Network Working Group                                      C. Daboo, Ed.
Internet-Draft                                                Apple Inc.
Obsoletes: 2446 (if approved)                          February 24,                               May 11, 2008
Intended status: Standards Track
Expires: August 27, November 12, 2008

    iCalendar Transport-Independent Interoperability Protocol (iTIP)
                     draft-ietf-calsify-2446bis-05
                     draft-ietf-calsify-2446bis-06

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document specifies a protocol using the iCalendar object
   specification to provide scheduling interoperability between
   different calendar systems.  This is done without reference to a
   specific transport protocol so as to allow multiple methods of
   communication between systems.  Subsequent documents will define
   profiles of this protocol using specific interoperable methods of
   communications between systems.

   iTIP complements the iCalendar object specification by adding
   semantics for group scheduling methods commonly available in current
   calendar systems.  These scheduling methods permit two or more
   calendar systems to perform transactions such as publish, schedule,
   reschedule, respond to scheduling requests, negotiation of changes or
   cancel.

Table of Contents

   1.  Introduction and Overview . . . . . . . . . . . . . . . . . .   5
     1.1.  Formatting Conventions  . . . . . . . . . . . . . . . . .   5
     1.2.  Related Documents . . . . . . . . . . . . . . . . . . . .   6
     1.3.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.4.  Methods . . . . . . . . . . . . . . . . . . . . . . . . .   7
   2.  Interoperability Models . . . . . . . . . . . . . . . . . . .   8
     2.1.  Application Protocol  . . . . . . . . . . . . . . . . . .   9
       2.1.1.  Calendar Entry State  . . . . . . . . . . . . . . . .   9
       2.1.2.  Delegation  . . . . . . . . . . . . . . . . . . . . .  10
       2.1.3.  Acting on Behalf of other Calendar Users  . . . . . .  10
       2.1.4.  Component Revisions . . . . . . . . . . . . . . . . .  10
       2.1.5.  Message Sequencing  . . . . . . . . . . . . . . . . .  11
   3.  Application Protocol Elements . . . . . . . . . . . . . . . .  12
     3.1.  Common Component Restriction Tables . . . . . . . . . . .  13
       3.1.1.  VCALENDAR . . . . . . . . . . . . . . . . . . . . . .  13
       3.1.2.  VTIMEZONE . . . . . . . . . . . . . . . . . . . . . .  13
       3.1.3.  VALARM  . . . . . . . . . . . . . . . . . . . . . . .  14
     3.2.  Methods for VEVENT Calendar Components  . . . . . . . . .  15
       3.2.1.  PUBLISH . . . . . . . . . . . . . . . . . . . . . . .  16
       3.2.2.  REQUEST . . . . . . . . . . . . . . . . . . . . . . .  17
       3.2.3.  REPLY . . . . . . . . . . . . . . . . . . . . . . . .  22
       3.2.4.  ADD . . . . . . . . . . . . . . . . . . . . . . . . .  24
       3.2.5.  CANCEL  . . . . . . . . . . . . . . . . . . . . . . .  26
       3.2.6.  REFRESH . . . . . . . . . . . . . . . . . . . . . . .  28
       3.2.7.  COUNTER . . . . . . . . . . . . . . . . . . . . . . .  29
       3.2.8.  DECLINECOUNTER  . . . . . . . . . . . . . . . . . . .  31
     3.3.  Methods For VFREEBUSY Components  . . . . . . . . . . . .  32
       3.3.1.  PUBLISH . . . . . . . . . . . . . . . . . . . . . . .  33
       3.3.2.  REQUEST . . . . . . . . . . . . . . . . . . . . . . .  34
       3.3.3.  REPLY . . . . . . . . . . . . . . . . . . . . . . . .  36
     3.4.  Methods For VTODO Components  . . . . . . . . . . . . . .  37
       3.4.1.  PUBLISH . . . . . . . . . . . . . . . . . . . . . . .  38
       3.4.2.  REQUEST . . . . . . . . . . . . . . . . . . . . . . .  39
       3.4.3.  REPLY . . . . . . . . . . . . . . . . . . . . . . . .  44
       3.4.4.  ADD . . . . . . . . . . . . . . . . . . . . . . . . .  46
       3.4.5.  CANCEL  . . . . . . . . . . . . . . . . . . . . . . .  47
       3.4.6.  REFRESH . . . . . . . . . . . . . . . . . . . . . . .  50
       3.4.7.  COUNTER . . . . . . . . . . . . . . . . . . . . . . .  52
       3.4.8.  DECLINECOUNTER  . . . . . . . . . . . . . . . . . . .  53
     3.5.  Methods For VJOURNAL Components . . . . . . . . . . . . .  56
       3.5.1.  PUBLISH . . . . . . . . . . . . . . . . . . . . . . .  56
       3.5.2.  ADD . . . . . . . . . . . . . . . . . . . . . . . . .  58
       3.5.3.  CANCEL  . . . . . . . . . . . . . . . . . . . . . . .  60
     3.6.  Status Replies  . . . . . . . . . . . . . . . . . . . . .  62
     3.7.  Implementation Considerations . . . . . . . . . . . . . .  64  65
       3.7.1.  Working With Recurrence Instances . . . . . . . . . .  64  65
       3.7.2.  Attendee Property Considerations  . . . . . . . . . .  65  66
       3.7.3.  X-Tokens  . . . . . . . . . . . . . . . . . . . . . .  66  67
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  66  67
     4.1.  Published Event Examples  . . . . . . . . . . . . . . . .  66  67
       4.1.1.  A Minimal Published Event . . . . . . . . . . . . . .  67  68
       4.1.2.  Changing A Published Event  . . . . . . . . . . . . .  67  68
       4.1.3.  Canceling A Published Event . . . . . . . . . . . . .  68  69
       4.1.4.  A Rich Published Event  . . . . . . . . . . . . . . .  69
       4.1.5.  Anniversaries or Events attached to entire days . . .  70  71
     4.2.  Group Event Examples  . . . . . . . . . . . . . . . . . .  70  71
       4.2.1.  A Group Event Request . . . . . . . . . . . . . . . .  71  72
       4.2.2.  Reply To A Group Event Request  . . . . . . . . . . .  72  73
       4.2.3.  Update An Event . . . . . . . . . . . . . . . . . . .  73
       4.2.4.  Countering an Event Proposal  . . . . . . . . . . . .  73  74
       4.2.5.  Delegating an Event . . . . . . . . . . . . . . . . .  76  77
       4.2.6.  Delegate Accepts the Meeting  . . . . . . . . . . . .  78  79
       4.2.7.  Delegate Declines the Meeting . . . . . . . . . . . .  79  80
       4.2.8.  Forwarding an Event Request . . . . . . . . . . . . .  80  81
       4.2.9.  Cancel A Group Event  . . . . . . . . . . . . . . . .  80  81
       4.2.10. Removing Attendees  . . . . . . . . . . . . . . . . .  81  82
       4.2.11. Replacing the Organizer . . . . . . . . . . . . . . .  83  84
     4.3.  Busy Time Examples  . . . . . . . . . . . . . . . . . . .  84  85
       4.3.1.  Request Busy Time . . . . . . . . . . . . . . . . . .  85  86
       4.3.2.  Reply To A Busy Time Request  . . . . . . . . . . . .  86  87
     4.4.  Recurring Event and Time Zone Examples  . . . . . . . . .  86  87
       4.4.1.  A Recurring Event Spanning Time Zones . . . . . . . .  86  87
       4.4.2.  Modify A Recurring Instance . . . . . . . . . . . . .  88  89
       4.4.3.  Cancel an Instance  . . . . . . . . . . . . . . . . .  90  91
       4.4.4.  Cancel Recurring Event  . . . . . . . . . . . . . . .  91  92
       4.4.5.  Change All Future Instances . . . . . . . . . . . . .  91  92
       4.4.6.  Add A New Instance To A Recurring Event . . . . . . .  92  93
       4.4.7.  Add A New Series of Instances To A Recurring Event  .  93  94
       4.4.8.  Counter An Instance Of A Recurring Event  . . . . . .  98  99
       4.4.9.  Error Reply To A Request  . . . . . . . . . . . . . .  99 100
     4.5.  Group To-do Examples  . . . . . . . . . . . . . . . . . . 101 102
       4.5.1.  A VTODO Request . . . . . . . . . . . . . . . . . . . 102 103
       4.5.2.  A VTODO Reply . . . . . . . . . . . . . . . . . . . . 102 103
       4.5.3.  A VTODO Request for Updated Status  . . . . . . . . . 103 104
       4.5.4.  A Reply: Percent-Complete . . . . . . . . . . . . . . 103 104
       4.5.5.  A Reply: Completed  . . . . . . . . . . . . . . . . . 104 105
       4.5.6.  An Updated VTODO Request  . . . . . . . . . . . . . . 104 105
       4.5.7.  Recurring VTODOs  . . . . . . . . . . . . . . . . . . 105 106
     4.6.  Journal Examples  . . . . . . . . . . . . . . . . . . . . 106 107
     4.7.  Other Examples  . . . . . . . . . . . . . . . . . . . . . 107 108
       4.7.1.  Event Refresh . . . . . . . . . . . . . . . . . . . . 107 108
       4.7.2.  Bad RECURRENCE-ID . . . . . . . . . . . . . . . . . . 107 108
   5.  Application Protocol Fallbacks  . . . . . . . . . . . . . . . 110 111
     5.1.  Partial Implementation  . . . . . . . . . . . . . . . . . 110 111
       5.1.1.  Event-Related Fallbacks . . . . . . . . . . . . . . . 110 111
       5.1.2.  Free/Busy-Related Fallbacks . . . . . . . . . . . . . 112 113
       5.1.3.  To-Do-Related Fallbacks . . . . . . . . . . . . . . . 113 114
       5.1.4.  Journal-Related Fallbacks . . . . . . . . . . . . . . 115 116
     5.2.  Latency Issues  . . . . . . . . . . . . . . . . . . . . . 116 117
       5.2.1.  Cancellation of an Unknown Calendar Component.  . . . 116 117
       5.2.2.  Unexpected Reply from an Unknown Delegate . . . . . . 117 118
     5.3.  Sequence Number . . . . . . . . . . . . . . . . . . . . . 117 118
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . 117 118
     6.1.  Security Threats  . . . . . . . . . . . . . . . . . . . . 117 118
       6.1.1.  Spoofing the "Organizer"  . . . . . . . . . . . . . . 117 118
       6.1.2.  Spoofing the "Attendee" . . . . . . . . . . . . . . . 117 118
       6.1.3.  Unauthorized Replacement of the Organizer . . . . . . 118 119
       6.1.4.  Eavesdropping . . . . . . . . . . . . . . . . . . . . 118 119
       6.1.5.  Flooding a Calendar . . . . . . . . . . . . . . . . . 118 119
       6.1.6.  Unauthorized REFRESH Requests . . . . . . . . . . . . 118 119
     6.2.  Recommendations . . . . . . . . . . . . . . . . . . . . . 118 119
       6.2.1.  Use of [RFC1847] to secure iTIP transactions  . . . . 118 119
       6.2.2.  Implementation Controls . . . . . . . . . . . . . . . 119 120
   7.  IANA Consideration  . . . . . . . . . . . . . . . . . . . . . 119 120
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 120 121
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . 120 121
     8.2.  Informative References  . . . . . . . . . . . . . . . . . 120 121
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . . 120 121
   Appendix B.  Change History (to be removed prior to
                publication as an RFC) . . . . . . . . . . . . . . . 120 121
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 122 123
   Intellectual Property and Copyright Statements  . . . . . . . . . 123 124

1.  Introduction and Overview

   This document specifies how calendaring systems use iCalendar
   [I-D.ietf-calsify-rfc2445bis] objects to interoperate with other
   calendar systems.  In particular, it specifies how to schedule
   events, to-dos, or daily journal entries.  It further specifies how
   to search for available busy time information.  It does so in a
   general way without specifying how communication between different
   systems actually takes place.  Subsequent documents specify transport
   bindings between systems that use this protocol.

