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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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/