Internet Engineering Task Force (IETF) M. Douglass

Internet Engineering Task Force (IETF) M. Douglass
Request for Comments: 9253 Bedework
Updates: 5545 August 2022
Category: Standards Track
ISSN: 2070-1721

Support for iCalendar Relationships

Abstract

This specification updates the iCalendar RELATED-TO property defined
in RFC 5545 by adding new relation types 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 is an Internet Standards Track document.

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). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9253.

Copyright Notice

Copyright (c) 2022 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
(https://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. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Structured iCalendar Relationships
1.2. Grouped iCalendar Relationships
1.3. Concept Relationships
1.4. Linked Relationships
1.5. Caching and Offline Use
1.6. Conventions Used in This Document
2. LINK Property Reference Types
3. Link Relation Types
4. New Temporal RELTYPE Parameter Values
5. Additional New RELTYPE Parameter Values
6. New Property Parameters
6.1. Link Relation
6.2. Gap
7. New Value Data Types
8. New Properties
8.1. Concept
8.2. Link
8.3. Refid
9. Updates to RFC 5545
9.1. RELATED-TO
10. Security Considerations
11. IANA Considerations
11.1. iCalendar Property Registrations
11.2. iCalendar Property Parameter Registrations
11.3. iCalendar Value Data Type Registrations
11.4. iCalendar RELTYPE Value Registrations
12. References
12.1. Normative References
12.2. Informative References
Acknowledgements
Author's Address

1. Introduction

iCalendar entities defined in [RFC5545] often need to be related to
each other or to associated metadata. 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, or 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.

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 iCalendar [RFC5545] RELATED-TO property has no support for
temporal relationships as used by 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 all related to the referring, aggregating
component and the retrieval of components based on this key. For
example, this may be used to identify the tasks associated with a
given project without having to communicate the task structure of the
project. A further example is the grouping of all sub-tasks
associated with the delivery of 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 categorization, 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,
as defined in [W3C.REC-skos-reference-20090818]. 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.

The current CATEGORIES property defined in [RFC5545] is used as a
free-form 'tagging' field. These values have some meaning to those
who apply them but not necessarily to any consumer. As such, it is
difficult to establish formal relationships between components based
on their category.

Rather than attempt to add semantics to the CATEGORIES 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
[RFC8288], which defines the generic concept of Web Linking, as well
as its expression in the HTTP LINK header field.

The LINK property defines a typed reference or relation to external
metadata 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.

Calendar components are often grouped into collections to represent a
calendar or a series of tasks, for example, Calendaring Extensions to
WebDAV (CalDAV) calendar collections [RFC4791].

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 unique identifier (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 website. 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 that make up a project. The LINK property
provides such a mechanism.

The LINK property MUST NOT be treated as just another attachment.
The ATTACH property defined in [RFC5545] has been extended by
[RFC8607] to handle server-side management and stripping of inline
data and to provide additional data about the attachment (size,
filename, etc.).

Additionally, clients may choose to handle attachments differently
from the LINK property, as attachments are often an integral part of
the message, for example, the agenda.

1.5. Caching and Offline Use

In general, the calendar entity should be self explanatory without
the need to download referenced metadata, such as a web page.

However, to facilitate offline display, the link type may identify
important pieces of data that should be downloaded in advance.

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.

The notation used in this memo to (re-)define iCalendar elements is
the ABNF notation of [RFC5234], as used by [RFC5545]. Any syntax
elements shown below that are not explicitly defined in this
specification come from iCalendar [RFC5545].

2. LINK Property Reference Types

The reference value in the LINK property defined below can take three
forms specified by the VALUE parameter:

URI: This is a URI referring to the target.

UID: This allows for linking within a single collection of calendar
components, and the value MUST refer to another component within
the same collection.

XML-REFERENCE: In an XML environment, it may be necessary to refer
to a fragment of an external XML artifact. This value is a URI
with an XPointer anchor value. The XPointer is defined in
[W3C.WD-xptr-xpointer-20021219], and its use as an anchor is
defined in [W3C.REC-xptr-framework-20030325].