   This protocol is based on messages sent from an originator to one or
   more recipients.  For certain types of messages, a recipient may
   reply, in order to update their status and may also return
   transaction/request status information.  The protocol supports the
   ability for the message originator to modify or cancel the original
   message.  The protocol also supports the ability for recipients to
   suggest changes to the originator of a message.  The elements of the
   protocol also define the user roles for its transactions.

1.1.  Formatting Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Calendaring and scheduling roles are referred to in quoted-strings of
   text with the first character of each word in upper case.  For
   example, "Organizer" refers to a role of a "Calendar User" (CU)
   within the scheduling protocol.

   Calendar components defined by [I-D.ietf-calsify-rfc2445bis] are
   referred to with capitalized, quoted-strings of text.  All calendar
   components start with the letter "V".  For example, "VEVENT" refers
   to the event calendar component, "VTODO" refers to the to-do calendar
   component and "VJOURNAL" refers to the daily journal calendar
   component.

   Scheduling methods are referred to with capitalized, quoted-strings
   of text.  For example, "REQUEST" refers to the method for requesting
   a scheduling calendar component be created or modified, "REPLY"
   refers to the method a recipient of a request uses to update their
   status with the "Organizer" of the calendar component.

   Properties defined by [I-D.ietf-calsify-rfc2445bis] are referred to
   with capitalized, quoted-strings of text, followed by the word
   "property".  For example, "ATTENDEE" property refers to the iCalendar
   property used to convey the calendar address of a "Calendar User".

   Property parameters defined by this specification are referred to
   with capitalized, quoted-strings of text, followed by the word
   "parameter".  For example, "VALUE" parameter refers to the iCalendar
   property parameter used to override the default data type for a
   property value.

   Enumerated values defined by this specification are referred to with
   capitalized text, either alone or followed by the word "value".

   In tables, the quoted-string text is specified without quotes in
   order to minimize the table length.

1.2.  Related Documents

   Implementers will need to be familiar with several other
   specifications that, along with this one, describe the Internet
   calendaring and scheduling standards.  The related documents are:

      [I-D.ietf-calsify-rfc2445bis] - specifies the objects, data types,
      properties and property parameters used in the protocols, along
      with the methods for representing and encoding them.

      [I-D.ietf-calsify-rfc2447bis] specifies an Internet email binding
      for iTIP.

   This specification does not attempt to repeat the concepts or
   definitions from these other specifications.  Where possible,
   explicit references are made to the other specifications.

1.3.  Roles

   Exchanges of iCalendar objects for the purposes of group calendaring
   and scheduling occur between "Calendar Users" (CUs).  CUs take on one
   of two roles in iTIP:

   +-----------+-------------------------------------------------------+
   | Role      | Description                                           |
   +-----------+-------------------------------------------------------+
   | Organizer | The CU who initiates an exchange takes on the role of |
   |           | "Organizer".  For example, the CU who proposes a      |
   |           | group meeting is the "Organizer".                     |
   |           |                                                       |
   | Attendee  | The CUs asked to participate in the group meeting by  |
   |           | the "Organizer" take on the role of "Attendee".       |
   +-----------+-------------------------------------------------------+

   Note that "ROLE" is also a descriptive parameter to the iCalendar
   "ATTENDEE" property.  Its use is to convey descriptive context about
   an "Attendee" such as "chair", "required participant" or "non-
   required participant" and has nothing to do with the calendaring
   workflow.

1.4.  Methods

   The ITIP methods are listed below and their usage and semantics are
   defined in section 3 of this document.

   +----------------+--------------------------------------------------+
   | Method         | Description                                      |
   +----------------+--------------------------------------------------+
   | PUBLISH        | Used to publish a calendar entry to one or more  |
   |                | Calendar Users.  There is no interactivity       |
   |                | between the publisher and any other calendar     |
   |                | user.  An example might include a baseball team  |
   |                | publishing its schedule to the public.           |
   |                |                                                  |
   | REQUEST        | Used to schedule a calendar entry with other     |
   |                | Calendar Users.  Requests are interactive in     |
   |                | that they require the receiver to respond using  |
   |                | the Reply methods.  Meeting Requests, Busy Time  |
   |                | requests and the assignment of VTODOs to other   |
   |                | Calendar Users are all examples.  Requests are   |
   |                | also used by the "Organizer" to update the       |
   |                | status of a calendar entry.                      |
   |                |                                                  |
   | REPLY          | A Reply is used in response to a Request to      |
   |                | convey "Attendee" status to the "Organizer".     |
   |                | Replies are commonly used to respond to meeting  |
   |                | and task requests.                               |
   |                |                                                  |
   | ADD            | Add one or more new instances to an existing     |
   |                | VEVENT, VTODO, or VJOURNAL.                      |
   |                |                                                  |
   | CANCEL         | Cancel one or more instances of an existing      |
   |                | VEVENT, VTODO, or VJOURNAL.                      |
   |                |                                                  |
   | REFRESH        | The Refresh method is used by an "Attendee" to   |
   |                | request the latest version of a calendar entry.  |
   |                |                                                  |
   | COUNTER        | The Counter method is used by an "Attendee" to   |
   |                | negotiate a change in the calendar entry.        |
   |                | Examples include the request to change a         |
   |                | proposed Event time or change the due date for a |
   |                | VTODO.                                           |
   |                |                                                  |
   | DECLINECOUNTER | Used by the "Organizer" to decline the proposed  |
   |                | counter-proposal.                                |
   +----------------+--------------------------------------------------+

   Group scheduling in iTIP is accomplished using the set of "request"
   and "response" methods described above.  The following table shows
   the methods broken down by who can send them.

   +------------+------------------------------------------------------+
   | Originator | Methods                                              |
   +------------+------------------------------------------------------+
   | Organizer  | PUBLISH, REQUEST, ADD, CANCEL, DECLINECOUNTER        |
   |            |                                                      |
   | Attendee   | REPLY, REFRESH, COUNTER, REQUEST (only when          |
   |            | delegating)                                          |
   +------------+------------------------------------------------------+

   Note that for some calendar component types, the allowable methods
   are a subset of the above set.

2.  Interoperability Models

   There are two distinct protocols relevant to interoperability: an
   "Application Protocol" and a "Transport Protocol".  The Application
   Protocol defines the content of the iCalendar objects sent between
   sender and receiver to accomplish the scheduling transactions listed
   above.  The Transport Protocol defines how the iCalendar objects are
   sent between the sender and receiver.  This document focuses on the
   Application Protocol.  Binding documents such as
   [I-D.ietf-calsify-rfc2447bis] focus on the Transport Protocol.

   The connection between Sender and Receiver in the diagram below
   refers to the Application Protocol.  The iCalendar objects passed
   from the Sender to the Receiver are presented in Section 3,
   Application Protocol Elements.

           +----------+                +----------+
           |          |      iTIP      |          |
           |  Sender  |<-------------->| Receiver |
           |          |                |          |
           +----------+                +----------+

   There are several variations of this diagram in which the Sender and
   Receiver take on various roles of a "Calendar User Agent" (CUA) or a
   "Calendar Service" (CS).

   The architecture of iTIP is depicted in the diagram below.  An
   application written to this specification may work with bindings for
   the store-and-forward transport, the real time transport, or both.
   Also note that iTIP could be bound to other transports.

      +--------------------------------------------------------+
      |                     iTIP Protocol                      |
      +--------------------------------------------------------+
      |                       Transport                        |
      +  -  -  -  -  -  +  -  -  -  -  -  -  +  -  -  -  -  -  +
      | Real-time       | Store-and-Forward  | Others          |
      +-----------------+--------------------+-----------------+

2.1.  Application Protocol

   In the iTIP model, a calendar entry is created and managed by an
   "Organizer".  The "Organizer" interacts with other CUs by sending one
   or more of the iTIP messages listed above.  "Attendees" use the
   "REPLY" method to communicate their status.  "Attendees" do not make
   direct changes to the master calendar entry.  They can, however, use
   the "COUNTER" method to suggest changes to the "Organizer".  In any
   case, the "Organizer" has complete control over the master calendar
   entry.

2.1.1.  Calendar Entry State

   There are two distinct states relevant to calendar entries: the
   overall state of the entry and the state associated with an
   "Attendee" to that entry.

   The state of an entry is defined by the "STATUS" property and is
   controlled by the "Organizer."  There is no default value for the
   "STATUS" property.  The "Organizer" sets the "STATUS" property to the
   appropriate value for each calendar entry.

   The state of a particular "Attendee" relative to an entry is defined
   by the "PARTSTAT" parameter in the "ATTENDEE" property for each
   "Attendee".  When an "Organizer" issues the initial entry, "Attendee"
   status is unknown.  The "Organizer" specifies this by setting the
   "PARTSTAT" parameter to "NEEDS-ACTION".  Each "Attendee" modifies
   their "ATTENDEE" property "PARTSTAT" parameter to an appropriate
   value as part of a "REPLY" message sent back to the "Organizer".

2.1.2.  Delegation

   Delegation is defined as the process by which an "Attendee" grants
   another CU (or several CUs) the right to attend on their behalf.  The
   "Organizer" is made aware of this change because the delegating
   "Attendee" informs the "Organizer".  These steps are detailed in the
   REQUEST method section.

2.1.3.  Acting on Behalf of other Calendar Users

   In many organizations one user will act on behalf of another to
   organize and/or respond to meeting requests.  ITIP provides two
   mechanisms that support these activities.

