Linkset: Media Types and a Link Relation Type for Link Sets
draft-ietf-httpapi-linkset-03

Document Type Active Internet-Draft (httpapi WG)
Authors Erik Wilde  , Herbert Van de Sompel 
Last updated 2021-07-07 (latest revision 2021-07-04)
Replaces draft-wilde-linkset
Stream Internet Engineering Task Force (IETF)
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Send notices to rsalz@akamai.com
Network Working Group                                           E. Wilde
Internet-Draft                                                     Axway
Intended status: Informational                          H. Van de Sompel
Expires: 5 January 2022            Data Archiving and Networked Services
                                                             4 July 2021

      Linkset: Media Types and a Link Relation Type for Link Sets
                     draft-ietf-httpapi-linkset-03

Abstract

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

Note to Readers

   Please discuss this draft on the "Building Blocks for HTTP APIs"
   mailing list (https://www.ietf.org/mailman/listinfo/httpapi).

   Online access to all versions and files is available on GitHub
   (https://github.com/ietf-wg-httpapi/linkset).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on 5 January 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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  . . . . . . . . . . . . . . .  10
       4.2.5.  JSON Extensibility  . . . . . . . . . . . . . . . . .  14
   5.  The "profile" attribute for media types to Represent Sets of
           Links . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  The "linkset" Relation Type for Linking to a Set of Links . .  15
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Set of Links Provided as application/linkset  . . . . . .  16
     7.2.  Set of Links Provided as application/linkset+json . . . .  17
     7.3.  Discovering a Link Set via the "linkset" Link Relation
           Type  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  Link Relation Type: linkset . . . . . . . . . . . . . . .  20
     8.2.  Media Type: application/linkset . . . . . . . . . . . . .  20
     8.3.  Media Type: application/linkset+json  . . . . . . . . . .  21
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  23
   11. Informative References  . . . . . . . . . . . . . . . . . . .  24
   Appendix A.  JSON-LD Context  . . . . . . . . . . . . . . . . . .  25
   Appendix B.  Implementation Status  . . . . . . . . . . . . . . .  30
     B.1.  GS1 . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     B.2.  FAIR Signposting Profile  . . . . . . . . . . . . . . . .  31
     B.3.  Open Journal Systems (OJS)  . . . . . . . . . . . . . . .  31
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

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1.  Introduction

   Resources on the Web often use typed Web Links [RFC8288], either
   embedded in resource representations, for example using the <link>
   element for HTML documents, or conveyed in the HTTP "Link" header
   field for documents of any media type.  In some cases, however,
   providing links in this manner is impractical or impossible and
   delivering a set of links as a stand-alone document is preferable.

   Therefore, this specification defines two document formats and
   associated media types to represent sets of links.  It also defines
   the "linkset" relation type that supports discovery of any resource
   that conveys a set of links as a stand-alone document.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This specification uses the terms "link context" and "link target" as
   defined in [RFC8288].

   In the examples provided in this document, links in the HTTP "Link"
   header field are shown on separate lines in order to improve
   readability.  Note, however, that as per Section 5.5 of
   [I-D.ietf-httpbis-semantics], line breaks are deprecated in values
   for HTTP fields; only whitespaces and tabs are supported as
   separators.

3.  Scenarios

   The following sections outline scenarios in which providing links by
   means of a standalone document instead of in an HTTP "Link" header
   field or as links embedded in the resource representation is
   advantageous or necessary.

   For all scenarios, links could be provided by means of a stand-alone
   document that is formatted according to the JSON-based serialization,
   the serialization aligned with the HTTP "Link" field format, or both.
   The former serialization is motivated by the widespread use of JSON
   and related tools, which suggests that handling sets of links
   expressed as JSON documents should be attractive to developers.  The
   latter serialization is provided for compatibility with the existing
   serialization used in the HTTP "Link" field and to allow reuse of
   tools created to handle it.

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   It is important to keep in mind that when providing links by means of
   a standalone representation, other links can still be provided using
   other approaches, i.e. it is possible combine various mechanisms to
   convey links.

3.1.  Third-Party Links

   In some cases it is useful that links pertaining to a resource are
   provided by a server other than the one that hosts the resource.  For
   example, this allows:

   *  Providing links in which the resource is involved not just as link
      context but also as link target.

   *  Providing links pertaining to the resource that the server hosting
      that resource is not aware of.

   *  External management of links pertaining to the resource in a
      special-purpose link management service.

   In such cases, links pertaining to a resource can be provided by
   another, specific resource.  That specific resource may be managed by
   the same or by another custodian as the resource to which the links
   pertain.  For clients intent on consuming links provided in that
   manner, it would be beneficial if the following conditions were met:

   *  Links are provided in a document that uses a well-defined media
      type.

   *  The resource to which the provided links pertain is able to link
      to the resource that provides these links using a well-known link
      relation type.

   These requirements are addressed in this specification through the
   definition of two media types and a link relation type, respectively.

