Linkset: Media Types and a Link Relation Type for Link Sets
draft-wilde-linkset-01

Network Working Group                                           E. Wilde
Internet-Draft                                           CA Technologies
Intended status: Informational                          H. Van de Sompel
Expires: March 10, 2018                   Los Alamos National Laboratory
                                                       September 6, 2017


      Linkset: Media Types and a Link Relation Type for Link Sets
                         draft-wilde-linkset-01

Abstract

   This specification defines two media types and a link relation type
   for sets of links.  The media types can be used to represents links
   in a standalone fashion, in one case in the native format used in the
   HTTP Link header, and in the other case in a JSON-based format.  The
   link relation can be used to reference these kind of standalone link
   sets, so that a resource can indicate that additional links putting
   it into context are available someplace else.  One typical scenario
   is when the number of links to put in an HTTP Link header field
   becomes too big, and thus these links should be provided by a
   distinct resource.

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
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   This Internet-Draft will expire on March 10, 2018.




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Copyright Notice

   Copyright (c) 2017 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
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   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 Link Sets  . . . . . . . . . . . . . . .   5
     4.1.  HTTP Link Document Format: application/linkset  . . . . .   6
     4.2.  JSON Document Format: application/linkset+json  . . . . .   6
       4.2.1.  Link Sets . . . . . . . . . . . . . . . . . . . . . .   6
       4.2.2.  Links . . . . . . . . . . . . . . . . . . . . . . . .   6
       4.2.3.  Link Target Attributes  . . . . . . . . . . . . . . .   8
       4.2.4.  Extension Target Attributes . . . . . . . . . . . . .  10
   5.  The "linkset" Relation Type for Linking to Link Sets  . . . .  10
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Links Provided in the Header of the Link Set Resource . .  11
     6.2.  Links Provided in the Body of the Link Set Resource,
           Link Set Serialized as application/linkset+json . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Link Relation Type: linkset . . . . . . . . . . . . . . .  15
     7.2.  Media Type: application/linkset . . . . . . . . . . . . .  16
       7.2.1.  IANA Considerations . . . . . . . . . . . . . . . . .  16
     7.3.  Media Type: application/linkset+json  . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19








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1.  Introduction

   Resources on the Web often convey typed Web Links
   [I-D.nottingham-rfc5988bis] as a part of resource representations,
   for example, using the <link> element for HTML representations, or
   the "Link" header field in HTTP response headers for representations
   of any media type.  In some cases, however, providing links by value
   is impractical or impossible.  In these cases, an approach to provide
   links by reference (instead of by value) can solve the problem.

   To that end, this specification defines two document formats and
   associated media types to represent a set of links.  It also defines
   the "linkset" relation type that a resource can use to support
   discovery of another resource that conveys links in which the former
   resource participates.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This specification uses the terms "link context" and "link target" as
   defined in [I-D.nottingham-rfc5988bis].  (These terms respectively
   correspond with "Context IRI" and "Target IRI" as used in RFC 5988
   [RFC5988]).  Although defined as IRIs, in common scenarios they are
   also URIs.

   Additionally, this specification uses the following terms for types
   of resources involved in providing links by reference:

   o  A "link set resource" is a resource that conveys a set of links.
      Section 4 defines two representations for a set of links, based on
      the abstract link model defined in [I-D.nottingham-rfc5988bis].

   o  An "origin resource" is a resource that participates in one or
      more links provided by a link set resource.  An origin resource
      can support discovery of an associated link set resource by using
      the relation type defined in Section 5.  As such, from the
      perspective of the origin resource, the links conveyed by the link
      set resource are provided by reference.

3.  Scenarios

   The following sections outline some scenarios in which it is useful
   to have the ability to separate resources from the links that pertain
   to them.




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   These are all scenarios in which providing links by reference is
   advantageous or necessary.  It is important to keep in mind that,
   when providing some links by reference, some/other links can still be
   provided by value.

3.1.  Third-Party Links

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

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

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

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

   In such cases, a third-party link set resource provides links
   pertaining to the origin resource.  This link set resource may be
   managed by the same custodian as the origin resource, or by a third
   party.

