wdiff rfc9264v2.txt rfc9264.txt

Internet Engineering Task Force (IETF) E. Wilde
Request for Comments: 9264 Axway
Category: Standards Track H. Van de Sompel
ISSN: 2070-1721 Data Archiving and Networked Services
July 2022

Linkset: Media Types and a Link Relation Type for Link Sets

Abstract

This specification defines two formats and associated media types for
representing sets of links as standalone documents. One format is
based on JSON, and the other is aligned with the format for
representing links in the HTTP "Link" header field. This
specification also introduces a link relation type to support the
discovery of sets of links.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9264.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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in the Revised BSD License.

Table of Contents

1. Introduction
2. Terminology
3. Use Cases and Motivation
3.1. Third-Party Links
3.2. Challenges Writing to the HTTP "Link" Header Field
3.3. Large Number of Links
4. Document Formats for Sets of Links
4.1. HTTP Link Document Format: application/linkset
4.2. JSON Document Format: application/linkset+json
4.2.1. Set of Links
4.2.2. Link Context Object
4.2.3. Link Target Object
4.2.4. Link Target Attributes
4.2.5. JSON Extensibility
5. The "profile" Parameter for Media Types to Represent Sets of
Links
6. The "linkset" Relation Type for Linking to a Set of Links
7. Examples
7.1. Set of Links Provided as "application/linkset"
7.2. Set of Links Provided as "application/linkset+json"
7.3. Discovering a Link Set via the "linkset" Link Relation Type
7.4. Link Set Profiles
7.4.1. Using a "profile" Attribute with a "linkset" Link
7.4.2. Using a "profile" Parameter with a Link Set Media Type
7.4.3. Using a Link with a "profile" Link Relation Type
8. IANA Considerations
8.1. Link Relation Type: linkset
8.2. Media Type: application/linkset
8.3. Media Type: application/linkset+json
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. JSON-LD Context
Acknowledgements
Authors' Addresses

1. Introduction

Resources on the Web often use typed Web Links [RFC8288], either
(1) embedded in resource representations -- for example, using the
<link> element for HTML documents or (2) 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 standalone document is preferable.

Therefore, this specification defines two formats for representing
sets of Web Links and their attributes as standalone documents. One
serializes links in the same format as the format used in the HTTP
"Link" header field, and the other serializes links in JSON. It also
defines associated media types to represent sets of links, and the
"linkset" relation type to support the discovery of any resource that
conveys a set of links as a standalone 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" in
the same manner that "Web Linking" [RFC8288] uses them.

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 "HTTP
Semantics" [RFC9110], line breaks are deprecated in values for HTTP
fields; only whitespaces and tabs are supported as separators.

3. Use Cases and Motivation

The following sections describe use cases 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 standalone
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 the reuse of
tools created to handle it.

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 to 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 a
link context but also as a link target, with a different resource
being the link context.

* 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 custodian 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 the 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.

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 in a
manner 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 Semantics" [RFC9110] defines error codes related
to excess communication by the user agent ("413 Content Too Large"
and "414 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 has been truncated. As a result,
applications take countermeasures 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 (to the web server implementation) 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], which 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 nearly identical to the field
value of the HTTP "Link" header field as specified in Section 3 of
[RFC8288].

* The format defined in Section 4.2 is expressed in JSON [RFC8259].

Links provided in the HTTP "Link" header field are intended to be
used in the context of an HTTP interaction, and contextual
information that is available during an interaction is used to
correctly interpret them. Links provided in link sets, however, can
be reused outside of an HTTP interaction, when no such contextual
information is available. As a result, implementers of link sets
should strive to make them self-contained by adhering to the
following recommendations.

For links provided in the HTTP "Link" header field that have no
anchor or that use relative references, the URI of the resource that
delivers the links provides the contextual information that is needed
for their correct interpretation. In order to support use cases
where link set documents are reused 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 the 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, the interpretation of
links in link set documents will depend on which URI is used as the
context.

For a "title" attribute provided on a link in the HTTP "Link" header
field, the language in which the title is expressed is provided by
the "Content-Language" header field of the HTTP interaction with the
resource that delivers the links. This does not apply to "title"
attributes provided for links in link set documents because that
would constrain all links in a link set to having a single title
language and would not support determining title languages when a
link set is used outside of an HTTP interaction. In order to support
use cases where link set documents are reused outside the context of
an HTTP interaction, it is RECOMMENDED to make them self-contained by
using the "title*" attribute instead of the "title" attribute because
"title*" allows expressing the title language as part of its value by
means of a language tag. Note that, in this regard, language tags
are matched case insensitively (see Section 2.1.1 of [RFC5646]). If
this recommendation is not followed, accurately determining the
language of titles provided on links in link set documents will not
be possible.

