Internet DRAFT - draft-wilde-linkset
draft-wilde-linkset
Network Working Group E. Wilde
Internet-Draft Axway
Intended status: Informational H. Van de Sompel
Expires: April 19, 2021 Data Archiving and Networked Services
October 16, 2020
Linkset: Media Types and a Link Relation Type for Link Sets
draft-wilde-linkset-07
Abstract
This specification defines two document formats and respective media
types for representing sets of links as stand-alone resources. One
format is JSON-based, the other aligned with the format for
representing links in the HTTP "Link" header field. This
specification also introduces a link relation type to support
discovery of sets of links.
Note to Readers
Please discuss this draft on the ART mailing list
(<https://www.ietf.org/mailman/listinfo/art>).
Online access to all versions and files is available on GitHub
(<https://github.com/dret/I-D/tree/master/linkset>).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 19, 2021.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Third-Party Links . . . . . . . . . . . . . . . . . . . . 4
3.2. Challenges Writing to HTTP Link Header Field . . . . . . 4
3.3. Large Number of Links . . . . . . . . . . . . . . . . . . 5
4. Document Formats for Sets of Links . . . . . . . . . . . . . 5
4.1. HTTP Link Document Format: application/linkset . . . . . 6
4.2. JSON Document Format: application/linkset+json . . . . . 6
4.2.1. Set of Links . . . . . . . . . . . . . . . . . . . . 7
4.2.2. Link Context Object . . . . . . . . . . . . . . . . . 7
4.2.3. Link Target Object . . . . . . . . . . . . . . . . . 8
4.2.4. Link Target Attributes . . . . . . . . . . . . . . . 9
5. The "linkset" Relation Type for Linking to a Set of Links . . 13
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Set of Links Provided as application/linkset . . . . . . 14
6.2. Set of Links Provided as application/linkset+json . . . . 15
6.3. Discovering a Link Set via the "linkset" Link Relation
Type . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 18
7.1. GS1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Open Journal Systems (OJS) . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8.1. Link Relation Type: linkset . . . . . . . . . . . . . . . 19
8.2. Media Type: application/linkset . . . . . . . . . . . . . 20
8.2.1. IANA Considerations . . . . . . . . . . . . . . . . . 20
8.3. Media Type: application/linkset+json . . . . . . . . . . 21
9. Security Considerations . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . 24
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Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 24
Appendix B. JSON-LD Context . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
Resources on the Web often use typed Web Links [RFC8288], either
embedded in resource representations, for example using the <link>
element for HTML documents, or conveyed in the HTTP "Link" header 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.
Therefor, this specification defines two document formats and
associated media types to represent sets of links. It also defines
the "linkset" relation type that supports discovery of any resource
that conveys a set of links as a stand-alone document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This specification uses the terms "link context" and "link target" as
defined in [RFC8288]. These terms respectively correspond with
"Context IRI" and "Target IRI" as used in [RFC5988]. Although
defined as IRIs, in common scenarios they are also URIs.
In the examples provided in this document, links in the HTTP "Link"
header are shown on separate lines in order to improve readability.
Note, however, that as per Section 3.2 of [RFC7230], line breaks are
not allowed in values for HTTP headers; only whitespaces and tabs are
supported as seperators.
3. Scenarios
The following sections outline scenarios in which providing links by
means of a standalone document instead of in an HTTP "Link" header
field or as links embedded in the resource representation is
advantageous or necessary.
For all scenarios, links could be provided by means of a stand-alone
document that is formatted according to the JSON-based serialization,
the serialization aligned with the HTTP "Link" header format, or
both. The former serialization is motivated by the widespread use of
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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" header and to allow 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 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:
o Providing links in which the resource is involved not just as link
context but also as link target.
o Providing links pertaining to the resource that the server hosting
that resource is not aware of.
o External management of links pertaining to the resource in a
special-purpose link management service.
In such cases, links pertaining to a resource can be provided by
another, specific resource. That specific resource may be managed by
the same or by another custodian as the resource to which the links
pertain. For clients intent on consuming links provided in that
manner, it would be beneficial if the following conditions were met:
o Links are provided in a document that uses a well-defined media
type.
o The resource to which the provided links pertain is able to link
to the resource that provides these links using a well-known link
relation type.
