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RFC 5546

 
 
 

iCalendar Transport-Independent Interoperability Protocol (iTIP)

Part 2 of 5, p. 13 to 36
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3.  Application Protocol Elements

   iTIP messages are "text/calendar" MIME entities that contain
   calendaring and scheduling information.  The particular type of
   iCalendar message is referred to as the "method type".  Each method
   type is identified by a "METHOD" property specified as part of the
   "text/calendar" content type.  The table below shows various

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   combinations of calendar components and the method types that this
   specification supports.

        +----------------+--------+-------+----------+-----------+
        |                | VEVENT | VTODO | VJOURNAL | VFREEBUSY |
        +----------------+--------+-------+----------+-----------+
        | PUBLISH        | Yes    | Yes   | Yes      | Yes       |
        | REQUEST        | Yes    | Yes   | No       | Yes       |
        | REFRESH        | Yes    | Yes   | No       | No        |
        | CANCEL         | Yes    | Yes   | Yes      | No        |
        | ADD            | Yes    | Yes   | Yes      | No        |
        | REPLY          | Yes    | Yes   | No       | Yes       |
        | COUNTER        | Yes    | Yes   | No       | No        |
        | DECLINECOUNTER | Yes    | Yes   | No       | No        |
        +----------------+--------+-------+----------+-----------+

   Each method type is defined in terms of its associated components and
   properties.  Some components and properties are required, some are
   optional, and others are excluded.  The restrictions are expressed in
   this document using a simple "restriction table".  The first column
   indicates the name of a component or property.  Properties of the
   iCalendar object are not indented.  Properties of a component are
   indented.  The second column (the "Presence" column) indicates
   whether or not a component or property should be present and, if
   present, how many times it can occur.  The third column contains
   comments for further clarification.

   The presence column uses the following values to assert whether a
   property is required or optional, and the number of times it may
   appear in the iCalendar object.

   +----------------+--------------------------------------------------+
   | Presence Value | Description                                      |
   +----------------+--------------------------------------------------+
   | 1              | One instance MUST be present.                    |
   | 1+             | At least one instance MUST be present.           |
   | 0              | Instances of this property MUST NOT be present.  |
   | 0+             | Multiple instances MAY be present.               |
   | 0 or 1         | Up to 1 instance of this property MAY be         |
   |                | present.                                         |
   +----------------+--------------------------------------------------+

   The tables also call out "IANA-PROPERTY", "X-PROPERTY", "IANA-
   COMPONENT", and "X-COMPONENT" to show where registered and
   experimental property and component extensions can appear.  The
   tables do not lay out the restrictions of property parameters.  Those
   restrictions are defined in [RFC5545].

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3.1.  Common Component Restriction Tables

3.1.1.  VCALENDAR

   The restriction table below applies to properties of the iCalendar
   object.  That is, the properties at the outermost scope.

          +-----------------------------------------------------+
          | Constraints for Properties in a VCALENDAR Component |
          +-----------------------------------------------------+

          +--------------------+----------+--------------------+
          | Component/Property | Presence | Comment            |
          +--------------------+----------+--------------------+
          | CALSCALE           | 0 or 1   |                    |
          | PRODID             | 1        |                    |
          | VERSION            | 1        | Value MUST be 2.0. |
          | IANA-PROPERTY      | 0+       |                    |
          | X-PROPERTY         | 0+       |                    |
          +--------------------+----------+--------------------+

3.1.2.  VTIMEZONE

   "VTIMEZONE" components may be referred to by other components via a
   "TZID" parameter on a "DATETIME" value type.  The property
   restrictions in the table below apply to any "VTIMEZONE" component in
   an iTIP message.

                 +--------------------------------------+
                 | Constraints for VTIMEZONE Components |
                 +--------------------------------------+