Note also 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 may be represented as direct links using the
"rel" construct and by switching the roles of the resources involved
in the link.

4.1. HTTP Link Document Format: application/linkset

This document format is nearly identical to the field value of the
HTTP "Link" header field as defined in Section 3 of [RFC8288], more
specifically by its ABNF [RFC5234] production rule for "Link" and its
subsequent rules. It differs from the format for field values of the
HTTP "Link" header field only in that not only spaces and horizontal
tabs are allowed as separators but also newline characters as a means
to improve readability for humans. The use of non-ASCII characters
in the field value of the HTTP "Link" header field is not allowed and
as such is also not allowed in "application/linkset" link sets.

The assigned media type for this format is "application/linkset".

When converting an "application/linkset" document to a field value
for the HTTP "Link" header field, newline characters MUST be removed
or MUST be replaced by whitespace (SP) in order to comply with
Section 5.5 of [RFC9110].

Implementers of "application/linkset" link sets should strive to make
them self-contained by following the recommendations provided in
Section 4 regarding their use outside the context of an HTTP
interaction.

It should be noted that the "application/linkset" format specified
here is different from the "application/link-format" format specified
in [RFC6690] in that the former fully matches the field value 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 (CoRE).

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
Section 2 of [RFC8288].

The assigned media type for this format is "application/
linkset+json".

In the interests of interoperability, "application/linkset+json" link
sets MUST be encoded using UTF-8 as per Section 8.1 of [RFC8259].

Implementers of "application/linkset+json" link sets should strive to
make them self-contained by following the recommendations provided in
Section 4 regarding their use outside the context of an HTTP
interaction.

The "application/linkset+json" serialization allows for OPTIONAL
support of a JSON-LD serialization. This can be achieved by adding
an appropriate context to the "application/linkset+json"
serialization using the approach described in Section 6.1 of
[W3C.REC-json-ld]. Communities of practice can decide which context
best meets their application needs. Appendix A shows an example of a
possible context that, when added to a JSON serialization, allows it
to be interpreted as Resource Description Framework (RDF) data
[W3C.REC-rdf11-concepts].

4.2.1. Set of Links

In the JSON representation of a set of links:

* A set of links is represented in JSON as an object that MUST
contain "linkset" as its sole member.

* The value of the "linkset" member is an array in which a distinct
JSON object -- the "link context object" (see Section 4.2.2) -- is
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.

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 contain 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
defined in Section 4.1 of [RFC3986].

* For each distinct relation type that the link context has with
link targets, a link context object MUST contain an additional
member. The value of 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.

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 contain 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 defined in
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 as its value ("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.

{ "linkset":
[
{ "anchor": "https://example.net/bar",
"next": [
{"href": "https://example.com/foo"}
]
}
]
}

Figure 1: Simple Linkset Example linkset example

The following example of a JSON-serialized set of links represents
two links that share a link context and relation type but have
different link targets.

{ "linkset":
[
{ "anchor": "https://example.net/bar",
"item": [
{"href": "https://example.com/foo1"},
{"href": "https://example.com/foo2"}
]
}
]
}

Figure 2: Linkset with Two Links two links with the Same Context same context

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, and the other is an extension
relation type.

{ "linkset":
[
{ "anchor": "https://example.net/bar",
"next": [
{"href": "https://example.com/foo1"}
]
},
{ "anchor": "https://example.net/boo",
"https://example.com/relations/baz" : [
{"href": "https://example.com/foo2"}
]
}
]
}

Figure 3: Linkset with Two Links two links with Different Contexts different contexts

4.2.4. Link Target Attributes

A link may be further qualified by target attributes as defined by
Section 2 of [RFC8288]. Three types of attributes exist:

* Serialization-defined attributes as described in Section 3.4.1 of
[RFC8288].

* Extension attributes defined and used by communities as allowed by
Section 3.4.2 of [RFC8288].

* Internationalized versions of the "title" attribute as 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:

"hreflang": The "hreflang" target attribute, defined as optional and
repeatable by [RFC8288], MUST be represented by an "hreflang"
member, its value MUST be an array (even if there is only 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 -- that follows the same model as the syntax
discussed in [RFC8288].

"media": The "media" target attribute, defined as optional and not
repeatable by [RFC8288], 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 the syntax discussed in [RFC8288].