These requirements are addressed in this specification through the
definition of two media types and a link relation type, respectively.
3.2. Challenges Writing to HTTP Link Header Field
In some cases, it is not straightforward to write links to the HTTP
"Link" header field from an application. This can, for example, be
the case because not all required link information is available to
the application or because the application does not have the
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capability to directly write HTTP headers. 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 header to become unpredictable.
This can be the case when links are determined dynamically dependent
on a range of contextual factors. It is possible to statically
configure a web server to correctly handle large HTTP response
headers by specifying an upper bound for their size. But when the
number of links is unpredictable, estimating a reliable upper bound
is challenging.
HTTP [RFC7231] 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 header content exceeds the upper bound that can be handled
by the server, and thus it has been truncated. As a result,
applications take counter measures aimed at controlling the size of
the HTTP "Link" header field, for example by limiting the links they
provide to those with select relation types, thereby limiting the
value of the HTTP "Link" header field to clients. Providing links by
means of a standalone document overcomes challenges related to the
unpredictable nature of the size of HTTP "Link" header fields.
4. Document Formats for Sets of Links
This section specifies two document formats to convey a set of links.
Both are based on the abstract model specified in Section 2 of Web
Linking [RFC8288] that defines a link as consisting of a "link
context", a "link relation type", a "link target", and optional
"target attributes":
o The format defined in Section 4.1 is identical to the payload of
the HTTP "Link" header field as specified in Web Linking
[RFC8288].
o The format defined in Section 4.2 is based on JSON [RFC8259].
Note that [RFC8288] deprecates the "rev" construct that was provided
by [RFC5988] as a means to express links with a directionality that
is the inverse of direct links that use the "rel" construct. In both
serializations for link sets defined here, inverse links SHOULD be
represented as direct links using the "rel" construct and by
switching the position of the resources involved in the link.
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4.1. HTTP Link Document Format: application/linkset
This document format is identical to the payload of the HTTP "Link"
header field as defined in Section 3 of [RFC8288], more specifically
by its ABNF production rule for "Link" and subsequent ones.
The assigned media type for this format is "application/linkset".
In order to support use cases where "application/linkset" documents
are re-used outside the context of an HTTP interaction, it is
RECOMMENDED to make them self-contained by adhering to the following
guidelines:
o For every link provided in the set of links, explicitly provide
the link context using the "anchor" attribute.
o For link context ("anchor" attribute) and link target ("href"
attribute), use absolute URIs (as defined in Section 4.3 of
[RFC3986]).
If these recommendations are not followed, interpretation of links in
"application/linkset" documents will depend on which URI is used as
context.
4.2. JSON Document Format: application/linkset+json
This document format uses JSON [RFC8259] as the syntax to represent a
set of links. The set of links follows the abstract model defined by
Web Linking [RFC8288].
The assigned media type for this format is "application/
linkset+json".
In order to support use cases where "application/linkset+json"
documents are re-used outside the context of an HTTP interaction, it
is RECOMMENDED to make them self-contained by adhering to the
following guidelines:
o For every link provided in the set of links, explicitly provide
the link context using the "anchor" member.
o For link context ("anchor" member) and link target ("href"
member), use absolute URIs (as defined in Section 4.3 of
[RFC3986]).
If these recommendations are not followed, interpretation of
"application/linkset+json" will depend on which URI is used as
context URI.
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The "application/linkset+json" serialization is designed such that it
can directly be used as the content of a JSON-LD serialization by
adding an appropriate context. Appendix B shows an example of a
possible context that, when added to a JSON serialization, allows it
to be interpreted as RDF.
4.2.1. Set of Links
In the JSON representation of a set of links:
o A set of links MUST be represented as a JSON object which MUST
have "linkset" as its sole member.
o The "linkset" member is an array in which a distinct JSON object -
the "link context object" (see Section 4.2.2) - MUST be used to
represent links that have the same link context.
o If necessary, the "linkset" member MAY contain information in
addition to link context objects, in which case that information
MUST NOT change the semantics of the links provided by those link
context objects.
o Even if there is only one link context object, it MUST be wrapped
in an array. Members other than link context objects MUST NOT be
included in this array.
