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

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

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
) 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.
Brown, et al. Informational [Page 1]
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Table of Contents
. Introduction ....................................................
. Terminology .....................................................
. Link Relations ..................................................
3.1
. 'version-history' ..........................................
3.2
. 'latest-version' ...........................................
3.3
. 'working-copy' .............................................
3.4
. 'working-copy-of' ..........................................
3.5
. 'predecessor-version' ......................................
3.6
. 'successor-version' ........................................
. IANA Considerations .............................................
4.1
. 'version-history' Link Relation Registration ...............
4.2
. 'latest-version' Link Relation Registration ................
4.3
. 'working-copy' Link Relation Registration ..................
4.4
. 'working-copy-of' Link Relation Registration ...............
4.5
. 'predecessor-version' Link Relation Registration ...........
4.6
. 'successor-version' Link Relation Registration .............
. Security Considerations .........................................
. Acknowledgments .................................................
. References ......................................................
7.1
. Normative References .......................................
7.2
. Informative References .....................................
Appendix A
. Relationship to Java Content Repository (JCR) and
WebDAV ................................................
A.1
. Example: Use of Link Relations in HTTP Link Header .......
10
Brown, et al. Informational [Page 2]
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. 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.
. 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").
Brown, et al. Informational [Page 3]
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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.
. 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.
Brown, et al. Informational [Page 4]
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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.
. 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
Brown, et al. Informational [Page 5]
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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
. 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.
Brown, et al. Informational [Page 6]
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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.
. 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.
. 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, <
v1.0/cs01/cmis-spec-v1.0.html
>.

Latest version available at
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,
>.

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.
Brown, et al. Informational [Page 7]
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Version Navigation Link Relations April 2010
WEB-LINKING
Nottingham, M., "Web Linking", Work in Progress,
March 2010.
Brown, et al. Informational [Page 8]
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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).
Brown, et al. Informational [Page 9]
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Version Navigation Link Relations April 2010
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: ; rel=latest-version;
anchor=
Link: ; rel=version-history;
anchor=

(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.)
Brown, et al. Informational [Page 10]
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Version Navigation Link Relations April 2010
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: ; rel=latest-version
Link: ; 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: ; rel=predecessor-version
Brown, et al. Informational [Page 11]
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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:
Brown, et al. Informational [Page 12]
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