rfc9074
Internet Engineering Task Force (IETF) C. Daboo
Request for Comments: 9074 Apple
Updates: 5545 K. Murchison, Ed.
Category: Standards Track Fastmail
ISSN: 2070-1721 August 2021
"VALARM" Extensions for iCalendar
Abstract
This document defines a set of extensions to the iCalendar "VALARM"
component to enhance the use of alarms and improve interoperability
between clients and servers.
This document updates RFC 5545.
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/rfc9074.
Copyright Notice
Copyright (c) 2021 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
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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.
Table of Contents
1. Introduction
2. Conventions Used in This Document
3. Extensible Syntax for VALARM
4. Alarm Unique Identifier
5. Alarm Related To
6. Alarm Acknowledgement
6.1. Acknowledged Property
7. Snoozing Alarms
7.1. Relationship Type Property Parameter
7.2. Example
8. Alarm Proximity Trigger
8.1. Proximity Property
8.2. Example
9. Security Considerations
10. Privacy Considerations
11. IANA Considerations
11.1. Property Registrations
11.2. Relationship Types Registry
11.3. Proximity Values Registry
12. References
12.1. Normative References
12.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The iCalendar specification [RFC5545] defines a set of components
used to describe calendar data. One of those is the "VALARM"
component, which appears as a subcomponent of the "VEVENT" and
"VTODO" components. The "VALARM" component is used to specify a
reminder for an event or task. Different alarm actions are possible,
as are different ways to specify how the alarm is triggered.
As iCalendar has become more widely used and as client-server
protocols, such as Calendaring Extensions to WebDAV (CalDAV)
[RFC4791], have become more prevalent, several issues with "VALARM"
components have arisen. Most of these relate to the need to extend
the existing "VALARM" component with new properties and behaviors to
allow clients and servers to accomplish specific tasks in an
interoperable manner. For example, clients typically need a way to
specify that an alarm has been dismissed by a calendar user or has
been "snoozed" by a set amount of time. To date, this has been done
through the use of custom "X-" properties specific to each client
implementation, leading to poor interoperability.
This specification defines a set of extensions to "VALARM" components
to cover common requirements for alarms not currently addressed in
iCalendar. Each extension is defined in a separate section below.
For the most part, each extension can be supported independently of
the others; though, in some cases, one extension will require
another. In addition, this specification describes mechanisms by
which clients can interoperably implement common features, such as
"snoozing".
2. 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].
When XML element types in the namespaces "DAV:" and
"urn:ietf:params:xml:ns:caldav" are referenced in this document
outside of the context of an XML fragment, the string "DAV:" and
"CALDAV:" will be prefixed to the element type names, respectively.
3. Extensible Syntax for VALARM
Section 3.6.6 of [RFC5545] defines the syntax for "VALARM" components
and properties within them. However, as written, it is hard to
extend this, e.g., by adding a new property common to all types of
alarms. Since many of the extensions defined in this document need
to extend the base syntax, an alternative form for the base syntax is
defined here, with the goal of simplifying specification of the
extensions while augmenting the existing functionality defined in
[RFC5545] to allow for nested subcomponents (as required by proximity
alarm triggers (Section 8)).
A "VALARM" calendar component is redefined by the following notation:
alarmcext = "BEGIN" ":" "VALARM" CRLF
*alarmprop *alarm-subcomp
"END" ":" "VALARM" CRLF
alarmprop = (
;
; the following are REQUIRED
; but MUST NOT occur more than once
;
action / trigger /
;
; one set of action properties MUST be
; present and MUST match the action specified
; in the ACTION property
;
actionprops /
;
; the following are OPTIONAL
; and MAY occur more than once
;
x-prop / iana-prop
;
)
actionprops = *audiopropext / *disppropext / *emailpropext
audiopropext = (
;
; 'duration' and 'repeat' are both OPTIONAL
; and MUST NOT occur more than once each,
; but if one occurs, so MUST the other
;
duration / repeat /
;
; the following is OPTIONAL
; but MUST NOT occur more than once
;
attach
;
)
disppropext = (
;
; the following are REQUIRED
; but MUST NOT occur more than once
;
description /
;
; 'duration' and 'repeat' are both OPTIONAL
; and MUST NOT occur more than once each,
; but if one occurs, so MUST the other
;
duration / repeat
;
)
emailpropext = (
;
; the following are all REQUIRED
; but MUST NOT occur more than once
;
description / summary /
;
; the following is REQUIRED
; and MAY occur more than once
;
attendee /
;
; 'duration' and 'repeat' are both OPTIONAL
; and MUST NOT occur more than once each,
; but if one occurs, so MUST the other
;
duration / repeat
;
; the following is OPTIONAL
; and MAY occur more than once
;
attach
;
)
alarm-subcomp = (
;
; the following are OPTIONAL
; and MAY occur more than once
;
x-comp / iana-comp
;
)
4. Alarm Unique Identifier
This extension adds a "UID" property to "VALARM" components to allow
a unique identifier to be specified. The value of this property can
then be used to refer uniquely to the "VALARM" component.