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   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to timezone.               |
   |   DAYLIGHT         | 0+       | MUST be one or more of either     |
   |                    |          | STANDARD or DAYLIGHT.             |
   |     COMMENT        | 0+       |                                   |
   |     DTSTART        | 1        | MUST be local time format.        |
   |     RDATE          | 0+       |                                   |
   |     RRULE          | 0 or 1   |                                   |
   |     TZNAME         | 0+       |                                   |
   |     TZOFFSETFROM   | 1        |                                   |
   |     TZOFFSETTO     | 1        |                                   |
   |     IANA-PROPERTY  | 0+       |                                   |
   |     X-PROPERTY     | 0+       |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   STANDARD         | 0+       | MUST be one or more of either     |
   |                    |          | STANDARD or DAYLIGHT.             |
   |     COMMENT        | 0+       |                                   |
   |     DTSTART        | 1        | MUST be local time format.        |
   |     RDATE          | 0+       | If present, RRULE MUST NOT be     |
   |                    |          | present.                          |
   |     RRULE          | 0 or 1   | If present, RDATE MUST NOT be     |
   |                    |          | present.                          |
   |     TZNAME         | 0+       |                                   |
   |     TZOFFSETFROM   | 1        |                                   |
   |     TZOFFSETTO     | 1        |                                   |
   |     IANA-PROPERTY  | 0+       |                                   |
   |     X-PROPERTY     | 0+       |                                   |
   |   TZID             | 1        |                                   |
   |   TZURL            | 0 or 1   |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   +--------------------+----------+-----------------------------------+

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3.1.3.  VALARM

   The property restrictions in the table below apply to any "VALARM"
   component in an iTIP message.

                   +-----------------------------------+
                   | Constraints for VALARM Components |
                   +-----------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | VALARM             | 0+       |                                   |
   |   ACTION           | 1        |                                   |
   |   ATTACH           | 0+       |                                   |
   |   ATTENDEE         | 0+       |                                   |
   |   DESCRIPTION      | 0 or 1   |                                   |
   |   DURATION         | 0 or 1   | If present, REPEAT MUST be        |
   |                    |          | present.                          |
   |   REPEAT           | 0 or 1   | If present, DURATION MUST be      |
   |                    |          | present.                          |
   |   SUMMARY          | 0 or 1   |                                   |
   |   TRIGGER          | 1        |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   +--------------------+----------+-----------------------------------+

3.2.  Methods for VEVENT Calendar Components

   This section defines the property set restrictions for the method
   types that are applicable to the "VEVENT" calendar component.  Each
   method is defined using a table that clarifies the property
   constraints that define the particular method.

   The following summarizes the methods that are defined for the
   "VEVENT" calendar component.

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   +----------------+--------------------------------------------------+
   | Method         | Description                                      |
   +----------------+--------------------------------------------------+
   | PUBLISH        | Post notification of an event.  Used primarily   |
   |                | as a method of advertising the existence of an   |
   |                | event.                                           |
   |                |                                                  |
   | REQUEST        | Make a request for an event.  This is an         |
   |                | explicit invitation to one or more Attendees.    |
   |                | Event requests are also used to update or change |
   |                | an existing event.  Clients that cannot handle   |
   |                | REQUEST MAY degrade the event to view it as a    |
   |                | PUBLISH.                                         |
   |                |                                                  |
   | REPLY          | Reply to an event request.  Clients may set      |
   |                | their status (PARTSTAT) to ACCEPTED, DECLINED,   |
   |                | TENTATIVE, or DELEGATED.                         |
   |                |                                                  |
   | ADD            | Add one or more instances to an existing event.  |
   |                |                                                  |
   | CANCEL         | Cancel one or more instances of an existing      |
   |                | event.                                           |
   |                |                                                  |
   | REFRESH        | A request is sent to an Organizer by an Attendee |
   |                | asking for the latest version of an event to be  |
   |                | resent to the requester.                         |
   |                |                                                  |
   | COUNTER        | Counter a REQUEST with an alternative proposal.  |
   |                | Sent by an Attendee to the Organizer.            |
   |                |                                                  |
   | DECLINECOUNTER | Decline a counter proposal.  Sent to an Attendee |
   |                | by the Organizer.                                |
   +----------------+--------------------------------------------------+

3.2.1.  PUBLISH

   The "PUBLISH" method in a "VEVENT" calendar component is an
   unsolicited posting of an iCalendar object.  Any CU may add published
   components to their calendar.  The "Organizer" MUST be present in a
   published iCalendar component.  "Attendees" MUST NOT be present.  Its
   expected usage is for encapsulating an arbitrary event as an
   iCalendar object.  The "Organizer" may subsequently update (with
   another "PUBLISH" method), add instances to (with an "ADD" method),
   or cancel (with a "CANCEL" method) a previously published "VEVENT"
   calendar component.

