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Versions: (draft-douglass-ical-relations) 00
01 02 03 04 05 06
Network Working Group M. Douglass
Internet-Draft Bedework
Updates: 5545 (if approved) December 31, 2020
Intended status: Standards Track
Expires: July 4, 2021
Support for iCalendar Relationships
draft-ietf-calext-ical-relations-06
Abstract
This specification updates RELATED-TO defined in [RFC5545] and
introduces new iCalendar properties LINK, CONCEPT and REFID to allow
better linking and grouping of iCalendar components and related data.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 4, 2021.
Copyright Notice
Copyright (c) 2020 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Structured iCalendar relationships . . . . . . . . . . . 3
1.2. Grouped iCalendar relationships . . . . . . . . . . . . . 3
1.3. Concept relationships . . . . . . . . . . . . . . . . . . 3
1.4. Linked relationships . . . . . . . . . . . . . . . . . . 4
1.5. Caching and offline use . . . . . . . . . . . . . . . . . 5
1.6. Conventions Used in This Document . . . . . . . . . . . . 5
2. Reference Types . . . . . . . . . . . . . . . . . . . . . . . 5
3. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 5
4. New temporal RELTYPE Parameter values . . . . . . . . . . . . 6
5. Additional New RELTYPE Parameter Values . . . . . . . . . . . 7
6. New Property Parameters . . . . . . . . . . . . . . . . . . . 8
6.1. Link Relation . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. New Value Data Types . . . . . . . . . . . . . . . . . . . . 9
8. New Properties . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Concept . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.2. Link . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.3. Refid . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. Redefined RELATED-TO Property . . . . . . . . . . . . . . . . 12
9.1. RELATED-TO . . . . . . . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . . 14
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
11.1. iCalendar Property Registrations . . . . . . . . . . . . 14
11.2. iCalendar Property Parameter Registrations . . . . . . . 15
11.3. iCalendar Value Data Type Registrations . . . . . . . . 15
11.4. iCalendar RELTYPE Value Registrations . . . . . . . . . 15
11.5. New Reference Type Registration . . . . . . . . . . . . 16
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
13. Normative References . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
iCalendar entities often need to be related to each other or to
associated meta-data. The specifications below support relationships
of the following forms:
Structured iCalendar: iCalendar entities can be related to each
other in some structured way, for example as parent, sibling,
before, after.
Grouped iCalendar: iCalendar entities can be related to each other
as a group. CATEGORIES are often used for this purpose but are
problematic for application developers due to their lack of
consistency and use as a free-form tag.
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Linked: Entities can be linked to other entities such as vcards
through a URI and associated REL and FMTTYPE parameters.
1.1. Structured iCalendar relationships
The currently existing iCalendar [RFC5545] RELATED-TO property has no
support for temporal relationships as used by standard project
management tools.
The RELTYPE parameter is extended to take new values defining
temporal relationships, a GAP parameter is defined to provide lead
and lag values, and RELATED-TO is extended to allow URI values.
These changes allow the RELATED-TO property to define a richer set of
relationships useful for project management.
1.2. Grouped iCalendar relationships
This specification defines a new REFID property which allows
arbitrary groups of entities to be associated with the same key
value.
REFID is used to identify a key allowing the association of
components that are related to the same object and retrieval of a
component based on this key. Two examples of how this may be used
are to identify the tasks associated with a given project without
having to communicate the task structure of the project, and to group
all tasks associated to a specific package in a package delivery
system.
As such, the presence of a REFID property imparts no meaning to the
component. It is merely a key to allow retrieval. This is distinct
from categorisation which, while allowing grouping also adds meaning
to the component to which it is attached.
1.3. Concept relationships
The name CONCEPT is used by the Simple Knowledge Organization System
defined in [W3C.CR-skos-reference-20090317]. The term "concept" more
accurately defines what we often mean by a category. It's not the
text string that is important but the meaning attached to it. For
example, the term "football" can mean very different sports.
The introduction of CONCEPT allows a more structured approach to
categorization, with the possibility of namespaced and path-like
values. Unlike REFID the CONCEPT property imparts some meaning. It
is assumed that the value of this property will reference a well
defined category.
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The current [RFC5545] CATEGORY property is used as a free form
'tagging' field. As such it is difficult to establish formal
relationships between components based on their category.