   First, the "Organizer" is treated as a special entity, separate from
   "Attendees".  All responses from "Attendees" flow to the "Organizer",
   making it easy to separate a calendar user organizing a meeting from
   calendar users attending the meeting.  Additionally, iCalendar
   provides descriptive roles for each "Attendee".  For instance, a role
   of "chair" may be ascribed to one or more "Attendees".  The "chair"
   and the "Organizer" may or may not be the same calendar user.  This
   maps well to scenarios where an assistant may manage meeting
   logistics for another individual who chairs a meeting.

   Second, a "SENT-BY" parameter may be specified in either the
   "Organizer" or "Attendee" properties.  When specified, the "SENT-BY"
   parameter indicates that the responding CU acted on behalf of the
   specified "Attendee" or "Organizer".

2.1.4.  Component Revisions

   The "SEQUENCE" property is used by the "Organizer" to indicate
   revisions to the calendar component.  The rules for incrementing the
   "SEQUENCE" number are defined in [I-D.ietf-calsify-rfc2445bis].  For
   clarity, these rules are paraphrased here in terms of how they are
   applied in iTIP.  For a given "UID" in a calendar component:

   o  For the "PUBLISH" and "REQUEST" methods, the "SEQUENCE" property
      value is incremented according to the rules defined in
      [I-D.ietf-calsify-rfc2445bis].

   o  The "SEQUENCE" property value MUST be incremented each time the
      "Organizer" uses the "ADD" or "CANCEL" methods.

   o  The "SEQUENCE" property value MUST NOT be incremented when using
      "REPLY", "REFRESH", "COUNTER", "DECLINECOUNTER", or when sending a
      delegation "REQUEST".

   In some circumstances the "Organizer" may not have received responses
   to the final revision sent out.  In this situation, the "Organizer"
   may wish to send an update "REQUEST", and set "RSVP=TRUE" for all
   "Attendees", so that current responses can be collected.

   The value of the "SEQUENCE" property contained in a response from an
   "Attendee" may not always match the "Organizer's" revision.
   Implementations may choose to have the CUA indicate to the CU that
   the response is to an entry that has been revised and allow the CU to
   decide whether or not to accept the response.

   Whilst a change in SEQUENCE number is indicative of a significant
   change to a previously scheduled item, ATTENDEE CUAs SHOULD NOT rely
   solely on a change in SEQUENCE as a means of detecting a significant
   change.  Instead CUAs SHOULD compare the old and new versions of the
   calendar components and determine the exact nature of the changes and
   make decisions, possibly based on calendar user preferences, as to
   whether the user needs to be explicitly informed of the change.

2.1.5.  Message Sequencing

   CUAs that handle the iTIP application protocol must often correlate a
   component in a calendar store with a component received in the iTIP
   message.  For example, an event may be updated with a later revision
   of the same event.  To accomplish this, a CUA must correlate the
   version of the event already in its calendar store with the version
   sent in the iTIP message.  In addition to this correlation, there are
   several factors that can cause iTIP messages to arrive in an
   unexpected order.  That is, an "Organizer" could receive a reply to
   an earlier revision of a component AFTER receiving a reply to a later
   revision.

   To maximize interoperability and to handle messages that arrive in an
   unexpected order, use the following rules:

   1.  The primary key for referencing a particular iCalendar component
       is the "UID" property value.  To reference an instance of a
       recurring component, the primary key is composed of the "UID" and
       the "RECURRENCE-ID" properties.

   2.  The secondary key for referencing a component is the "SEQUENCE"
       property value.  For components where the "UID" is the same, the
       component with the highest numeric value for the "SEQUENCE"
       property obsoletes all other revisions of the component with
       lower values.

   3.  "Attendees" send "REPLY" messages to the "Organizer".  For
       replies where the "UID" property value is the same, the value of
       the "SEQUENCE" property indicates the revision of the component
       to which the "Attendee" is replying.  The reply with the highest
       numeric value for the "SEQUENCE" property obsoletes all other
       replies with lower values.

   4.  In situations where the "UID" and "SEQUENCE" properties match,
       the "DTSTAMP" property is used as the tie-breaker.  The component
       with the latest "DTSTAMP" overrides all others.  Similarly, for
       "Attendee" responses where the "UID" property values match and
       the "SEQUENCE" property values match, the response with the
       latest "DTSTAMP" overrides all others.

   Hence, CUAs must persist the following component properties: "UID",
   "RECURRENCE-ID", "SEQUENCE", and "DTSTAMP".  Furthermore, for each
   "ATTENDEE" property of a component CUAs must persist the "SEQUENCE"
   and "DTSTAMP" property values associated with the "Attendee's"
   response.

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
   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 a component or property should be present or not, 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, is 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 "X-PROPERTY" and "X-COMPONENT" to show where
   vendor-specific properties and components can appear.  The tables do
   not lay out the restrictions of property parameters.  Those
   restrictions are defined in [I-D.ietf-calsify-rfc2445bis].

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.

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

   +--------------------+----------+-----------------------------------+
   | 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+       | if present RRULE MUST NOT be      |
   |                    |          | present                           |
   |     RRULE          | 0+       | if present RDATE MUST NOT be      |
   |                    |          | present                           |
   |     TZNAME         | 0+       |                                   |
   |     TZOFFSETFROM   | 1        |                                   |
   |     TZOFFSETTO     | 1        |                                   |
   |     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+       | if present RDATE MUST NOT be      |
   |                    |          | present                           |
   |     TZNAME         | 0+       |                                   |
   |     TZOFFSETFROM   | 1        |                                   |
   |     TZOFFSETTO     | 1        |                                   |
   |     X-PROPERTY     | 0+       |                                   |
   |   TZID             | 1        |                                   |
   |   TZURL            | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   +--------------------+----------+-----------------------------------+

3.1.3.  VALARM

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

   +--------------------+----------+-----------------------------------+
   | 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        |                                   |
   |   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.

   +----------------+--------------------------------------------------+
   | 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 an   |
   |                | 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:

   +--------------------+----------+-----------------------------------+
   | 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+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of                     |
   |                    |          | TENTATIVE/CONFIRMED/CANCELLED     |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   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              |
   |                    |          |                                   |
   | 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, changing 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".

   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 reconfirm of the
   "VEVENT" calendar component.

   For the "REQUEST" method, multiple "VEVENT" components in a single
   iCalendar object are only permitted when 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:

   +--------------------+----------+-----------------------------------+
   | 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                           |
   |   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+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of TENTATIVE/CONFIRMED |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | 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.

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 no 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".

   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.

   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 value of the "ORGANIZER" property has been changed to
   the calendar address of 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"
   was 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 an event may invite another uninvited CU to
   the event.  The invited "Attendee" accomplishes this by forwarding
   the original "REQUEST" method to the uninvited CU.  The "Organizer"
   decides whether or not the uninvited CU is added to the attendee
   list.  If the "Organizer" decides not to add the uninvited CU no
   further action is required, however the "Organizer" MAY send the
   uninvited CU a "CANCEL" message.  If the "Organizer" decides to add
   an uninvited CU, a new "ATTENDEE" property is added for the uninvited
   CU with its property parameters set as the "Organizer" deems
   appropriate.  When forwarding a "REQUEST" to another CU, the
   forwarding "Attendee" MUST NOT make changes to the VEVENT property
   set.

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 may also be used to respond to an unsuccessful
   "REQUEST" method.  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" can 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.

   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:

   +--------------------+----------+-----------------------------------+
   | 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   | MUST 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   |                                   |
   |   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+       |                                   |
   |   STATUS           | 0 or 1   |                                   |
   |   SUMMARY          | 0 or 1   |                                   |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0 or 1   | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 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".

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

   +--------------------+----------+-----------------------------------+
   | 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                           |
   |   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+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of TENTATIVE/CONFIRMED |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   RECURRENCE-ID    | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | 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
   "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 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
       THISANDPRIOR (or THISANDFUTURE)
       THISANDFUTURE to indicate cancellation of the specified "VEVENT"
       calendar component and all instances before
       (or after); after; or

   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.

   When a "VEVENT" is cancelled, the "SEQUENCE" property value MUST be
   incremented.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "CANCEL"                  |
   |                    |          |                                   |
   | VEVENT             | 1+       | All must have the same UID        |
   |   ATTENDEE         | 0+       | MUST include all "Attendees"      |
   |                    |          | being removed the event.  MUST    |
   |                    |          | include 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   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | MUST be present if referring to   |
   |                    |          | one or more or more recurring     |
   |                    |          | instances.  Otherwise it MUST NOT |
   |                    |          | be present                        |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   STATUS           | 0 or 1   | MUST be set to CANCELLED.  If     |
   |                    |          | uninviting specific "Attendees"   |
   |                    |          | then MUST NOT be included.        |
   |   SUMMARY          | 0 or 1   |                                   |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 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:

   +--------------------+----------+-----------------------------------+
   | 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   | MUST only if referring to an      |
   |                    |          | instance of a recurring calendar  |
   |                    |          | component.  Otherwise it must NOT |
   |                    |          | be present.                       |
   |   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        |                                   |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 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 description.  The "Attendee" sends this
   message to the "Organizer" of the event.

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

   The "Organizer" rejects the counter proposal by sending the
   "Attendee" a VEVENT "DECLINECOUNTER" method.  The "Organizer" accepts
   the counter proposal by rescheduling the event as described in
   section 3.2.2.1 Rescheduling an Event.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "COUNTER"                 |
   |                    |          |                                   |
   | VEVENT             | 1        |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |
   |   ORGANIZER        | 1        | MUST be the "Organizer" of the    |
   |                    |          | original event                    |
   |   SEQUENCE         | 1        | MUST be present if value is       |
   |                    |          | greater than 0, MAY be present if |
   |                    |          | 0                                 |
   |   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   | MUST 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+       |                                   |
   |   STATUS           | 0 or 1   | Value must be one of              |
   |                    |          | CONFIRMED/TENATIVE/ CANCELLED     |
   |   TRANSP           | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | 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:

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "DECLINECOUNTER"          |
   |                    |          |                                   |
   | VEVENT             | 1        |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   UID              | 1        | MUST, same UID specified in       |
   |                    |          | original REQUEST and subsequent   |
   |                    |          | COUNTER                           |
   |   COMMENT          | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | MUST only if referring to an      |
   |                    |          | instance of a recurring calendar  |
   |                    |          | component.  Otherwise it MUST NOT |
   |                    |          | be present.                       |
   |   REQUEST-STATUS   | 0+       |                                   |
   |   SEQUENCE         | 0 or 1   | MUST be present if value is       |
   |                    |          | greater than 0, MAY be present if |
   |                    |          | 0                                 |
   |   X-PROPERTY       | 0+       |                                   |
   |   ATTACH           | 0        |                                   |
   |   ATTENDEE         | 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        |                                   |
   |   RESOURCES        | 0        |                                   |
   |   RRULE            | 0        |                                   |
   |   STATUS           | 0        |                                   |
   |   SUMMARY          | 0        |                                   |
   |   TRANSP           | 0        |                                   |
   |   URL              | 0        |                                   |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.3.  Methods For VFREEBUSY Components

   This section defines the property set for the methods that are
   applicable to the "VFREEBUSY" calendar component.  Each of the
   methods is defined using a restriction table.