Note that UID references may need updating on import. An example is
data to be imported from a file containing VTODO and VEVENT
components, with a VTODO referring to VEVENT components by UID. When
imported into a CalDAV system, the VTODO components are typically
placed in a different collection from the VEVENT components. This
would require the UID reference to be replaced with a URI.

3. Link Relation Types

Two forms of relation types are defined in [RFC8288]: 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].

4. New Temporal RELTYPE Parameter Values

This section defines the usual temporal relationships for use with
the RELTYPE parameter defined 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 (a negative
value) or lag (a positive value) 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,
which is the target of the relationship. This is indicated by the
direction of the arrows in the diagrams below.

Also, each relationship may be modified by the addition of a GAP
parameter to the relationship that applies to the targeted component.

RELTYPE=FINISHTOSTART: Task-B cannot start until Task-A finishes.
For example, when painting is complete, carpet laying can begin.

============
| Task-A |
============
|
V
============
| Task-B |
============

Figure 1: Finish-to-Start Relationship

RELTYPE=FINISHTOFINISH: Task-B can only be completed after Task-A is
finished. The related tasks may run in parallel before
completion.

For example, in the development of two related pieces of software
(e.g., the API and the implementation), the design of the
implementation (Task-B) cannot be completed until the design of
the API (Task-A) has been completed.

==================
| Task-A |--+
================== |
|
============ |
| Task-B |<-+
============

Figure 2: Finish-to-Finish Relationship

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 after 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=FIRST: This indicates that the referenced calendar component
is the first in a series the referencing calendar component is
part of.

RELTYPE=NEXT: This indicates that the referenced calendar component
is the next in a series the referencing calendar component is part
of.

RELTYPE=DEPENDS-ON: This 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: This establishes a reference from the current
component to components with a REFID property that matches the
value given in the associated RELATED-TO property.

RELTYPE=CONCEPT: This establishes a reference from the current
component to components with a CONCEPT property that matches the
value given in the associated RELATED-TO property.

Note that the relationship types of PARENT, CHILD, and SIBLING
establish a hierarchical relationship. The new types of FIRST and
NEXT are an ordering relationship.

6. New Property Parameters

6.1. Link Relation

Parameter name: LINKREL

Purpose: This property specifies the relationship of data referenced
by a LINK property.

Format Definition: This parameter is defined by the following
notation:

linkrelparam = "LINKREL" "="
(DQUOTE uri DQUOTE
/ iana-token) ; Other IANA registered type

Description: This parameter MUST be specified on all LINK properties
and define the type of reference. This allows programs consuming
this data to automatically scan for references they support.
There is no default relation type.

Any link relation in the link registry established by [RFC8288],
or new link relations, may be used. It is expected that link
relation types seeing significant usage in calendaring will have
the calendaring usage described in an RFC.

LINKREL=latest-version: This identifies the latest version of the
event information.

Registration: These relation types are registered in [RFC8288].

6.2. Gap

Parameter name: GAP

Purpose: This property specifies the length of the gap, positive or
negative, between two components with a temporal relationship.

Format Definition: This parameter is defined by the following
notation, where dur-value is defined in Section 3.3.6 of
[RFC5545]. :

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.

An example of lag time might be if Task-A is "paint the room" and
Task-B is "lay the carpets". Then, Task-A may be related to
Task-B with RELTYPE=FINISHTOSTART with a gap of 1 day -- long
enough for the paint to dry.

====================
| paint the room |--+
==================== |
|(lag of one day)
|
| ===================
+->| lay the carpet |
===================

Figure 5: Finish-to-Start Relationship with Lag

For an example of lead time, in constructing a two-story building,
the electrical work must be done before painting. However, the
painter can move in to the first floor as the electricians move
upstairs.

=====================
| electrical work |--+
===================== |
+-------------+
|(lead of estimated time)
| ==================
+->| painting |
==================

Figure 6: Finish-to-Start Relationship with Lead

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.

XML-REFERENCE: VALUE=XML-REFERENCE indicates that the associated
value references an associated XML artifact and is a URI with an
XPointer anchor value. The XPointer is defined in
[W3C.WD-xptr-xpointer-20021219], and its use as an anchor is
defined in [W3C.REC-xptr-framework-20030325].