3.2.  Challenges Writing to HTTP Link Header Field

   In some cases, it is not straightforward to write links to the HTTP
   "Link" header field from an application.  This can, for example, be
   the case because not all required link information is available to
   the application or because the application does not have the
   capability to directly write HTTP fields.  In such cases, providing
   links by means of a standalone document can be a solution.  Making
   the resource that provides these links discoverable can be achieved
   by means of a typed link.

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3.3.  Large Number of Links

   When conveying links in an HTTP "Link" header field, it is possible
   for the size of the HTTP response fields to become unpredictable.
   This can be the case when links are determined dynamically dependent
   on a range of contextual factors.  It is possible to statically
   configure a web server to correctly handle large HTTP response fields
   by specifying an upper bound for their size.  But when the number of
   links is unpredictable, estimating a reliable upper bound is
   challenging.

   Section 15 of HTTP [I-D.ietf-httpbis-semantics] defines error codes
   related to excess communication by the user agent ("413 Request
   Entity Too Large" and "414 Request-URI Too Long"), but no specific
   error codes are defined to indicate that response field content
   exceeds the upper bound that can be handled by the server, and thus
   it has been truncated.  As a result, applications take counter
   measures aimed at controlling the size of the HTTP "Link" header
   field, for example by limiting the links they provide to those with
   select relation types, thereby limiting the value of the HTTP "Link"
   header field to clients.  Providing links by means of a standalone
   document overcomes challenges related to the unpredictable nature of
   the size of HTTP "Link" header fields.

4.  Document Formats for Sets of Links

   This section specifies two document formats to convey a set of links.
   Both are based on the abstract model specified in Section 2 of Web
   Linking [RFC8288] that defines a link as consisting of a "link
   context", a "link relation type", a "link target", and optional
   "target attributes":

   *  The format defined in Section 4.1 is identical to the payload of
      the HTTP "Link" header field as specified in Web Linking Section 3
      of [RFC8288].

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

   Note that Section 3.3 of [RFC8288] deprecates the "rev" construct
   that was provided by [RFC5988] as a means to express links with a
   directionality that is the inverse of direct links that use the "rel"
   construct.  In both serializations for link sets defined here,
   inverse links SHOULD be represented as direct links using the "rel"
   construct and by switching the position of the resources involved in
   the link.

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4.1.  HTTP Link Document Format: application/linkset

   This document format is identical to the payload of the HTTP "Link"
   header field as defined in Section 3 of [RFC8288], more specifically
   by its ABNF production rule for "Link" and subsequent ones.  The use
   of non-ASCII characters in the payload of the HTTP "Link" Header
   field is not interoperable.

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

   In order to support use cases where "application/linkset" documents
   are re-used outside the context of an HTTP interaction, it is
   RECOMMENDED to make them self-contained by adhering to the following
   guidelines:

   *  For every link provided in the set of links, explicitly provide
      the link context using the "anchor" attribute.

   *  For link context ("anchor" attribute) and link target ("href"
      attribute), use URI References that are not relative references
      (as defined in Section 4.1 of [RFC3986]).

   If these recommendations are not followed, interpretation of links in
   "application/linkset" documents will depend on which URI is used as
   context.

   It should be noted that the "application/linkset" format specified
   here is different than the "application/link-format" format specified
   in [RFC6690] in that the former fully matches the payload of the HTTP
   "Link" header field as defined in Section 3 of [RFC8288], whereas the
   latter introduces constraints on that definition to meet requirements
   for Constrained RESTful Environments.

4.2.  JSON Document Format: application/linkset+json

   This document format uses JSON [RFC8259] as the syntax to represent a
   set of links.  The set of links follows the abstract model defined by
   Web Linking Section 2 of [RFC8288].

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

   In order to support use cases where "application/linkset+json"
   documents are re-used outside the context of an HTTP interaction, it
   is RECOMMENDED to make them self-contained by adhering to the
   following guidelines:

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   *  For every link provided in the set of links, explicitly provide
      the link context using the "anchor" member.

   *  For link context ("anchor" member) and link target ("href"
      member), use URI References that are not relative references (as
      defined in Section 4.1 of [RFC3986]).

   If these recommendations are not followed, interpretation of
   "application/linkset+json" will depend on which URI is used as
   context URI.

   The "application/linkset+json" serialization is designed such that it
   can directly be used as the content of a JSON-LD serialization by
   adding an appropriate context.  Appendix A shows an example of a
   possible context that, when added to a JSON serialization, allows it
   to be interpreted as RDF.

4.2.1.  Set of Links

   In the JSON representation of a set of links:

   *  A set of links MUST be represented as a JSON object which MUST
      have "linkset" as its sole member.

   *  The "linkset" member is an array in which a distinct JSON object -
      the "link context object" (see Section 4.2.2) - MUST be used to
      represent links that have the same link context.

   *  Even if there is only one link context object, it MUST be wrapped
      in an array.  Members other than link context objects MUST NOT be
      included in this array.