4.2.2. Link Context Object
In the JSON representation one or more links that have the same link
context are represented by a JSON object, the link context object. A
link context object adheres to the following rules:
o Each link context object MUST have an "anchor" member with a value
that represents the link context. This value SHOULD be an
absolute URI as defined in Section 4.3 of [RFC3986]. Cases
whereby no value is to be provided for the "anchor" member (i.e.
the resource providing the set of links is the link context for
each link in the link context object) MUST be handled by providing
an "anchor" member with null value ("anchor": "").
o For each distinct relation type that the link context has with
link targets, a link context object MUST have an additional
member. This member is an array in which a distinct JSON object -
the "link target object" (see Section 4.2.3) - MUST be used for
each link target for which the relationship with the link context
(value of the encompassing anchor member) applies. The name of
this member expresses the relation type of the link as follows:
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o
* For registered relation types [RFC8288], the name of this
member is the registered name of the relation type.
* For extension relation types [RFC8288], the name of this member
is the URI that uniquely represents the relation type.
o Even if there is only one link target object it MUST be wrapped in
an array. Members other than link target objects MUST NOT be
included in this array.
4.2.3. Link Target Object
In the JSON representation a link target is represented by a JSON
object, the link target object. A link target object adheres to the
following rules:
o Each link target object MUST have an "href" member with a value
that represents the link target. This value SHOULD be an absolute
URI as defined in Section 4.3 of [RFC3986]. Cases whereby no
value is to be provided for the "href" member (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
null value ("href": "").
o 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": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo"}
]
}
]
}
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The following example of a JSON-serialized set of links represents
two links that share link context and relation type but have
different link targets.
{
"linkset":
[
{ "anchor": "http://example.net/bar",
"item": [
{"href": "http://example.com/foo1"},
{"href": "http://example.com/foo2"}
]
}
]
}
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.
{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo1"}
]
},
{ "anchor": "http://example.net/boo",
"http://example.com/relations/baz" : [
{"href": "http://example.com/foo2"}
]
}
]
}
4.2.4. Link Target Attributes
A link may be further qualified by target attributes. Three types of
attributes exist:
o Attributes defined by the serialization of Web Linking [RFC8288].
o Extension attributes defined and used by communities as allowed by
[RFC8288].
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o Internationalized versions of the "title" attribute defined by
[RFC8288] and of extension attributes allowed by [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
RFC 8288 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:
o "hreflang": The optional and repeatable "hreflang" target
attribute MUST be represented by an array (even if there only is
one value to be represented), and each value in that array MUST be
a string - representing one value of the "hreflang" target
attribute for a link - which follows the same model as in the
[RFC8288] syntax.
o "media": The optional and not repeatable "media" target attribute
MUST be represented by a "media" member in the link target object,
and its value MUST be a string that follows the same model as in
the [RFC8288] syntax.
o "type": The optional and not repeatable "type" target attribute
MUST be represented by a "type" member in the link target object,
and its value MUST be a string that follows the same model as in
the [RFC8288] syntax.
o "title": The optional and not repeatable "title" target attribute
MUST be represented by a "title" member in the link target object,
and its value MUST be a string that follows the same model as in
the [RFC8288] syntax.
o "title*": The optional and not repeatable "title*" target
attribute is motivated by character encoding and language issues
and follows the model defined in [RFC8187]. The details of the
JSON representation that applies to title* are described in
Section 4.2.4.2.
The following example illustrates how the repeatable "hreflang" and
the not repeatable "type" target attributes are represented in a link
target object.
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{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo",
"type": "text/html",
"hreflang": [ "en" , "de" ]
}
]
}
]
}
4.2.4.2. Internationalized Target Attributes
In addition to the target attributes described in Section 4.2.4.1,
[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:
o An internationalized target attribute is represented as a member
of the link context object with the same name (including the *) of
the attribute.
o The character encoding information as prescribed by [RFC8187] is
not preserved; instead, the content of the internationalized
attribute is represented in the character encoding used for the
JSON set of links.
o The value of the internationalized target attribute is an array
that contains one or more JSON objects. The name of the first
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 the optional
second member of such JSON object is "language" and its value is
the language tag [RFC5646] for the language in which the attribute
content is conveyed.
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The following example illustrates how the "title*" target attribute
defined by [RFC8288] is represented in a link target object.