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.

This categorization is distinct from the more informal "tagging"
of components provided by the existing CATEGORIES property. It is
expected that the value of the CONCEPT property will reference an
external resource that 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, the Library of Congress, or an open source of
categorization 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:https://example.com/event-types/arts/music

8.2. Link

Property name: LINK

Purpose: This property provides a reference to external information
related to a component.

Value type: URI, UID, or XML-REFERENCE

Property Parameters: The VALUE parameter is required. Non-standard,
link relation type, format type, label, and language parameters
can also be specified on this property. The LABEL parameter is
defined in [RFC7986].

Conformance: This property can be specified zero or more times 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.

Format Definition: This property is defined by the following
notation:

link = "LINK" linkparam ":"
( uri / ; for VALUE=XML-REFERENCE
uri / ; for VALUE=URI
text ) ; for VALUE=UID
CRLF

linkparam = (";" "VALUE" "=" ("XML-REFERENCE" /
"URI" /
"UID"))
1*(";" linkrelparam)
1*(";" fmttypeparam)
1*(";" labelparam)
1*(";" languageparam)
*(";" other-param)
; the elements herein may appear in any order,
; and the order is not significant.

This property is a serialization of the model in [RFC8288], where
the link target is carried in the property value, the link context
is the containing calendar entity, and the link relation type and
any target attributes are carried in iCalendar property
parameters.

The LINK property parameters map to [RFC8288] attributes as
follows:

LABEL: This parameter maps to the "title" attribute defined in
Section 3.4.1 of [RFC8288].

LANGUAGE: This parameter maps to the "hreflang" attribute defined
in Section 3.4.1 of [RFC8288].

LINKREL: This parameter maps to the link relation type defined in
Section 2.1 of [RFC8288].

FMTTYPE: This parameter maps to the "type" attribute defined in
Section 3.4.1 of [RFC8288].

There is no mapping for "title*", "anchor", "rev", or "media"
[RFC8288].

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:
https://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:
https://example.com/tasks/01234567-abcd1234.ics

Example: The following is an example of this property, which
provides a reference to a fragment of an XML document. The link
relation is a vendor-defined value.

LINK;LINKREL="https://example.com/linkrel/costStructure";
VALUE=XML-REFERENCE:
https://example.com/xmlDocs/bidFramework.xml
#xpointer(descendant::CostStruc/range-to(
following::CostStrucEND[1]))

8.3. Refid

Property name: REFID

Purpose: This property value acts as a key for associated iCalendar
entities.

Value type: TEXT

Property Parameters: Non-standard parameters can be specified on
this property.

Conformance: This property can be specified zero or more 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. Updates to RFC 5545

This specification updates the RELATED-TO property defined in
Section 3.8.4.5 of [RFC5545]. The contents of Section 9.1 replace
that section.

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.

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.

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.

The FIRST and NEXT define ordering relationships between calendar
components.

The DEPENDS-ON relationship 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.

The REFID and CONCEPT relationships establish a reference from the
current component to the referenced component.

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 and times changed in a corresponding way.
Similarly, if a PARENT calendar component is canceled 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.

Deletion of the target component, for example, the target of a
FIRST, NEXT, or temporal relationship, can result in broken links.

It is up to the target calendar system to maintain any property
implications of these relationships.

Format Definition: This property is defined by the following
notation:

related = "RELATED-TO" relparam ":"
( text / ; 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:[email protected]

RELATED-TO:[email protected]

RELATED-TO;VALUE=URI;RELTYPE=STARTTOFINISH:
https://example.com/caldav/user/jb/cal/
19960401-080045-4000F192713.ics

10. Security Considerations

All of the security considerations of Section 7 of [RFC5545] apply to
this specification.

Applications using the LINK property need to be aware of the risks
entailed in using the URIs provided as values. See Section 7 of
[RFC3986] for a discussion of the security considerations relating to
URIs.

In particular, note Section 7.1 (Reliability and Consistency) of
[RFC3986], which points out the lack of a stability guarantee for
referenced resources.

