Independent Submission                                          J. Snell
Request for Comments: 6903                                    March 2013
Category: Informational
ISSN: 2070-1721


                     Additional Link Relation Types

Abstract

   This specification defines a number of additional link relation types
   that can used for a range of purposes in a variety of applications
   types.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6903.

Copyright Notice

   Copyright (c) 2013 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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  "about" . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  "preview" . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  "privacy-policy"  . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  The "privacy-policy" Link Relation and P3P  . . . . . . .   4
   5.  "terms-of-service"  . . . . . . . . . . . . . . . . . . . . .   4
   6.  "type"  . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The fundamental model for web links and the "Link Relations" registry
   are established by [RFC5988].  This specification defines and adds
   the following additional link relation types to the registry:
   "about", "preview", "privacy-policy", "terms-of-service", and "type".

2.  "about"

   The "about" link relation can be used to refer to a resource that is
   the subject or topic of the link's context.  Multiple subjects can be
   indicated through the use of multiple "about" link relations.

   For example, if the context resource is a review about a particular
   product, the "about" link can be used to reference the URL of the
   product:

     HTTP/1.1 200 OK
     Content-Type: application/json
     Link: <http://store.example.org/product/abc>; rel="about"

     {...}

3.  "preview"

   The "preview" link relation can be used to refer to a resource that
   serves as a preview of the link's context, likely with reduced
   quality or limited content.  For instance, the preview link might
   reference a screen capture of a video, a brief snippet of audio from
   a song, or a thumbnail representation of an image.

   For example, issuing an HTTP HEAD request to a URI representing a
   large video or image file might return a link to a short or lower-
   quality preview of the original:

     HTTP/1.1 200 OK
     Content-Text: video/mpeg
     Link: <http://example.org/preview/10-second-preview.mpg>;
       rel="preview"; type="video/mpeg"

4.  "privacy-policy"

   The "privacy-policy" link relation can be used to refer to a resource
   describing the privacy policy associated with the link's context.
   The privacy policy can be any resource that discloses what personal
   information about the user is collected and how that personal
   information is stored, used, managed, and disclosed to other parties.

   For example, an HTTP server that collects personal information about
   a user throughout the course of the user's interaction with the
   service can include "privacy-policy" links within all HTTP responses
   using any combination of Link headers or links embedded in the
   response payload:

     HTTP/1.1 200 OK
     Content-Type: text/html

     <html>
       <head>
         ...
         <link rel="privacy-policy" href="/privacy-policy.html" />
         ...
       </head>
       <body>
         ...
       </body>
     </html>


   Note that in the absence of clear legal obligations placed on an
   entity, either through contract or law, the presence of a "privacy-
   policy" link does not constitute a legally binding obligation on the
   part of the service.  The linked resource can only be interpreted as
   a description of the expected practice.

   It is recommended that publishers of privacy-policy resources linked
   to using the "privacy-policy" link relation provide a clear and
   simple mechanism for signaling when changes to the privacy-policy
   resource have been made, such as generating a new Entity Tag for the

   resource or generating a hash over the privacy policy's content.  The
   extent to which mechanisms are utilized is out of the scope of this
   specification, however.

4.1.  The "privacy-policy" Link Relation and P3P

   The Platform for Privacy Preferences [P3P] is a W3C Recommendation
   that defines a data format for the expression of privacy policy
   information.  While the "privacy-policy" link relation can be used to
   reference P3P documents, there is no intended relationship, normative
   or otherwise, between this specification and the P3P Recommendation.
   As far as this specification is concerned, P3P documents are just one
   possible type of resource that "privacy-policy" links can reference.

5.  "terms-of-service"

   The "terms-of-service" link relation can be used to refer to a
   resource describing the terms of service associated with the link's
   context.  The terms of service can be any resource that describes the
   rules to which a consumer of the service must agree to follow when
   using the service provided by the link's context.

   For example, an HTTP server can include "terms-of-service" links
   within all HTTP responses using any combination of Link headers or
   links embedded in the response payload:

     HTTP/1.1 200 OK
     Content-Type: text/html

     <html>
       <head>
         ...
         <link rel="terms-of-service" href="/tos.html">
         ...
       </head>
       <body>
         ...
       </body>
     </html>

   It must be noted that the terms of service linked to using this link
   relation carry no legal weight and can be ignored with impunity in
   the absence of an explicit, legally enforceable contract.  The linked
   terms of service are simply a notice of the terms that may be
   expected to apply once a contract is established.

6.  "type"

   The "type" link relation can be used to indicate that the context
   resource is an instance of the resource identified by the target
   Internationalized Resource Identifier (IRI).

     HTTP/1.1 200 OK
     Content-Type: text/plain
     Link: <http://example.org/Person/givenName>; rel="type"

     Sally

   When used within the header of an HTTP message, the type specified by
   the "type" link relation cannot be confused with the content type of
   the payload as given by the Content-Type header.  The "type" link
   relation references the payload's abstract semantic type, whereas the
   Content-Type header identifies the specific serialization format of
   the payload.

   If the context can be considered to be an instance of multiple
   semantic types, multiple "type" link relations can be used.

7.  IANA Considerations

   The "Link Relation Types" registry has been updated with the
   following entries:

   o  Relation Name: about
   o  Description: Refers to a resource that is the subject of the
      link's context.
   o  Reference: This specification, Section 2

   o  Relation Name: preview
   o  Description: Refers to a resource that provides a preview of the
      link's context.
   o  Reference: This specification, Section 3

   o  Relation Name: privacy-policy
   o  Description: Refers to a privacy policy associated with the link's
      context.
   o  Reference: This specification, Section 4

   o  Relation Name: terms-of-service
   o  Description: Refers to the terms of service associated with the
      link's context.
   o  Reference: This specification, Section 5

   o  Relation Name: type
   o  Description: Refers to a resource identifying the abstract
      semantic type of which the link's context is considered to be an
      instance.
   o  Reference: This specification, Section 6

8.  Security Considerations

   There are no additional security concerns introduced by this
   document.

9.  References

9.1.  Normative References

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

9.2.  Informative References

   [P3P]      W3C, "P3P: The Platform for Privacy Preferences", November
              2007, <http://www.w3.org/P3P/>.

Author's Address

   James M Snell

   EMail: jasnell@gmail.com