   This method type is an iCalendar object that conforms to the
   following property constraints:

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             +----------------------------------------------+
             | Constraints for a METHOD:PUBLISH of a VEVENT |
             +----------------------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST equal PUBLISH.               |
   |                    |          |                                   |
   | VEVENT             | 1+       |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   SUMMARY          | 1        | Can be null.                      |
   |   UID              | 1        |                                   |
   |   RECURRENCE-ID    | 0 or 1   | Only if referring to an instance  |
   |                    |          | of a recurring calendar           |
   |                    |          | component.  Otherwise, it MUST    |
   |                    |          | NOT be present.                   |
   |   SEQUENCE         | 0 or 1   | MUST be present if value is       |
   |                    |          | greater than 0; MAY be present if |
   |                    |          | 0.                                |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0 or 1   |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   | Can be null.                      |
   |   DTEND            | 0 or 1   | If present, DURATION MUST NOT be  |
   |                    |          | present.                          |
   |   DURATION         | 0 or 1   | If present, DTEND MUST NOT be     |
   |                    |          | present.                          |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0 or 1   |                                   |
   |   STATUS           | 0 or 1   | MAY be one of                     |
   |                    |          | TENTATIVE/CONFIRMED/CANCELLED.    |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |

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   |   ATTENDEE         | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   |   VALARM           | 0+       |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone.             |
   |                    |          |                                   |
   | IANA-COMPONENT     | 0+       |                                   |
   | X-COMPONENT        | 0+       |                                   |
   +--------------------+----------+-----------------------------------+

3.2.2.  REQUEST

   The "REQUEST" method in a "VEVENT" component provides the following
   scheduling functions:

   o  Invite "Attendees" to an event.

   o  Reschedule an existing event.

   o  Response to a "REFRESH" request.

   o  Update the details of an existing event, without rescheduling it.

   o  Update the status of "Attendees" of an existing event, without
      rescheduling it.

   o  Reconfirm an existing event, without rescheduling it.

   o  Forward a "VEVENT" to another uninvited CU.

   o  For an existing "VEVENT" calendar component, delegate the role of
      "Attendee" to another CU.

   o  For an existing "VEVENT" calendar component, change the role of
      "Organizer" to another CU.

   The "Organizer" originates the "REQUEST".  The recipients of the
   "REQUEST" method are the CUs invited to the event, the "Attendees".
   "Attendees" use the "REPLY" method to convey attendance status to the
   "Organizer".

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   The "UID" and "SEQUENCE" properties are used to distinguish the
   various uses of the "REQUEST" method.  If the "UID" property value in
   the "REQUEST" is not found on the recipient's calendar, then the
   "REQUEST" is for a new "VEVENT" calendar component.  If the "UID"
   property value is found on the recipient's calendar, then the
   "REQUEST" is for a rescheduling, an update, or a reconfirmation of
   the "VEVENT" calendar component.

   For the "REQUEST" method, multiple "VEVENT" components in a single
   iCalendar object are only permitted for components with the same
   "UID" property.  That is, a series of recurring events may have
   instance-specific information.  In this case, multiple "VEVENT"
   components are needed to express the entire series.

   This method type is an iCalendar object that conforms to the
   following property constraints:

             +----------------------------------------------+
             | Constraints for a METHOD:REQUEST of a VEVENT |
             +----------------------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be REQUEST.                  |
   |                    |          |                                   |
   | VEVENT             | 1+       | All components MUST have the same |
   |                    |          | UID.                              |
   |   ATTENDEE         | 1+       |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   SEQUENCE         | 0 or 1   | MUST be present if value is       |
   |                    |          | greater than 0; MAY be present if |
   |                    |          | 0.                                |
   |   SUMMARY          | 1        | Can be null.                      |
   |   UID              | 1        |                                   |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   | Can be null.                      |
   |   DTEND            | 0 or 1   | If present, DURATION MUST NOT be  |
   |                    |          | present.                          |

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   |   DURATION         | 0 or 1   | If present, DTEND MUST NOT be     |
   |                    |          | present.                          |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | Only if referring to an instance  |
   |                    |          | of a recurring calendar           |
   |                    |          | component.  Otherwise, it MUST    |
   |                    |          | NOT be present.                   |
   |   RELATED-TO       | 0+       |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0 or 1   |                                   |
   |   STATUS           | 0 or 1   | MAY be one of                     |
   |                    |          | TENTATIVE/CONFIRMED.              |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   |   VALARM           | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone.             |
   |                    |          |                                   |
   | IANA-COMPONENT     | 0+       |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.2.2.1.  Rescheduling an Event

   The "REQUEST" method may be used to reschedule an event.  A
   rescheduled event involves a change to the existing event in terms of
   its time or recurrence intervals and possibly the location or
   description.  If the recipient CUA of a "REQUEST" method finds that
   the "UID" property value already exists on the calendar but that the
   "SEQUENCE" (or "DTSTAMP") property value in the "REQUEST" method is
   greater than the value for the existing event, then the "REQUEST"
   method describes a rescheduling of the event.