{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo",
"type": "text/html",
"hreflang": [ "en" , "de" ],
"title": "Next chapter",
"title*": [ { "value": "nachstes Kapitel" , "language" : "de" } ]
}
]
}
]
}
The above example assumes that the German title contains an umlaut
character (in the native syntax it would be encoded as title*=UTF-
8'de'n%c3%a4chstes%20Kapitel), which gets encoded in its unescaped
form in the JSON representation. This is not shown in the above
example due to the limitations of RFC publication. Implementations
MUST properly decode/encode internationalized target attributes that
follow the model of [RFC8187] when transcoding between the
"application/linkset" and the "application/linkset+json" formats.
4.2.4.3. Extension Target Attributes
Extension target attributes are attributes that are not defined by
RFC 8288 (as listed in Section 4.2.4.1), but are nevertheless used to
qualify links. They can be defined by communities in any way deemed
necessary, and it is up to them to make sure their usage is
understood by target applications. However, lacking standardization,
there is no interoperable understanding of these extension
attributes. One important consequence is that their cardinality is
unknown to generic applications. Therefore, in the JSON
serialization, all extension target attributes are treated as
repeatable.
The JSON serialization for these target attributes MUST be as
follows:
o An extension target attribute is represented as a member of the
link context object with the same name of the attribute, including
the * if applicable.
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o The value of an extension attribute MUST be represented by an
array, even if there only is one value to be represented.
o If the extension target attribute does not have a name with a
trailing asterisk, then each value in that array MUST be a string
that represents one value of the attribute.
o If the extension attribute has a name with a trailing asterisk (it
follows the content model of [RFC8187]), then each value in that
array MUST be a JSON object. The value of each such JSON object
MUST be structured as described in Section 4.2.4.2.
The example shows a link target object with three extension target
attributes. The value for each extension target attribute is an
array. The two first are regular extension target attributes, with
the first one ("foo") having only one value and the second one
("bar") having two. The last extension target attribute ("baz*")
follows the naming rule of [RFC8187] and therefore is encoded
according to the serialization described in Section 4.2.4.2.
{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{ "href": "http://example.com/foo",
"type": "text/html",
"foo": [ "foovalue" ],
"bar": [ "barone", "bartwo" ],
"baz*": [ { "value": "bazvalue" , "language" : "en" } ]
}
]
}
]
}
5. 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
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
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expressed using different media types, or to different sets of links,
potentially provided by different third-party services.
A user agent that follows a "linkset" link MUST be aware that the set
of links provided by the resource that is the target of the link can
contain links in which the resource that is the context of the link
does not participate; it MAY decide to ignore those links.
A user agent that follows a "linkset" link and obtains links for
which anchors and targets are not expressed as absolute URIs MUST
properly determine what the context is for these links; it SHOULD
ignore links for which it is unable to unambiguously make that
determination.
6. Examples
Section 6.1 and Section 6.2 show examples whereby the set of links
are provided as "application/linkset" and "application/linkset+json"
documents, respectively. Section 6.3 illustrates the use of the
"linkset" link relation type to support discovery of sets of links.
6.1. Set of Links Provided as application/linkset
Figure 1 shows a client issuing an HTTP GET request against resource
<http://example.org/resource1>.
GET /resource1 HTTP/1.1
Host: example.org
Connection: close
Figure 1: Client HTTP GET request
Figure 2 shows the response to the GET request of Figure 1. The
response contains a Content-Type header specifying that the media
type of the response is "application/linkset". A set of links,
including links that pertain to the responding resource, is provided
in the response body.
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HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:35:51 GMT
Server: Apache-Coyote/1.1
Content-Length: 729
Content-Type: application/linkset
Connection: close
<http://authors.example.net/johndoe>
; rel="author"
; type="application/rdf+xml"
; anchor="http://example.org/resource1",
<http://authors.example.net/janedoe>
; rel="author"
; type="application/rdf+xml"
; anchor="http://example.org/resource1",
<http://example.org/resource1/items/AF48EF.pdf>
; rel="item"
; type="application/pdf"
; anchor="http://example.org/resource1",
<http://example.org/resource1/items/CB63DA.html>
; rel="item"
; type="text/html"
; anchor="http://example.org/resource1",
<http://example.org/resource1>
; rel="latest-version"
; anchor="http://example.org/resource41/",
<http://example.org/resource40>
; rel="prev"
; anchor="http://example.org/resource41/"
Figure 2: Response to HTTP GET includes a set of links
6.2. Set of Links Provided as application/linkset+json
Figure 3 shows the client issuing an HTTP GET request against
<http://example.com/links/article/7507>. In the request, the client
uses an "Accept" header to indicate it prefers a response in the
"application/linkset+json" format.
