resulting from
applying the JSON-LD context
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Note that the JSON-LD context of Figure 14 does not handle (meta)link
relations of type ""linkset"" as they are in conflict with the top-
level JSON key. A workaround is to rename the top-level key to
""_linkset"" in the "application/linkset+json" before transforming a
link set to JSON-LD.
Appendix B. Implementation Status
This section is to be removed before publishing as an RFC.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC 6982
[RFC6982]. The description of implementations in this section is
intended to assist the IETF in its decision processes in progressing
drafts to RFCs. Please note that the listing of any individual
implementation here does not imply endorsement by the IETF.
Furthermore, no effort has been spent to verify the information
presented here that was supplied by IETF contributors. This is not
intended as, and must not be construed to be, a catalog of available
implementations or their features. Readers are advised to note that
other implementations may exist.
According to RFC 6982, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
B.1. GS1
GS1 is a provider of barcodes (GS1 GTINs and EAN/UPC) for retail
products and manages an ecology of services and standards to leverage
them at a global scale. GS1 has indicated that it will implement
this "linkset" specification as a means to allow requesting and
representing links pertaining to products from various retailers.
Currently, the GS1 Digital Link specification makes an informative
reference to version 03 of the "linkset" I-D. GS1 expresses
confidence that this will become a normative reference in the next
iteration of that specification, likely to be ratified as a GS1
standard around February 2021.
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B.2. FAIR Signposting Profile
The FAIR Signposting Profile is a community specification aimed at
improving machine navigation of scholarly objects on the web through
the use of typed web links pointing at e.g. web resources that are
part of a specific object, persistent identifiers for the object and
its authors, license information pertaining to the object. The
specification encourages the use of Linksets and initial
implementations are ongoing, for example, for the open source
Dataverse data repository platform that was initiated by Harvard
University and is meanwhile used by research institutions, worldwide.
B.3. Open Journal Systems (OJS)
Open Journal Systems (OJS) is an open-source software for the
management of peer-reviewed academic journals, and is created by the
Public Knowledge Project (PKP), released under the GNU General Public
License. Open Journal Systems (OJS) is a journal management and
publishing system that has been developed by PKP through its
federally funded efforts to expand and improve access to research.
The OJS platform has implemented "linkset" support as an alternative
way to provide links when there are more than a configured limit
(they consider using about 10 as a good default, for testing purpose
it is currently set to 8).
Acknowledgements
Thanks for comments and suggestions provided by Phil Archer,
Dominique Guinard, Mark Nottingham, Julian Reschke, Stian Soiland-
Reyes, and Sarven Capadisli.
Authors' Addresses
Erik Wilde
Axway
Email: erik.wilde@dret.net
URI: http://dret.net/netdret/
Herbert Van de Sompel
Data Archiving and Networked Services
Email: herbert.van.de.sompel@dans.knaw.nl
URI: https://orcid.org/0000-0002-0715-6126
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