Rather than attempt to add semantics to the CATEGORY property it
seems best to continue its usage as an informal tag and establish a
new CONCEPT property with more constraints.
1.4. Linked relationships
The currently existing iCalendar standard [RFC5545] lacks a general
purpose method for referencing additional, external information
relating to calendar components.
This document proposes a method for referencing typed external
information that can provide additional information about an
iCalendar component. This new LINK property is closely aligned to
the LINK header defined in [RFC8288]
The LINK property defines a typed reference or relation to external
meta-data or related resources. By providing type and format
information as parameters, clients and servers are able to discover
interesting references and make use of them, perhaps for indexing or
the presentation of interesting links for the user.
It is also often necessary to reference calendar components in other
collections. For example, a VEVENT might refer to a VTODO from which
it was derived. The PARENT, SIBLING and CHILD relationships defined
for the RELATED-TO property only allow for a UID which is inadequate
for many purposes. Allowing other value types for those
relationships may help but would cause backward compatibility issues.
The link property can link components in different collections or
even on different servers.
When publishing events it is useful to be able to refer back to the
source of that information. The actual event may have been consumed
from a feed or an ics file on a web site. A LINK property can
provide a reference to the originator of the event.
Beyond the need to relate elements temporally, project management
tools often need to be able to specify the relationships between the
various events and tasks which make up a project. The LINK property
provides such a mechanism.
The LINK property SHOULD NOT be treated as just another attachment.
The ATTACH property is being extended to handle server-side
management and stripping of inline data. Clients may choose to
handle attachments differently as they are often an integral part of
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the message - for example, the agenda. See
[I-D.daboo-caldav-attachments]
1.5. Caching and offline use
To facilitate offline display the link type may identify important
pieces of data which should be downloaded in advance.
In general, the calendar entity should be self explanatory without
the need to download referenced meta-data such as a web page.
1.6. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY" and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Reference Types
The actual reference value can take three forms specified by the type
parameter
URI: The default type. This is a URI referring to the target.
UID: This allows for linking within a single collection and the
value MUST be another component within that collection.
REFERENCE: An XPointer. In an XML environment it may be necessary
to refer to an external XML artifact. The XPointer is defined in
[W3C.WD-xptr-xpointer-20021219] and allows addressing portions of
XML documents.
3. Link Relation Types
[RFC8288] defines two forms of relation type: registered and
extension. Registered relation types are added to the Link Relations
registry as specified in Section 2.1.1 of [RFC8288]. Extension
relation types, defined in Section 2.1.2 of [RFC8288], are specified
as unique URIs that are not registered in the registry.
The relation types defined in Section 6.1 will be registered with
IANA in accordance with the specifications in [RFC8288].
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4. New temporal RELTYPE Parameter values
This section defines the usual temporal relationships for use with
the RELTYPE parameter redefined in Section 3.2.15 of [RFC5545]:
FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH or STARTTOSTART.
The [RFC5545] RELATED-TO property with one or more of these temporal
relationships will be present in the predecessor entity and will
refer to the successor entity.
The GAP parameter (see Section 6.2) specifies the lead or lag time
between the predecessor and the successor. In the description of
each temporal relationship below we refer to Task-A, which contains
and controls the relationship, and Task-B the target of the
relationship.
RELTYPE=FINISHTOSTART: Task-B cannot start until Task-A finishes.
For example, when sanding is complete, painting can begin.
============
| Task-A |--+
============ |
|
V
============
| Task-B |
============
Figure 1: Finish to start relationship
RELTYPE=FINISHTOFINISH: Task-B cannot finish before Task-A is
finished, that is the end of Task-A defines the end of Task-B.
For example, we start the potatoes, then the meat then the peas
but they should all be cooked at the same time.
============
| Task-A |--+
============ |
|
============ |
| Task-B |<-+
============
Figure 2: Finish to finish relationship
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RELTYPE=STARTTOFINISH: The start of Task-A (which occurs after Task-
B) controls the finish of Task-B. For example, ticket sales
(Task-B) end when the game starts (Task-A).
============
+--| Task-A |
| ============
|
============ |
| Task-B |<-+
============
Figure 3: Start to finish relationship
RELTYPE=STARTTOSTART: The start of Task-A triggers the start of
Task-B, that is Task-B can start anytime after Task-A starts.