The "UID" property defined here follows the definition in
Section 3.8.4.7 of [RFC5545] with the security and privacy updates in
Section 5.3 of [RFC7986]. In particular, it MUST be a globally
unique identifier that does not contain any security- or privacy-
sensitive information.
The "VALARM" component defined in Section 3 is extended here as:
alarmprop =/ (
;
; the following is OPTIONAL
; but MUST NOT occur more than once
;
uid
;
)
5. Alarm Related To
It is often convenient to relate one or more "VALARM" components to
other "VALARM" components (e.g., see Section 7). This can be
accomplished if the "VALARM" components each have their own "UID"
property (as per Section 4).
This specification updates the usage of the "RELATED-TO" property
defined in Section 3.8.4.5 of [RFC5545] to enable its use with
"VALARM" components. Specific types of relationships between
"VALARM" components can be identified by registering new values for
the "RELTYPE" property parameter defined in Section 3.2.15 of
[RFC5545].
The "VALARM" component defined in Section 3 is extended here as:
alarmprop =/ (
;
; the following is OPTIONAL
; and MAY occur more than once
;
related
;
)
6. Alarm Acknowledgement
There is currently no way for a "VALARM" component to indicate
whether it has been triggered and acknowledged. With the advent of a
standard client/server protocol for calendaring and scheduling data
([RFC4791]), it is quite possible for an event with an alarm to exist
on multiple clients in addition to the server. If each of those is
responsible for performing the action when an alarm triggers, then
multiple "alerts" are generated by different devices. In such a
situation, a calendar user would like to be able to "dismiss" the
alarm on one device and have it automatically dismissed on the
others, too.
Also, with recurring events that have alarms, it is important to know
when the last alarm in the recurring set was acknowledged so that the
client can determine whether past alarms have been missed.
To address these needs, this specification adds an "ACKNOWLEDGED"
property to "VALARM" components to indicate when the alarm was last
acknowledged (or sent, if acknowledgement is not possible). This is
defined by the syntax below.
alarmprop =/ (
;
; the following is OPTIONAL
; but MUST NOT occur more than once
;
acknowledged
;
)
6.1. Acknowledged Property
Property Name: ACKNOWLEDGED
Purpose: This property specifies the UTC date and time at which the
corresponding alarm was last sent or acknowledged.
Value Type: DATE-TIME
Property Parameters: IANA and nonstandard property parameters can be
specified on this property.
Conformance: This property can be specified within "VALARM" calendar
components.
Description: This property is used to specify when an alarm was last
sent or acknowledged. This allows clients to determine when a
pending alarm has been acknowledged by a calendar user so that any
alerts can be dismissed across multiple devices. It also allows
clients to track repeating alarms or alarms on recurring events or
to-dos to ensure that the right number of missed alarms can be
tracked.
Clients SHOULD set this property to the current date-time value in
UTC when a calendar user acknowledges a pending alarm. Certain
kinds of alarms, such as email-based alerts, might not provide
feedback as to when the calendar user sees them. For those kinds
of alarms, the client SHOULD set this property when the alarm is
triggered and the action is successfully carried out.
