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draft-ietf-httpapi-linkset-10.xml
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<?xml version="1.0" encoding="UTF-8"?>
<?rfc compact="yes" ?>
<?rfc subcompact="no" ?>
<?rfc toc="yes" ?>
<?rfc sortrefs="yes" ?>
<?rfc symrefs="yes" ?>
<rfc category="std" ipr="trust200902" docName="draft-ietf-httpapi-linkset-10" version="3" xmlns:xi="http://www.w3.org/2001/XInclude">
<front>
<title abbrev="Linkset">Linkset: Media Types and a Link Relation Type for Link Sets</title>
<author initials="E." surname="Wilde" fullname="Erik Wilde">
<organization>Axway</organization>
<address>
<email>[email protected]</email>
<uri>http://dret.net/netdret/</uri>
</address>
</author>
<author initials="H." surname="Van de Sompel" fullname="Herbert Van de Sompel">
<organization>Data Archiving and Networked Services</organization>
<address>
<email>[email protected]</email>
<uri>https://orcid.org/0000-0002-0715-6126</uri>
</address>
</author>
<date day="5" month="May" year="2022"/>
<abstract>
<t>This specification defines two formats and respective media types for representing sets of links as stand-alone documents. 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.</t>
</abstract>
<note title="Note to Readers">
<t>Please discuss this draft on the "Building Blocks for HTTP APIs" mailing list (<eref target="https://www.ietf.org/mailman/listinfo/httpapi"/>).</t>
<t>Online access to all versions and files is available on GitHub (<eref target="https://github.com/ietf-wg-httpapi/linkset"/>).</t>
</note>
</front>
<middle>
<section title="Introduction" anchor="introduction">
<t>Resources on the Web often use typed Web Links <xref target="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.</t>
<t>Therefore, this specification defines two formats for representing sets of Web Links and their attributes as stand-alone documents. One serializes links in the same format as 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 that supports discovery of any resource that conveys a set of links as a stand-alone document.</t>
</section>
<section title="Terminology" anchor="terminology">
<t>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 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only when, they appear in all capitals, as shown here.</t>
<t>This specification uses the terms "link context" and "link target" in the same manner as <xref target="RFC8288"/>.</t>
<t>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 <xref target="I-D.ietf-httpbis-semantics" section="5.5"/>, line breaks are deprecated in values for HTTP fields; only whitespaces and
tabs are supported as separators.</t>
<!--
<t>Additionally, this specification uses the following terms for types of resources involved
in providing links by reference:</t>
<t><list style="symbols">
<t>A "linkset resource" is a resource that conveys a set of links. <xref target="linkset-formats"/>
defines two representations for a set of links, based on the abstract link model defined in
<xref target="RFC8288"/>.</t>
<t>An "origin resource" is a resource that participates in one or more links provided
by a linkset resource. An origin resource can support discovery of an associated
linkset resource by using the relation type defined in <xref target="linkset-link-relation"/>.
As such, from the perspective of the origin resource, the links conveyed by the linkset resource
are provided by reference.</t>
</list></t>
-->
</section>
<section title="Use Cases and Motivation" anchor="use-cases">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<section title="Third-Party Links" anchor="third-party">
<t>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:</t>
<ul>
<li>Providing links in which the resource is involved not just as link context but
also as link target, with a different resource being the link context.</li>
<li>Providing links pertaining to the resource that the server hosting that
resource is not aware of.</li>
<li>External management of links pertaining to the resource in a special-purpose link
management service.</li>
</ul>
<t>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:</t>
<ul>
<li>Links are provided in a document that uses a well-defined media type.</li>
<li>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.</li>
</ul>
<t>These requirements are addressed in this specification through the definition of two media types and a link relation type, respectively.</t>
<!--
<t>In order for the server hosting the resource to provide an up-to-date and complete set of links
in which that resource participates,
the server would need to obtain the links from the third-party resource,
and embed them in the resource's
representations prior to responding to a client. Doing so would increase latency and load,
which may be unnecessary if a client is not intent on consuming these links. Providing links by means of a standalone document,
removes the server-to-server communication and resulting overhead required to obtain
links. Instead, the consumer of the resource can decide if they need the additional links as context
for the resource and only access the third-party resource that provides the links if they do.</t>
-->
</section>
<section title="Challenges Writing to HTTP Link Header Field" anchor="header-writing">
<t>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.</t>
</section>
<section title="Large Number of Links" anchor="link-volume">
<t>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.</t>
<t><xref target="I-D.ietf-httpbis-semantics" section="15">HTTP</xref> 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 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
(to the web server implementation) nature of the size of HTTP "Link" header fields.</t>
<!--
<t>In more extreme scenarios it is conceivable that the number of links to be conveyed becomes so
large that even a standalone document would become too large.