   This document only addresses the transfer of busy time information.
   Applications desiring free time information MUST infer this from
   available busy time information.

   The busy time information within the iCalendar object MAY be grouped
   into more than one "VFREEBUSY" calendar component.  This capability
   allows busy time periods to be grouped according to some common
   periodicity, such as a calendar week, month, or year.  In this case,
   each "VFREEBUSY" calendar component MUST include the "ATTENDEE",
   "DTSTART" and "DTEND" properties in order to specify the source of
   the busy time information and the date and time interval over which
   the busy time information covers.

   The "FREEBUSY" property value MAY include a list of values, separated
   by the COMMA character (US-ASCII decimal 44).  Alternately, multiple
   busy time periods MAY be specified with multiple instances of the
   "FREEBUSY" property.  Both forms MUST be supported by implementations
   conforming to this document.  Duplicate busy time periods SHOULD NOT
   be specified in an iCalendar object.  However, two different busy
   time periods MAY overlap.

   "FREEBUSY" properties should be sorted such that their values are in
   ascending order, based on the start time, and then the end time, with
   the earliest periods first.  For example, today's busy time
   information should appear after yesterday's busy time information.
   And the busy time for this half-hour should appear after the busy
   time for earlier today.

   Since events may span a day boundary, free busy time period may also
   span a day boundary.  Individual "A" requests busy time from
   individuals "B", "C" and "D".  Individual "B" and "C" replies with
   busy time data to individual "A".  Individual "D" does not support
   busy time requests and does not reply with any data.  If the
   transport binding supports exception messages, then individual "D"
   returns an "unsupported capability" message to individual "A".

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

             +---------+-------------------------------------+
             | Method  | Description                         |
             +---------+-------------------------------------+
             | PUBLISH | Publish unsolicited busy time data. |
             |         |                                     |
             | REQUEST | Request busy time data.             |
             |         |                                     |
             | REPLY   | Reply to a busy time request.       |
             +---------+-------------------------------------+

3.3.1.  PUBLISH

   The "PUBLISH" method in a "VFREEBUSY" calendar component is used to
   publish busy time data.  The method may be sent from one CU to any
   other.  The purpose of the method is to provide a message for sending
   unsolicited busy time data.  That is, the busy time data is not being
   sent as a "REPLY" to the receipt of a "REQUEST" method.

   The "ATTENDEE" "ORGANIZER" property must be specified in the busy time
   information.  The value is the CU address of the originator of the
   busy time information.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "PUBLISH"                 |
   |                    |          |                                   |
   | VFREEBUSY          | 1+       |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        | DateTime values must be in UTC    |
   |   DTEND            | 1        | DateTime values must be in UTC    |
   |   FREEBUSY         | 0+       | MUST be BUSYTIME.  Multiple       |
   |                    |          | instances are allowed.  Multiple  |
   |                    |          | instances must be sorted in       |
   |                    |          | ascending order                   |
   |   ORGANIZER        | 1        | MUST contain the address of       |
   |                    |          | originator of busy time data.     |
   |   UID              | 1        |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   URL              | 0 or 1   | Specifies busy time URL           |
   |   ATTENDEE         | 0        |                                   |
   |   DURATION         | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.3.2.  REQUEST

   The "REQUEST" method in a "VFREEBUSY" calendar component is used to
   ask a "Calendar User" for their busy time information.  The request
   may be for a busy time information bounded by a specific date and
   time interval.

   This message only permits requests for busy time information.  The
   message is sent from a "Calendar User" requesting the busy time
   information to one or more intended recipients.

   If the originator of the "REQUEST" method is not authorized to make a
   busy time request on the recipient's calendar system, then an
   exception message SHOULD be returned in a "REPLY" method, but no busy
   time data need be returned.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "REQUEST"                 |
   |                    |          |                                   |
   | VFREEBUSY          | 1        |                                   |
   |   ATTENDEE         | 1+       | contain the address of the        |
   |                    |          | calendar store                    |
   |   DTEND            | 1        | DateTime values must be in UTC    |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        | DateTime values must be in UTC    |
   |   ORGANIZER        | 1        | MUST be the request originator's  |
   |                    |          | address                           |
   |   UID              | 1        |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   FREEBUSY         | 0        |                                   |
   |   DURATION         | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |   URL              | 0        |                                   |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.3.3.  REPLY

   The "REPLY" method in a "VFREEBUSY" calendar component is used to
   respond to a busy time request.  The method is sent by the recipient
   of a busy time request to the originator of the request.

   The "REPLY" method may also be used to respond to an unsuccessful
   "REQUEST" method.  Depending on the "REQUEST-STATUS" value, no busy
   time information may be returned.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "REPLY"                   |
   |                    |          |                                   |
   | VFREEBUSY          | 1        |                                   |
   |   ATTENDEE         | 1        | (address of recipient replying)   |
   |   DTSTAMP          | 1        |                                   |
   |   DTEND            | 1        | DateTime values must be in UTC    |
   |   DTSTART          | 1        | DateTime values must be in UTC    |
   |   FREEBUSY         | 0+       | (values MUST all be of the same   |
   |                    |          | data type.  Multiple instances    |
   |                    |          | are allowed.  Multiple instances  |
   |                    |          | MUST be sorted in ascending       |
   |                    |          | order.  Values MUST NOT overlap)  |
   |   ORGANIZER        | 1        | MUST be the request originator's  |
   |                    |          | address                           |
   |   UID              | 1        |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0 or 1   |                                   |
   |   REQUEST-STATUS   | 0+       |                                   |
   |   URL              | 0 or 1   | (specifies busy time URL)         |
   |   X-PROPERTY       | 0+       |                                   |
   |   DURATION         | 0        |                                   |
   |   SEQUENCE         | 0        |                                   |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.4.  Methods For VTODO Components

   This section defines the property set for the methods that are
   applicable to the "VTODO" calendar component.  Each of the methods is
   defined using a restriction table that specifies the property
   constraints that define the particular method.

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

   +----------------+--------------------------------------------------+
   | Method         | Description                                      |
   +----------------+--------------------------------------------------+
   | PUBLISH        | Post notification of a VTODO.  Used primarily as |
   |                | a method of advertising the existence of a       |
   |                | VTODO.                                           |
   |                |                                                  |
   | REQUEST        | Assign a VTODO.  This is an explicit assignment  |
   |                | to one or more Calendar Users.  The REQUEST      |
   |                | method is also used to update or change an       |
   |                | existing VTODO.  Clients that cannot handle      |
   |                | REQUEST MAY degrade the method to treat it as a  |
   |                | PUBLISH.                                         |
   |                |                                                  |
   | REPLY          | Reply to a VTODO request.  Attendees MAY set     |
   |                | PARTSTAT to ACCEPTED, DECLINED, TENTATIVE,       |
   |                | DELEGATED, PARTIAL, and COMPLETED.               |
   |                |                                                  |
   | ADD            | Add one or more instances to an existing to-do.  |
   |                |                                                  |
   | CANCEL         | Cancel one or more instances of an existing      |
   |                | to-do.                                           |
   |                |                                                  |
   | REFRESH        | A request sent to a VTODO Organizer asking for   |
   |                | the latest version of a VTODO.                   |
   |                |                                                  |
   | COUNTER        | Counter a REQUEST with an alternative proposal.  |
   |                |                                                  |
   | DECLINECOUNTER | Decline a counter proposal by an Attendee.       |
   +----------------+--------------------------------------------------+

3.4.1.  PUBLISH

   The "PUBLISH" method in a "VTODO" calendar component has no
   associated response.  It is simply a posting of an iCalendar object
   that maybe added to a calendar.  It MUST have an "Organizer".  It
   MUST NOT have "Attendees".  Its expected usage is for encapsulating
   an arbitrary "VTODO" calendar component 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 "VTODO" calendar component.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        |                                   |
   |                    |          | MUST be "PUBLISH"                 |
   | VTODO              | 1+       |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   PRIORITY         | 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+       |                                   |
   |   COMPLETED        | 0 or 1   |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   | Can be null                       |
   |   DUE              | 0 or 1   | If present DURATION MUST NOT be   |
   |                    |          | present                           |
   |   DURATION         | 0 or 1   | If present DUE MUST NOT be        |
   |                    |          | present                           |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PERCENT-COMPLETE | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | MUST only if referring to an      |
   |                    |          | instance of a recurring calendar  |
   |                    |          | component.  Otherwise it MUST NOT |
   |                    |          | be present.                       |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of COMPLETED/NEEDS     |
   |                    |          | ACTION/IN- PROCESS/CANCELLED      |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   ATTENDEE         | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.4.2.  REQUEST

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

   o  Assign a to-do to one or more "Calendar Users"
   o  Reschedule an existing to-do
   o  Update the details of an existing to-do, without rescheduling it
   o  Update the completion status of "Attendees" of an existing to-do,
      without rescheduling it
   o  Reconfirm an existing to-do, without rescheduling it
   o  Delegate/reassign an existing to-do to another "Calendar User"

   The assigned "Calendar Users" are identified in the "VTODO" calendar
   component by individual "ATTENDEE;ROLE=REQ-PARTICIPANT" property
   value sequences.

   The originator of a "REQUEST" is the "Organizer" of the to-do.  The
   recipient of a "REQUEST" is the "Calendar User" assigned the to-do.
   The "Attendee" uses the "REPLY" method to convey their acceptance and
   completion status to the "Organizer" of the "REQUEST".

   The "UID", "SEQUENCE", and "DTSTAMP" 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 to-do.  If the "UID"
   property value is found on the recipient's calendar, then the
   "REQUEST" is a rescheduling, an update, or a reconfirm of the "VTODO"
   calendar object.

   If the "Organizer" of the "REQUEST" method is not authorized to make
   a to-do request on the "Attendee's" calendar system, then an
   exception is returned in the "REQUEST-STATUS" property of a
   subsequent "REPLY" method, but no scheduling action is performed.