When an alarm is triggered on a client, clients can check to see
if an "ACKNOWLEDGED" property is present. If it is, and the value
of that property is greater than or equal to the computed trigger
time for the alarm, then the client SHOULD NOT trigger the alarm.
Similarly, if an alarm has been triggered and an "alert" has been
presented to a calendar user, clients can monitor the iCalendar
data to determine whether an "ACKNOWLEDGED" property is added or
changed in the alarm component. If the value of any
"ACKNOWLEDGED" property in the alarm changes and is greater than
or equal to the trigger time of the alarm, then clients SHOULD
dismiss or cancel any "alert" presented to the calendar user.
Format Definition: This property is defined by the following
notation:
acknowledged = "ACKNOWLEDGED" *acknowledgedparam ":" datetime CRLF
acknowledgedparam = (
;
; the following is OPTIONAL
; and MAY occur more than once
;
(";" other-param)
;
)
Example: The following is an example of this property:
ACKNOWLEDGED:20090604T084500Z
7. Snoozing Alarms
Users often want to "snooze" an alarm, and this specification defines
a standard approach to accomplish that.
To "snooze" an alarm that has been triggered, clients MUST do the
following:
1. Set the "ACKNOWLEDGED" property (see Section 6.1) on the
triggered alarm.
2. Create a new "VALARM" component (the "snooze" alarm) within the
parent component of the triggered alarm (i.e., as a "sibling"
component of the triggered alarm).
a. The new "snooze" alarm MUST be set to trigger at the user's
chosen "snooze" interval after the original alarm is
triggered. Clients SHOULD use an absolute "TRIGGER" property
with a "DATE-TIME" value specified in UTC.
b. The new "snooze" alarm MUST have a "RELATED-TO" property (see
Section 5) with a value set to the "UID" property value of
the original "VALARM" component that was triggered. If the
triggered "VALARM" component does not already have a "UID"
property, the client MUST add one. The "RELATED-TO" property
added to the new "snooze" alarm MUST include a "RELTYPE"
property parameter with a value set to "SNOOZE" (see
Section 7.1).
3. When the "snooze" alarm is triggered, the client MUST do the
following:
a. Update the "ACKNOWLEDGED" property on the original related
alarm.
b. If the "snooze" alarm is itself "snoozed", the client MUST
remove the "snooze" alarm component and return to step 2.
Otherwise, if the "snooze" alarm is dismissed, the client
MUST do one of the following:
* Set the "ACKNOWLEDGED" property on the "snooze" alarm.
* Remove the "snooze" alarm component.
Note that regardless of the final disposition of the "snooze" alarm
when triggered, the original "VALARM" component is left unchanged
other than updating its "ACKNOWLEDGED" property.
7.1. Relationship Type Property Parameter
This specification adds the "SNOOZE" relationship type for use with
the "RELTYPE" property defined in Section 3.2.15 of [RFC5545]. This
is used when relating a "snoozed" "VALARM" component to the original
alarm that the "snooze" was generated for.
7.2. Example
The following example shows the "snoozing", "re-snoozing", and
dismissal of an alarm. Note that the encompassing "VCALENDAR"
component has been omitted for brevity and that the line breaks
surrounding the "VALARM" components are for clarity only and would
not be present in the actual iCalendar data.