   In order for the server hosting the origin resource to provide an up-
   to-date and complete set of links in which the origin resource
   participates, the server would need to obtain the links from the link
   set resource, and embed them in the origin resource's representations
   prior to responding to a client.  Doing so would increase latency and
   load, which may be unnecessary if a client is not intent on consuming
   these links.  Providing links by reference, instead of by value,
   removes the server-to-server communication and resulting overhead
   required to obtain links.  Instead, the consumer of the origin
   resource can decide if they need the additional links as context for
   the resource.

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 headers.  In such cases, providing links by
   reference can be a solution because a link to a link set resource
   associated with an origin resource can typically be added by means of
   a web server rewrite rule that leverages the resource's URI.




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3.3.  Large Number of Links

   When conveying links in the HTTP "Link" header, 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 boundary for their size.  But when the number of
   links is unpredictable, estimating a reliable upper boundary 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 boundary 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, for example by limiting the links they
   provide to those with select relation types, thereby limiting the
   value of the HTTP "Link" header to clients.  Providing links by
   reference, instead of by value, overcomes challenges related to the
   unpredictable nature of the extent of HTTP "Link" headers.

   In more extreme scenarios it is conceivable that the number of links
   pertaining to the origin resource becomes so large that the response
   from the associated link set resource becomes too large.  This could
   be the case for highly popular origin resources, when the link set
   includes links in which the origin resource participates as both link
   context and link target.  In such cases, the link set resource could
   deliver responses incrementally, for example, using a paged resource
   model that clients can consume as required.

4.  Document Formats for Link Sets

   Link set resources can be represented by serializations that support
   conveying both link contexts and link targets.  This section
   specifies two document formats based on the abstract model specified
   in Section 2 of [I-D.nottingham-rfc5988bis] that defines a link as
   consisting of a "link context", a "link relation type", a "link
   target", and optional "target attributes".  In those terms, a link
   set resource is a resource that conveys a collection of one or more
   such links.

   The format defined in Section 4.1 is identical to the payload of the
   HTTP Link header as specified in [I-D.nottingham-rfc5988bis].  The
   format defined in Section 4.2 is based on JSON and thus does not have
   the character encoding limitations of HTTP header fields.




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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.  It is defined in Section 3 of [I-D.nottingham-rfc5988bis],
   more specifically by its ABNF production rule for "Link" and
   subsequent ones.  The assigned media type for this format is
   "application/linkset".

4.2.  JSON Document Format: application/linkset+json

   This document format is based on the native Link header field
   representation defined by [I-D.nottingham-rfc5988bis], but uses JSON
   [RFC7159] as the syntax to represent the information contained in the
   native format.  It is described in this section.  The assigned media
   type for this format is "application/linkset+json".

4.2.1.  Link Sets

   In the JSON representation of a link set:

   o  A link set MUST be represented as a JSON array.

   o  A distinct JSON object - the "link object" - in this array MUST be
      used to represent an individual link or multiple links that have
      the same link context and link target.  (see Section 4.2.2).

   o  Implementations MUST wrap link objects in an array, even if the
      link set contains only one link.

   o  Implementations MUST NOT include any members other than link
      objects in the array that represents a link set.

4.2.2.  Links

   In the JSON representation, individual links (as well as multiple
   links that have the same link context and link target) are
   represented by a JSON object, the link object.  A link object adheres
   to the following rules:

   o  Each link object MUST have an "href" member with a value that
      represents the link target.

   o  Each link object MAY have an "anchor" member with a value that
      represents the link context.  If this member is not provided, the
      link context is the link set resource.

   o  The value of both the "href" and "anchor" members MUST be a URI-
      reference as defined in Section 4.1 of [RFC3986].  Note that for



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      relative references, the baseURI is the URI of the link set
      resource.

   o  Each link object MUST have a "rel" or "rev" member (with the
      latter one deprecated) with a value that represents one or more
      link relation types, represented as an array of strings.  The
      value of each "rel"/"rev" link relation type MUST either be a
      registered relation type or an extension relation type (URI), as
      described in Sections 2.1.1 and 2.1.2 of
      [I-D.nottingham-rfc5988bis], respectively.

   o  Each link object MAY have additional members representing target
      attributes (see Section 4.2.3).

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

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

   The following example of a JSON-serialized link set represents two
   links with the core components of a link: link context, link relation
   type, and link target.  The syntax of this example takes advantage of
   the fact that multiple relation types can be used in one link object
   to represent multiple links that have the same link context and link
   target.