============
+--| Task-A |
| ============
|
| ============
+->| Task-B |
============
Figure 4: Start to start relationship
5. Additional New RELTYPE Parameter Values
This section defines the additional relationships below:
RELTYPE=DEPENDS-ON: Indicates that the current calendar component
depends on the referenced calendar component in some manner. For
example a task may be blocked waiting on the other, referenced,
task.
RELTYPE=REFID: Establishes a reference from the current component to
components with a REFID property which matches the value given in
the associated RELATED-TO property.
RELTYPE=CONCEPT: Establishes a reference from the current component
to components with a CONCEPT property which matches the value
given in the associated RELATED-TO property.
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6. New Property Parameters
6.1. Link Relation
Parameter name: LINKREL
Purpose: To specify the relationship of data referenced by a LINK
property.
Format Definition:
This parameter is defined by the following notation:
linkrelparam = "REL" "="
("SOURCE" ; Link to source of this component
/ DQUOTE uri DQUOTE
/ iana-token) ; Other IANA registered type
Description: This parameter MUST be specified on all LINK
properties, and defines the type of reference. This allows
programs consuming this data to automatically scan for references
they support. In addition to the values defined here any value
defined in [RFC8288] may be used. There is no default relation
type.
REL=SOURCE: identifies the source of the event information.
Registration: These relation types are registered in [RFC8288]
6.2. Gap
Parameter name: GAP
Purpose: To specify the length of the gap, positive or negative,
between two temporaly related components.
Format Definition:
This parameter is defined by the following notation:
gapparam = "GAP" "=" dur-value
Description: This parameter MAY be specified on the RELATED-TO
property, and defines the duration of time between the predecessor
and successor in an interval. When positive it defines the lag
time between a task and its logical successor. When negative it
defines the lead time.
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An example of lag time might be if task A is "paint the room" and
task B is "hang the drapes" then task A may be related to task B
with RELTYPE=FINISHTOSTART with a gap long enough for the paint to
dry.
An example of lead time might be to relate a 1 week task A to the
end of task B with RELTYPE=STARTTOFINISH and a negative gap of 1
week so they finish at the same time.
7. New Value Data Types
This specification defines the following new value types to be used
with the VALUE property parameter:
UID VALUE=UID indicates that the associated value is the UID for a
component.
REFERENCE VALUE=REFERENCE indicates that the associated value is an
XPointer referencing an associated XML artifact.
8. New Properties
8.1. Concept
Property name: CONCEPT
Purpose: This property defines the formal categories for a calendar
component.
Value type: URI
Property Parameters: IANA, and non-standard parameters can be
specified on this property.
Conformance: This property can be specified zero or more times in
any iCalendar component.
Description: This property is used to specify formal categories or
classifications of the calendar component. The values are useful
in searching for a calendar component of a particular type and
category.
Within the "VEVENT", "VTODO", or "VJOURNAL" calendar components,
more than one formal category can be specified by using multiple
CONCEPT properties.
This categorization is distinct from the more informal "tagging"
of components provided by the existing CATEGORIES property. It is
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expected that the value of the CONCEPT property will reference an
external resource which provides information about the
categorization.
In addition, a structured URI value allows for hierarchical
categorization of events.
Possible category resources are the various proprietary systems,
for example Library of Congress, or an open source of
categorisation data.
Format Definition:
This property is defined by the following notation:
concept = "CONCEPT" conceptparam ":"
uri CRLF
conceptparam = *(";" other-param)
Example:
The following is an example of this property. It points to a server
acting as the source for the calendar object.
CONCEPT:http://example.com/event-types/arts/music
8.2. Link
Property name: LINK
Purpose: This property provides a reference to external information
about a component.
Value type: URI, TEXT or REFERENCE
Property Parameters: The VALUE parameter is required. Non-standard,
reference type or format type parameters can also be specified on
this property. The LABEL parameter is defined in [RFC7986]
Conformance: This property MAY be specified in any iCalendar
component.
Description: When used in a component the value of this property
points to additional information related to the component. For
example, it may reference the originating web server.