"title": The "title" target attribute, defined as optional and not
repeatable by [RFC8288], MUST be represented by a "title" member
in the link target object, and its value MUST be a JSON string.

"title*": The "title*" target attribute, defined as optional and not
repeatable by [RFC8288], 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.

"type": The "type" target attribute, defined as optional and not
repeatable by [RFC8288], 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 the syntax discussed in [RFC8288].

The following example illustrates how the "hreflang" (repeatable)
target attribute and the "type" (not repeatable) target attribute are
represented in a link target object.

{ "linkset":
[
{ "anchor": "https://example.net/bar",
"next": [
{ "href": "https://example.com/foo",
"type": "text/html",
"hreflang": [ "en" , "de" ]
}
]
}
]
}

Figure 4: Linkset with "hreflang" and "type" Target Attributes target attributes

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 "*")
as the attribute.

* The character encoding information as prescribed by [RFC8187] is
not preserved; instead, the content of the internationalized
attribute is represented as a JSON string.

* 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 objects 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 objects 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
as defined by Section 3.4.1 of [RFC8288] is represented in a link
target object.

{ "linkset":
[
{ "anchor": "https://example.net/bar",
"next": [
{ "href": "https://example.com/foo",
"type": "text/html",
"hreflang": [ "en" , "de" ],
"title": "Next chapter",
"title*": [ { "value": "nächstes Kapitel" ,
"language" : "de" } ]
}
]
}
]
}

Figure 5: Linkset with "title" and "title*" Target Attributes target attributes

The above example assumes that the German title contains an umlaut
character (in the original 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"
format and the "application/linkset+json" format.

4.2.4.3. Extension Target Attributes

Extension target attributes (e.g., as listed in Section 4.2.4.1) are
attributes that are not defined by Section 3.4.1 of [RFC8288] 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 target object with the same name as the attribute, including
the "*" if applicable.

* The value of an extension attribute MUST be represented by an
array, even if there is only 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 JSON
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 following example shows a link target object with three extension
target attributes. The value for each extension target attribute is
an array. The first two 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": "https://example.net/bar",
"next": [
{ "href": "https://example.com/foo",
"type": "text/html",
"foo": [ "foovalue" ],
"bar": [ "barone", "bartwo" ],
"baz*": [ { "value": "bazvalue" ,
"language" : "en" } ]
}
]
}
]
}

Figure 6: Linkset with Extension Target Attributes extension target attributes

4.2.5. JSON Extensibility

The Web Linking model [RFC8288] provides for the use of extension
target attributes as discussed in Section 4.2.4.3. The use of other
forms of extensions is NOT RECOMMENDED. Limiting the JSON format in
this way allows unambiguous round trips 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.

Cases may exist in which the use of extensions other than those
discussed in Section 4.2.4.3 may be useful -- for example, when a
link set publisher needs to include descriptive or technical metadata
for internal consumption. If such extensions are used, they MUST NOT
change the semantics of the JSON members defined in this
specification. Agents that consume JSON linkset documents can safely
ignore such extensions.

5. The "profile" Parameter 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" parameter
MAY be used in conjunction with the media types "application/linkset"
and "application/linkset+json" as detailed in Sections 4.1 and 4.2,
respectively. For example, the parameter could be used to indicate
that a link set uses a specific, limited set of link relation types.

The value of the "profile" parameter 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. When providing
multiple profile URIs, care should be taken that the corresponding
profiles are not conflicting. Profile URIs MAY be registered in the
IANA's "Profile URIs" registry in the manner specified by [RFC7284].

The presence of a "profile" parameter 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.

Section 7.4.2 shows an example of using the "profile" parameter in
conjunction with the "application/linkset+json" media type.

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.

The set of links provided by the resource that is the target of a
"linkset" link may contain links in which the resource that is the
context of the "linkset" link does not participate. User agents MUST
process each link in the link set independently, including processing
of the link context and link target, and MAY ignore links from the
link set in which the context of the "linkset" link does not
participate.

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.

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 "linkset" link relation type.
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's "Profile URIs" registry in the manner specified by
[RFC7284]. Section 7.4.1 shows an example of using the "profile"
attribute on a link with the "linkset" relation type, making both the
link set and the profile(s) to which it complies discoverable.

7. Examples

Sections 7.1 and 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 the discovery of sets of links, and
Section 7.4 shows how to convey profile information pertaining to a
link set.

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 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".

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 includes a Set set of Links 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 that it prefers a response
in the "application/linkset+json" format.