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

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "REQUEST"                 |
   |                    |          |                                   |
   | VTODO              | 1+       | All components must have the same |
   |                    |          | UID                               |
   |   ATTENDEE         | 1+       |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   PRIORITY         | 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+       |                                   |
   |   COMPLETED        | 0 or 1   |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   | Can be null                       |
   |   DUE              | 0 or 1   | If present DURATION MUST NOT be   |
   |                    |          | present                           |
   |   DURATION         | 0 or 1   | If present DUE MUST NOT be        |
   |                    |          | present                           |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PERCENT-COMPLETE | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | present if referring to an        |
   |                    |          | instance of a recurring calendar  |
   |                    |          | component.  Otherwise it MUST NOT |
   |                    |          | be present.                       |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of COMPLETED/NEEDS     |
   |                    |          | ACTION/IN- PROCESS                |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.4.2.1.  REQUEST for Rescheduling a VTODO

   The "REQUEST" method may be used to reschedule a "VTODO" calendar
   component.

   Rescheduling a "VTODO" calendar component involves a change to the
   existing "VTODO" calendar component in terms of its start or due time
   or recurrence intervals and possibly the description.  If the
   recipient CUA of a "REQUEST" method finds that the "UID" property
   value already exists on the calendar, but that the "SEQUENCE"
   property value in the "REQUEST" is greater than the value for the
   existing VTODO, then the "REQUEST" method describes a rescheduling of
   the "VTODO" calendar component.

3.4.2.2.  REQUEST for Update or Reconfirmation of a VTODO

   The "REQUEST" method may be used to update or reconfirm a "VTODO"
   calendar component.  Reconfirmation is merely an update of "Attendee"
   completion status or overall "VTODO" calendar component status.

   An update to an existing "VTODO" calendar component does not involve
   changes to the start or due time or recurrence intervals, nor
   generally to the description for the "VTODO" calendar component.  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 "VTODO"
   calendar component details, but not a rescheduling of the "VTODO"
   calendar component.

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

   Unsolicited "REQUEST" methods MAY be sent by the "Organizer" of a
   "VTODO" calendar component.  The unsolicited "REQUEST" methods are
   used to update the details of the "VTODO" (without rescheduling it or
   updating the completion status of "Attendees") or the "VTODO"
   calendar component itself (i.e., reconfirm the "VTODO").

3.4.2.3.  REQUEST for Delegating a VTODO

   The "REQUEST" method is also used to delegate or reassign ownership
   of a "VTODO" calendar component to another "Calendar User".  For
   example, it may be used to delegate an "Attendee's" role (i.e.
   "chair", or "participant") for a "VTODO" calendar component.  The
   "REQUEST" method is sent by one of the "Attendees" of an existing
   "VTODO" calendar component to some other individual.  An "Attendee"
   of a "VTODO" calendar component MUST NOT delegate to the "Organizer"
   of the event.

   For the purposes of this description, the "Attendee" delegating the
   "VTODO" calendar component is referred to as the "Delegator".  The
   "Attendee" receiving the delegation request is referred to as the
   "Delegate".

   The "Delegator" of a "VTODO" calendar component MUST forward the
   existing "REQUEST" method for a "VTODO" calendar component to the
   "Delegate".  The "VTODO" calendar component description MUST include
   the "Delegator's" up-to-date "VTODO" calendar component definition.
   The "REQUEST" method MUST also include an "ATTENDEE" property with
   the calendar address of the "Delegate".  The "Delegator" MUST also
   send a "REPLY" method back 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".  A response to the delegation
   "REQUEST" is sent from the "Delegate" to the "Organizer" and
   optionally, to the "Delegator".  The "REPLY" method from the
   "Delegate" SHOULD include the "ATTENDEE" property with their calendar
   address and the "DELEGATED-FROM" parameter with the value of the
   "Delegator's" calendar address.

   The delegation "REQUEST" method MUST assign a value for the "RSVP"
   property parameter associated with the "Delegator's" "Attendee"
   property to that of the "Delegate's" "ATTENDEE" property.  For
   example if the "Delegator's" "ATTENDEE" property specifies
   "RSVP=TRUE", then the "Delegate's" "ATTENDEE" property MUST specify
   "RSVP=TRUE".

3.4.2.4.  REQUEST Forwarded To An Uninvited Calendar User

   An "Attendee" assigned a "VTODO" calendar component may send the
   "VTODO" calendar component to another new CU, not previously
   associated with the "VTODO" calendar component.  The current
   "Attendee" assigned the "VTODO" 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 "VTODO" 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 to-do 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 to-do, the new "ATTENDEE" property is not added
   to the "VTODO" calendar component.  The "Organizer" MAY send the CU a
   "CANCEL" message to indicate that they will not be added to the to-
   do.  If the "Organizer" decides to add the new CU, the new "ATTENDEE"
   property is added to the "VTODO" calendar component.  Furthermore,
   the "Organizer" is free to change any "ATTENDEE" property parameter
   from the values supplied by the new CU to something the "Organizer"
   considers appropriate.

3.4.2.5.  REQUEST Updated Attendee Status

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

3.4.3.  REPLY

   The "REPLY" method in a "VTODO" calendar component is used to respond
   (e.g., accept or decline) to a request or to reply to a delegation
   request.  It is also used by an "Attendee" to update their completion
   status.  When used to provide a delegation response, the "Delegator"
   MUST include the calendar address of the "Delegate" in the
   "DELEGATED-TO" parameter of the "Delegator's" "ATTENDEE" property.
   The "Delegate" MUST include the calendar address of the "Delegator"
   on the "DELEGATED-FROM" parameter of the "Delegate's" "ATTENDEE"
   property.

   The "REPLY" method MAY also be used to respond to an unsuccessful
   "VTODO" calendar component "REQUEST" method.  Depending on the
   "REQUEST-STATUS" value, no scheduling action may have been performed.

   The "Organizer" of a "VTODO" calendar component MAY receive a "REPLY"
   method from a "Calendar User" not in the original "REQUEST".  For
   example, a "REPLY" method MAY be received from a "Delegate" of a
   "VTODO" calendar component.  In addition, the "REPLY" method MAY be
   received from an unknown "Calendar User", having been forwarded the
   "REQUEST" by an original "Attendee" of the "VTODO" calendar
   component.  This uninvited "Attendee" MAY be accepted, or the
   "Organizer" MAY cancel the "VTODO" calendar component for the
   uninvited "Attendee" by sending them a "CANCEL" method.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "REPLY"                   |
   |                    |          |                                   |
   | VTODO              | 1+       | All component MUST have the same  |
   |                    |          | UID                               |
   |   ATTENDEE         | 1+       |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   REQUEST-STATUS   | 1+       |                                   |
   |   UID              | 1        | MUST must be the address of the   |
   |                    |          | replying attendee                 |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   COMPLETED        | 0 or 1   |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   |                                   |
   |   DTSTART          | 0 or 1   |                                   |
   |   DUE              | 0 or 1   | If present DURATION MUST NOT be   |
   |                    |          | present                           |
   |   DURATION         | 0 or 1   | If present DUE MUST NOT be        |
   |                    |          | present                           |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PERCENT-COMPLETE | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | MUST only if referring to an      |
   |                    |          | instance of a Recurring calendar  |
   |                    |          | component.  Otherwise it MUST NOT |
   |                    |          | be present                        |
   |   SEQUENCE         | 0 or 1   | MUST be the sequence number of    |
   |                    |          | the original REQUEST if greater   |
   |                    |          | than 0.  MAY be present if 0.     |
   |   STATUS           | 0 or 1   |                                   |
   |   SUMMARY          | 0 or 1   | Can be null                       |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0 or 1   | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.4.4.  ADD

   The "ADD" method in a "VTODO" calendar component is used to add one
   or more instances to an existing to-do.

   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
   "VTODO".  If an "Attendee" implementation does not support the "ADD"
   method it should respond with a "REQUEST-STATUS" value of 5.3 and ask
   for a "REFRESH".

   The "SEQUENCE" property value is incremented as the sequence of to-
   dos has changed.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "ADD"                     |
   |                    |          |                                   |
   | VTODO              | 1        |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   PRIORITY         | 1        |                                   |
   |   SEQUENCE         | 1        | MUST be greater than 0            |
   |   SUMMARY          | 1        | Can be null                       |
   |   UID              | 1        | MUST match that of the original   |
   |                    |          | to-do                             |
   |   ATTACH           | 0+       |                                   |
   |   ATTENDEE         | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   COMPLETED        | 0 or 1   |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   | Can be null                       |
   |   DTSTART          | 0 or 1   |                                   |
   |   DUE              | 0 or 1   | If present DURATION MUST NOT be   |
   |                    |          | present                           |
   |   DURATION         | 0 or 1   | If present DUE MUST NOT be        |
   |                    |          | present                           |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PERCENT-COMPLETE | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of COMPLETED/NEEDS     |
   |                    |          | ACTION/IN- PROCESS                |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   RECURRENCE-ID    | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VJOURNAL           | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.4.5.  CANCEL

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

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

   a.  the "RECURRENCE-ID" property for an instance in the sequence MUST
       be specified with the "RANGE" property parameter value of
       THISANDPRIOR (or THISANDFUTURE)
       THISANDFUTURE to indicate cancellation of the specified "VTODO"
       calendar component and all instances before (or
       after) after

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

   When a "VTODO" is cancelled, the "SEQUENCE" property value MUST be
   incremented.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "CANCEL"                  |
   |                    |          |                                   |
   | VTODO              | 1        |                                   |
   |   ATTENDEE         | 0+       | include all "Attendees" being     |
   |                    |          | removed from the todo.  MUST      |
   |                    |          | include all "Attendees" if the    |
   |                    |          | entire todo is cancelled.         |
   |   UID              | 1        | MUST echo original UID            |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   SEQUENCE         | 1        |                                   |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   COMPLETED        | 0 or 1   |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   |                                   |
   |   DTSTART          | 0 or 1   |                                   |
   |   DUE              | 0 or 1   | If present DURATION MUST NOT be   |
   |                    |          | present                           |
   |   DURATION         | 0 or 1   | If present DUE MUST NOT be        |
   |                    |          | present                           |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PERCENT-COMPLETE | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | MUST only if referring to one or  |
   |                    |          | more instances of a recurring     |
   |                    |          | calendar component.  Otherwise it |
   |                    |          | MUST NOT be present.              |
   |   RELATED-TO       | 0+       |                                   |
   |   RESOURCES        | 0+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   STATUS           | 0 or 1   | If present it MUST be set to      |
   |                    |          | "CANCELLED".  MUST NOT be used if |
   |                    |          | purpose is to remove "ATTENDEES"  |
   |                    |          | rather than cancel the entire     |
   |                    |          | VTODO.                            |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0 or 1   | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.4.6.  REFRESH

   The "REFRESH" method in a "VTODO" calendar component is used by
   "Attendees" of an existing "VTODO" calendar component to request an
   updated description from the "Organizer" of the "VTODO" calendar
   component.  The "Organizer" of the "VTODO" calendar component MAY use
   this method to request an updated status from the "Attendees".  The
   "REFRESH" method MUST specify the "UID" property corresponding to the
   "VTODO" calendar component needing update.