For example, this could be the case for highly interlinked resources and when links are provided in which such
resources participates as both link context and link target.
In such cases, the links could be delivered incrementally, for example, by means of a paged resource model.</t>
-->
</section>
</section>
<section title="Document Formats for Sets of Links" anchor="linkset-formats">
<t>This section specifies two document formats to convey a set of links. Both are based on the abstract model
specified in <xref target="RFC8288" section="2">Web Linking</xref>
that defines a link as consisting of a "link context", a "link relation type", a "link target",
and optional "target attributes":</t>
<ul>
<li>The format defined in <xref target="linkset-native"/> is nearly identical to the field value
of the HTTP "Link" header field as specified in <xref target="RFC8288" section="3"/>.</li>
<li>The format defined in <xref target="linkset-json"/> is expressed in <xref target="RFC8259">JSON</xref>.</li>
</ul>
<t>Links provided in the HTTP Link header 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 re-used 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.</t>
<t>For links provided in the HTTP Link header 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 re-used outside the context
of an HTTP interaction, it is RECOMMENDED to make them self-contained by adhering to the following guidelines:</t>
<ul>
<li>For every link provided in the set of links, explicitly provide the link context
using the "anchor" attribute.</li>
<li>For link context ("anchor" attribute) and link target ("href" attribute), use URI references that are not relative references (as defined in <xref target="RFC3986" section="4.1"/>).
</li>
</ul>
<t>If these recommendations are not followed, interpretation of links in link set documents will depend on which URI is used as context.</t>
<t>For a "title" attribute provided on a link in the HTTP Link header, the language in which the title is expressed is provided by
the Content-Language header 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
re-used 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.
With this regard, note that language tags are matched case-insensitively (see <xref target="RFC5646" section="2.1.1"/>).
If this recommendation is not followed, accurately determining the language of titles provided on links in link set documents will not be possible.</t>
<t>Note also that <xref target="RFC8288" section="3.3"/> deprecates the "rev" construct that was provided by <xref target="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.</t>
<section title="HTTP Link Document Format: application/linkset" anchor="linkset-native">
<t>This document format is nearly identical to the field value of the
HTTP "Link" header field as defined in
<xref target="RFC8288" section="3"/>, more specifically by
its ABNF <xref target="RFC5234"/> production rule for "Link" and subsequent ones. It differs from the format for field values of the
HTTP "Link" header 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.</t>
<t>The assigned media type for this format is "application/linkset".</t>
<t>When converting an "application/linkset" document to a field value for the
HTTP "Link" header, newline characters MUST be removed or MUST be replaced by white space (SP) in order to comply with
<xref target="I-D.ietf-httpbis-semantics" section="5.5"/>.</t>
<t>Implementers of "application/linkset" link sets should strive to make them self-contained
by following the recommendations regarding their use outside the context of an HTTP interaction provided in
<xref target="linkset-formats"/>.</t>
<t>It should be noted that the "application/linkset" format specified here is different from the "application/link-format"
format specified in <xref target="RFC6690"/> in that the former fully matches the
field value of the HTTP "Link" header field as defined in <xref target="RFC8288" section="3"/>, whereas
the latter introduces constraints on that definition to meet requirements for Constrained RESTful Environments (CoRE).</t>
</section>
<section title="JSON Document Format: application/linkset+json" anchor="linkset-json">
<t>This document format uses JSON <xref target="RFC8259"/> as the syntax to represent
a set of links. The set of links follows the abstract model defined by Web Linking <xref target="RFC8288" section="2"/>.</t>
<t>The assigned media type for this format is "application/linkset+json".</t>
<t>In the interests of interoperability "application/linkset+json" link sets MUST be encoded using UTF-8 as per
<xref target="RFC8259" section="8.1"/>.</t>
<t>Implementers of "application/linkset+json" link sets should strive to make them self-contained
by following the recommendations regarding their use outside the context of an HTTP interaction
provided in <xref target="linkset-formats"/>.</t>
<t>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 6.8. of <xref target="W3C.REC-json-ld-20140116"/>.