GET links/resource1 HTTP/1.1
Host: example.org
Accept: application/linkset+json

Figure 9: Client HTTP GET Request Expressing Preference request expressing preference for an
"application/linkset+json" Response 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: 1246

{ "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"
}
]
},
{ "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 client's request for the Linkset linkset

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 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".

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 request

7.4. Link Set Profiles

The examples in this section illustrate the use of the "profile"
attribute for a link with the "linkset" link relation type and the
"profile" attribute for a link set media type. The examples are
inspired by the implementation of link sets by GS1 (the standards
body behind many of the world's barcodes).

7.4.1. Using a "profile" Attribute with a "linkset" Link

Figure 13 shows a client issuing an HTTP HEAD request against trade
item 09506000134352 at <https://id.gs1.org/01/9506000134352>.

HEAD /01/9506000134352 HTTP/1.1
Host: id.gs1.org

Figure 13: Client HTTP HEAD Request request

Figure 14 shows the server's response to the request of Figure 13,
including a "linkset" link with a "profile" attribute that has the
profile URI <https://www.gs1.org/voc/?show=linktypes> as its value.
Dereferencing that URI yields a profile document that lists all the
link relation types that a client can expect when requesting the link
set made discoverable by the "linkset" link. The link relation types
are presented in abbreviated form, e.g., <gs1:activityIdeas>, whereas
the actual link relation type URIs are available as hyperlinks on the
abbreviations, e.g., <https://www.gs1.org/voc/activityIdeas>. For
posterity, that profile document was saved in the Internet Archive at
<https://web.archive.org/web/20210927160406/https://www.gs1.org/
voc/?show=linktypes> on 27 September 2021.

HTTP/1.1 307 Temporary Redirect
Date: Mon, 27 Sep 2021 16:03:07 GMT
Server: nginx
Link: <https://id.gs1.org/01/9506000134352?linkType=all>
; rel="linkset"
; type="application/linkset+json"
; profile="https://www.gs1.org/voc/?show=linktypes"
Location: https://example.com/risotto-rice-with-mushrooms/

Figure 14: Response to the Client's client's HEAD Request, request, including a
"profile" Attribute attribute for the "linkset" Link link

7.4.2. Using a "profile" Parameter with a Link Set Media Type

Figure 15 shows a client issuing an HTTP HEAD request against the
link set <https://id.gs1.org/01/9506000134352?linkType=all> that was
discovered through the HTTP interactions shown in Section 7.4.1.

HEAD /01/9506000134352?linkType=all HTTP/1.1
Host: id.gs1.org

Figure 15: Client HTTP HEAD Request request

Figure 16 shows the server's response to the request of Figure 15.
Note the "profile" parameter for the "application/linkset+json" media
type, which has as its value the same profile URI
<https://www.gs1.org/voc/?show=linktypes> as was used in Figure 14.

HTTP/1.1 200 OK
Date: Mon, 27 Sep 2021 16:03:33 GMT
Server: nginx
Content-Type: application/linkset+json;
profile="https://www.gs1.org/voc/?show=linktypes"
Content-Length: 396

Figure 16: Response to the Client's client's HEAD Request, request, including a
"profile" Parameter parameter for the "application/linkset+json" Media Type media type

7.4.3. Using a Link with a "profile" Link Relation Type

Note that the response shown in Figure 16 from the link set resource
is equivalent to the response shown in Figure 17, which leverages the
"profile" link relation type defined in [RFC6906].

HTTP/1.1 200 OK
Date: Mon, 27 Sep 2021 16:03:33 GMT
Server: nginx
Content-Type: application/linkset+json
Link: <https://www.gs1.org/voc/?show=linktypes>; rel="profile"
Content-Length: 396

Figure 17: Response to the Client's client's HEAD Request, request, including a
"profile" Link link

A link with a "profile" link relation type as shown in Figure 17 can
also be conveyed in the link set document itself. This is
illustrated by Figure 18. Following the recommendation that all
links in a link set document should have an explicit anchor, such a
link has the URI of the link set itself as the anchor and the profile
URI as the target. Multiple profile URIs are handled by using
multiple "href" members.