   A refresh of a recurrence instance of a "VTODO" calendar component
   may be requested by specifying the "RECURRENCE-ID" property
   corresponding to the associated "VTODO" calendar component.  The
   "Organizer" responds with the latest description and rendition of the
   "VTODO" calendar component.  In most cases this will be a REQUEST
   unless the "VTODO" has been cancelled, in which case the ORGANIZER
   MUST send a "CANCEL".  This method is intended to facilitate machine
   processing of requests for updates to a "VTODO" calendar component.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "REFRESH"                 |
   |                    |          |                                   |
   | VTODO              | 1        |                                   |
   |   ATTENDEE         | 1        |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   UID              | 1        | MUST echo original UID            |
   |   RECURRENCE-ID    | 0 or 1   | MUST only if referring to an      |
   |                    |          | instance of a Recurring calendar  |
   |                    |          | component.  Otherwise it MUST NOT |
   |                    |          | be present                        |
   |   X-PROPERTY       | 0+       |                                   |
   |   ATTACH           | 0        |                                   |
   |   CATEGORIES       | 0        |                                   |
   |   CLASS            | 0        |                                   |
   |   COMMENT          | 0        |                                   |
   |   COMPLETED        | 0        |                                   |
   |   CONTACT          | 0        |                                   |
   |   CREATED          | 0        |                                   |
   |   DESCRIPTION      | 0        |                                   |
   |   DTSTART          | 0        |                                   |
   |   DUE              | 0        |                                   |
   |   DURATION         | 0        |                                   |
   |   EXDATE           | 0        |                                   |
   |   GEO              | 0        |                                   |
   |   LAST-MODIFIED    | 0        |                                   |
   |   LOCATION         | 0        |                                   |
   |   ORGANIZER        | 0        |                                   |
   |   PERCENT-COMPLETE | 0        |                                   |
   |   PRIORITY         | 0        |                                   |
   |   RDATE            | 0        |                                   |
   |   RELATED-TO       | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |   RESOURCES        | 0        |                                   |
   |   RRULE            | 0        |                                   |
   |   SEQUENCE         | 0        |                                   |
   |   STATUS           | 0        |                                   |
   |   URL              | 0        |                                   |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.4.7.  COUNTER

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

   The counter proposal is an iCalendar object consisting of a "VTODO"
   calendar component describing the complete description of the
   alternate "VTODO" calendar component.

   The "Organizer" rejects the counter proposal by sending the
   "Attendee" a "DECLINECOUNTER" method.  The "Organizer" accepts the
   counter proposal by sending all of the "Attendees" of the "VTODO"
   calendar component a "REQUEST" method rescheduling the "VTODO"
   calendar component.  In the latter case, the "Organizer" SHOULD reset
   the individual "RSVP" property parameter values to TRUE on each
   "ATTENDEE" property; in order to force a response by the "Attendees".

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "COUNTER"                 |
   |                    |          |                                   |
   | VTODO              | 1        |                                   |
   |   ATTENDEE         | 1+       |                                   |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   PRIORITY         | 1        |                                   |
   |   SUMMARY          | 1        | Can be null                       |
   |   UID              | 1        |                                   |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   COMPLETED        | 0 or 1   |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   | Can be null                       |
   |   DTSTART          | 0 or 1   |                                   |
   |   DUE              | 0 or 1   | If present DURATION MUST NOT be   |
   |                    |          | present                           |
   |   DURATION         | 0 or 1   | If present DUE MUST NOT be        |
   |                    |          | present                           |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PERCENT-COMPLETE | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | MUST only 3.5if 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   |                                   |
   |   SEQUENCE         | 0 or 1   | MUST echo the original SEQUENCE   |
   |                    |          | number.  MUST be present if       |
   |                    |          | non-zero.  MAY be present if      |
   |                    |          | zero.                             |
   |   STATUS           | 0 or 1   | MAY be one of COMPLETED/NEEDS     |
   |                    |          | ACTION/IN- PROCESS/CANCELLED      |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0 or 1   | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.4.8.  DECLINECOUNTER

   The "DECLINECOUNTER" method in a "VTODO" calendar component is used
   by an "Organizer" of "VTODO" calendar component to reject a counter
   proposal offered by one of the "Attendees".  The "Organizer" sends
   the 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:

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "DECLINECOUNTER"          |
   |                    |          |                                   |
   | VTODO              | 1        |                                   |
   |   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+       |                                   |
   |   COMPLETED        | 0 or 1   |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   |                                   |
   |   DTSTART          | 0 or 1   |                                   |
   |   DUE              | 0 or 1   | If present DURATION MUST NOT be   |
   |                    |          | present                           |
   |   DURATION         | 0 or 1   | If present DUE MUST NOT be        |
   |                    |          | present                           |
   |   EXDATE           | 0+       |                                   |
   |   GEO              | 0 or 1   |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   LOCATION         | 0 or 1   |                                   |
   |   PERCENT-COMPLETE | 0 or 1   |                                   |
   |   PRIORITY         | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | MUST 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+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of COMPLETED/NEEDS     |
   |                    |          | ACTION/IN- PROCESS                |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.5.  Methods For VJOURNAL Components

   This section defines the property set for the methods that are
   applicable to the "VJOURNAL" calendar component.

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

   +---------+---------------------------------------------------------+
   | Method  | Description                                             |
   +---------+---------------------------------------------------------+
   | PUBLISH | Post a journal entry.  Used primarily as a method of    |
   |         | advertising the existence of a journal entry.           |
   |         |                                                         |
   | ADD     | Add one or more instances to an existing journal entry. |
   |         |                                                         |
   | CANCEL  | Cancel one or more instances of an existing journal     |
   |         | entry.                                                  |
   +---------+---------------------------------------------------------+

3.5.1.  PUBLISH

   The "PUBLISH" method in a "VJOURNAL" calendar component has no
   associated response.  It is simply a posting of an iCalendar object
   that may be added to a calendar.  It MUST have an "Organizer".  It
   MUST NOT have "Attendees".  The expected usage is for encapsulating
   an arbitrary journal entry as an iCalendar object.  The "Organizer"
   MAY subsequently update (with another "PUBLISH" method) or cancel
   (with a "CANCEL" method) a previously published journal entry.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "PUBLISH"                 |
   |                    |          |                                   |
   | VJOURNAL           | 1+       |                                   |
   |   DESCRIPTION      | 1        | Can be null.                      |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   UID              | 1        |                                   |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   EXDATE           | 0+       |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RECURRENCE-ID    | 0 or 1   | MUST only if referring to an      |
   |                    |          | instance of a recurring calendar  |
   |                    |          | component.  Otherwise it MUST NOT |
   |                    |          | be present.                       |
   |   RELATED-TO       | 0+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   SEQUENCE         | 0 or 1   | MUST echo the original SEQUENCE   |
   |                    |          | number.  MUST be present if       |
   |                    |          | non-zero.  MAY be present if      |
   |                    |          | zero.                             |
   |   STATUS           | 0 or 1   | MAY be one of                     |
   |                    |          | DRAFT/FINAL/CANCELLED             |
   |   SUMMARY          | 0 or 1   | Can be null                       |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   ATTENDEE         | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.5.2.  ADD

   The "ADD" method in a "VJOURNAL" calendar component is used to add
   one or more instances to an existing "VJOURNAL" entry.  There is no
   response to the "Organizer".

   If the "UID" property value in the "ADD" is not found on the
   recipient's calendar, then the recipient MAY treat the "ADD" as a
   "PUBLISH".

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "ADD"                     |
   |                    |          |                                   |
   | VJOURNAL           | 1        |                                   |
   |   DESCRIPTION      | 1        | Can be null                       |
   |   DTSTAMP          | 1        |                                   |
   |   DTSTART          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   SEQUENCE         | 1        | MUST be greater than 0            |
   |   UID              | 1        | MUST match that of the original   |
   |                    |          | journal                           |
   |   ATTACH           | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   EXDATE           | 0+       |                                   |
   |   LAST-MODIFIED    | 0 or 1   |                                   |
   |   RDATE            | 0+       |                                   |
   |   RELATED-TO       | 0+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   STATUS           | 0 or 1   | MAY be one of                     |
   |                    |          | DRAFT/FINAL/CANCELLED             |
   |   SUMMARY          | 0 or 1   | Can be null                       |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   ATTENDEE         | 0        |                                   |
   |   RECURRENCE-ID    | 0        |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VALARM             | 0+       |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0 or 1   | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.5.3.  CANCEL

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

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

   a.  the "RECURRENCE-ID" property for an instance in the sequence MUST
       be specified with the "RANGE" property parameter value of
       THISANDPRIOR (or THISANDFUTURE)
       THISANDFUTURE to indicate cancellation of the specified "VTODO"
       calendar component and all instances before (or
       after) after

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

   When a "VJOURNAL" is cancelled, the "SEQUENCE" property value MUST be
   incremented.

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

   +--------------------+----------+-----------------------------------+
   | Component/Property | Presence | Comment                           |
   +--------------------+----------+-----------------------------------+
   | METHOD             | 1        | MUST be "CANCEL"                  |
   |                    |          |                                   |
   | VJOURNAL           | 1+       | All MUST have the same UID        |
   |   DTSTAMP          | 1        |                                   |
   |   ORGANIZER        | 1        |                                   |
   |   SEQUENCE         | 1        |                                   |
   |   UID              | 1        | MUST be the UID of the original   |
   |                    |          | REQUEST                           |
   |   ATTACH           | 0+       |                                   |
   |   ATTENDEE         | 0+       |                                   |
   |   CATEGORIES       | 0+       |                                   |
   |   CLASS            | 0 or 1   |                                   |
   |   COMMENT          | 0+       |                                   |
   |   CONTACT          | 0+       |                                   |
   |   CREATED          | 0 or 1   |                                   |
   |   DESCRIPTION      | 0 or 1   |                                   |
   |   DTSTART          | 0 or 1   |                                   |
   |   EXDATE           | 0+       |                                   |
   |   LAST-MODIFIED    | 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+       |                                   |
   |   RRULE            | 0+       |                                   |
   |   STATUS           | 0 or 1   | MAY be present, must be           |
   |                    |          | "CANCELLED" if present            |
   |   SUMMARY          | 0 or 1   |                                   |
   |   URL              | 0 or 1   |                                   |
   |   X-PROPERTY       | 0+       |                                   |
   |   REQUEST-STATUS   | 0        |                                   |
   |                    |          |                                   |
   | VTIMEZONE          | 0+       | MUST be present if any date/time  |
   |                    |          | refers to a timezone              |
   |                    |          |                                   |
   | X-COMPONENT        | 0+       |                                   |
   |                    |          |                                   |
   | VALARM             | 0        |                                   |
   |                    |          |                                   |
   | VEVENT             | 0        |                                   |
   |                    |          |                                   |
   | VFREEBUSY          | 0        |                                   |
   |                    |          |                                   |
   | VTODO              | 0        |                                   |
   +--------------------+----------+-----------------------------------+

3.6.  Status Replies

   The "REQUEST-STATUS" property may include is used to convey status information
   about a REPLY, COUNTER or DECLINE-COUNTER iTIP message.  The codes
   listed in the table below SHOULD be used.  Additional codes MAY be
   used provided they are defined in a Standards Track or Experimental
   RFC.