Communities of practice can decide which context best meets their application needs.
<xref target="appendix-1"/> shows an example of a possible context that, when added to a JSON serialization,
allows it to be interpreted as Resource Description Framework (RDF) <xref target="W3C.REC-rdf11-concepts-20140225"/> data.</t>
<section title="Set of Links" anchor="linkset-in-json">
<t>In the JSON representation of a set of links:</t>
<ul>
<li>A set of links is represented in JSON as an object which MUST contain "linkset" as its sole member.</li>
<li>The value of the "linkset" member is an array in which a distinct JSON object -
the "link context object" (see <xref target="linkset-json-link-context-object"/>) -
is used to represent links that have the same link context.</li>
<li>Even if there is only one link context object, it MUST be wrapped in an array.</li>
</ul>
</section>
<section title="Link Context Object" anchor="linkset-json-link-context-object">
<t>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:</t>
<ul>
<li>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 <xref target="RFC3986" section="4.1"/>.</li>
<li><t>
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 <xref target="linkset-json-link-target-object"/>) -
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:</t>
<ul>
<li>For registered relation types (<xref target="RFC8288" section="2.1.1"/>),
the name of this member is the registered name of the relation type.</li>
<li>For extension relation types (<xref target="RFC8288" section="2.1.2"/>),
the name of this member is the URI that uniquely represents the relation type.</li>
</ul></li>
<li>Even if there is only one link target object it MUST be wrapped in an array.</li>
</ul>
</section>
<section title="Link Target Object" anchor="linkset-json-link-target-object">
<t>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:</t>
<ul>
<li>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 <xref target="RFC3986" section="4.1"/>. 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": "").</li>
<li>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 <xref target="json-target-attributes"/>.</li>
</ul>
<t>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.</t>
<figure>
<sourcecode type="json"><![CDATA[
{ "linkset":
[
{ "anchor": "https://example.net/bar",
"next": [
{"href": "https://example.com/foo"}
]
}
]
}]]>
</sourcecode>
</figure>
<t>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.</t>
<figure>
<sourcecode type="json"><![CDATA[
{ "linkset":
[
{ "anchor": "https://example.net/bar",
"item": [
{"href": "https://example.com/foo1"},
{"href": "https://example.com/foo2"}
]
}
]
}]]>
</sourcecode>
</figure>
<t>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.</t>
<figure>
<sourcecode type="json"><![CDATA[
{ "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"}
]
}
]
}]]>
</sourcecode>
</figure>
</section>
<section title="Link Target Attributes" anchor="json-target-attributes">
<t>A link may be further qualified by target attributes as defined by <xref target="RFC8288" section="2">Web Linking</xref>.
Three types of attributes exist:</t>
<ul>
<li>Serialisation-defined attributes described in <xref target="RFC8288" section="3.4.1">Web Linking</xref>.</li>
<li>Extension attributes defined and used by communities as allowed by
<xref target="RFC8288" section="3.4.2"/>.</li>
<li>Internationalized versions of the "title" attribute defined by <xref target="RFC8288"/> and of extension attributes
allowed by <xref target="RFC8288" section="3.4"/>.</li>
<!--
<li>Attributes defined by <xref target="I-D.nottingham-link-hint">Link Hints</xref> and the associated HTTP Link Hint Registry.</li>
-->
</ul>
<t>The handling of these different types of attributes is described in the sections below.</t>
<section title="Target Attributes Defined by Web Linking" anchor="json-target-attributes-existing">
<t><xref target="RFC8288" section="3.4.1"/> 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:</t>
<ul>
<li>"hreflang": The "hreflang" target attribute,
defined as optional and repeatable by <xref target="RFC8288"/>,
MUST be represented by an "hreflang" member, and its value MUST be 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 <xref target="RFC8288"/> syntax.</li>
<li>"media": The "media" target attribute,
defined as optional and not repeatable by <xref target="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 in the <xref target="RFC8288"/> syntax.</li>
<li>"type": The "type" target attribute,
defined as optional and not repeatable by <xref target="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 in the <xref target="RFC8288"/> syntax.</li>
<li>"title": The "title" target attribute,
defined as optional and not repeatable by <xref target="RFC8288"/>,
MUST be represented by a "title"
member in the link target object, and its value MUST be a JSON string.</li>
<li>"title*": The "title*" target attribute,
defined as optional and not repeatable by <xref target="RFC8288"/>,
is motivated by character encoding
and language issues and follows the model defined in <xref target="RFC8187"/>.