Assume that we have the following event with an alarm set to trigger
15 minutes before the meeting:
BEGIN:VEVENT
CREATED:20210302T151004Z
UID:AC67C078-CED3-4BF5-9726-832C3749F627
DTSTAMP:20210302T151004Z
DTSTART;TZID=America/New_York:20210302T103000
DTEND;TZID=America/New_York:20210302T113000
SUMMARY:Meeting
BEGIN:VALARM
UID:8297C37D-BA2D-4476-91AE-C1EAA364F8E1
TRIGGER:-PT15M
DESCRIPTION:Event reminder
ACTION:DISPLAY
END:VALARM
END:VEVENT
When the alarm is triggered, the user decides to "snooze" it for 5
minutes. The client acknowledges the original alarm and creates a
new "snooze" alarm as a sibling of, and relates it to, the original
alarm (note that both occurrences of "VALARM" reside within the same
"parent" VEVENT):
BEGIN:VEVENT
CREATED:20210302T151004Z
UID:AC67C078-CED3-4BF5-9726-832C3749F627
DTSTAMP:20210302T151516Z
DTSTART;TZID=America/New_York:20210302T103000
DTEND;TZID=America/New_York:20210302T113000
SUMMARY:Meeting
BEGIN:VALARM
UID:8297C37D-BA2D-4476-91AE-C1EAA364F8E1
TRIGGER:-PT15M
DESCRIPTION:Event reminder
ACTION:DISPLAY
ACKNOWLEDGED:20210302T151514Z
END:VALARM
BEGIN:VALARM
UID:DE7B5C34-83FF-47FE-BE9E-FF41AE6DD097
TRIGGER;VALUE=DATE-TIME:20210302T152000Z
RELATED-TO;RELTYPE=SNOOZE:8297C37D-BA2D-4476-91AE-C1EAA364F8E1
DESCRIPTION:Event reminder
ACTION:DISPLAY
END:VALARM
END:VEVENT
When the "snooze" alarm is triggered, the user decides to "snooze" it
again for an additional 5 minutes. The client once again
acknowledges the original alarm, removes the triggered "snooze"
alarm, and creates another new "snooze" alarm as a sibling of, and
relates it to, the original alarm (note the different UID for the new
"snooze" alarm):
BEGIN:VEVENT
CREATED:20210302T151004Z
UID:AC67C078-CED3-4BF5-9726-832C3749F627
DTSTAMP:20210302T152026Z
DTSTART;TZID=America/New_York:20210302T103000
DTEND;TZID=America/New_York:20210302T113000
SUMMARY:Meeting
BEGIN:VALARM
UID:8297C37D-BA2D-4476-91AE-C1EAA364F8E1
TRIGGER:-PT15M
DESCRIPTION:Event reminder
ACTION:DISPLAY
ACKNOWLEDGED:20210302T152024Z
END:VALARM
BEGIN:VALARM
UID:87D690A7-B5E8-4EB4-8500-491F50AFE394
TRIGGER;VALUE=DATE-TIME:20210302T152500Z
RELATED-TO;RELTYPE=SNOOZE:8297C37D-BA2D-4476-91AE-C1EAA364F8E1
DESCRIPTION:Event reminder
ACTION:DISPLAY
END:VALARM
END:VEVENT
When the second "snooze" alarm is triggered, the user decides to
dismiss it. The client acknowledges both the original alarm and the
second "snooze" alarm:
BEGIN:VEVENT
CREATED:20210302T151004Z
UID:AC67C078-CED3-4BF5-9726-832C3749F627
DTSTAMP:20210302T152508Z
DTSTART;TZID=America/New_York:20210302T103000
DTEND;TZID=America/New_York:20210302T113000
SUMMARY:Meeting
BEGIN:VALARM
UID:8297C37D-BA2D-4476-91AE-C1EAA364F8E1
TRIGGER:-PT15M
DESCRIPTION:Event reminder
ACTION:DISPLAY
ACKNOWLEDGED:20210302T152507Z
END:VALARM
BEGIN:VALARM
UID:87D690A7-B5E8-4EB4-8500-491F50AFE394
TRIGGER;VALUE=DATE-TIME:20210302T152500Z
RELATED-TO;RELTYPE=SNOOZE:8297C37D-BA2D-4476-91AE-C1EAA364F8E1
DESCRIPTION:Event reminder
ACTION:DISPLAY
ACKNOWLEDGED:20210302T152507Z
END:VALARM
END:VEVENT
8. Alarm Proximity Trigger
Currently, a "VALARM" is triggered when a specific date-time value is
reached. It is also desirable to be able to trigger alarms based on
location, e.g., when arriving at or departing from a particular
location.
This specification adds the following elements to "VALARM" components
to indicate when an alarm can be triggered based on location.