   This specification allows for multiple "REQUEST-STATUS" properties to
   be returned in iCalendar components in the appropriate iTIP messages.
   When multiple "REQUEST-STATUS" properties are present, the following
   restrictions apply:

   1.  Within any one component, the "top-level" numeric value of the
       "short return status code" MUST be the same for all REQUEST-
       STATUS properties. i.e. there cannot be a mixture of e.g., 2.xx
       and 5.xx codes within a single component.

   2.  Across all components in the iTIP message, the following applies:

       A.  If any one component would have a 3.xx code, then all
           components MUST have a code in that range or "REQUEST-STATUS"
           MUST NOT be present in the other components if a 3.xx code is
           not appropriate for those components.

       B.  If any one component would have a 5.xx code, then all
           components MUST have a code in that range or "REQUEST-STATUS"
           MUST NOT be present in the other components if a 5.xx code is
           not appropriate for those components.  This rule overrides
           (a) above.

       C.  2.xx and 4.xx codes can be used in different components
           provided that each component follows the restriction in (1)
           above.

   The following values: "REQUEST-STATUS" codes are defined:

   +----------+------------------------+-------------------------------+
   | Short    | Longer Return Status   | Offending Data                |
   | Return   | Description            |                               |
   | Status   |                        |                               |
   | Code     |                        |                               |
   +----------+------------------------+-------------------------------+
   | 2.0      | Success.               | None.                         |
   |          |                        |                               |
   | 2.1      | Success but fallback   | Property name and value MAY   |
   |          | taken on one or more   | be specified.                 |
   |          | property values.       |                               |
   |          |                        |                               |
   | 2.2      | Success, invalid       | Property name MAY be          |
   |          | property ignored.      | specified.                    |
   |          |                        |                               |
   | 2.3      | Success, invalid       | Property parameter name and   |
   |          | property parameter     | value MAY be specified.       |
   |          | ignored.               |                               |
   |          |                        |                               |
   | 2.4      | Success, unknown       | Non-standard property name    |
   |          | non-standard property  | MAY be specified.             |
   |          | ignored.               |                               |
   |          |                        |                               |
   | 2.5      | Success, unknown       | Property and non-standard     |
   |          | non-standard property  | value MAY be specified.       |
   |          | value ignored.         |                               |
   |          |                        |                               |
   | 2.6      | Success, invalid       | Calendar component sentinel   |
   |          | calendar component     | (e.g., BEGIN:ALARM) MAY be    |
   |          | ignored.               | specified.                    |
   |          |                        |                               |
   | 2.7      | Success, request       | Original and forwarded        |
   |          | forwarded to Calendar  | caluser addresses MAY be      |
   |          | User.                  | specified.                    |
   |          |                        |                               |
   | 2.8      | Success, repeating     | RRULE or RDATE property name  |
   |          | event ignored.         | and value MAY be specified.   |
   |          | Scheduled as a single  |                               |
   |          | component.             |                               |
   |          |                        |                               |
   | 2.9      | Success, truncated end | DTEND property value MAY be   |
   |          | date time to date      | specified.                    |
   |          | boundary.              |                               |
   |          |                        |                               |
   | 2.10     | Success, repeating     | RRULE or RDATE property name  |
   |          | VTODO ignored.         | and value MAY be specified.   |
   |          | Scheduled as a single  |                               |
   |          | VTODO.                 |                               |
   |          |                        |                               |
   | 2.11     | Success, unbounded     | RRULE property name and value |
   |          | RRULE clipped at some  | MAY be specified.  Number of  |
   |          | finite number of       | instances MAY also be         |
   |          | instances              | specified.                    |
   |          |                        |                               |
   | 3.0      | Invalid property name. | Property name MAY be          |
   |          |                        | specified.                    |
   |          |                        |                               |
   | 3.1      | Invalid property       | Property name and value MAY   |
   |          | value.                 | be specified.                 |
   |          |                        |                               |
   | 3.2      | Invalid property       | Property parameter name and   |
   |          | parameter.             | value MAY be specified.       |
   |          |                        |                               |
   | 3.3      | Invalid property       | Property parameter name and   |
   |          | parameter value.       | value MAY be specified.       |
   |          |                        |                               |
   | 3.4      | Invalid calendar       | Calendar component sentinel   |
   |          | component sequence.    | MAY be specified (e.g.,       |
   |          |                        | BEGIN:VTIMEZONE).             |
   |          |                        |                               |
   | 3.5      | Invalid date or time.  | Date/time value(s) MAY be     |
   |          |                        | specified.                    |
   |          |                        |                               |
   | 3.6      | Invalid rule.          | Rule value MAY be specified.  |
   |          |                        |                               |
   | 3.7      | Invalid Calendar User. | Attendee property value MAY   |
   |          |                        | be specified.                 |
   |          |                        |                               |
   | 3.8      | No authority.          | METHOD and Attendee property  |
   |          |                        | values MAY be specified.      |
   |          |                        |                               |
   | 3.9      | Unsupported version.   | VERSION property name and     |
   |          |                        | value MAY be specified.       |
   |          |                        |                               |
   | 3.10     | Request entity too     | None.                         |
   |          | large.                 |                               |
   |          |                        |                               |
   | 3.11     | Required component or  | Component or property name    |
   |          | property missing.      | MAY be specified.             |
   |          |                        |                               |
   | 3.12     | Unknown component or   | Component or property name    |
   |          | property found         | MAY be specified              |
   |          |                        |                               |
   | 3.13     | Unsupported component  | Component or property name    |
   |          | or property found      | MAY be specified              |
   |          |                        |                               |
   | 3.14     | Unsupported capability | Method or action MAY be       |
   |          |                        | specified                     |
   |          |                        |                               |
   | 4.0      | Event conflict.        | DTSTART and DTEND property    |
   |          | Date/time is busy.     | name and values MAY be        |
   |          |                        | specified.                    |
   |          |                        |                               |
   | 5.0      | Request MAY supported. | Method property value MAY be  |
   |          |                        | specified.                    |
   |          |                        |                               |
   | 5.1      | Service unavailable.   | ATTENDEE property value MAY   |
   |          |                        | be specified.                 |
   |          |                        |                               |
   | 5.2      | Invalid calendar       | ATTENDEE property value MAY   |
   |          | service.               | be specified.                 |
   |          |                        |                               |
   | 5.3      | No scheduling support  | ATTENDEE property value MAY   |
   |          | for user.              | be specified.                 |
   +----------+------------------------+-------------------------------+

3.7.  Implementation Considerations

3.7.1.  Working With Recurrence Instances

   iCalendar includes a recurrence grammar to represent recurring
   events.  The benefit of such a grammar is the ability to represent a
   number of events in a single object.  However, while this simplifies
   creation of a recurring event, meeting instances still need to be
   referenced.  For instance, an "Attendee" may decline the third
   instance of a recurring Friday event.  Similarly, the "Organizer" may
   change the time or location to a single instance of the recurring
   event.

   Since implementations may elect to store recurring events as either a
   single event object or a collection of discreet, related event
   objects, the protocol is designed so that each recurring instance may
   be both referenced and versioned.  Hence, implementations that choose
   to maintain per-instance properties (such as "ATTENDEE" property
   "PARTSTAT" parameter) may do so.  However, the protocol does not
   require per-instance recognition unless the instance itself must be
   renegotiated.

   The scenarios for recurrence instance referencing are listed below.
   For purposes of simplification a change to an event refers to a
   "trigger property."  That is, a property that has a substantive
   effect on the meeting itself such as start time, location, due date
   (for "VTODO" calendar component components) and possibly description.

   "Organizer" initiated actions:

   o  "Organizer" deletes or changes a single instance of a recurring
      event
   o  "Organizer" makes changes that affect all future instances
   o  "Organizer" makes changes that affect all previous instances
   o  "Organizer" deletes or modifies a previously changed instance

   "Attendee" initiated actions:

   o  "Attendee" changes status for a particular recurrence instance
   o  "Attendee" sends Event-Counter for a particular recurrence
      instance

   An instance of a recurring event is assigned a unique identification,
   "RECURRENCE-ID" property, when that instance is renegotiated.
   Negotiation may be necessary when a substantive change to the event
   or to-do has be made (such as changing the start time, end time, due
   date or location).  The "Organizer" can identify a specific
   recurrence instance using the "RECURRENCE-ID" property.  The property
   value is equal to the date/time of the instance.  If the "Organizer"
   wishes to change the "DTSTART", the original "DTSTART" value is used
   for "RECURRENCE-ID" property and the new "DTSTART" and "DTEND" values
   reflect the change.  Note that after the change has occurred, the
   "RECURRENCE-ID" has changed to the new "DTSTART" value.

3.7.2.  Attendee Property Considerations

   The "ORGANIZER" property is required on published events, to-dos, and
   journal entries for two reasons.  First, only the "Organizer" is
   allowed to update and redistribute an event or to-do component.  It
   follows that the "ORGANIZER" property MUST be present in the event,
   to-do, or journal entry component so that the CUA has a basis for
   authorizing an update.  Second, it is prudent to provide a point of
   contact for anyone who receives a published component in case of
   problems.

   There are valid [RFC2822] addresses that represent groups.  Sending
   email to such an address results in mail being sent to multiple
   recipients.  Such an address may be used as the value of an
   "ATTENDEE" property.  Thus, it is possible that the recipient of a
   "REQUEST" does not appear explicitly in the list.

   It is recommended that the general approach to finding a "Calendar
   User" in an attendee list be as follows:

   1.  Search for the "Calendar User" in the attendee list where
       "CUTYPE=INDIVIDUAL"

   2.  Failing (1) look for attendees where "CUTYPE=GROUP" or
       'CUTYPE=UNKNOWN".  The CUA then determines if the "Calendar User"
       is a member of one of these groups.  If so, the "REPLY" method
       sent to the "Organizer" MUST contain a new "ATTENDEE" property in
       which:

       *  the "TYPE" property parameter is set to INDIVIDUAL
       *  the "MEMBER" property parameter is set to the name of the
          group

   3.  Failing (2) the CUA MAY ignore or accept the request as the
       "Calendar User" wishes.

3.7.3.  X-Tokens

   To make iCalendar objects extensible, new property types MAY be
   inserted into components.  These properties are called X-Tokens as
   they are prefixed with "X-".  A client is not required to make sense
   of X-Tokens.  Clients are not required to save X-Tokens or use them
   in replies.