The details of the JSON
representation that applies to title* are described in
<xref target="linkset-json-i18n-link-parameter"/>.
</li>
</ul>
<t>The following example illustrates how the repeatable "hreflang" and the
not repeatable "type" target attributes are represented in a link target object.</t>
<figure>
<sourcecode type="json"><![CDATA[
{ "linkset":
[
{ "anchor": "https://example.net/bar",
"next": [
{ "href": "https://example.com/foo",
"type": "text/html",
"hreflang": [ "en" , "de" ]
}
]
}
]
}]]>
</sourcecode>
</figure>
</section>
<section title="Internationalized Target Attributes" anchor="linkset-json-i18n-link-parameter">
<t>In addition to the target attributes described in <xref target="json-target-attributes-existing"/>,
<xref target="RFC8288" section="3.4"/> also supports
attributes that follow the content model of <xref target="RFC8187"/>.
In <xref target="RFC8288"/>, these target
attributes are recognizable by the use of a trailing asterisk in the attribute name,
such as "title*".
The content model of <xref target="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.</t>
<t>The JSON serialization for these target attributes MUST be
as follows:</t>
<ul>
<li>An internationalized target attribute is represented as a member of the link context object with
the same name (including the *) as the attribute.</li>
<li>The character encoding information
as prescribed by <xref target="RFC8187"/> is not preserved; instead, the
content of the internationalized attribute is represented as a JSON string.</li>
<li>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 <xref target="RFC5646"/>
for the language in which the attribute content is conveyed.
</li>
</ul>
<t>The following example illustrates how the "title*" target attribute
defined by <xref target="RFC8288" section="3.4.1"/> is represented in a link target object.</t>
<figure>
<sourcecode type="json"><![CDATA[
{ "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" } ]
}
]
}
]
}]]>
</sourcecode>
</figure>
<t>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 <xref target="RFC8187"/> when transcoding between the "application/linkset" and the "application/linkset+json" formats.</t>
</section>
<section title="Extension Target Attributes" anchor="json-target-attributes-extension">
<t>Extension target attributes are attributes that are not defined by <xref target="RFC8288" section="3.4.1"/>
(as listed in <xref target="json-target-attributes-existing"/>), 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.</t>
<t>The JSON serialization for these target attributes MUST be
as follows:</t>
<ul>
<li>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.</li>
<li>The value of an extension attribute MUST be represented by an array, even if there only is one value to be represented.</li>
<li>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.</li>
<li>If the extension attribute has a name with a trailing asterisk
(it follows the content model of <xref target="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 <xref target="linkset-json-i18n-link-parameter"/>.</li>
</ul>
<t>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 <xref target="RFC8187"/> and therefore is encoded according to the serialization described in <xref target="linkset-json-i18n-link-parameter"/>.</t>
<figure>
<sourcecode type="json"><![CDATA[
{ "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" } ]
}
]
}
]
}]]>
</sourcecode>
</figure>
</section>
<!--
<section title="Target Attributes in the HTTP Link Hints Registry" anchor="json-target-attributes-hints">
<t>In order to augment interoperability regarding target attributes,
<xref target="I-D.nottingham-link-hint">Link Hints</xref> defines a registration mechanism for
target attributes, an associated HTTP Link Hint Registry, and a set of attributes to initially populate that registry.
The initial registry, for example, includes the "allow" attribute that can be used to convey the HTTP methods that are supported
to interact with the target resource.</t>
<t>The JSON serialization for these target attributes MUST be as follows:</t>
<t><list style="symbols">
<t>A Link Hint target attribute is represented as a member of the link context object with the same name of the attribute as registered in the HTTP Link Hint Registry.</t>
<t>The value of a Link Hint target attribute MUST adhere to the content model specified for the attribute in the HTTP Link Hint Registry.</t>
<t>Internationalization of Link Hint target attributes is not applicable.</t>
</list></t>
<t>The example shows a link target object with the "Type" attribute defined by <xref target="RFC8187"/> and the "allow" attribute that is registered in the HTTP Link Hint Registry.
