Link Relation Types for Simple Version Navigation between Web Resources
RFC 5829

Internet Engineering Task Force (IETF)                          A. Brown
Request for Comments: 5829                                      G. Clemm
Category: Informational                                              IBM
ISSN: 2070-1721                                          J. Reschke, Ed.
                                                              greenbytes
                                                              April 2010

Link Relation Types for Simple Version Navigation between Web Resources

Abstract

   This specification defines a set of link relation types that may be
   used on Web resources for navigation between a resource and other
   resources related to version control, such as past versions and
   working copies.

Status of This Memo

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

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are 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/rfc5829.

Copyright Notice

   Copyright (c) 2010 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
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   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.

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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Link Relations ..................................................4
      3.1. 'version-history' ..........................................4
      3.2. 'latest-version' ...........................................4
      3.3. 'working-copy' .............................................4
      3.4. 'working-copy-of' ..........................................4
      3.5. 'predecessor-version' ......................................4
      3.6. 'successor-version' ........................................5
   4. IANA Considerations .............................................5
      4.1. 'version-history' Link Relation Registration ...............5
      4.2. 'latest-version' Link Relation Registration ................5
      4.3. 'working-copy' Link Relation Registration ..................5
      4.4. 'working-copy-of' Link Relation Registration ...............6
      4.5. 'predecessor-version' Link Relation Registration ...........6
      4.6. 'successor-version' Link Relation Registration .............6
   5. Security Considerations .........................................6
   6. Acknowledgments .................................................7
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................7
   Appendix A.  Relationship to Java Content Repository (JCR) and
                WebDAV ................................................9
      A.1.  Example: Use of Link Relations in HTTP Link Header .......10

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1.  Introduction

   This specification defines a set of link relation types that may be
   used on Web resources that exist in a system that supports versioning
   to navigate among the different resources available, such as past
   versions and working copies.

   These link relations are used in the AtomPub ([RFC5023]) bindings of
   the "Content Management Interoperability Services" (CMIS).  See
   Section 3.4.3.3 of [CMIS] for further information.

2.  Terminology

   Versioned Resource

      When a resource is put under version control, it becomes a
      "versioned resource".  Many servers protect versioned resources
      from modifications by considering them "checked in", and by
      requiring a "checkout" operation before modification, and a
      "checkin" operation to get back to the "checked-in" state.  Other
      servers allow modification, in which case the checkout/checkin
      operation may happen implicitly.

   Version History

      A "version history" resource is a resource that contains all the
      versions of a particular versioned resource.

   Predecessor, Successor

      When a versioned resource is checked out and then subsequently
      checked in, the version that was checked out becomes a
      "predecessor" of the version created by the checkin.  A client can
      specify multiple predecessors for a new version if the new version
      is logically a merge of those predecessors.  The inverse of the
      predecessor relation is the "successor" relation.  Therefore, if X
      is a predecessor of Y, then Y is a successor of X.

   Working Copy

      A "working copy" is a resource at a server-defined URL that can be
      used to create a new version of a versioned resource.

   Checkout

      A "checkout" is an operation on a versioned resource that creates
      a working copy, or changes the versioned resource to be a working
      copy as well ("in-place versioning").

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   Checkin

      A "checkin" is an operation on a working copy that creates a new
      version of its corresponding versioned resource.

      Note: the operations for putting a resource under version control
      and for checking in and checking out depend on the protocol in use
      and are beyond the scope of this document; see [CMIS], [RFC3253],
      and [JSR-283] for examples.

3.  Link Relations

   The following link relations are defined.

3.1.  'version-history'

   When included on a versioned resource, this link points to a resource
   containing the version history for this resource.

3.2.  'latest-version'

   When included on a versioned resource, this link points to a resource
   containing the latest (e.g., current) version.

   The latest version is defined by the system.  For linear versioning
   systems, this is probably the latest version by timestamp.  For
   systems that support branching, there will be multiple latest
   versions, one for each branch in the version history.

   Some systems may allow more than one of these link relations.

3.3.  'working-copy'

   When included on a versioned resource, this link points to a working
   copy for this resource.

   Some systems may allow more than one of these link relations.

3.4.  'working-copy-of'

   When included on a working copy, this link points to the versioned
   resource from which this working copy was obtained.