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

   The previous example is equivalent to the following one, which does
   not use the syntax shortcut and therefore represents the two links
   individually.

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








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4.2.3.  Link Target Attributes

   In many cases, a link is further qualified by target attributes,
   either defined by the serialization of [I-D.nottingham-rfc5988bis] or
   extension attributes defined and used by communities.

4.2.3.1.  JSON Representation of RFC 5988bis Target Attributes

   RFC 5988bis 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 and MUST be handled in the JSON representation 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
      attributes for a link - which follows the same model as in the
      [I-D.nottingham-rfc5988bis] syntax.

   o  "media": The optional and not repeatable "media" target attribute
      MUST be represented by a "media" member in the link object, and
      its value MUST be a string that follows the same model as in the
      [I-D.nottingham-rfc5988bis] syntax.

   o  "type": The optional and not repeatable "type" target attribute
      MUST be represented by a "type" member in the link object, and its
      value MUST be a string that follows the same model as in the
      [I-D.nottingham-rfc5988bis] syntax.

   o  "title": The optional and not repeatable "title" target attribute
      MUST be represented by a "title" member in the link object, and
      its value MUST be a string that follows the same model as in the
      [I-D.nottingham-rfc5988bis] 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 [RFC5987].  The details of the
      JSON representation that applies to title* are described in
      Section 4.2.3.2.

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






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

4.2.3.2.  Internationalized Target Attributes for a Link

   In addition to the target attributes described in Section 4.2.3,
   [I-D.nottingham-rfc5988bis] also supports attributes that follow the
   content model of [RFC5987].  In [I-D.nottingham-rfc5988bis], these
   target attributes are recognizable by the use of a trailing asterisk
   in the attribute name, such as "title*".  The content model of
   [RFC5987] 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 attributes is as follows:

   o  Internationalized target attributes use the attribute name
      (including the *) as their name.

   o  The character encoding information as prescribed by [RFC5987] is
      not preserved; instead, the attribute value is represented in the
      character encoding used for the JSON link set.

   o  The attribute value is an array that contains one or more arrays.
      In each of these arrays, the first value is a string representing
      the attribute value (in its unescaped version), and the optional
      second array value is a language tag [RFC5646] associated with
      that attribute value.

   The following example illustrates how the "title*" target attribute
   defined by [I-D.nottingham-rfc5988bis] is represented in a link
   object.

   [ { "href"   : "http://example.com/foo",
       "anchor" : "http://example.net/bar",
       "rel"    : [ "next" ] ,
       "title*" : [ [ "nachstes Kapitel", "de" ] ] } ]

   The above example assumes that the german title contains an umlaut
   character (which in the native syntax would be encoded as title*=UTF-
   8'de'n%c3%a4chstes%20Kapitel), which gets encoded in its unescaped
   form (but not shown here due to the limitations of RFC publication)
   in the JSON representation.  Implementations MUST properly decode/




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   encode RFC 5987 style target attributes when transcoding between the
   native and the JSON syntax.

4.2.4.  Extension Target Attributes

   Extension target attributes are attributes that are not defined by
   RFC 5988bis (as listed in Section 4.2.3.1), 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:

   o  Extension target attributes use the attribute name (including the
      *, if applicable) as their name.

   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 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 extension target attribute.

   o  If the extension attribute has a name with a trailing asterisk (it
      follows the content model of [RFC5987]), then each value in that
      array MUST be an array.  The value of each array MUST be
      structured as described in Section 4.2.3.2.

5.  The "linkset" Relation Type for Linking to Link Sets

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

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

   More than one link with a "linkset" relation type MAY be provided.
   Multiple such links can refer to the same set of links expressed
   using different representations, or to different sets of links
   (potentially provided by different services).





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   The use of a link with the "linkset" relation type does not preclude
   the provision of links with other relation types, i.e. the origin
   resource can provide typed links other than a "linkset" link.
   Therefore, the effective set of links pertaining to the origin
   resource is the union of the links that the resource itself provides,
   and of all links - provided by the link set resource - in which it
   participates.

   The link set resource MAY provide the links that pertain to the
   origin resource in its HTTP response header and/or body.

   In common scenarios (the origin resource is distinct from the link
   set resource), it is essential for link set representations to make
   the URI of the origin resource explicit for those links in which the
   origin resource acts as link context.