The registry specifies the content model for this attribute as an array of strings. Therefor, that same content model is used in the "application/linkset+json"
serialization of sets of links.</t>
<figure>
<sourcecode type="json"><![CDATA[
{ "linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{ "href": "http://example.com/foo",
"type": "text/html",
"allow": [ "GET", "POST" ]
}
]
}
]
}]]>
</sourcecode>
</figure>
</section>
-->
</section>
<section title="JSON Extensibility" anchor="json-extensibility">
<t>The Web linking model (<xref target="RFC8288"/>) provides for the use of extension target attributes as discussed in
<xref target="json-target-attributes-extension"/>.
The use of other forms of extensions is NOT RECOMMENDED.
Limiting the JSON format in this way 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.
</t>
<t>Cases may exist in which the use of extensions other than those of <xref target="json-target-attributes-extension"/> may be useful.
For example, when a link set publisher needs to include descriptive or technical metadata for internal consumption.
In case 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.</t>
</section>
</section>
</section>
<section title='The "profile" parameter for media types to Represent Sets of Links' anchor="profile-attribute">
<t>As a means to convey specific constraints or conventions (as per <xref target="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" detailed in
<xref target="linkset-native"/> and <xref target="linkset-json"/>, respectively.
For example, the parameter could be used to indicate that a link set uses a specific, limited set of link relation
types.</t>
<t>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 Profile URI Registry in the manner specified by <xref target="RFC7284"/>.</t>
<t>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.</t>
<t><xref target="profile-parameter-example"/> shows an example of using the "profile" parameter in conjunction with the
"application/linkset+json" media type.</t>
</section>
<section title='The "linkset" Relation Type for Linking to a Set of Links' anchor="linkset-link-relation">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<!-- <t>The use of a link with the "linkset" relation
type does not preclude the provision of links with other relation types,
i.e. the resource that is the link context can provide typed links other than a "linkset" link.
Therefore, the
effective set of links provided by that resource is the union of the links that the
resource itself provides, and the links provided by the resource that is the target of the "linkset" link -
in which it participates.</t> -->
<t>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 link context and link target, and MAY ignore links from
the link set in which the context of the "linkset" link does not participate.</t>
<t>A user agent that follows a "linkset" link and obtains links for which anchors and targets are expressed as
relative references (as per <xref target="RFC3986" section="4.1"/>) MUST determine what the context is for these links; it SHOULD ignore links for which it is unable to
unambiguously make that determination.</t>
<t>As a means to convey specific constraints or conventions (as per <xref target="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 Profile URI Registry in the manner specified by <xref target="RFC7284"/>.
<xref target="profile-attribute-example"/> 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.</t>
<!--
<t>There is no constraint on the target URI of a link with the "linkset" relation type;
designing and using these URIs is left to the discretion of implementers.</t>
<t>In common scenarios (the origin resource is distinct from the
linkset resource), it is essential for linkset representations to
make the URI of the origin resource explicit for those links in
which the origin resource acts as link context.</t>
-->
<!--
<t>If an origin resource provides a "linkset" link pointing at a linkset resource,
and that linkset resource provides a "linkset" link in turn,
then this latter link points at links pertaining to the linkset resource.
This means that in the context of the latter link, the linkset resource is an origin
resource. This means that linkset relations are not transitive, and it is up to a client
to decide whether they follow chains of "linkset" links or not.</t>
-->
</section>
<section title="Examples" anchor="linkset-link-examples">
<t><xref target="linkset-link-link-format"/> and <xref target="linkset-link-json-format"/>
show examples whereby a set of links is provided as "application/linkset" and "application/linkset+json" documents, respectively.