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3.2.2.2.  Updating or Reconfirmation of an Event

   The "REQUEST" method may be used to update or reconfirm an event.  An
   update to an existing event does not involve changes to the time or
   recurrence intervals, and might not involve a change to the location
   or description for the event.  If the recipient CUA of a "REQUEST"
   method finds that the "UID" property value already exists on the
   calendar and that the "SEQUENCE" property value in the "REQUEST" is
   the same as the value for the existing event, then the "REQUEST"
   method describes an update of the event details, but not a
   rescheduling of the event.

   The update "REQUEST" method is the appropriate response to a
   "REFRESH" method sent from an "Attendee" to the "Organizer" of an
   event.

   The "Organizer" of an event may also send unsolicited "REQUEST"
   methods.  The unsolicited "REQUEST" methods may be used to update the
   details of the event without rescheduling it, to update the
   "PARTSTAT" parameter of "Attendees", or to reconfirm the event.

3.2.2.3.  Delegating an Event to Another CU

   Some calendar and scheduling systems allow "Attendees" to delegate
   their presence at an event to another "Calendar User". iTIP supports
   this concept using the following workflow.  Any "Attendee" may
   delegate their right to participate in a calendar "VEVENT" to another
   CU.  The implication is that the delegate participates in lieu of the
   original "Attendee", NOT in addition to the "Attendee".  The
   delegator MUST notify the "Organizer" of this action using the steps
   outlined below.  Implementations may support or restrict delegation
   as they see fit.  For instance, some implementations may restrict a
   delegate from delegating a "REQUEST" to another CU.

   The "Delegator" of an event forwards the existing "REQUEST" to the
   "Delegate".  The "REQUEST" method MUST include an "ATTENDEE" property
   with the calendar address of the "Delegate".  The "Delegator" MUST
   also send a "REPLY" method to the "Organizer" with the "Delegator's"
   "ATTENDEE" property "PARTSTAT" parameter value set to "DELEGATED".
   In addition, the "DELEGATED-TO" parameter MUST be included with the
   calendar address of the "Delegate".  Also, a new "ATTENDEE" property
   for the "Delegate" MUST be included and must specify the calendar
   user address set in the "DELEGATED-TO" parameter, as above.

   In response to the request, the "Delegate" MUST send a "REPLY" method
   to the "Organizer", and optionally to the "Delegator".  The "REPLY"
   method SHOULD include the "ATTENDEE" property with the "DELEGATED-
   FROM" parameter value of the "Delegator's" calendar address.

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   The "Delegator" may continue to receive updates to the event even
   though they will not be attending.  This is accomplished by the
   "Delegator" setting their "role" attribute to "NON-PARTICIPANT" in
   the "REPLY" to the "Organizer".

3.2.2.4.  Changing the Organizer

   The situation may arise where the "Organizer" of a "VEVENT" is no
   longer able to perform the "Organizer" role and abdicates without
   passing on the "Organizer" role to someone else.  When this occurs,
   the "Attendees" of the "VEVENT" may use out-of-band mechanisms to
   communicate the situation and agree upon a new "Organizer".  The new
   "Organizer" should then send out a new "REQUEST" with a modified
   version of the "VEVENT" in which the "SEQUENCE" number has been
   incremented and the "ORGANIZER" property has been changed to the new
   "Organizer".

3.2.2.5.  Sending on Behalf of the Organizer

   There are a number of scenarios that support the need for a "Calendar
   User" to act on behalf of the "Organizer" without explicit role
   changing.  This might be the case if the CU designated as "Organizer"
   is sick or unable to perform duties associated with that function.
   In these cases, iTIP supports the notion of one CU acting on behalf
   of another.  Using the "SENT-BY" parameter, a "Calendar User" could
   send an updated "VEVENT" "REQUEST".  In the case where one CU sends
   on behalf of another CU, the "Attendee" responses are still directed
   back towards the CU designated as "Organizer".

