| Internet-Draft | VALARM Extensions | March 2021 |
| Daboo & Murchison | Expires 23 September 2021 | [Page] |
This document defines a set of extensions to the iCalendar VALARM component to enhance use of alarms and improve interoperability between clients and servers.¶
This document updates RFC5545.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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The iCalendar [RFC5545] specification defines a set of components used to describe calendar data. One of those is the "VALARM" component which appears as a sub-component of "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 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".¶
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.¶
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.¶
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 by adding, e.g., a new property common to all types of alarm. 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 sub-components (as required by proximity alarm triggers (Section 8)).¶
A "VALARM" calendar component is re-defined 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
)
¶
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
)
¶
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
)
¶
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
)
¶
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 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" 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.¶
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)
)
¶
The following is an example of this property:¶
ACKNOWLEDGED:20090604T084500Z¶
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:¶
Set the "ACKNOWLEDGED" property (see Section 6.1) on the triggered alarm.¶
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).¶
When the "snooze" alarm is triggered, the client MUST do the following:¶
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.¶
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.¶
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 VALARMs 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¶
VALARMs are currently triggered when a specific date-time 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.¶
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.¶
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
¶
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¶
In addition to the security properties of iCalendar (see Section 7 of [RFC5545]), VALARMs, if not monitored properly, can be used to disturb users and/or leak personal information. For instance, an undesirable audio alert could cause embarassment. 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 iTIP [RFC5546], a subscription feed, or a shared calendar) SHOULD remove all VALARMs from the data prior to storing in their calendar system.¶
Security considerations related to unique identifiers for VALARMs are discussed in Section 4.¶
Proximity VALARMs, if not used carefully, can leak a user's past, present, or future location. For instance, storing an iCalendar resource containing proxmity VALARMs 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, 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 acknowledgment on a remote server.¶
Privacy considerations related to unique identifiers for VALARMs are discussed in Section 4.¶
This document defines the following new iCalendar properties to be added to the registry defined in Section 8.2.3 of [RFC5545] and located here: https://www.iana.org/assignments/icalendar#properties¶
| Property | Status | Reference |
|---|---|---|
| ACKNOWLEDGED | Current | RFCXXXX, Section 6.1 |
| PROXIMITY | Current | RFCXXXX, Section 8.1 |
This document defines the following new iCalendar relationship type to be added to the registry defined in Section 8.3.8 of [RFC5545] and located here: https://www.iana.org/assignments/icalendar#relationship-types¶
| Relationship Type | Status | Reference |
|---|---|---|
| SNOOZE | Current | RFCXXXX, Section 7.1 |
This document creates a new iCalendar registry for values of the "PROXIMITY" property located here: https://www.iana.org/assignments/icalendar#proximity-values¶
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 | RFCXXXX, Section 8.1 |
| DEPART | Current | RFCXXXX, Section 8.1 |
| CONNECT | Current | RFCXXXX, Section 8.1 |
| DISCONNECT | Current | RFCXXXX, Section 8.1 |
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.¶
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