<xref target="linkset-rel-example"/> illustrates the use of the "linkset" link relation type to support discovery of sets of links and
<xref target="profile-examples"/> shows how to convey profile information pertaining to a link set.</t>
<section title="Set of Links Provided as application/linkset" anchor="linkset-link-link-format">
<t><xref target="Request1"/> shows a client issuing an
HTTP GET request against resource <https://example.org/links/resource1>.</t>
<figure title="Client HTTP GET request" anchor="Request1">
<artwork type="http-message"><![CDATA[
GET /links/resource1 HTTP/1.1
Host: example.org
]]>
</artwork>
</figure>
<t><xref target="Response1.1"/> shows the response to the GET request of
<xref target="Request1"/>. 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".</t>
<figure title="Response to HTTP GET includes a set of links" anchor="Response1.1">
<artwork type="http-message"><![CDATA[
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"
]]>
</artwork>
</figure>
</section>
<section title="Set of Links Provided as application/linkset+json" anchor="linkset-link-json-format">
<t><xref target="Request4"/> 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.</t>
<figure title="Client HTTP GET request expressing preference for "application/linkset+json" response" anchor="Request4">
<artwork type="http-message"><![CDATA[
GET links/resource1 HTTP/1.1
Host: example.org
Accept: application/linkset+json
]]></artwork>
</figure>
<t><xref target="Response4.1"/> shows the response to the HTTP GET request of <xref target="Request4"/>.
The set of links is serialized according to the media type "application/linkset+json".</t>
<figure title="Response to the client's request for the set of links" anchor="Response4.1">
<artwork type="http-message"><![CDATA[
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"}
]
}
]
}]]>
</artwork>
</figure>
</section>
<section title="Discovering a Link Set via the "linkset" Link Relation Type" anchor="linkset-rel-example">
<t><xref target="Request3"/> shows a client issuing an
HTTP HEAD request against resource
<https://example.org/resource1>.</t>
<figure title="Client HTTP HEAD request" anchor="Request3">
<artwork type="http-message"><![CDATA[
HEAD resource1 HTTP/1.1
Host: example.org
]]>
</artwork>
</figure>
<t><xref target="Response3.1"/> shows the response to the HEAD request of
<xref target="Request3"/>. 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".</t>
<figure title="Response to HTTP HEAD request" anchor="Response3.1">
<artwork type="http-message"><![CDATA[
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
]]>
</artwork>
</figure>
</section>
<section title="Link Set Profiles" anchor="profile-examples">
<t>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).</t>
<section title="Using a "profile" Attribute with a "linkset" Link" anchor="profile-attribute-example">
<t><xref target="Request_pr_at"/> shows a client issuing an
HTTP HEAD request against trade item 09506000134352 at <https://id.gs1.org/01/9506000134352>.</t>
<figure title="Client HTTP HEAD request" anchor="Request_pr_at">
<artwork type="http-message"><![CDATA[
HEAD /01/9506000134352 HTTP/1.1
Host: id.gs1.org
]]></artwork>
</figure>
<t><xref target="Response_pr_at"/> shows the server's response to the request of
<xref target="Request_pr_at"/>, 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.</t>
<figure title="Response to the client's HEAD request including a "profile" attribute for the "linkset" link" anchor="Response_pr_at">
<artwork type="http-message"><![CDATA[
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/
]]>
</artwork>
</figure>
</section>
<section title="Using a "profile" Parameter with a Link Set Media Type" anchor="profile-parameter-example">
<t><xref target="Request_pr_par"/> 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 <xref target="profile-attribute-example"/>.</t>
<figure title="Client HTTP HEAD request" anchor="Request_pr_par">
<artwork type="http-message"><![CDATA[
HEAD /01/9506000134352?linkType=all HTTP/1.1
Host: id.gs1.org
]]></artwork>
</figure>
<t><xref target="Response_pr_par"/> shows the server's response to the request of <xref target="Request_pr_par"/>. Note the "profile" parameter for the application/linkset+json media type, which has as value the same Profile URI <https://www.gs1.org/voc/?show=linktypes> as was used in <xref target="Response_pr_at"/>.</t>
<figure title="Response to the client's HEAD request including a "profile" parameter for the "application/linkset+json" media type" anchor="Response_pr_par">
<artwork type="http-message"><![CDATA[
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
]]></artwork>
</figure>
</section>
<section title="Using a Link with a "profile" Link Relation Type" anchor="profile-link-example">
<t>Note that the response <xref target="Response_pr_par"/> from the link set resource is equivalent to the response shown in <xref target="Response_pr_link"/>, which leverages the "profile" link relation type defined in <xref target="RFC6906"/>.</t>
<figure title="Response to the client's HEAD request including a "profile" link" anchor="Response_pr_link">
<artwork type="http-message"><![CDATA[