4.2.2.  Link Context Object

   In the JSON representation one or more links that have the same link
   context are represented by a JSON object, the link context object.  A
   link context object adheres to the following rules:

   *  Each link context object MAY have an "anchor" member with a value
      that represents the link context.  If present, this value MUST be
      a URI Reference and SHOULD NOT be a relative reference as per
      Section 4.1 of [RFC3986].

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   *  For each distinct relation type that the link context has with
      link targets, a link context object MUST have an additional
      member.  This member is an array in which a distinct JSON object -
      the "link target object" (see Section 4.2.3) - MUST be used for
      each link target for which the relationship with the link context
      (value of the encompassing anchor member) applies.  The name of
      this member expresses the relation type of the link as follows:

      -  For registered relation types (Section 2.1.1 of [RFC8288]), the
         name of this member is the registered name of the relation
         type.

      -  For extension relation types (Section 2.1.2 of [RFC8288]), the
         name of this member is the URI that uniquely represents the
         relation type.

   *  Even if there is only one link target object it MUST be wrapped in
      an array.  Members other than link target objects MUST NOT be
      included in this array.

4.2.3.  Link Target Object

   In the JSON representation a link target is represented by a JSON
   object, the link target object.  A link target object adheres to the
   following rules:

   *  Each link target object MUST have an "href" member with a value
      that represents the link target.  This value MUST be a URI
      Reference and SHOULD NOT be a relative reference as per
      Section 4.1 of [RFC3986].  Cases where the href member is present,
      but no value is provided for it (i.e. the resource providing the
      set of links is the target of the link in the link target object)
      MUST be handled by providing an "href" member with an empty string
      ("href": "").

   *  In many cases, a link target is further qualified by target
      attributes.  Various types of attributes exist and they are
      conveyed as additional members of the link target object as
      detailed in Section 4.2.4.

   The following example of a JSON-serialized set of links represents
   one link with its core components: link context, link relation type,
   and link target.

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

                                  Figure 1

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

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

                                  Figure 2

   The following example shows a set of links that represents two links,
   each with a different link context, link target, and relation type.
   One relation type is registered, the other is an extension relation
   type.

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

                                  Figure 3

4.2.4.  Link Target Attributes

   A link may be further qualified by target attributes.  Three types of
   attributes exist:

   *  Attributes defined in Section 3.4.1 of Web Linking [RFC8288].

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

   *  Internationalized versions of the "title" attribute defined by
      [RFC8288] and of extension attributes allowed by Section 3.4 of
      [RFC8288].

   The handling of these different types of attributes is described in
   the sections below.

4.2.4.1.  Target Attributes Defined by Web Linking

   Section 3.4.1 of [RFC8288] defines the following target attributes
   that may be used to annotate links: "hreflang", "media", "title",
   "title*", and "type"; these target attributes follow different
   occurrence and value patterns.  In the JSON representation, these
   attributes MUST be conveyed as additional members of the link target
   object as follows:

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   *  "hreflang": The optional and repeatable "hreflang" target
      attribute MUST be represented by an array (even if there only is
      one value to be represented), and each value in that array MUST be
      a string - representing one value of the "hreflang" target
      attribute for a link - which follows the same model as in the
      [RFC8288] syntax.

   *  "media": The optional and not repeatable "media" target attribute
      MUST be represented by a "media" member in the link target object,
      and its value MUST be a string that follows the same model as in
      the [RFC8288] syntax.

   *  "type": The optional and not repeatable "type" target attribute
      MUST be represented by a "type" member in the link target object,
      and its value MUST be a string that follows the same model as in
      the [RFC8288] syntax.

   *  "title": The optional and not repeatable "title" target attribute
      MUST be represented by a "title" member in the link target object,
      and its value MUST be a string that follows the same model as in
      the [RFC8288] syntax.

   *  "title*": The optional and not repeatable "title*" target
      attribute is motivated by character encoding and language issues
      and follows the model defined in [RFC8187].  The details of the
      JSON representation that applies to title* are described in
      Section 4.2.4.2.

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

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

                                  Figure 4

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4.2.4.2.  Internationalized Target Attributes

   In addition to the target attributes described in Section 4.2.4.1,
   Section 3.4 of [RFC8288] also supports attributes that follow the
   content model of [RFC8187].  In [RFC8288], these target attributes
   are recognizable by the use of a trailing asterisk in the attribute
   name, such as "title*".  The content model of [RFC8187] uses a
   string-based microsyntax that represents the character encoding, an
   optional language tag, and the escaped attribute value encoded
   according to the specified character encoding.

   The JSON serialization for these target attributes MUST be as
   follows:

   *  An internationalized target attribute is represented as a member
      of the link context object with the same name (including the *) of
      the attribute.

   *  The character encoding information as prescribed by [RFC8187] is
      not preserved; instead, the content of the internationalized
      attribute is represented in the character encoding used for the
      JSON set of links.