   A user agent that follows a "linkset" link from an origin resource
   MUST be aware that the link set resource can provide links in which
   the origin resource does not participate and MAY decide to ignore
   those links.

   There is no constraint on the target URI of a link with the "linkset"
   relation type; designing and using these URIs is left to the
   discretion of implementers.

   If an origin resource provides a "linkset" link pointing at a link
   set resource, and that link set resource provides a "linkset" link in
   turn, then this latter link points at links pertaining to the link
   set resource.  This means that in the context of the latter link, the
   link set resource is an origin resource.  This means that linkset
   relations are not transitive, and it is up to a client to decide
   whether they follow "nested chains" of "linkset" links or not.

6.  Examples

   Sections Section 6.1 and Section 6.2 show examples whereby the link
   set resource provides links pertaining to the origin resource, in its
   response header and body, respectively.

6.1.  Links Provided in the Header of the Link Set Resource

   Figure 1 shows a client issuing an HTTP head request against origin
   resource http://example.org/resource1.








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   HEAD /resource1 HTTP/1.1
   Host: example.org
   Connection: close


                    Figure 1: Client HTTP HEAD Request

   Figure 2 shows the response to the HEAD request of Figure 1.  The
   response contains a Link header with a link that uses the "linkset"
   relation type.  It indicates that links pertaining to the origin
   resource are provided by link set resource http://example.com/
   links?uri=http%3A%2F%2Fexample.org%2Fresource.

HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:37:51 GMT
Server: Apache-Coyote/1.1
Link: <http://example.com/links?uri=http%3A%2F%2Fexample.org%2Fresource>
      ; rel="linkset"
      ; type="text/html"
Content-Length: 5214
Content-Type: text/html;charset=utf-8
Connection: close


            Figure 2: Response to HTTP HEAD on Origin Resource

   While in this example the URI of the link set resource uses a pattern
   that represents the URI of the origin resource, this is opaque to the
   client, which simply follows target URI to retrieving the link set
   resource.

   Figure 3 shows the client issuing an HTTP GET request against the
   link set resource provided in Figure 2.

   GET /links?uri=http%3A%2F%2Fexample.org%2Fresource HTTP/1.1
   Host: example.com
   Connection: close


          Figure 3: Client HTTP GET against the Link Set Resource

   Figure 4 shows the response headers to the HTTP GET request of
   Figure 3.  The links pertaining to the origin resource are provided
   in the Link response header of the link set resource.  As can be
   seen, in order to support an unambiguous determination of the link
   context of each link, the "anchor" attribute is always provided.
   Note that most, but not all, links have the origin resource as link
   context (anchor).



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   HTTP/1.1 200 OK
   Date: Mon, 28 Nov 2016 14:40:02 GMT
   Server: Apache-Coyote/1.1
   Link: <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.net/resource41/>
      ; rel="related"
      ; type="application/pdf"
      ; anchor="http://example.org/resource1/items/AF48EF.pdf"
   Content-Type: text/html
   Content-Length: 3018

       Figure 4: Response to HTTP GET against the Link Set Resource

6.2.  Links Provided in the Body of the Link Set Resource, Link Set
      Serialized as application/linkset+json

   Figure 5 is an example of a client issuing an HTTP head request
   against origin resource http://example.org/article?id=10.1371/
   journal.pone.0167475

   HEAD article?id=10.1371/journal.pone.0167475 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 links pertaining to the origin
   resource are provided by link set resource
   http://example.com/links/10.1371/journal.pone.0167475, which provides
   a representation with media type application/linkset+json.



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   HTTP/1.1 200 OK
   Date: Mon, 28 Nov 2016 14:37:51 GMT
   Server: Apache-Coyote/1.1
   Link: <http://example.com/links/10.1371/journal.pone.0167475>
         ; rel="linkset"
         ; type="application/linkset+json"
   Content-Length: 236
   Content-Type: text/html;charset=utf-8
   Connection: close


            Figure 6: Response to HTTP HEAD on Origin Resource

   In this example, the URI of the linkset resource does not directly
   represent the URI of the origin resource anymore.  There still is an
   association possible through the use of a URI pattern that is
   includes identifying information.  But as in the example above, the
   URI of the link set resource is opaque to the client that simply
   accesses the URI to retrieve the link set resource.