3.5.  'predecessor-version'

   When included on a versioned resource, this link points to a resource
   containing the predecessor version in the version history.

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   Some systems may allow more than one of these link relations in the
   case of multiple branches merging.

3.6.  'successor-version'

   When included on a versioned resource, this link points to a resource
   containing the successor version in the version history.

   Some systems may allow more than one of these link relations in order
   to support branching.

4.  IANA Considerations

   The link relations below have been registered by IANA per Section 7.1
   of [RFC4287]:

4.1.  'version-history' Link Relation Registration

   Attribute Value:  version-history

   Description:  See Section 3.1.

   Expected display characteristics:  Undefined; this relation can be
      used for background processing or to provide extended
      functionality without displaying its value.

   Security considerations:  See Section 5.

4.2.  'latest-version' Link Relation Registration

   Attribute Value:  latest-version

   Description:  See Section 3.2.

   Expected display characteristics:  Undefined; this relation can be
      used for background processing or to provide extended
      functionality without displaying its value.

   Security considerations:  See Section 5.

4.3.  'working-copy' Link Relation Registration

   Attribute Value:  working-copy

   Description:  See Section 3.3.

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   Expected display characteristics:  Undefined; this relation can be
      used for background processing or to provide extended
      functionality without displaying its value.

   Security considerations:  See Section 5.

4.4.  'working-copy-of' Link Relation Registration

   Attribute Value:  working-copy-of

   Description:  See Section 3.4.

   Expected display characteristics:  Undefined; this relation can be
      used for background processing or to provide extended
      functionality without displaying its value.

   Security considerations:  See Section 5.

4.5.  'predecessor-version' Link Relation Registration

   Attribute Value:  predecessor-version

   Description:  See Section 3.5.

   Expected display characteristics:  Undefined; this relation can be
      used for background processing or to provide extended
      functionality without displaying its value.

   Security considerations:  See Section 5.

4.6.  'successor-version' Link Relation Registration

   Attribute Value:  successor-version

   Description:  See Section 3.6.

   Expected display characteristics:  Undefined; this relation can be
      used for background processing or to provide extended
      functionality without displaying its value.

   Security considerations:  See Section 5.

5.  Security Considerations

   Automated agents should take care when these relations cross
   administrative domains (e.g., the URI has a different authority than
   the current document).  Such agents should also take care to detect
   circular references.

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   Care should be applied when versioned resources are subject to
   differing access policies.  In this case, exposing links may leak
   information even if the linked resource itself is properly secured.
   In particular, the syntax of the link target could expose sensitive
   information (see Section 16.2 of [RFC3253] for a similar
   consideration in WebDAV Versioning).  Note that this applies to
   exposing link metadata in general, not only to links related to
   versioning.

6.  Acknowledgments

   Thanks to the members of Content Management Interoperability Services
   (CMIS) Technical Committee (TC) at OASIS for the initial proposal,
   and to Jan Algermissen for feedback during IETF review.

7.  References

7.1.  Normative References

   [RFC4287]   Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
               Syndication Format", RFC 4287, December 2005.

7.2.  Informative References

   [CMIS]      Brown, A., Gur-Esh, E., McVeigh, R., and F.  Mueller,
               "Content Management Interoperability Services (CMIS)
               Version 1.0", OASIS Content Management Interoperability
               Services (CMIS) Version 1.0 Committee Specification 01,
               March 2010,  <http://docs.oasis-open.org/cmis/CMIS/
               v1.0/cs01/cmis-spec-v1.0.html>.

               Latest version available at
               <http://docs.oasis-open.org/cmis/CMIS/v1.0/
               cmis-spec-v1.0.html>

   [JSR-283]   Day Software, Nuescheler, D., and P. Piegaze, "Content
               Repository API for Java(tm) Technology Specification",
               Java Specification Request 283, August 2009,
               <http://www.day.com/specs/jcr/2.0/>.

   [RFC3253]   Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J.
               Whitehead, "Versioning Extensions to WebDAV (Web
               Distributed Authoring and Versioning)", RFC 3253,
               March 2002.

   [RFC5023]   Gregorio, J. and B. de hOra, "The Atom Publishing
               Protocol", RFC 5023, October 2007.

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   [WEB-LINKING]
               Nottingham, M., "Web Linking", Work in Progress,
               March 2010.