   *  The value of the internationalized target attribute is an array
      that contains one or more JSON objects.  The name of one member of
      such JSON object is "value" and its value is the actual content
      (in its unescaped version) of the internationalized target
      attribute, i.e. the value of the attribute from which the encoding
      and language information are removed.  The name of another,
      optional, member of such JSON object is "language" and its value
      is the language tag [RFC5646] for the language in which the
      attribute content is conveyed.

   The following example illustrates how the "title*" target attribute
   defined by Section 3.4.1 of [RFC8288] is represented in a link target
   object.

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

                                  Figure 5

   The above example assumes that the German title contains an umlaut
   character (in the native syntax it would be encoded as title*=UTF-
   8'de'n%c3%a4chstes%20Kapitel), which gets encoded in its unescaped
   form in the JSON representation.  Implementations MUST properly
   decode/encode internationalized target attributes that follow the
   model of [RFC8187] when transcoding between the "application/linkset"
   and the "application/linkset+json" formats.

4.2.4.3.  Extension Target Attributes

   Extension target attributes are attributes that are not defined by
   Section 3.4.1 of [RFC8288] (as listed in Section 4.2.4.1), but are
   nevertheless used to qualify links.  They can be defined by
   communities in any way deemed necessary, and it is up to them to make
   sure their usage is understood by target applications.  However,
   lacking standardization, there is no interoperable understanding of
   these extension attributes.  One important consequence is that their
   cardinality is unknown to generic applications.  Therefore, in the
   JSON serialization, all extension target attributes are treated as
   repeatable.

   The JSON serialization for these target attributes MUST be as
   follows:

   *  An extension target attribute is represented as a member of the
      link context object with the same name of the attribute, including
      the * if applicable.

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   *  The value of an extension attribute MUST be represented by an
      array, even if there only is one value to be represented.

   *  If the extension target attribute does not have a name with a
      trailing asterisk, then each value in that array MUST be a string
      that represents one value of the attribute.

   *  If the extension attribute has a name with a trailing asterisk (it
      follows the content model of [RFC8187]), then each value in that
      array MUST be a JSON object.  The value of each such JSON object
      MUST be structured as described in Section 4.2.4.2.

   The example shows a link target object with three extension target
   attributes.  The value for each extension target attribute is an
   array.  The two first are regular extension target attributes, with
   the first one ("foo") having only one value and the second one
   ("bar") having two.  The last extension target attribute ("baz*")
   follows the naming rule of [RFC8187] and therefore is encoded
   according to the serialization described in Section 4.2.4.2.

   {
     "linkset":
       [
         { "anchor": "http://example.net/bar",
           "next": [
                 { "href": "http://example.com/foo",
                   "type": "text/html",
                   "foo":  [ "foovalue" ],
                   "bar":  [ "barone", "bartwo" ],
                   "baz*": [ { "value": "bazvalue" ,
                               "language" : "en" } ]
                 }
           ]
         }
       ]
   }

                                  Figure 6

4.2.5.  JSON Extensibility

   The extensibility of the JSON document format for representing a set
   of links is restricted to the extensibility provided by [RFC8288].
   The Web linking model provides for the use of extension target
   attributes as discussed in Section 4.2.4.3.  Extensions based on the
   JSON syntax MUST NOT be used, and MUST be ignored when found in a
   JSON linkset document.

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   This limitation of the JSON format allows to unambiguously round trip
   between links provided in the HTTP "Link" header field, sets of links
   serialized according to the "application/linkset" format, and sets of
   links serialized according to the "application/linkset+json" format.

5.  The "profile" attribute for media types to Represent Sets of Links

   As a means to convey specific constraints or conventions (as per
   [RFC6906]) that apply to a link set document, the "profile" attribute
   MAY be used in conjunction with the media types "application/linkset"
   and "application/linkset+json" detailed in Section 4.1 and
   Section 4.2, respectively.  For example, the attribute could be used
   to indicate that a link set uses a specific, limited set of link
   relation types.

   The value of the "profile" attribute MUST be a non-empty list of
   space-separated URIs, each of which identifies specific constraints
   or conventions that apply to the link set document.  Profile URIs MAY
   be registered in the IANA Profile URI Registry in the manner
   specified by [RFC7284].

   The presence of a "profile" attribute in conjunction with the
   "application/linkset" and "application/linkset+json" media types does
   not change the semantics of a link set.  As such, clients with and
   without knowledge of profile URIs can use the same representation.

6.  The "linkset" Relation Type for Linking to a Set of Links

   The target of a link with the "linkset" relation type provides a set
   of links, including links in which the resource that is the link
   context participates.

   A link with the "linkset" relation type MAY be provided in the header
   field and/or the body of a resource's representation.  It may also be
   discovered by other means, such as through client-side information.

   A resource MAY provide more than one link with a "linkset" relation
   type.  Multiple such links can refer to the same set of links
   expressed using different media types, or to different sets of links,
   potentially provided by different third-party services.