GET links/article/7507 HTTP/1.1
Host: example.com
Accept: application/linkset+json
Connection: close
Figure 3: Client HTTP GET request expressing preference for
"application/linkset+json" response
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Figure 4 shows the response to the HTTP GET request of Figure 3. The
set of links is serialized according to the media type "application/
linkset+json".
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HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:46:22 GMT
Server: Apache-Coyote/1.1
Content-Type: application/linkset+json
Content-Length: 802
{
"linkset": [
{
"anchor": "https://example.org/article/view/7507",
"author": [
{
"href": "https://orcid.org/0000-0002-1825-0097",
}
],
"item": [
{
"href": "https://example.org/article/7507/item/1",
"type": "application/pdf"
},
{
"href": "https://example.org/article/7507/item/2",
"type": "text/csv"
}
],
"cite-as": [
{
"href": "https://doi.org/10.5555/12345680",
"title": "A Methodology for the Emulation of Architecture"
}
]
},
{
"anchor": "https://example.com/links/article/7507",
"alternate": [
{
"href": "https://mirror.example.com/links/article/7507",
"type": "application/linkset"
}
]
}
]
}
Figure 4: Response to the client's request for the set of links
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6.3. Discovering a Link Set via the "linkset" Link Relation Type
Figure 5 shows a client issuing an HTTP HEAD request against resource
<http://example.org/article/view/7507>.
HEAD article/view/7507 HTTP/1.1
Host: example.org
Connection: close
Figure 5: Client HTTP HEAD request
Figure 6 shows the response to the HEAD request of Figure 5. The
response contains a "Link" header with a link that has the "linkset"
relation type. It indicates that a set of links is provided by
resource <http://example.com/links/article/7507>, 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: <http://example.com/links/article/7507>
; rel="linkset"
; type="application/linkset+json"
Content-Length: 236
Content-Type: text/html;charset=utf-8
Connection: close
Figure 6: Response to HTTP HEAD request
Section 6.2 shows a client obtaining a set of links by issuing an
HTTP GET on the target of the link with the "linkset" relation type,
<http://example.com/links/article/7507>.
7. Implementation Status
Note to RFC Editor: Please remove this section before publication.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC 6982
[RFC6982]. The description of implementations in this section is
intended to assist the IETF in its decision processes in progressing
drafts to RFCs. Please note that the listing of any individual
implementation here does not imply endorsement by the IETF.
Furthermore, no effort has been spent to verify the information
presented here that was supplied by IETF contributors. This is not
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intended as, and must not be construed to be, a catalog of available
implementations or their features. Readers are advised to note that
other implementations may exist.
According to RFC 6982, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
7.1. GS1
GS1 is a provider of barcodes (GS1 GTINs and EAN/UPC) for retail
products and manages an ecology of services and standards to leverage
them at a global scale. GS1 has indicated that it will implement
this "linkset" specification as a means to allow requesting and
representing links pertaining to products from various retailers.
Currently, the GS1 Digital Link specification makes an informative
reference to version 03 of the "linkset" I-D. GS1 expresses
confidence that this will become a normative reference in the next
iteration of that specification, likely to be ratified as a GS1
standard around February 2021.
7.2. Open Journal Systems (OJS)
Open Journal Systems (OJS) is an open-source software for the
management of peer-reviewed academic journals, and is created by the
Public Knowledge Project (PKP), released under the GNU General Public
License. Open Journal Systems (OJS) is a journal management and
publishing system that has been developed by PKP through its
federally funded efforts to expand and improve access to research.
The OJS platform has implemented "linkset" support as an alternative
way to provide links when there are more than a configured limit
(they consider using about 10 as a good default, for testing purpose
it is currently set to 8).
8. IANA Considerations
8.1. Link Relation Type: linkset
The link relation type below has been registered by IANA per
Section 6.2.1 of Web Linking [RFC8288]:
Relation Name: linkset
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Description: The Target IRI of a link with the "linkset" relation
type provides a set of links, including links in which the Context
IRI of the link participates.