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
]]></artwork>
</figure>
<t>A link with a "profile" link relation type as shown in <xref target="Response_pr_link"/> can also be conveyed in the link set document itself. This is illustrated by <xref target="Response_pr_linkset"/>. 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 anchor and the Profile URI as target. Multiple Profile URIs are handled by using multiple "href" members.</t>
<figure title="A Link Set that declares the profile it complies with using a "profile" link" anchor="Response_pr_linkset">
<sourcecode type="json"><![CDATA[
{ "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"}
]
}
]
}]]>
</sourcecode>
</figure>
</section>
</section>
</section>
<section title="IANA Considerations" anchor="iana-considerations">
<section title="Link Relation Type: linkset">
<t>The link relation type below should be registered by IANA in the Link Relation Type Registry
as per <xref target="RFC8288" section="4.2">Web Linking</xref>:</t>
<ul empty="true">
<li>Relation Name: linkset</li>
<li>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.</li>
<li>Reference: [[ This document ]]</li>
</ul>
</section>
<section title="Media Type: application/linkset">
<t>The Internet media type application/linkset for a linkset encoded as described in <xref target="linkset-native"/>
should be registered by IANA in the Media Type Registry as per <xref target="RFC6838"/>.</t>
<ul empty="true">
<li>Type name: application</li>
<li>Subtype name: linkset</li>
<li>Required parameters: N/A</li>
<li>Optional parameters: profile</li>
<li>Encoding considerations: Linksets are encoded according to the definition of <xref target="RFC8288"/>. The encoding of <xref target="RFC8288"/> is based on the general encoding rules of <xref target="I-D.ietf-httpbis-semantics"/>, with the addition of allowing indicating character encoding and language for specific parameters as defined by <xref target="RFC8187"/>.</li>
<li>Security considerations: The security considerations of [[ This document ]] apply.</li>
<li>Interoperability considerations: N/A</li>
<li>Published specification: [[ This document ]]</li>
<li>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.</li>
<li><t>Additional information:</t>
<ul empty="true">
<li>Magic number(s): N/A</li>
<li>File extension(s): This media type does not propose a specific extension.</li>
<li>Macintosh file type code(s): TEXT</li>
</ul>
</li>
<li>Person & email address to contact for further information: Erik Wilde <[email protected]></li>
<li>Intended usage: COMMON</li>
<li>Restrictions on usage: none</li>
<li>Author: Erik Wilde <[email protected]></li>
<li>Change controller: IETF</li>
</ul>
</section>
<section title="Media Type: application/linkset+json">
<t>The Internet media type application/linkset+json for a linkset encoded as described in <xref target="linkset-json"/>
should be registered by IANA in the Media Type Registry as per <xref target="RFC6838"/>.</t>
<ul empty="true">
<li>Type name: application</li>
<li>Subtype name: linkset+json</li>
<li>Required parameters: N/A</li>
<li>Optional parameters: profile</li>
<li>Encoding considerations: The encoding considerations of <xref target="RFC8259"/> apply</li>
<li>Security considerations: The security considerations of [[ This document ]] apply.</li>
<li>Interoperability considerations: The interoperability considerations of <xref target="RFC8259"/> apply.</li>
<li>Published specification: [[ This document ]]</li>
<li>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.</li>
<li><t>Additional information:</t>
<ul empty="true">
<li>Magic number(s): N/A</li>
<li>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.</li>
<li>Macintosh file type code(s): TEXT</li>
</ul>
</li>
<li>Person & email address to contact for further information: Erik Wilde <[email protected]></li>
<li>Intended usage: COMMON</li>
<li>Restrictions on usage: none</li>
<li>Author: Erik Wilde <[email protected]></li>
<li>Change controller: IETF</li>
</ul>
</section>
</section>
<section title="Security Considerations" anchor="security-considerations">
<t>The security considerations of <xref target="RFC3986" section="7"/> apply, as well as those of Web Linking <xref target="RFC8288"/> as long as the latter are not specifically discussing the risks of exposing information in HTTP header fields.</t>
<t>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 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.</t>
<t>For the specific mechanisms defined in this specification, two security considerations should be taken into account:</t>
<ul>
<li>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.</li>
<li>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.</li>
</ul>
</section>
</middle>
<back>
<references title="Normative References">
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3986.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5234.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5646.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml4/reference.W3C.REC-json-ld-20140116.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6838.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>