3.2.2.6.  Forwarding to an Uninvited CU

   An "Attendee" invited to a "VEVENT" calendar component may send the
   "VEVENT" calendar component to another new CU not previously
   associated with the "VEVENT" calendar component.  The current
   "Attendee" invited to the "VEVENT" calendar component does this by
   forwarding the original "REQUEST" method to the new CU.  The new CU
   can send a "REPLY" to the "Organizer" of the "VEVENT" calendar
   component.  The reply contains an "ATTENDEE" property for the new CU.

   The "Organizer" ultimately decides whether or not the new CU becomes
   part of the event and is not obligated to do anything with a "REPLY"
   from a new (uninvited) CU.  If the "Organizer" does not want the new
   CU to be part of the event, the new "ATTENDEE" property is not added
   to the "VEVENT" calendar component.  The "Organizer" MAY send the CU
   a "CANCEL" message to indicate that they will not be added to the
   event.  If the "Organizer" decides to add the new CU, the new
   "ATTENDEE" property is added to the "VEVENT" calendar component.
   Furthermore, the "Organizer" is free to change any "ATTENDEE"

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   property parameter from the values supplied by the new CU to
   something the "Organizer" considers appropriate.  The "Organizer"
   SHOULD send the new CU a "REQUEST" message to inform them that they
   have been added.

   When forwarding a "REQUEST" to another CU, the forwarding "Attendee"
   MUST NOT make changes to the original message.

3.2.2.7.  Updating Attendee Status

   The "Organizer" of an event may also request updated status from one
   or more "Attendees".  The "Organizer" sends a "REQUEST" method to the
   "Attendee" and sets the "ATTENDEE;RSVP=TRUE" property parameter.  The
   "SEQUENCE" property for the event is not changed from its previous
   value.  A recipient will determine that the only change in the
   "REQUEST" is that their "RSVP" property parameter indicates a request
   for updated status.  The recipient SHOULD respond with a "REPLY"
   method indicating their current status with respect to the "REQUEST".

3.2.3.  REPLY

   The "REPLY" method in a "VEVENT" calendar component is used to
   respond (e.g., accept or decline) to a "REQUEST" or to reply to a
   delegation "REQUEST".  When used to provide a delegation response,
   the "Delegator" SHOULD include the calendar address of the "Delegate"
   on the "DELEGATED-TO" property parameter of the "Delegator's"
   "ATTENDEE" property.  The "Delegate" SHOULD include the calendar
   address of the "Delegator" on the "DELEGATED-FROM" property parameter
   of the "Delegate's" "ATTENDEE" property.

   The "REPLY" method is also used when processing of a "REQUEST" fails.
   Depending on the value of the "REQUEST-STATUS" property, no
   scheduling action may have been performed.

   The "Organizer" of an event may receive the "REPLY" method from a CU
   not in the original "REQUEST".  For example, a "REPLY" may be
   received from a "Delegate" to an event.  In addition, the "REPLY"
   method may be received from an unknown CU (a "Party Crasher").  This
   uninvited "Attendee" may be accepted, or the "Organizer" may cancel
   the event for the uninvited "Attendee" by sending a "CANCEL" method
   to the uninvited "Attendee".

   An "Attendee" MAY include a message to the "Organizer" using the
   "COMMENT" property.  For example, if the user indicates tentative
   acceptance and wants to let the "Organizer" know why, the reason can
   be expressed in the "COMMENT" property value.

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   The "Organizer" may also receive a "REPLY" from one CU on behalf of
   another.  Like the scenario enumerated above for the "Organizer",
   "Attendees" may have another CU respond on their behalf.  This is
   done using the "SENT-BY" parameter.

   The optional properties listed in the table below (those listed as
   "0+" or "0 or 1") MUST NOT be changed from those of the original
   request.  If property changes are desired, the "COUNTER" message must
   be used.

   This method type is an iCalendar object that conforms to the
   following property constraints:

              +--------------------------------------------+
              | Constraints for a METHOD:REPLY of a VEVENT |
              +--------------------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be REPLY.                    |
   |                    |          |                                   |
   | VEVENT             | 1+       | All components MUST have the same |
   |                    |          | UID.                              |
   |   ATTENDEE         | 1        | MUST be the address of the        |
   |                    |          | Attendee replying.                |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   RECURRENCE-ID    | 0 or 1   | Only if referring to an instance  |
   |                    |          | of a recurring calendar           |
   |                    |          | component.  Otherwise, it MUST    |
   |                    |          | NOT be present.                   |
   |   UID              | 1        | MUST be the UID of the original   |
   |                    |          | REQUEST.                          |
   |   SEQUENCE         | 0 or 1   | If non-zero, MUST be the sequence |
   |                    |          | number of the original REQUEST.   |
   |                    |          | MAY be present if 0.              |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   |                                   |
   |   DTEND            | 0 or 1   | If present, DURATION MUST NOT be  |
   |                    |          | present.                          |
   |   DTSTART          | 0 or 1   |                                   |