   A user agent that follows a "linkset" link MUST be aware that the set
   of links provided by the resource that is the target of the link can
   contain links in which the resource that is the context of the link
   does not participate; it MAY decide to ignore those links.

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   A user agent that follows a "linkset" link and obtains links for
   which anchors and targets are expressed as relative references (as
   per Section 4.1 of [RFC3986]) MUST determine what the context is for
   these links; it SHOULD ignore links for which it is unable to
   unambiguously make that determination.

7.  Examples

   Section 7.1 and Section 7.2 show examples whereby a set of links is
   provided as "application/linkset" and "application/linkset+json"
   documents, respectively.  Section 7.3 illustrates the use of the
   "linkset" link relation type to support discovery of sets of links.

7.1.  Set of Links Provided as application/linkset

   Figure 7 shows a client issuing an HTTP GET request against resource
   <https://example.org/links/resource1>.

   GET /links/resource1 HTTP/1.1
   Host: example.org

                     Figure 7: Client HTTP GET request

   Figure 8 shows the response to the GET request of Figure 7.  The
   response contains a Content-Type header field specifying that the
   media type of the response is "application/linkset".  A set of links,
   revealing authorship and versioning related to resource
   <https://example.org/resource1>, is provided in the response body.
   The HTTP "Link" header field indicates the availability of an
   alternate representation of the set of links using media type
   "application/linkset+json".

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   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:35:51 GMT
   Server: Apache-Coyote/1.1
   Content-Length: 1023
   Content-Type: application/linkset
   Link: <https://example.org/links/resource1>
         ; rel="alternate"
         ; type="application/linkset+json"

   <https://authors.example.net/johndoe>
      ; rel="author"
      ; type="application/rdf+xml"
      ; anchor="https://example.org/resource1",
   <https://example.org/resource1?version=3>
      ; rel="latest-version"
      ; type="text/html"
      ; anchor="https://example.org/resource1",
   <https://example.org/resource1?version=2>
      ; rel="predecessor-version"
      ; type="text/html"
      ; anchor="https://example.org/resource1?version=3",
   <https://example.org/resource1?version=1>
      ; rel="predecessor-version"
      ; type="text/html"
      ; anchor="https://example.org/resource1?version=2",
   <https://example.org/resource1?version=1>
      ; rel="memento"
      ; type="text/html"
      ; datetime="Thu, 13 Jun 2019 09:34:33 GMT"
      ; anchor="https://example.org/resource1",
   <https://example.org/resource1?version=2>
      ; rel="memento"
      ; type="text/html"
      ; datetime="Sun, 21 Jul 2019 12:22:04 GMT"
      ; anchor="https://example.org/resource1",
   <https://authors.example.net/alice>
      ; rel="author"
      ; anchor="https://example.org/resource1#comment=1"

           Figure 8: Response to HTTP GET includes a set of links

7.2.  Set of Links Provided as application/linkset+json

   Figure 9 shows the client issuing an HTTP GET request against
   <https://example.org/links/resource1>.  In the request, the client
   uses an "Accept" header field to indicate it prefers a response in
   the "application/linkset+json" format.

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

        Figure 9: Client HTTP GET request expressing preference for
                    "application/ linkset+json" response

   Figure 10 shows the response to the HTTP GET request of Figure 9.
   The set of links is serialized according to the media type
   "application/linkset+json".

   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:46:22 GMT
   Server: Apache-Coyote/1.1
   Content-Type: application/linkset+json
   Link: <https://example.org/links/resource1>
         ; rel="alternate"
         ; type="application/linkset"
   Content-Length: 1349

   {
     "linkset": [
       {
         "anchor": "https://example.org/resource1",
         "author": [
           {
             "href": "https://authors.example.net/johndoe",
             "type": "application/rdf+xml"
           }
         ],
         "memento": [
           {
             "href": "https://example.org/resource1?version=1",
             "type": "text/html",
             "datetime": "Thu, 13 Jun 2019 09:34:33 GMT"
           },
           {
             "href": "https://example.org/resource1?version=2",
             "type": "text/html",
             "datetime": "Sun, 21 Jul 2019 12:22:04 GMT"
           }
         ],
         "latest-version": [
           {
             "href": "https://example.org/resource1?version=3",
             "type": "text/html"
           }
         ]

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       },
       {
         "anchor": "https://example.org/resource1?version=3",
         "predecessor-version": [
           {
             "href": "https://example.org/resource1?version=2",
             "type": "text/html"
           }
         ]
       },
       {
         "anchor": "https://example.org/resource1?version=2",
         "predecessor-version": [
           {
             "href": "https://example.org/resource1?version=1",
             "type": "text/html"
           }
         ]
       },
       {
         "anchor": "https://example.org/resource1#comment=1",
         "author": [
           {
             "href": "https://authors.example.net/alice"
           }
         ]
       }
     ]
   }

      Figure 10: Response to the client's request for the set of links

7.3.  Discovering a Link Set via the "linkset" Link Relation Type

   Figure 11 shows a client issuing an HTTP HEAD request against
   resource <https://example.org/resource1>.