"PROXIMITY" property: indicates that a location-based trigger is to
be used and which action is used for the trigger
"VLOCATION" component(s) [RFC9073]: used to indicate the actual
location(s) to trigger off of, specified with a URL property
containing a 'geo' URI [RFC5870], which allows for two or three
coordinate values with an optional uncertainty
alarmprop =/ (
;
; the following is OPTIONAL
; but MUST NOT occur more than once
;
proximity
;
)
alarm-subcomp =/ (
;
; the following is OPTIONAL
; and MAY occur more than once but only
; when a PROXIMITY property is also present
;
locationc
;
)
Typically, when a "PROXIMITY" property is used, there is no need to
specify a time-based trigger using the "TRIGGER" property. However,
since "TRIGGER" is defined as a required property for a "VALARM"
component, for backwards compatibility, it has to be present but
ignored. To indicate a "TRIGGER" that is to be ignored, clients
SHOULD use a value a long time in the past. A value of
"19760401T005545Z" has been commonly used for this purpose.
8.1. Proximity Property
Property Name: PROXIMITY
Purpose: This property indicates that a location-based trigger is
applied to an alarm.
Value Type: TEXT
Property Parameters: IANA and nonstandard property parameters can be
specified on this property.
Conformance: This property can be specified within "VALARM" calendar
components.
Description: This property is used to indicate that an alarm has a
location-based trigger. Its value identifies the action that will
trigger the alarm.
When the property value is set to "ARRIVE", the alarm is triggered
when the calendar user agent arrives in the vicinity of one or
more locations. When set to "DEPART", the alarm is triggered when
the calendar user agent departs from the vicinity of one or more
locations. Each location MUST be specified with a "VLOCATION"
component. Note that the meaning of "vicinity" in this context is
implementation defined.
When the property value is set to "CONNECT", the alarm is
triggered when the calendar user agent connects to an automobile
to which it has been paired via Bluetooth [BTcore]. When set to
"DISCONNECT", the alarm is triggered when the calendar user agent
disconnects from an automobile to which it has been paired via
Bluetooth. Note that neither current implementations of proximity
alarms nor this document have a mechanism to target a particular
automobile. Such a mechanism may be specified in a future
extension.
Format Definition: This property is defined by the following
notation:
proximity = "PROXIMITY" *proximityparam ":" proximityvalue CRLF
proximityparam = (
;
; the following is OPTIONAL
; and MAY occur more than once
;
(";" other-param)
;
)
proximityvalue = "ARRIVE" / "DEPART" /
"CONNECT" / "DISCONNECT" / iana-token / x-name
8.2. Example
The following example shows a "VALARM" component with a proximity
trigger set to trigger when the device running the calendar user
agent leaves the vicinity defined by the URL property in the
"VLOCATION" component. Note use of the "u=" parameter with the 'geo'
URI to define the uncertainty of the location determination.
BEGIN:VALARM
UID:77D80D14-906B-4257-963F-85B1E734DBB6
ACTION:DISPLAY
TRIGGER;VALUE=DATE-TIME:19760401T005545Z
DESCRIPTION:Remember to buy milk
PROXIMITY:DEPART
BEGIN:VLOCATION
UID:123456-abcdef-98765432
NAME:Office
URL:geo:40.443,-79.945;u=10
END:VLOCATION
END:VALARM
9. Security Considerations
In addition to the security properties of iCalendar (see Section 7 of
[RFC5545]), a "VALARM", if not monitored properly, can be used to
disturb users and/or leak personal information. For instance, an
undesirable audio alert could cause embarrassment; an unwanted
display alert could be considered an annoyance; or an email alert
could be used to leak a user's location to a third party or to send
unsolicited email to multiple users. Therefore, CalDAV clients and
servers that accept iCalendar data from a third party (e.g., via
iCalendar Transport-Independent Interoperability Protocol (iTIP)
[RFC5546], a subscription feed, or a shared calendar) SHOULD remove
each "VALARM" from the data prior to storing in their calendar
system.
Security considerations related to unique identifiers for "VALARM"
are discussed in Section 4.
10. Privacy Considerations
A proximity "VALARM", if not used carefully, can leak a user's past,
present, or future location. For instance, storing an iCalendar
resource containing proximity "VALARM"s to a shared calendar on
CalDAV server can expose to anyone that has access to that calendar
the user's intent to leave from or arrive at a particular location at
some future time. Furthermore, if a CalDAV client updates the shared
iCalendar resource with an "ACKNOWLEDGED" property when the alarm is
triggered, this will leak the exact date and time that the user left
from or arrived at the location. Therefore, CalDAV clients that
implement proximity alarms SHOULD give users the option of storing
and/or acknowledging the alarms on the local device only and not
storing the alarm and/or acknowledgement on a remote server.