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Format Definition:
This property is defined by the following notation:
link = "LINK" linkparam ":"
( text / ; for VALUE=REFERENCE
uri / ; for VALUE=URI
text ) ; for VALUE=TEXT
CRLF
linkparam = ; the elements herein may appear in any order,
; and the order is not significant.
(";" "VALUE" "=" ("UID" /
"URI" /
"TEXT"))
1*(";" linkrelparam)
(";" fmttypeparam)
(";" labelparam)
*(";" other-param)
Example:
The following is an example of this property which provides a
reference to the source for the calendar object.
LINK;LINKREL=SOURCE;LABEL=Venue;VALUE=URI:http://example.com/events
Example:
The following is an example of this property which provides a
reference to an entity from which this one was derived. The link
relation is a vendor defined value
LINK;LINKREL="https://example.com/linkrel/derivedFrom";VALUE=URI:
http://example.com/tasks/01234567-abcd1234.ics
8.3. Refid
Property name: REFID
Purpose: This property value acts as a key for associated iCalendar
entities.
Value type: TEXT
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Property Parameters: Non-standard parameters can be specified on
this property.
Conformance: This property MAY be specified multiple times in any
iCalendar component.
Description: The value of this property is free-form text that
creates an identifier for associated components. All components
that use the same REFID value are associated through that value
and can be located or retrieved as a group. For example, all of
the events in a travel itinerary would have the same REFID value,
so as to be grouped together.
Format Definition:
This property is defined by the following notation:
refid = "REFID" refidparam ":" text CRLF
refidparam = *(";" other-param)
Example:
The following is an example of this property.
REFID:itinerary-2014-11-17
9. Redefined RELATED-TO Property
9.1. RELATED-TO
Property name: RELATED-TO
Purpose: This property is used to represent a relationship or
reference between one calendar component and another. The
definition here extends the definition in Section 3.8.4.5 of
[RFC5545] by allowing URI or UID values and a GAP parameter.
Value type: URI, UID or TEXT
Property Parameters: Relationship type, IANA and non-standard
property parameters can be specified on this property.
Conformance: This property MAY be specified in any iCalendar
component.
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Description: By default or when VALUE=UID is specified, the property
value consists of the persistent, globally unique identifier of
another calendar component. This value would be represented in a
calendar component by the "UID" property.
By default, the property value points to another calendar
component that has a PARENT relationship to the referencing
object. The "RELTYPE" property parameter is used to either
explicitly state the default PARENT relationship type to the
referenced calendar component or to override the default PARENT
relationship type and specify either a CHILD or SIBLING
relationship or a temporal relationship.
The PARENT relationship indicates that the calendar component is a
subordinate of the referenced calendar component. The CHILD
relationship indicates that the calendar component is a superior
of the referenced calendar component. The SIBLING relationship
indicates that the calendar component is a peer of the referenced
calendar component.
To preserve backwards compatibility the value type MUST be UID
when the PARENT, SIBLING or CHILD relationships are specified.
The FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH or STARTTOSTART
relationships define temporal relationships as specified in the
reltype parameter definition.
Changes to a calendar component referenced by this property can
have an implicit impact on the related calendar component. For
example, if a group event changes its start or end date or time,
then the related, dependent events will need to have their start
and end dates changed in a corresponding way. Similarly, if a
PARENT calendar component is cancelled or deleted, then there is
an implied impact to the related CHILD calendar components. This
property is intended only to provide information on the
relationship of calendar components. It is up to the target
calendar system to maintain any property implications of this
relationship.
Format Definition:
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This property is defined by the following notation:
related = "RELATED-TO" relparam ":"
( uid / ; for VALUE=UID
uri / ; for VALUE=URI
text ) ; for VALUE=TEXT or default
CRLF
relparam = ; the elements herein may appear in any order,
; and the order is not significant.
[";" "VALUE" "=" ("UID" /
"URI" /
"TEXT")]
[";" reltypeparam]
[";" gapparam]
*(";" other-param)
Example:
The following are examples of this property.
RELATED-TO:jsmith.part7.19960817T083000.xyzMail@example.com
RELATED-TO:19960401-080045-4000F192713-0052@example.com
RELATED-TO;VALUE=URI;RELTYPE=STARTTOFINISH:
http://example.com/caldav/user/jb/cal/
19960401-080045-4000F192713.ics
10. Security Considerations
Applications using the LINK property need to be aware of the risks
entailed in using the URIs provided as values. See [RFC3986] for a
discussion of the security considerations relating to URIs.