When the value is an XML-REFERENCE type, the targeted data is an XML
document or portion thereof. Consumers need to be aware of the
security issues related to XML processing -- in particular, those
related to XML entities. See Section 20.6 of [RFC4918].
Additionally, note that the reference may be invalid or become so
over time.

The CONCEPT and redefined RELATED-TO properties have the same issues
in that values may be URIs.

Extremely large values for the GAP parameter may lead to unexpected
behavior.

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]. IANA has also added a reference to this document, where
the properties originally defined in [RFC5545] have been updated by
this document.

+============+=========+=============================+
| Property | Status | Reference |
+============+=========+=============================+
| CONCEPT | Current | Section 8.1 |
+------------+---------+-----------------------------+
| LINK | Current | Section 8.2 |
+------------+---------+-----------------------------+
| REFID | Current | Section 8.3 |
+------------+---------+-----------------------------+
| RELATED-TO | Current | [RFC5545], Section 3.8.4.5; |
| | | RFC 9253, Section 9.1 |
+------------+---------+-----------------------------+

Table 1

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 |
+===========+=========+=============+
| GAP | Current | Section 6.2 |
+-----------+---------+-------------+
| LINKREL | Current | Section 6.1 |
+-----------+---------+-------------+

Table 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 |
+=================+=========+===========+
| XML-REFERENCE | Current | Section 7 |
+-----------------+---------+-----------+
| UID | Current | Section 7 |
+-----------------+---------+-----------+

Table 3

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].

+===================+=========+===========+
| Relationship Type | Status | Reference |
+===================+=========+===========+
| CONCEPT | Current | Section 5 |
+-------------------+---------+-----------+
| DEPENDS-ON | Current | Section 5 |
+-------------------+---------+-----------+
| FINISHTOFINISH | Current | Section 4 |
+-------------------+---------+-----------+
| FINISHTOSTART | Current | Section 4 |
+-------------------+---------+-----------+
| FIRST | Current | Section 5 |
+-------------------+---------+-----------+
| NEXT | Current | Section 5 |
+-------------------+---------+-----------+
| REFID | Current | Section 5 |
+-------------------+---------+-----------+
| STARTTOFINISH | Current | Section 4 |
+-------------------+---------+-----------+
| STARTTOSTART | Current | Section 4 |
+-------------------+---------+-----------+

Table 4

12. References

12.1. Normative References

[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>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.

[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918,
DOI 10.17487/RFC4918, June 2007,
<https://www.rfc-editor.org/info/rfc4918>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[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.REC-skos-reference-20090818]
Miles, A. and S. Bechhofer, "SKOS Simple Knowledge
Organization System Reference", W3C Recommendation REC-
skos-reference-20090818, 18 August 2009,
<https://www.w3.org/TR/2009/REC-skos-reference-20090818>.

[W3C.REC-xptr-framework-20030325]
Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
Framework", W3C Recommendation REC-xptr-framework-
20030325, 25 March 2003,
<https://www.w3.org/TR/2003/REC-xptr-framework-20030325>.

[W3C.WD-xptr-xpointer-20021219]
DeRose, S., Maler, E., and R. Daniel, "XPointer xpointer()
Scheme", W3C WD WD-xptr-xpointer-20021219, 19 December
2002,
<http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219>.

12.2. Informative References

[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
DOI 10.17487/RFC4791, March 2007,
<https://www.rfc-editor.org/info/rfc4791>.

[RFC8607] Daboo, C., Quillaud, A., and K. Murchison, Ed.,
"Calendaring Extensions to WebDAV (CalDAV): Managed
Attachments", RFC 8607, DOI 10.17487/RFC8607, June 2019,
<https://www.rfc-editor.org/info/rfc8607>.

Acknowledgements

The author would like to thank the members of CalConnect, the
Calendaring and Scheduling Consortium technical committees, and the
following individuals for contributing their ideas, support, and
comments:

Adrian Apthorp, Cyrus Daboo, Marten Gajda, and Ken Murchison

The author would also like to thank CalConnect and the Calendaring
and Scheduling Consortium for advice with this specification.

Author's Address

Michael Douglass
Bedework
226 3rd Street
Troy, NY 12180
United States of America
Email: [email protected]
URI: https://bedework.com