Privacy considerations related to unique identifiers for "VALARM" are
discussed in Section 4.
11. IANA Considerations
11.1. Property Registrations
This document defines the following new iCalendar properties that
have been added to the "Properties" registry defined in Section 8.2.3
of [RFC5545] and located here: <https://www.iana.org/assignments/
icalendar>.
+==============+=========+=======================+
| Property | Status | Reference |
+==============+=========+=======================+
| ACKNOWLEDGED | Current | RFC 9074, Section 6.1 |
+--------------+---------+-----------------------+
| PROXIMITY | Current | RFC 9074, Section 8.1 |
+--------------+---------+-----------------------+
Table 1: Additions to the Properties Registry
11.2. Relationship Types Registry
This document defines the following new iCalendar relationship type
that has been added to the "Relationship Types" registry defined in
Section 8.3.8 of [RFC5545] and located here:
<https://www.iana.org/assignments/icalendar>.
+===================+=========+=======================+
| Relationship Type | Status | Reference |
+===================+=========+=======================+
| SNOOZE | Current | RFC 9074, Section 7.1 |
+-------------------+---------+-----------------------+
Table 2: Addition to the Relationship Types Registry
11.3. Proximity Values Registry
A new iCalendar registry for values of the "PROXIMITY" property has
been created and is located here: <https://www.iana.org/assignments/
icalendar>.
Additional values MAY be used, provided the process described in
Section 8.2.1 of [RFC5545] is used to register them, using the
template in Section 8.2.6 of [RFC5545].
The following table has been used to initialize the Proximity Value
Registry.
+============+=========+=======================+
| Value | Status | Reference |
+============+=========+=======================+
| ARRIVE | Current | RFC 9074, Section 8.1 |
+------------+---------+-----------------------+
| DEPART | Current | RFC 9074, Section 8.1 |
+------------+---------+-----------------------+
| CONNECT | Current | RFC 9074, Section 8.1 |
+------------+---------+-----------------------+
| DISCONNECT | Current | RFC 9074, Section 8.1 |
+------------+---------+-----------------------+
Table 3: Initial Contents of the Proximity
Values Registry
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>.
[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>.
[RFC5870] Mayrhofer, A. and C. Spanring, "A Uniform Resource
Identifier for Geographic Locations ('geo' URI)",
RFC 5870, DOI 10.17487/RFC5870, June 2010,
<https://www.rfc-editor.org/info/rfc5870>.
[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>.
[RFC9073] Douglass, M., "Event Publishing Extensions to iCalendar",
RFC 9073, DOI 10.17487/RFC9073, August 2021,
<https://www.rfc-editor.org/info/rfc9073>.
12.2. Informative References
[BTcore] Bluetooth Special Interest Group, "Bluetooth Core
Specification Version 5.0 Feature Overview", December
2016, <https://www.bluetooth.com/bluetooth-resources/
bluetooth-5- go-faster-go-further/>.
[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>.
[RFC5546] Daboo, C., Ed., "iCalendar Transport-Independent
Interoperability Protocol (iTIP)", RFC 5546,
DOI 10.17487/RFC5546, December 2009,
<https://www.rfc-editor.org/info/rfc5546>.
Acknowledgements
This specification came about via discussions at The Calendaring and
Scheduling Consortium. Also, thanks to the following for providing
feedback: Bernard Desruisseaux, Mike Douglass, Jacob Farkas, Jeffrey
Harris, Ciny Joy, Barry Leiba, and Daniel Migault.
Authors' Addresses
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
United States of America
Email: cyrus@daboo.name
URI: http://www.apple.com/
Kenneth Murchison (editor)
Fastmail US LLC
Suite 1201
1429 Walnut St
Philadelphia, PA 19102
United States of America
Email: murch@fastmailteam.com
URI: http://www.fastmail.com/
ERRATA