The CONCEPT and redefined RELATED-TO property have the same issues in
that values may be URIs.
11. IANA Considerations
11.1. iCalendar Property Registrations
The following iCalendar property names have been added to the
iCalendar Properties Registry defined in Section 8.3.2 of [RFC5545]
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+----------+---------+-------------+
| Property | Status | Reference |
+----------+---------+-------------+
| CONCEPT | Current | Section 8.1 |
| LINK | Current | Section 8.2 |
| REFID | Current | Section 8.3 |
+----------+---------+-------------+
11.2. iCalendar Property Parameter Registrations
The following iCalendar property parameter names have been added to
the iCalendar Parameters Registry defined in Section 8.3.3 of
[RFC5545]
+-----------+---------+-------------+
| Parameter | Status | Reference |
+-----------+---------+-------------+
| REL | Current | Section 6.1 |
| GAP | Current | Section 6.2 |
+-----------+---------+-------------+
11.3. iCalendar Value Data Type Registrations
The following iCalendar property parameter names have been added to
the iCalendar Value Data Types Registry defined in Section 8.3.4 of
[RFC5545]
+-----------------+---------+-----------+
| Value Data Type | Status | Reference |
+-----------------+---------+-----------+
| UID | Current | Section 7 |
| REFERENCE | Current | Section 7 |
+-----------------+---------+-----------+
11.4. iCalendar RELTYPE Value Registrations
The following iCalendar "RELTYPE" values have been added to the
iCalendar Relationship Types Registry defined in Section 8.3.8 of
[RFC5545]
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+-------------------+---------+-----------+
| Relationship Type | Status | Reference |
+-------------------+---------+-----------+
| DEPENDS-ON | Current | Section 5 |
| REFID | Current | Section 5 |
| CONCEPT | Current | Section 5 |
| FINISHTOSTART | Current | Section 4 |
| FINISHTOFINISH | Current | Section 4 |
| STARTTOFINISH | Current | Section 4 |
| STARTTOSTART | Current | Section 4 |
+-------------------+---------+-----------+
11.5. New Reference Type Registration
The following link relation values have been added to the Reference
Types Registry defined in Section 6.2.2 of [RFC8288]
+--------+---------+-------------+
| Name | Status | Reference |
+--------+---------+-------------+
| SOURCE | Current | Section 6.1 |
+--------+---------+-------------+
12. Acknowledgements
The author would like to thank the members of the Calendaring and
Scheduling Consortium technical committees and the following
individuals for contributing their ideas, support and comments:
Adrian Apthorp, Cyrus Daboo, Marten Gajda, Ken Murchison
The author would also like to thank CalConnect, the Calendaring and
Scheduling Consortium for advice with this specification.
13. Normative References
[I-D.daboo-caldav-attachments]
Daboo, C. and A. Quillaud, "CalDAV Managed Attachments",
draft-daboo-caldav-attachments-03 (work in progress),
February 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and
Scheduling Core Object Specification (iCalendar)",
RFC 5545, DOI 10.17487/RFC5545, September 2009,
<https://www.rfc-editor.org/info/rfc5545>.
[RFC7986] Daboo, C., "New Properties for iCalendar", RFC 7986,
DOI 10.17487/RFC7986, October 2016,
<https://www.rfc-editor.org/info/rfc7986>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
[W3C.CR-skos-reference-20090317]
Bechhofer, S. and A. Miles, "SKOS Simple Knowledge
Organization System Reference", World Wide Web Consortium
CR CR-skos-reference-20090317, March 2009,
<http://www.w3.org/TR/2009/CR-skos-reference-20090317>.
[W3C.WD-xptr-xpointer-20021219]
DeRose, S., Daniel, R., and E. Maler, "XPointer xpointer()
Scheme", World Wide Web Consortium WD WD-xptr-xpointer-
20021219, December 2002,
<http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219>.
Author's Address
Michael Douglass
Bedework
226 3rd Street
Troy, NY 12180
USA
Email: mdouglass@bedework.com
URI: http://bedework.com
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