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Appendix A.  Relationship to Java Content Repository (JCR) and WebDAV

   The link relations defined in Section 3 correspond to various
   properties used in WebDAV Versioning [RFC3253] and JCR [JSR-283]:

   version-history

      WebDAV: the resource identified by the DAV:version-history
      property ([RFC3253], Sections 5.2.1 and 5.3.1).

      JCR: the node identified by jcr:versionHistory property
      ([JSR-283], Section 3.13.2.4) for versionable nodes, the parent
      folder for version nodes.

   latest-version

      WebDAV: for version-controlled resources, DAV:checked-in
      ([RFC3253], Section 3.2.1) or DAV:checked-out ([RFC3253], Section
      3.3.1), depending on checkin state.  For version resources, a
      successor version that itself does not have any successors.

      JCR: the version node identified by the jcr:baseVersion property
      ([JSR-283], Section 3.13.2.5) for versionable nodes; for version
      nodes, a successor version that itself does not have any
      successors.

   working-copy

      WebDAV: for version-controlled resources that are checked-out in
      place: the resource itself.  For version resources: each resource
      identified by a member of the DAV:checkout-set property (see
      [RFC3253], Section 3.4.3).

      JCR: for checked-out versionable nodes: the node itself.

   working-copy-of

      WebDAV: the resource identified by the DAV:checked-out property
      (see [RFC3253], Section 3.3.1).

      JCR: for checked-out versionable nodes: the node identified by the
      jcr:baseVersion property ([JSR-283], Section 3.13.12.5).

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   predecessor-version

      WebDAV: each resource identified by a member of DAV:predecessor-
      set ([RFC3253], Sections 3.3.2 and 3.4.1).

      JCR: each node identified by a member of jcr:predecessors
      ([JSR-283], Section 3.13.3.3).

   successor-version

      WebDAV: each resource identified by a member of DAV:successor-set
      ([RFC3253], Section 3.4.2).

      JCR: each node identified by a member of jcr:successors
      ([JSR-283], Section 3.13.3.4).

A.1.  Example: Use of Link Relations in HTTP Link Header

   The "Web Linking" specification ([WEB-LINKING]) generalizes Atom link
   relations, and also reintroduces the HTTP "Link" header as a way to
   expose link relations in HTTP responses.  This will make it possible
   to expose version links independently from a specific vocabulary, be
   it the Atom Feed Format ([RFC4287]) or WebDAV properties ([RFC3253]).

   For instance, a response to a VERSION-CONTROL request ([RFC3253],
   Section 3.5) could expose a newly created version-history and
   checked-in version as link relations:

   >> Request:

   VERSION-CONTROL /docs/test.txt HTTP/1.1
   Host: example.net

   >> Response:

   HTTP/1.1 204 No Content
   Link: </system/v/84345634/1>; rel=latest-version;
         anchor=</docs/test.txt>
   Link: </system/vh/84345634>; rel=version-history;
         anchor=</docs/test.txt>

   (Note that in this case, the anchor parameter is used, as the
   response to a VERSION-CONTROL request is not a representation of the
   resource at the Request-URI.)

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   A subsequent HEAD request on that resource could expose the version-
   history and latest-version relations as well:

   >> Request:

   HEAD /docs/test.txt HTTP/1.1
   Host: example.net

   >> Response:

   HTTP/1.1 200 OK
   Content-Type: text/plain; charset=UTF-8
   Content-Length: 12345
   Link: </system/v/84345634/1>; rel=latest-version
   Link: </system/vh/84345634>; rel=version-history

   After creating more versions, following the latest-version would then
   expose predecessors of a version:

   >> Request:

   HEAD /system/v/84345634/3 HTTP/1.1
   Host: example.net

   >> Response:

   HTTP/1.1 200 OK
   Content-Type: text/plain; charset=UTF-8
   Content-Length: 12323
   Link: </system/v/84345634/2>; rel=predecessor-version

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Authors' Addresses

   Al Brown
   IBM
   3565 Harbor Blvd
   Costa Mesa, California 92626
   USA

   EMail: albertcbrown@us.ibm.com

   Geoffrey Clemm
   IBM
   20 Maguire Road
   Lexington, MA 02421
   USA

   EMail: geoffrey.clemm@us.ibm.com

   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW 48155
   Germany

   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/

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