Reference: [[ This document ]]
8.2. Media Type: application/linkset
8.2.1. IANA Considerations
The Internet media type [RFC6838] for a natively encoded linkset is
application/linkset.
Type name: application
Subtype name: linkset
Required parameters: none
Optional parameters: none
Encoding considerations: Linksets are encoded according to the
definition of [RFC8288]. The encoding of [RFC8288] is based on
the general encoding rules of [RFC7230], with the addition of
allowing indicating character encoding and language for specific
parameters as defined by [RFC8187].
Security considerations: The security considerations of [[ This
document ]] apply.
Interoperability considerations: The interoperability
considerations of [RFC7230] apply.
Published specification: [[ This document ]]
Applications that use this media type: This media type is not
specific to any application, as it can be used by any application
that wants to interchange web links.
Additional information:
Magic number(s): N/A
File extension(s): This media type does not propose a specific
extension.
Macintosh file type code(s): TEXT
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Person & email address to contact for further information: Erik
Wilde <erik.wilde@dret.net>
Intended usage: COMMON
Restrictions on usage: none
Author: Erik Wilde <erik.wilde@dret.net>
Change controller: IETF
8.3. Media Type: application/linkset+json
The Internet media type [RFC6838] for a JSON-encoded linkset is
application/linkset+json.
Type name: application
Subtype name: linkset+json
Required parameters: none
Optional parameters: none
Encoding considerations: The encoding considerations of [RFC8259]
apply
Security considerations: The security considerations of [[ This
document ]] apply.
Interoperability considerations: The interoperability
considerations of [RFC8259] apply.
Published specification: [[ This document ]]
Applications that use this media type: This media type is not
specific to any application, as it can be used by any application
that wants to interchange web links.
Additional information:
Magic number(s): N/A
File extension(s): JSON documents often use ".json" as the file
extension, and this media type does not propose a specific
extension other than this generic one.
Macintosh file type code(s): TEXT
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Person & email address to contact for further information: Erik
Wilde <erik.wilde@dret.net>
Intended usage: COMMON
Restrictions on usage: none
Author: Erik Wilde <erik.wilde@dret.net>
Change controller: IETF
9. Security Considerations
The security considerations of Web Linking [RFC8288] apply, as long
as they are not specifically discussing the risks of exposing
information in HTTP header fields.
In general, links may cause information leakage when they expose
information (such as URIs) that can be sensitive or private. Links
may expose "hidden URIs" that are not supposed to be openly shared,
and may not be sufficiently protected. Ideally, none of the URIs
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:
o 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.
o 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.
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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>.
[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>.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013,
<https://www.rfc-editor.org/info/rfc6982>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[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>.
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[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>.
10.2. Informative References
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<https://www.rfc-editor.org/info/rfc5988>.
[W3C.REC-json-ld-20140116]
Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0",
World Wide Web Consortium Recommendation REC-json-ld-
20140116, January 2014,
<https://www.w3.org/TR/2014/REC-json-ld-20140116>.
10.3. URIs
[1] https://www.w3.org/TR/2014/REC-json-ld-20140116/#interpreting-
json-as-json-ld
[2] https://tools.ietf.org/html/rfc8288#appendix-A.2
Appendix A. Acknowledgements
Thanks for comments and suggestions provided by Phil Archer,
Dominique Guinard, Mark Nottingham, Stian Soiland-Reyes, and Sarven
Capadisli.
Appendix B. JSON-LD Context
A set of links rendered according to the JSON serialization defined
in Section 4.2 can be interpreted as RDF triples by adding a JSON-LD
context [W3C.REC-json-ld-20140116] that maps the JSON keys to
corresponding Linked Data terms. And, as per
[W3C.REC-json-ld-20140116] section 6.8 [1], 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
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such a JSON-LD context by using a link with the relation type
"http://www.w3.org/ns/json-ld#context" in the HTTP "Link" header.
Using the latter approach to support discovery of a JSON-LD Context,
the response to the GET request of Figure 3 against the URI of a set
of links would be as shown in Figure 7.
HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:48:22 GMT
Server: Apache-Coyote/1.1
Content-Type: application/linkset+json
Link: <https://example.org/contexts/linkset.jsonld>
; rel="http://www.w3.org/ns/json-ld#context"
; type="application/ld+json"
Content-Length: 846
{
"linkset": [
{
"anchor": "https://example.org/article/view/7507",
"author": [
{
"href": "https://orcid.org/0000-0002-1825-0097"
}
],
"item": [
{
"href": "https://example.org/article/7507/item/1",
"type": "application/pdf"
},
{
"href": "https://example.org/article/7507/item/2",
"type": "text/csv"
}
],
"cite-as": [
{
"href": "https://doi.org/10.5555/12345680",
"title": "A Methodology for the Emulation of Architecture"
}
]
},
{
"anchor": "https://example.com/links/article/7507",
"alternate": [
{
"href": "https://mirror.example.com/links/article/7507",
"type": "application/linkset"
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}
]
}
]
}
Figure 7: Using a typed link to support discovery of a JSON-LD
Context for a Set of Links
In order to obtain the JSON-LD Context conveyed by the server, the
user agent issues an HTTP GET against the link target of the link
with the "http://www.w3.org/ns/json-ld#context" relation type. The
response to this GET is shown in Figure 8. This particular JSON-LD
context maps "application/linkset+json" representations of link sets
to Dublin Core Terms. It also renders each link relation as an
absolute URI, inspired by the same approach used for Atom [RFC4287]
described in [RFC8288] appendix A.2 [2].
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HTTP/1.1 200 OK
Content-Type: application/ld+json
Content-Length: 638
{
"@context": [
{
"@vocab": "http://www.iana.org/assignments/relation/",
"anchor": "@id",
"href": "@id",
"linkset": "@graph",
"_linkset": "@graph",
"title": {
"@id": "http://purl.org/dc/terms/title"
},
"title*": {
"@id": "http://purl.org/dc/terms/title"
},
"type": {
"@id": "http://purl.org/dc/terms/format"
}
},
{
"language": "@language",
"value": "@value",
"hreflang": {
"@id": "http://purl.org/dc/terms/language",
"@container": "@set"
}
}
]
}
Figure 8: JSON-LD Context mapping to schema.org and IANA assignments
Applying the JSON-LD context of Figure 8 to the link set of Figure 7
allows transforming the "application/linkset+json" link set to an RDF
link set. Figure 9 shows the latter represented by means of the
"text/turtle" RDF serialization.
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<https://example.org/article/view/7507>
<http://www.iana.org/assignments/relation/author>
<https://orcid.org/0000-0002-1825-0097> .
<https://example.org/article/view/7507>
<http://www.iana.org/assignments/relation/item>
<https://example.org/article/7507/item/1> .
<https://example.org/article/7507/item/1>
<http://purl.org/dc/terms/format>
"application/pdf" .
<https://example.org/article/view/7507>
<http://www.iana.org/assignments/relation/item>
<https://example.org/article/7507/item/2> .
<https://example.org/article/7507/item/2>
<http://purl.org/dc/terms/format>
"text/csv" .
<https://example.org/article/view/7507>
<http://www.iana.org/assignments/relation/cite-as>
<https://doi.org/10.5555/12345680> .
<https://doi.org/10.5555/12345680>
<http://purl.org/dc/terms/title>
"A Methodology for the Emulation of Architecture" .
<https://example.com/links/article/7507>
<http://www.iana.org/assignments/relation/alternate>
<https://mirror.example.com/links/article/7507> .
<https://mirror.example.com/links/article/7507>
<http://purl.org/dc/terms/format>
"application/linkset" .
Figure 9: RDF serialization of the link set resulting from applying
the JSON-LD context
Note that the JSON-LD context of Figure 8 does not handle (meta)link
relations of type ""linkset"" as they are in conflict with the top-
level JSON key. A workaround is to rename the top-level key to
""_linkset"" in the "application/linkset+json" before transforming a
link set to JSON-LD.
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Authors' Addresses
Erik Wilde
Axway
Email: erik.wilde@dret.net
URI: http://dret.net/netdret/
Herbert Van de Sompel
Data Archiving and Networked Services
Email: herbert.van.de.sompel@dans.knaw.nl
URI: https://orcid.org/0000-0002-0715-6126
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