   HEAD resource1 HTTP/1.1
   Host: example.org

                    Figure 11: Client HTTP HEAD request

   Figure 12 shows the response to the HEAD request of Figure 11.  The
   response contains an HTTP "Link" header field with a link that has
   the "linkset" relation type.  It indicates that a set of links is
   provided by resource <https://example.org/links/resource1>, which
   provides a representation with media type "application/linkset+json".

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   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:45:54 GMT
   Server: Apache-Coyote/1.1
   Link: <https://example.org/links/resource1>
         ; rel="linkset"
         ; type="application/linkset+json"
   Content-Length: 236
   Content-Type: text/html;charset=utf-8

                  Figure 12: Response to HTTP HEAD request

   Section 7.2 shows a client obtaining a set of links by issuing an
   HTTP GET on the target of the link with the "linkset" relation type,
   <https://example.org/links/resource1>.

8.  IANA Considerations

8.1.  Link Relation Type: linkset

   The link relation type below should be registered by IANA per
   Section 6.2.1 of Web Linking [RFC8288]:

      Relation Name: linkset

      Description: The link target of a link with the "linkset" relation
      type provides a set of links, including links in which the link
      context of the link participates.

      Reference: [[ This document ]]

8.2.  Media Type: application/linkset

   The Internet media type [RFC6838] for a natively encoded linkset is
   application/linkset.

      Type name: application

      Subtype name: linkset

      Required parameters: none

      Optional parameters: profile

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      Encoding considerations: Linksets are encoded according to the
      definition of [RFC8288].  The encoding of [RFC8288] is based on
      the general encoding rules of [I-D.ietf-httpbis-semantics], with
      the addition of allowing indicating character encoding and
      language for specific parameters as defined by [RFC8187].

      Security considerations: The security considerations of [[ This
      document ]] apply.

      Interoperability considerations: N/A

      Published specification: [[ This document ]]

      Applications that use this media type: This media type is not
      specific to any application, as it can be used by any application
      that wants to interchange web links.

      Additional information:

         Magic number(s): N/A

         File extension(s): This media type does not propose a specific
         extension.

         Macintosh file type code(s): TEXT

      Person & email address to contact for further information: Erik
      Wilde <erik.wilde@dret.net>

      Intended usage: COMMON

      Restrictions on usage: none

      Author: Erik Wilde <erik.wilde@dret.net>

      Change controller: IETF

8.3.  Media Type: application/linkset+json

   The Internet media type [RFC6838] for a JSON-encoded linkset is
   application/linkset+json.

      Type name: application

      Subtype name: linkset+json

      Required parameters: none

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      Optional parameters: profile

      Encoding considerations: The encoding considerations of [RFC8259]
      apply

      Security considerations: The security considerations of [[ This
      document ]] apply.

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

      Published specification: [[ This document ]]

      Applications that use this media type: This media type is not
      specific to any application, as it can be used by any application
      that wants to interchange web links.

      Additional information:

         Magic number(s): N/A

         File extension(s): JSON documents often use ".json" as the file
         extension, and this media type does not propose a specific
         extension other than this generic one.

         Macintosh file type code(s): TEXT

      Person & email address to contact for further information: Erik
      Wilde <erik.wilde@dret.net>

      Intended usage: COMMON

      Restrictions on usage: none

      Author: Erik Wilde <erik.wilde@dret.net>

      Change controller: IETF

9.  Security Considerations

   The security considerations of Web Linking [RFC8288] apply, as long
   as they are not specifically discussing the risks of exposing
   information in HTTP header fields.

   In general, links may cause information leakage when they expose
   information (such as URIs) that can be sensitive or private.  Links
   may expose "hidden URIs" that are not supposed to be openly shared,
   and may not be sufficiently protected.  Ideally, none of the URIs

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   exposed in links should be supposed to be "hidden"; instead, if these
   URIs are supposed to be limited to certain users, then technical
   measures should be put in place so that accidentally exposing them
   does not cause any harm.

   For the specific mechanisms defined in this specification, two
   security considerations should be taken into account:

   *  The Web Linking model always has an "implicit context", which is
      the resource of the HTTP interaction.  This original context can
      be lost or can change when self-contained link representations are
      moved.  Changing the context can change the interpretation of
      links when they have no explicit anchor, or when they use relative
      URIs.  Applications may choose to ignore links that have no
      explicit anchor or that use relative URIs when these are exchanged
      in stand-alone resources.

   *  The model introduced in this specification supports "3rd party
      links", where one party can provide links that have another
      party's resource as an anchor.  Depending on the link semantics
      and the application context, it is important to verify that there
      is sufficient trust in that 3rd party to allow it to provide these
      links.  Applications may choose to treat 3rd party links
      differently than cases where a resource and the links for that
      resource are provided by the same party.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

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   [RFC8187]  Reschke, J., "Indicating Character Encoding and Language
              for HTTP Header Field Parameters", RFC 8187,
              DOI 10.17487/RFC8187, September 2017,
              <https://www.rfc-editor.org/info/rfc8187>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              DOI 10.17487/RFC6982, July 2013,
              <https://www.rfc-editor.org/info/rfc6982>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC6906]  Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
              DOI 10.17487/RFC6906, March 2013,
              <https://www.rfc-editor.org/info/rfc6906>.