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   |   DURATION         | 0 or 1   | If present, DTEND MUST NOT be     |
   |                    |          | present.                          |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   REQUEST-STATUS   | 0+       |                                   |
   |   RRULE            | 0 or 1   |                                   |
   |   STATUS           | 0 or 1   |                                   |
   |   SUMMARY          | 0 or 1   |                                   |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   |   VALARM           | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0 or 1   | MUST be present if any date/time  |
   |                    |          | refers to a timezone.             |
   |                    |          |                                   |
   | IANA-COMPONENT     | 0+       |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.2.4.  ADD

   The "ADD" method allows the "Organizer" to add one or more new
   instances to an existing "VEVENT" using a single iTIP message without
   having to send the entire "VEVENT" with all the existing instance
   data, as it would have to do if the "REQUEST" method were used.

   The "UID" must be that of the existing event.  If the "UID" property
   value in the "ADD" is not found on the recipient's calendar, then the
   recipient SHOULD send a "REFRESH" to the "Organizer" in order to be
   updated with the latest version of the "VEVENT".  If an "Attendee"
   implementation does not support the "ADD" method, it should respond
   with a "REQUEST-STATUS" value of 3.14 and ask for a "REFRESH".

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   When handling an "ADD" message, the "Attendee" treats each component
   in the "ADD" message as if it were referenced via an "RDATE" in the
   main component.

   This method type is an iCalendar object that conforms to the
   following property constraints:

               +------------------------------------------+
               | Constraints for a METHOD:ADD of a VEVENT |
               +------------------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be ADD.                      |
   |                    |          |                                   |
   | VEVENT             | 1        |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   SEQUENCE         | 1        | MUST be greater than 0.           |
   |   SUMMARY          | 1        | Can be null.                      |
   |   UID              | 1        | MUST match that of the original   |
   |                    |          | event.                            |
   |   ATTACH           | 0+       |                                   |
   |   ATTENDEE         | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   | Can be null.                      |
   |   DTEND            | 0 or 1   | If present, DURATION MUST NOT be  |
   |                    |          | present.                          |
   |   DURATION         | 0 or 1   | If present, DTEND MUST NOT be     |
   |                    |          | present.                          |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of                     |
   |                    |          | TENTATIVE/CONFIRMED.              |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |

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   |   EXDATE           | 0        |                                   |
   |   RECURRENCE-ID    | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |   RDATE            | 0        |                                   |
   |   RRULE            | 0        |                                   |
   |                    |          |                                   |
   |   VALARM           | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone.             |
   |                    |          |                                   |
   | IANA-COMPONENT     | 0+       |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.2.5.  CANCEL

   The "CANCEL" method in a "VEVENT" calendar component is used to send
   a cancellation notice of an existing event request to the affected
   "Attendees".  The message is sent by the "Organizer" of the event.
   For a recurring event, either the whole event or instances of an
   event may be cancelled.  To cancel the complete range of a recurring
   event, the "UID" property value for the event MUST be specified and a
   "RECURRENCE-ID" MUST NOT be specified in the "CANCEL" method.  In
   order to cancel an individual instance of the event, the
   "RECURRENCE-ID" property value for the event MUST be specified in the
   "CANCEL" method.

   There are two options for canceling a sequence of instances of a
   recurring "VEVENT" calendar component:

   a.  The "RECURRENCE-ID" property for an instance in the sequence MUST
       be specified with the "RANGE" property parameter value of
       "THISANDFUTURE" to indicate cancellation of the specified
       "VEVENT" calendar component and all instances after.

   b.  Individual recurrence instances may be cancelled by specifying
       multiple "VEVENT" components each with a "RECURRENCE-ID" property
       corresponding to one of the instances to be cancelled.

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   The "Organizer" MUST send a "CANCEL" message to each "Attendee"
   affected by the cancellation.  This can be done using a single
   "CANCEL" message for all "Attendees" or by using multiple messages
   with different subsets of the affected "Attendees" in each.

