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.

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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