{ "linkset":
[
{ "anchor": "https://id.gs1.org/01/9506000134352?linkType=all",
"profile": [
{"href": "https://www.gs1.org/voc/?show=linktypes"}
]
},
{ "anchor": "https://id.gs1.org/01/9506000134352",
"https://gs1.org/voc/whatsInTheBox": [
{"href": "https://example.com/en/packContents/GB"}
]
}
]
}

Figure 18: A Linkset That Declares linkset that declares the Profile It Complies profile it complies with,
Using
using a "profile" Link link

8. IANA Considerations

8.1. Link Relation Type: linkset

The link relation type below has been registered by IANA in the "Link
Relation Types" registry as per Section 4.2 of [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: RFC 9264

8.2. Media Type: application/linkset

The Internet media type "application/linkset" for a linkset encoded
as described in Section 4.1 has been registered by IANA in the "Media
Types" registry as per [RFC6838].

Type name: application

Subtype name: linkset

Required parameters: N/A

Optional parameters: profile

Encoding considerations: Linksets are encoded according to the
definitions provided in [RFC8288]. The encoding discussed in
[RFC8288] is based on the general encoding rules specified by HTTP
[RFC9110] and allows specific parameters to be extended by the
indication of character encoding and language as defined by
[RFC8187].

Security considerations: The security considerations of RFC 9264
apply.

Interoperability considerations: N/A

Published specification: RFC 9264

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 "application/linkset+json" for a linkset
encoded as described in Section 4.2 has been registered by IANA in
the "Media Types" registry as per [RFC6838].

Type name: application

Subtype name: linkset+json

Required parameters: N/A

Optional parameters: profile

Encoding considerations: The encoding considerations of [RFC8259]
apply.

Security considerations: The security considerations of RFC 9264
apply.

Interoperability considerations: The interoperability considerations
of [RFC8259] apply.

Published specification: RFC 9264

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 Section 7 of [RFC3986] apply, as well
as those of Web Linking [RFC8288] as long as the latter 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 that may not be sufficiently protected. Ideally, none of the
URIs 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 standalone resources.

* The model introduced in this specification supports "third-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 third party to allow it to provide
these links. Applications may choose to treat third-party links
differently than cases where a resource and the links for that
resource are provided by the same party.

10. References

10.1. 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>.

[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>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[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>.

[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>.

[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>.

[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>.

[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>.

[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[W3C.REC-json-ld]
Sporny, M., Ed., Kellogg, G., Ed., and M. Lanthaler, Ed.,
"JSON-LD 1.1: A JSON-based Serialization for Linked Data",
W3C Recommendation REC-json-ld-20140116, July 2020,
<https://www.w3.org/TR/json-ld/>.

10.2. Informative References

[DCMI-TERMS]
Dublin Core Metadata Initiative, "DCMI Metadata Terms",
January 2020, <https://www.dublincore.org/specifications/
dublin-core/dcmi-terms/>.

[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<https://www.rfc-editor.org/info/rfc5988>.

[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>.

[W3C.REC-rdf11-concepts]
Cyganiak, R., Ed., Wood, D., Ed., and M. Lanthaler, Ed.,
"RDF 1.1 Concepts and Abstract Syntax", W3C Consortium
Recommendation REC-rdf11-concepts, February 2014,
<https://www.w3.org/TR/rdf11-concepts/>.

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] that maps the JSON keys to corresponding
Linked Data terms. And, as per Section 6.1 of [W3C.REC-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 field.

Figure 19 shows the response to an HTTP GET against the URI of a link
set resource and illustrates this approach to support the discovery
of a JSON-LD context. This example is inspired by the GS1
implementation and shows a link set that uses relation types from the
GS1 vocabulary at <https://www.gs1.org/voc/> that are expressed as
HTTP URIs.

HTTP/1.1 200 OK
Date: Mon, 11 Oct 2021 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: 1532

{
"linkset": [
{
"anchor": "https://id.gs1.org/01/09506000149301",
"https://gs1.org/voc/pip": [
{
"href": "https://example.com/en/defaultPage",
"hreflang": [
"en"
],
"type": "text/html",
"title": "Product information"
},
{
"href": "https://example.com/fr/defaultPage",
"hreflang": [
"fr"
],
"title": "Information produit"
}
],
"https://gs1.org/voc/whatsInTheBox": [
{
"href": "https://example.com/en/packContents/GB",
"hreflang": [
"en"
],
"title": "What's in the box?"
},
{
"href": "https://example.com/fr/packContents/FR",
"hreflang": [
"fr"
],
"title": "Qu'y a-t-il dans la boite?"
},
{
"href": "https://example.com/fr/packContents/CH",
"hreflang": [
"fr"
],
"title": "Qu'y a-t-il dans la boite?"
}
],
"https://gs1.org/voc/relatedVideo": [
{
"href": "https://video.example",
"hreflang": [
"en",
"fr"
],
"title*": [
{
"value": "See it in action!",
"language": "en"
},
{
"value": "Voyez-le en action!",
"language": "fr"
}
]
}
]
}
]
}

Figure 19: Using a Typed Link typed link to Support support the Discovery discovery of a JSON-
LD Context context for a Linkset linkset

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 20. This particular JSON-LD
context maps "application/linkset+json" representations of link sets
to Dublin Core Terms terms [DCMI-TERMS]. Note that the "linkset" entry in
the JSON-LD context is introduced to support links with the "linkset"
relation type in link sets.