   When a "VEVENT" is cancelled, the "SEQUENCE" property value MUST be
   incremented as described in Section 2.1.4.

   This method type is an iCalendar object that conforms to the
   following property constraints:

              +---------------------------------------------+
              | Constraints for a METHOD:CANCEL of a VEVENT |
              +---------------------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be CANCEL.                   |
   |                    |          |                                   |
   | VEVENT             | 1+       | All must have the same UID.       |
   |   ATTENDEE         | 0+       | MUST include some or all          |
   |                    |          | Attendees being removed from the  |
   |                    |          | event.  MUST include some or all  |
   |                    |          | Attendees if the entire event is  |
   |                    |          | cancelled.                        |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   SEQUENCE         | 1        |                                   |
   |   UID              | 1        | MUST be the UID of the original   |
   |                    |          | REQUEST.                          |
   |   COMMENT          | 0+       |                                   |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   |                                   |
   |   DTEND            | 0 or 1   | If present, DURATION MUST NOT be  |
   |                    |          | present.                          |
   |   DTSTART          | 0 or 1   |                                   |
   |   DURATION         | 0 or 1   | If present, DTEND MUST NOT be     |
   |                    |          | present.                          |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |

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   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | Only if referring to an instance  |
   |                    |          | of a recurring calendar           |
   |                    |          | component.  Otherwise, it MUST    |
   |                    |          | NOT be present.                   |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0 or 1   |                                   |
   |   STATUS           | 0 or 1   | MUST be set to CANCELLED to       |
   |                    |          | cancel the entire event.  If      |
   |                    |          | uninviting specific Attendees,    |
   |                    |          | then MUST NOT be included.        |
   |   SUMMARY          | 0 or 1   |                                   |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   |   VALARM           | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone.             |
   |                    |          |                                   |
   | IANA-COMPONENT     | 0+       |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.2.6.  REFRESH

   The "REFRESH" method in a "VEVENT" calendar component is used by
   "Attendees" of an existing event to request an updated description
   from the event "Organizer".  The "REFRESH" method must specify the
   "UID" property of the event to update.  A recurrence instance of an
   event may be requested by specifying the "RECURRENCE-ID" property
   corresponding to the associated event.  The "Organizer" responds with
   the latest description and version of the event.

   This method type is an iCalendar object that conforms to the
   following property constraints:

Top      Up      ToC       Page 32 
             +----------------------------------------------+
             | Constraints for a METHOD:REFRESH of a VEVENT |
             +----------------------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be REFRESH.                  |
   |                    |          |                                   |
   | VEVENT             | 1        |                                   |
   |   ATTENDEE         | 1        | MUST be the address of requester. |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   UID              | 1        | MUST be the UID associated with   |
   |                    |          | original REQUEST.                 |
   |   COMMENT          | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | Only if referring to an instance  |
   |                    |          | of a recurring calendar           |
   |                    |          | component.  Otherwise, it MUST    |
   |                    |          | NOT be present.                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   ATTACH           | 0        |                                   |
   |   CATEGORIES       | 0        |                                   |
   |   CLASS            | 0        |                                   |
   |   CONTACT          | 0        |                                   |
   |   CREATED          | 0        |                                   |
   |   DESCRIPTION      | 0        |                                   |
   |   DTEND            | 0        |                                   |
   |   DTSTART          | 0        |                                   |
   |   DURATION         | 0        |                                   |
   |   EXDATE           | 0        |                                   |
   |   GEO              | 0        |                                   |
   |   LAST-MODIFIED    | 0        |                                   |
   |   LOCATION         | 0        |                                   |
   |   PRIORITY         | 0        |                                   |
   |   RDATE            | 0        |                                   |
   |   RELATED-TO       | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |   RESOURCES        | 0        |                                   |
   |   RRULE            | 0        |                                   |
   |   SEQUENCE         | 0        |                                   |
   |   STATUS           | 0        |                                   |
   |   SUMMARY          | 0        |                                   |
   |   TRANSP           | 0        |                                   |
   |   URL              | 0        |                                   |
   |                    |          |                                   |

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   |   VALARM           | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       |                                   |
   |                    |          |                                   |
   | IANA-COMPONENT     | 0+       |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.2.7.  COUNTER

   The "COUNTER" method for a "VEVENT" calendar component is used by an
   "Attendee" of an existing event to submit to the "Organizer" a
   counter proposal to the event.  The "Attendee" sends this message to
   the "Organizer" of the event.