   [RFC7284]  Lanthaler, M., "The Profile URI Registry", RFC 7284,
              DOI 10.17487/RFC7284, June 2014,
              <https://www.rfc-editor.org/info/rfc7284>.

   [I-D.ietf-httpbis-semantics]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-15, 30 March 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-15>.

11.  Informative References

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   [W3C.REC-json-ld-20140116]
              Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0",
              World Wide Web Consortium Recommendation REC-json-ld-
              20140116, 16 January 2014,
              <https://www.w3.org/TR/2014/REC-json-ld-20140116>.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
              December 2005, <https://www.rfc-editor.org/info/rfc4287>.

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

Appendix A.  JSON-LD Context

   A set of links rendered according to the JSON serialization defined
   in Section 4.2 can be interpreted as RDF triples by adding a JSON-LD
   context [W3C.REC-json-ld-20140116] that maps the JSON keys to
   corresponding Linked Data terms.  And, as per
   [W3C.REC-json-ld-20140116] section 6.8 (https://www.w3.org/TR/2014/
   REC-json-ld-20140116/#interpreting-json-as-json-ld), when delivering
   a link set that is rendered according to the "application/
   linkset+json" media type to a user agent, a server can convey the
   availability of such a JSON-LD context by using a link with the
   relation type "http://www.w3.org/ns/json-ld#context" in the HTTP
   "Link" header.

   Using the latter approach to support discovery of a JSON-LD Context,
   the response to the GET request of Figure 9 against the URI of a set
   of links would be as shown in Figure 13.

   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:48:22 GMT
   Server: Apache-Coyote/1.1
   Content-Type: application/linkset+json
   Link: <https://example.org/contexts/linkset.jsonld>
         ; rel="http://www.w3.org/ns/json-ld#context"
         ; type="application/ld+json"
   Content-Length: 1349

   {
     "linkset": [
       {
         "anchor": "https://example.org/resource1",
         "author": [
           {
             "href": "https://authors.example.net/johndoe",

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             "type": "application/rdf+xml"
           }
         ],
         "memento": [
           {
             "href": "https://example.org/resource1?version=1",
             "type": "text/html",
             "datetime": "Thu, 13 Jun 2019 09:34:33 GMT"
           },
           {
             "href": "https://example.org/resource1?version=2",
             "type": "text/html",
             "datetime": "Sun, 21 Jul 2019 12:22:04 GMT"
           }
         ],
         "latest-version": [
           {
             "href": "https://example.org/resource1?version=3",
             "type": "text/html"
           }
         ]
       },
       {
         "anchor": "https://example.org/resource1?version=3",
         "predecessor-version": [
           {
             "href": "https://example.org/resource1?version=2",
             "type": "text/html"
           }
         ]
       },
       {
         "anchor": "https://example.org/resource1?version=2",
         "predecessor-version": [
           {
             "href": "https://example.org/resource1?version=1",
             "type": "text/html"
           }
         ]
       },
       {
         "anchor": "https://example.org/resource1#comment=1",
         "author": [
           {
             "href": "https://authors.example.net/alice"
           }
         ]
       }

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     ]
   }

      Figure 13: Using a typed link to support discovery of a JSON-LD
                         Context for a Set of Links

   In order to obtain the JSON-LD Context conveyed by the server, the
   user agent issues an HTTP GET against the link target of the link
   with the "http://www.w3.org/ns/json-ld#context" relation type.  The
   response to this GET is shown in Figure 14.  This particular JSON-LD
   context maps "application/linkset+json" representations of link sets
   to Dublin Core Terms.  It also renders each link relation as a URI
   Reference, inspired by the same approach used for Atom [RFC4287]
   described in Appendix A.2 of [RFC8288].

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   HTTP/1.1 200 OK
   Content-Type: application/ld+json
   Content-Length: 708

   {
     "@context": [
       {
         "@vocab":  "http://www.iana.org/assignments/relation/",
         "anchor":  "@id",
         "href":    "@id",
         "linkset": "@graph",
         "_linkset": "@graph",
         "title":   {
           "@id":    "http://purl.org/dc/terms/title"
         },
         "title*":  {
           "@id":    "http://purl.org/dc/terms/title"
         },
         "type":    {
           "@id":    "http://purl.org/dc/terms/format"
         },
         "datetime":    {
           "@id":    "http://purl.org/dc/terms/date"
         }
       },
       {
         "language": "@language",
         "value":    "@value",
         "hreflang": {
           "@id":        "http://purl.org/dc/terms/language",
           "@container": "@set"
         }
       }
     ]
   }

      Figure 14: JSON-LD Context mapping to Dublin Core Terms and IANA
                                assignments

   Applying the JSON-LD context of Figure 14 to the link set of
   Figure 13 allows transforming the "application/linkset+json" link set
   to an RDF link set.  Figure 15 shows the latter represented by means
   of the "text/turtle" RDF serialization.

