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Versions: 00 01 02 draft-wilde-linkset

Network Working Group                                           E. Wilde
Internet-Draft                                           CA Technologies
Intended status: Informational                          H. Van de Sompel
Expires: December 15, 2017                Los Alamos National Laboratory
                                                           June 13, 2017


              Linkset: A Link Relation Type for Link Sets
                    draft-wilde-linkset-link-rel-02

Abstract

   This specification defines the "linkset" link relation type that can
   be used to link to a resource that provides a set of links.

Note to Readers

   Please discuss this draft on the ART mailing list
   (<https://www.ietf.org/mailman/listinfo/art>).

   Online access to all versions and files is available on GitHub
   (<https://github.com/dret/I-D/tree/master/linkset-link-rel>).

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 15, 2017.

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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Origin Resource . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Link Set Resource . . . . . . . . . . . . . . . . . . . .   3
   3.  Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Third-Party Links . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Large Number of Links . . . . . . . . . . . . . . . . . .   4
   4.  The "linkset" Relation Type for Linking to Link Sets  . . . .   5
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Links Provided in the Header of the Link Set Resource . .   6
     5.2.  Link Set Serialized as application/link-format+json . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Link Relation Type: linkset . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Resources on the Web often convey typed Web Links [RFC5988] 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.  This specification defines the
   "linkset" relation type that allows to link resources to sets of
   links, thereby making it possible to represent links by reference,
   and not by value.

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 section introduces two types of resources involved in providing
   links by reference, as well as the link relation type used to
   interlink them.



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2.1.  Origin Resource

   An "origin resource" is a resource that makes links in which it
   participates discoverable by providing a typed link that has a "link
   set resource" as the target.  From the perspective of the origin
   resource, the links in the "link set resource" are therefore provided
   by reference.

2.2.  Link Set Resource

   A "link set resource" is a resource - distinct from the origin
   resource, and possibly linked to from the origin resource - that
   provides one or more links in which the origin resource participates.

   Because the link set resource is distinct from the origin resource,
   links provided by the link set resource must represent both the
   source and target of each link to allow unambiguous interpretation.

3.  Scenarios

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

   These are all scenarios in which providing (some) links by reference
   is advantageous or necessary to accomplish certain goals.  It is
   important to keep in mind that even when using the pattern of "links
   by reference", it is still possible to provide links by value,
   allowing resources to decide and combine which of the two patterns
   they would like to use.

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 source but also as target.

   o  Providing links pertaining to the original 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



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   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 for it, it 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.  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
   a response header 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 incoming links as well.  In such cases, the link set
   resource could deliver responses incrementally, for example, using a
   paged resource model that clients could consume as required.







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

   A link with the "linkset" link relation type has as Context IRI the
   IRI of an origin resource, and as Target IRI the IRI of an associated
   link set resource.

   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 link sets
   (potentially provided by different services).

   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 in which it participates which are provided by the
   link set resources linked from it via "linkset" links.

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

   o  In cases whereby the link set resource provides these links in its
      Link HTTP response header, the payload of that header MUST comply
      with the syntax defined in Web Linking [RFC5988].  The media type
      of the response body is not constrained.

   o  In cases whereby the link set resource provides these links in its
      response body, the body SHOULD allow a client to determine the
      source and target of each provided link.  The media type of the
      response body is otherwise not constrained.

   There is no constraint on the Target IRI of a link with the "linkset"
   relation type; designing and using these links 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 also means that
   linkset relations are not transitive; it is up to a client to decide
   whether they follow "chains" of linkset links or not.



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5.  Examples

   Sections Section 5.1 and Section 5.2 show examples whereby the link
   set resource provides links pertaining to the origin resource, in its
   response header and body, respectively.

5.1.  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.

   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 IRI of the linkset resource uses a pattern
   that represents the IRI of the origin resource, this is opaque to the
   client, which simply follows the provided linkset IRI when retrieving
   the linkset resource.

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







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   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 Context
   IRI of each link, the "anchor" attribute is provided for each link.
   Note that most, but not all, links have the origin resource as
   Context IRI (anchor).

   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







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5.2.  Link Set Serialized as application/link-format+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 vendor media type application/
   vnd.example.org.linkset+json.

   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/vnd.example.org.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 IRI of the linkset resource does not directly
   represent the IRI of the origin resource anymore.  There still is an
   association possible through a IRI pattern that is including DOI
   information, but as in the example above, the linkset IRI is opaque
   to the client which simply accesses the IRI to retrieve the linkset
   resource.

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








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   GET /links/10.1371/journal.pone.0167475 HTTP/1.1
   Host: example.com
   Accept: application/vnd.example.org.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 vendor media type application/
   vnd.example.org.linkset+json.

HTTP/1.1 200 OK

Date: Mon, 28 Nov 2016 14:40:02 GMT
Server: Apache-Coyote/1.1
Content-Type: application/vnd.example.org.linkset+json
Content-Length: 729

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

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

6.  IANA Considerations

   The link relation type below has been registered by IANA per
   Section 6.2.1 of RFC 5988 [RFC5988]:

6.1.  Link Relation Type: linkset

      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 ]]

7.  Security Considerations

   ...





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8.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

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

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

   [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,
              <http://www.rfc-editor.org/info/rfc7231>.

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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