   The counter proposal is an iCalendar object consisting of a "VEVENT"
   calendar component that provides the complete description of the
   alternate event.

   The "Organizer" rejects the counter proposal by sending the
   "Attendee" a "DECLINECOUNTER" method.  The "Organizer" accepts the
   counter proposal by rescheduling the event as described in
   Section 3.2.2.1, "Rescheduling an Event".  The "Organizer's" CUA
   SHOULD send a "REQUEST" message to all "Attendees" affected by any
   change triggered by an accepted "COUNTER".

   This method type is an iCalendar object that conforms to the
   following property constraints:

             +----------------------------------------------+
             | Constraints for a METHOD:COUNTER of a VEVENT |
             +----------------------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be COUNTER.                  |
   |                    |          |                                   |
   | VEVENT             | 1        |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |

Top      Up      ToC       Page 34 
   |   ORGANIZER        | 1        | MUST be the Organizer of the      |
   |                    |          | original event.                   |
   |   SEQUENCE         | 1        | MUST echo the original SEQUENCE   |
   |                    |          | number.  MUST be present if       |
   |                    |          | non-zero.  MAY be present if      |
   |                    |          | zero.                             |
   |   SUMMARY          | 1        | Can be null.                      |
   |   UID              | 1        | MUST be the UID associated with   |
   |                    |          | the REQUEST being countered.      |
   |   ATTACH           | 0+       |                                   |
   |   ATTENDEE         | 0+       | Can also be used to propose other |
   |                    |          | Attendees.                        |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   |                                   |
   |   DTEND            | 0 or 1   | If present, DURATION MUST NOT be  |
   |                    |          | present.                          |
   |   DURATION         | 0 or 1   | If present, DTEND MUST NOT be     |
   |                    |          | present.                          |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | Only if referring to an instance  |
   |                    |          | of a recurring calendar           |
   |                    |          | component.  Otherwise, it MUST    |
   |                    |          | NOT be present.                   |
   |   RELATED-TO       | 0+       |                                   |
   |   REQUEST-STATUS   | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0 or 1   |                                   |
   |   STATUS           | 0 or 1   | Value must be one of              |
   |                    |          | CONFIRMED/TENATIVE/CANCELLED.     |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   |   VALARM           | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone.             |
   |                    |          |                                   |

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   | IANA-COMPONENT     | 0+       |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.2.8.  DECLINECOUNTER

   The "DECLINECOUNTER" method in a "VEVENT" calendar component is used
   by the "Organizer" of an event to reject a counter proposal submitted
   by an "Attendee".  The "Organizer" must send the "DECLINECOUNTER"
   message to the "Attendee" that sent the "COUNTER" method to the
   "Organizer".

   This method type is an iCalendar object that conforms to the
   following property constraints:

          +-----------------------------------------------------+
          | Constraints for a METHOD:DECLINECOUNTER of a VEVENT |
          +-----------------------------------------------------+

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be DECLINECOUNTER.           |
   |                    |          |                                   |
   | VEVENT             | 1+       | All components MUST have the same |
   |                    |          | UID.                              |
   |   ATTENDEE         | 1+       | MUST for all Attendees.           |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   SEQUENCE         | 1        | MUST echo the original SEQUENCE   |
   |                    |          | number.                           |
   |   UID              | 1        | MUST echo original UID.           |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   | Can be null.                      |
   |   DTSTART          | 0 or 1   |                                   |
   |   DTEND            | 0 or 1   | If present, DURATION MUST NOT be  |
   |                    |          | present.                          |

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   |   DURATION         | 0 or 1   | If present, DTEND MUST NOT be     |
   |                    |          | present.                          |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | Only if referring to an instance  |
   |                    |          | of a recurring calendar           |
   |                    |          | component.  Otherwise, it MUST    |
   |                    |          | NOT be present.                   |
   |   RELATED-TO       | 0+       |                                   |
   |   REQUEST-STATUS   | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0 or 1   |                                   |
   |   STATUS           | 0 or 1   | MAY be one of                     |
   |                    |          | TENTATIVE/CONFIRMED.              |
   |   SUMMARY          | 0 or 1   | Can be null.                      |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   IANA-PROPERTY    | 0+       |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone.             |
   |                    |          |                                   |
   | IANA-COMPONENT     | 0+       |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   |   VALARM           | 0        |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   +--------------------+----------+-----------------------------------+


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