   Figure 7 shows the client issuing an HTTP GET request against the
   link set resource provided in Figure 6.

   GET /links/10.1371/journal.pone.0167475 HTTP/1.1
   Host: example.com
   Accept: application/linkset+json
   Connection: close

          Figure 7: Client HTTP GET against the Link Set Resource

   Figure 8 shows the response headers to the HTTP GET request of
   Figure 7.  The links pertaining to the origin resource are provided
   in the response body of the link set resource and are serialized
   according to the media type application/linkset+json.

















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HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:40:02 GMT
Server: Apache-Coyote/1.1
Content-Type: application/linkset+json
Content-Length: 794

[{"href":"http://authors.example.net/johndoe",
  "anchor":"http://example.org/article?id=10.1371/journal.pone.0167475",
  "rel":["author"],"type":"application/rdf+xml"},
 {"href":"http://authors.example.net/janedoe",
  "anchor":"http://example.org/article?id=10.1371/journal.pone.0167475",
  "rel":["author"],"type":"application/rdf+xml"},
 {"href":"http://example.org/resource1/items/AF48EF.pdf",
  "anchor":"http://example.org/article?id=10.1371/journal.pone.0167475",
  "rel":["item"],"type":"text/html"},
 {"href":"http://example.org/resource1/items/CB63DA.html",
  "anchor":"http://example.org/article?id=10.1371/journal.pone.0167475",
  "rel":"[item"],"type":"application/pdf"},
 {"href":"http://example.net/resource41/",
  "anchor":"http://example.org/resource1/items/AF48EF.pdf",
  "rel":["related"],"type":"application/pdf"}]

       Figure 8: Response to HTTP GET against the Link Set Resource

   If Figure 6 would have provided a link to a link set with media type
   application/linkset, and if the client would have requested that link
   set, then the body of the response would have been similar to
   Figure 8.  But it would have had application/linkset as Content-Type,
   the payload of the HTTP Link header of Figure 4 as body, and an
   accordingly adjusted value for Content-Length.

7.  IANA Considerations

7.1.  Link Relation Type: linkset

   The link relation type below has been registered by IANA per
   Section 6.2.1 of Web Linking [I-D.nottingham-rfc5988bis]:

      Relation Name: linkset

      Description: The Target IRI of a link with the "linkset" relation
      type provides a set of links that pertain to the Context IRI of
      the link.

      Reference: [[ This document ]]






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7.2.  Media Type: application/linkset

7.2.1.  IANA Considerations

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

      Type name: application

      Subtype name: linkset

      Required parameters: none

      Optional parameters: none

      Encoding considerations: ...

      Security considerations: ...

      Interoperability considerations: ...

      Published specification: [[ This document ]]

      Applications that use this media type: ...

      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: Herbert
      Van de Sompel <herbertv@lanl.gov>

      Intended usage: COMMON

      Restrictions on usage: none

      Author: Herbert Van de Sompel <herbertv@lanl.gov>

      Change controller: IETF







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7.3.  Media Type: application/linkset+json

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

      Type name: application

      Subtype name: linkset+json

      Required parameters: none

      Optional parameters: none

      Encoding considerations: ...

      Security considerations: ...

      Interoperability considerations: ...

      Published specification: [[ This document ]]

      Applications that use this media type: ...

      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: Herbert
      Van de Sompel <herbertv@lanl.gov>

      Intended usage: COMMON

      Restrictions on usage: none

      Author: Herbert Van de Sompel <herbertv@lanl.gov>

      Change controller: IETF








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8.  Security Considerations

   ...

9.  Normative References

   [I-D.nottingham-rfc5988bis]
              Nottingham, M., "Web Linking", draft-nottingham-
              rfc5988bis-08 (work in progress), August 2017.

   [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>.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,
              <https://www.rfc-editor.org/info/rfc5987>.

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

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

   [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>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.






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   [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>.

   [W3C.REC-html401-19991224]
              Rivoal, F., "Media Queries", World Wide Web
              Consortium Recommendation REC-css3-mediaqueries-20120619,
              June 2012.

Authors' Addresses

   Erik Wilde
   CA Technologies

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


   Herbert Van de Sompel
   Los Alamos National Laboratory

   Email: herbertv@lanl.gov
   URI:   http://public.lanl.gov/herbertv/



























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