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Independent Submission                                          J. SnellRequest for Comments: 6903                                    March 2013Category: InformationalISSN: 2070-1721Additional Link Relation TypesAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Snell                         Informational                     [Page 1]

RFC 6903                Additional Link Relations             March 2013Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  "about" . . . . . . . . . . . . . . . . . . . . . . . . . . .23.  "preview" . . . . . . . . . . . . . . . . . . . . . . . . . .24.  "privacy-policy"  . . . . . . . . . . . . . . . . . . . . . .34.1.  The "privacy-policy" Link Relation and P3P  . . . . . . .45.  "terms-of-service"  . . . . . . . . . . . . . . . . . . . . .46.  "type"  . . . . . . . . . . . . . . . . . . . . . . . . . . .57.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .58.  Security Considerations . . . . . . . . . . . . . . . . . . .69.  References  . . . . . . . . . . . . . . . . . . . . . . . . .69.1.  Normative References  . . . . . . . . . . . . . . . . . .69.2.  Informative References  . . . . . . . . . . . . . . . . .61.  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.Snell                         Informational                     [Page 2]

RFC 6903                Additional Link Relations             March 2013   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 theSnell                         Informational                     [Page 3]

RFC 6903                Additional Link Relations             March 2013   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.Snell                         Informational                     [Page 4]

RFC 6903                Additional Link Relations             March 20136.  "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 5Snell                         Informational                     [Page 5]

RFC 6903                Additional Link Relations             March 2013   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 68.  Security Considerations   There are no additional security concerns introduced by this   document.9.  References9.1.  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.comSnell                         Informational                     [Page 6]