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   <https://authors.example.net/johndoe>
     <http://purl.org/dc/terms/format>
     "application/rdf+xml" .
   <https://example.org/resource1#comment=1>
     <http://www.iana.org/assignments/relation/author>
     <https://authors.example.net/alice> .
   <https://example.org/resource1>
     <http://www.iana.org/assignments/relation/author>
     <https://authors.example.net/johndoe> .
   <https://example.org/resource1>
     <http://www.iana.org/assignments/relation/latest-version>
     <https://example.org/resource1?version=3> .
   <https://example.org/resource1>
     <http://www.iana.org/assignments/relation/memento>
     <https://example.org/resource1?version=1> .
   <https://example.org/resource1>
     <http://www.iana.org/assignments/relation/memento>
     <https://example.org/resource1?version=2> .
   <https://example.org/resource1?version=1>
     <http://purl.org/dc/terms/date>
     "Thu, 13 Jun 2019 09:34:33 GMT" .
   <https://example.org/resource1?version=1>
     <http://purl.org/dc/terms/format>
     "text/html" .
   <https://example.org/resource1?version=2>
     <http://purl.org/dc/terms/date>
     "Sun, 21 Jul 2019 12:22:04 GMT" .
   <https://example.org/resource1?version=2>
     <http://purl.org/dc/terms/format>
     "text/html" .
   <https://example.org/resource1?version=2>
     <http://www.iana.org/assignments/relation/predecessor-version>
     <https://example.org/resource1?version=1> .
   <https://example.org/resource1?version=3>
     <http://purl.org/dc/terms/format>
     "text/html" .
   <https://example.org/resource1?version=3>
     <http://www.iana.org/assignments/relation/predecessor-version>
     <https://example.org/resource1?version=2> .

        Figure 15: RDF serialization of the link set resulting from
                        applying the JSON-LD context

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   Note that the JSON-LD context of Figure 14 does not handle (meta)link
   relations of type ""linkset"" as they are in conflict with the top-
   level JSON key.  A workaround is to rename the top-level key to
   ""_linkset"" in the "application/linkset+json" before transforming a
   link set to JSON-LD.

Appendix B.  Implementation Status

   This section is to be removed before publishing as an RFC.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 6982
   [RFC6982].  The description of implementations in this section is
   intended to assist the IETF in its decision processes in progressing
   drafts to RFCs.  Please note that the listing of any individual
   implementation here does not imply endorsement by the IETF.
   Furthermore, no effort has been spent to verify the information
   presented here that was supplied by IETF contributors.  This is not
   intended as, and must not be construed to be, a catalog of available
   implementations or their features.  Readers are advised to note that
   other implementations may exist.

   According to RFC 6982, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

B.1.  GS1

   GS1 is a provider of barcodes (GS1 GTINs and EAN/UPC) for retail
   products and manages an ecology of services and standards to leverage
   them at a global scale.  GS1 has indicated that it will implement
   this "linkset" specification as a means to allow requesting and
   representing links pertaining to products from various retailers.
   Currently, the GS1 Digital Link specification makes an informative
   reference to version 03 of the "linkset" I-D.  GS1 expresses
   confidence that this will become a normative reference in the next
   iteration of that specification, likely to be ratified as a GS1
   standard around February 2021.

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B.2.  FAIR Signposting Profile

   The FAIR Signposting Profile is a community specification aimed at
   improving machine navigation of scholarly objects on the web through
   the use of typed web links pointing at e.g.  web resources that are
   part of a specific object, persistent identifiers for the object and
   its authors, license information pertaining to the object.  The
   specification encourages the use of Linksets and initial
   implementations are ongoing, for example, for the open source
   Dataverse data repository platform that was initiated by Harvard
   University and is meanwhile used by research institutions, worldwide.

B.3.  Open Journal Systems (OJS)

   Open Journal Systems (OJS) is an open-source software for the
   management of peer-reviewed academic journals, and is created by the
   Public Knowledge Project (PKP), released under the GNU General Public
   License.  Open Journal Systems (OJS) is a journal management and
   publishing system that has been developed by PKP through its
   federally funded efforts to expand and improve access to research.

   The OJS platform has implemented "linkset" support as an alternative
   way to provide links when there are more than a configured limit
   (they consider using about 10 as a good default, for testing purpose
   it is currently set to 8).

Acknowledgements

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

Authors' Addresses

   Erik Wilde
   Axway

   Email: erik.wilde@dret.net
   URI:   http://dret.net/netdret/

   Herbert Van de Sompel
   Data Archiving and Networked Services

   Email: herbert.van.de.sompel@dans.knaw.nl
   URI:   https://orcid.org/0000-0002-0715-6126

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