HTTP/1.1 200 OK
Content-Type: application/ld+json
Content-Length: 658

{
"@context": [
{
"@version": 1.1,
"@vocab": "https://gs1.org/voc/",
"anchor": "@id",
"href": "@id",
"linkset": {
"@id": "@graph",
"@context": {
"linkset": "linkset"
}
},
"title": {
"@id": "http://purl.org/dc/terms/title"
},
"title*": {
"@id": "http://purl.org/dc/terms/title"
},
"type": {
"@id": "http://purl.org/dc/terms/format"
}
},
{
"language": "@language",
"value": "@value",
"hreflang": {
"@id": "http://purl.org/dc/terms/language",
"@container": "@set"
}
}
]
}

Figure 20: JSON-LD Context Mapping context mapping to Dublin Core Terms terms

Applying the JSON-LD context of Figure 20 to the link set of
Figure 19 allows transforming the "application/linkset+json" link set
to an RDF link set. Figure 21 shows the latter represented by means
of the "text/turtle" RDF serialization.

<https://example.com/en/defaultPage>
<http://purl.org/dc/terms/format>
"text/html" .
<https://example.com/en/defaultPage>
<http://purl.org/dc/terms/language>
"en" .
<https://example.com/en/defaultPage>
<http://purl.org/dc/terms/title>
"Product information" .
<https://example.com/en/packContents/GB>
<http://purl.org/dc/terms/language>
"en" .
<https://example.com/en/packContents/GB>
<http://purl.org/dc/terms/title>
"What's in the box?" .
<https://example.com/fr/defaultPage>
<http://purl.org/dc/terms/language>
"fr" .
<https://example.com/fr/defaultPage>
<http://purl.org/dc/terms/title>
"Information produit" .
<https://example.com/fr/packContents/CH>
<http://purl.org/dc/terms/language>
"fr" .
<https://example.com/fr/packContents/CH>
<http://purl.org/dc/terms/title>
"Qu'y a-t-il dans la boite?" .
<https://example.com/fr/packContents/FR>
<http://purl.org/dc/terms/language>
"fr" .
<https://example.com/fr/packContents/FR>
<http://purl.org/dc/terms/title>
"Qu'y a-t-il dans la boite?" .
<https://id.gs1.org/01/09506000149301>
<https://gs1.org/voc/pip>
<https://example.com/en/defaultPage> .
<https://id.gs1.org/01/09506000149301>
<https://gs1.org/voc/pip>
<https://example.com/fr/defaultPage> .
<https://id.gs1.org/01/09506000149301>
<https://gs1.org/voc/relatedVideo>
<https://video.example> .
<https://id.gs1.org/01/09506000149301>
<https://gs1.org/voc/whatsInTheBox>
<https://example.com/en/packContents/GB> .
<https://id.gs1.org/01/09506000149301>
<https://gs1.org/voc/whatsInTheBox>
<https://example.com/fr/packContents/CH> .
<https://id.gs1.org/01/09506000149301>
<https://gs1.org/voc/whatsInTheBox>
<https://example.com/fr/packContents/FR> .
<https://video.example>
<http://purl.org/dc/terms/language>
"en" .
<https://video.example>
<http://purl.org/dc/terms/language>
"fr" .
<https://video.example>
<http://purl.org/dc/terms/title>
"See it in action!"@en .
<https://video.example>
<http://purl.org/dc/terms/title>
"Voyez-le en action!"@fr .

Figure 21: RDF Serialization serialization of the Linkset Resulting linkset resulting from
Applying
applying the JSON-LD Context context

Acknowledgements

Thanks for comments and suggestions provided by Phil Archer,
Dominique Guinard, Mark Nottingham, Julian Reschke, Rob Sanderson,
Stian Soiland-Reyes, Sarven Capadisli, and Addison Phillips.

Authors' Addresses

Erik Wilde
Axway
Email: erik.wilde@dret.net

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