4.  Examples

4.1.  Published Event Examples

   In the calendaring and scheduling context, publication refers to the
   one way transfer of event information.  Consumers of published events
   simply incorporate the event into a calendar.  No reply is expected.
   Individual "A" publishes an event.  Individual "B" reads the event
   and incorporates it into their calendar.  Events are published in
   several ways including: embedding the event as an object in a web
   page, e- mailing the event to a distribution list, and posting the
   event to a newsgroup.

   The table below illustrates the sequence of events between the
   publisher and the consumers of a published event.

   +-----------------+-----------------------+-------------------------+
   | Action          | Organizer             | Receiver                |
   +-----------------+-----------------------+-------------------------+
   | Publish an      | "A" sends or posts a  | "B" reads a published   |
   | event           | PUBLISH message       | event                   |
   |                 |                       |                         |
   | Publish an      | "A" sends or posts a  | "B" reads the updated   |
   | updated event   | PUBLISH message       | event                   |
   |                 |                       |                         |
   | Cancel a        | "A" sends or posts a  | "B" reads the canceled  |
   | published event | CANCEL message        | event publication       |
   +-----------------+-----------------------+-------------------------+

4.1.1.  A Minimal Published Event

   The iCalendar object below describes a single event that begins on
   July 1, 1997 at 20:00 UTC.  This event contains the minimum set of
   properties for a "PUBLISH" for a "VEVENT" calendar component.

      BEGIN:VCALENDAR
      METHOD:PUBLISH
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      DTSTART:19970701T200000Z
      DTSTAMP:19970611T190000Z
      SUMMARY:ST. PAUL SAINTS -VS- DULUTH-SUPERIOR DUKES
      UID:0981234-1234234-23@example.com
      END:VEVENT
      END:VCALENDAR

4.1.2.  Changing A Published Event

   The iCalendar object below describes an update to the event described
   in 4.1.1, the time has been changed, an end time has been added, and
   the sequence number has been adjusted.

      BEGIN:VCALENDAR
      METHOD:PUBLISH
      VERSION:2.0
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      DTSTAMP:19970612T190000Z
      DTSTART:19970701T210000Z
      DTEND:19970701T230000Z
      SEQUENCE:1
      UID:0981234-1234234-23@example.com
      SUMMARY:ST. PAUL SAINTS -VS- DULUTH-SUPERIOR DUKES
      END:VEVENT
      END:VCALENDAR

   The "UID" property is used by the client to identify the event.  The
   "SEQUENCE" property indicates that this is a change to the event.
   The event with a matching UID and sequence number 0 is superseded by
   this event.

   The "SEQUENCE" property provides a reliable way to distinguish
   different versions of the same event.  Each time an event is
   published, its sequence number is incremented.  If a client receives
   an event with a sequence number 5 and finds it has the same event
   with sequence number 2, the event SHOULD be updated.  However, if the
   client received an event with sequence number 2 and finds it already
   has sequence number 5 of the same event, the event MUST NOT be
   updated.

4.1.3.  Canceling A Published Event

   The iCalendar object below cancels the event described in 4.1.1.
   This cancels the event with "SEQUENCE" property of 0, 1, and 2.

      BEGIN:VCALENDAR
      METHOD:CANCEL
      VERSION:2.0
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      COMMENT:DUKES forfeit the game
      SEQUENCE:2
      UID:0981234-1234234-23@example.com
      DTSTAMP:19970613T190000Z
      END:VEVENT
      END:VCALENDAR

4.1.4.  A Rich Published Event

   This example describes the same event as in 4.1.1, but in much
   greater detail.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:PUBLISH
      SCALE:GREGORIAN
      VERSION:2.0
      BEGIN:VTIMEZONE
      TZID:America-Chicago
      TZURL:http://zones.stds_r_us.net/tz/America-Chicago
      TZURL:http://example.com/tz/America-Chicago
      BEGIN:STANDARD
      DTSTART:19671029T020000
      RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
      TZOFFSETFROM:-0500
      TZOFFSETTO:-0600
      TZNAME:CST
      END:STANDARD
      BEGIN:DAYLIGHT
      DTSTART:19870405T020000
      RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4
      TZOFFSETFROM:-0600
      TZOFFSETTO:-0500
      TZNAME:CDT
      END:DAYLIGHT
      END:VTIMEZONE
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTACH:http://www.dukes.com/
      ATTACH:http://www.example.com/
      CATEGORIES:SPORTS EVENT,ENTERTAINMENT
      CLASS:PRIVATE
      DESCRIPTION:MIDWAY STADIUM\n
       Big time game. MUST see.\n
       Expected duration:2 hours\n
      DTEND;TZID=America-Chicago:19970701T180000
      DTSTART;TZID=America-Chicago:19970702T160000
      DTSTAMP:19970614T190000Z
      STATUS:CONFIRMED
      LOCATION;VALUE=URI:http://www.midwaystadium.com/
      LOCATION;VALUE=URI:http://stadium.example.com/
      PRIORITY:2
      RESOURCES:SCOREBOARD
      SEQUENCE:3
      SUMMARY:ST. PAUL SAINTS -VS- DULUTH-SUPERIOR DUKES
      UID:0981234-1234234-23@example.com
      RELATED-TO:0981234-1234234-14@example.com
      BEGIN:VALARM
      TRIGGER:-PT2H
      ACTION:DISPLAY
      DESCRIPTION:You should be leaving for the game now.
      END:VALARM
      BEGIN:VALARM
      TRIGGER:-PT30M
      ACTION:AUDIO
      END:VALARM
      END:VEVENT
      END:VCALENDAR

   The "RELATED-TO" field contains the "UID" property of a related
   calendar event.  The "SEQUENCE" property 3 indicates that this event
   supersedes versions 0, 1, and 2.

4.1.5.  Anniversaries or Events attached to entire days

   This example demonstrates the use of the "VALUE" parameter to tie a
   "VEVENT" to day rather than a specific time.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:PUBLISH
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      DTSTAMP:19970614T190000Z
      UID:0981234-1234234-23@example.com
      DTSTART;VALUE=DATE:19970714
      RRULE:FREQ=YEARLY;INTERVAL=1
      SUMMARY: Bastille Day
      END:VEVENT
      END:VCALENDAR

4.2.  Group Event Examples

   Group events are distinguished from published events in that they
   have "Attendees" and that there is interaction between the
   "Attendees" and the "Organizer" with respect to the event.
   Individual "A" requests a meeting between individuals "A", "B", "C"
   and "D".  Individual "B" confirms attendance to the meeting.
   Individual "C" declines attendance.  Individual "D" tentatively
   confirms attendance.  The following table illustrates the the message
   flow between these individuals.  A, the CU scheduling the meeting, is
   referenced as the "Organizer".

   +--------------+-----------------------+----------------------------+
   | Action       | "Organizer"           | Attendee                   |
   +--------------+-----------------------+----------------------------+
   | Initiate a   | "A" sends a REQUEST   |                            |
   | meeting      | message to "B", "C",  |                            |
   | request      | and "D"               |                            |
   |              |                       |                            |
   | Accept the   |                       | "B" sends a REPLY message  |
   | meeting      |                       | to "A" with its ATTENDEE   |
   | request      |                       | "PARTSTAT" parameter set   |
   |              |                       | to "ACCEPTED"              |
   |              |                       |                            |
   | Decline the  |                       | "C" sends a REPLY message  |
   | meeting      |                       | to "A" with its ATTENDEE   |
   | request      |                       | "PARTSTAT" parameter set   |
   |              |                       | to "DECLINED"              |
   |              |                       |                            |
   | Tentatively  |                       | "D" sends a REPLY message  |
   | accept the   |                       | to "A" with its ATTENDEE   |
   | meeting      |                       | "PARTSTAT" parameter set   |
   | request      |                       | to "tentative"             |
   |              |                       |                            |
   | Confirm      | "A" sends a REQUEST   |                            |
   | meeting      | message to "B" and    |                            |
   | status with  | "D" with updated      |                            |
   | attendees    | information.          |                            |
   +--------------+-----------------------+----------------------------+

4.2.1.  A Group Event Request

   A sample meeting request is sent from "A" to "B", "C", and "D".  "E"
   is also sent a copy of the request but is not expected to attend and
   need not reply.  "E" illustrates how CUAs might implement an "FYI"
   type feature.  Note the use of the "ROLE" parameter.  The default
   value for the "ROLE" parameter is "REQ-PARTICIPANT" and it need not
   be enumerated.  In this case we are using the value "NON-PARTICIPANT"
   to indicate "E" is a non-attending CU.  The parameter is not needed
   on other "Attendees" since "PARTICIPANT" is the default value.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED;CN=A:mailto:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL;CN=B:mailto:b@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL;CN=C:mailto:c@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL;CN=Hal:mailto:d@example.com
      ATTENDEE;RSVP=FALSE;CUTYPE=ROOM:conf_big@example.com
      ATTENDEE;ROLE=NON-PARTICIPANT;RSVP=FALSE:mailto:e@example.com
      DTSTAMP:19970611T190000Z
      DTSTART:19970701T200000Z
      DTEND:19970701T2000000Z
      SUMMARY:Conference
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

4.2.2.  Reply To A Group Event Request

   Attendee "B" accepts the meeting.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VEVENT
      ATTENDEE;PARTSTAT=ACCEPTED:mailto:b@example.com
      ORGANIZER:mailto:a@example.com
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      REQUEST-STATUS:2.0;Success
      DTSTAMP:19970612T190000Z
      END:VEVENT
      END:VCALENDAR

   "B" could have declined the meeting or indicated tentative acceptance
   by setting the "ATTENDEE" "PARTSTAT" parameter to "DECLINED" or
   "TENTATIVE", respectively.  Also, "REQUEST-STATUS" is not required in
   successful transactions.

4.2.3.  Update An Event

   The event is moved to a different time.  The combination of the "UID"
   property (unchanged) and the "SEQUENCE" (bumped to 1) properties
   indicate the update.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:b@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:c@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL;CN=Hal:mailto:d@example.com
      ATTENDEE;ROLE=NON-PARTICIPANT;RSVP=FALSE;
       CUTYPE=ROOM:mailto:conf@example.com
      ATTENDEE;ROLE=NON-PARTICIPANT;RSVP=FALSE:mailto:e@example.com
      DTSTART:19970701T180000Z
      DTEND:19970701T190000Z
      SUMMARY:Phone Conference
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:1
      DTSTAMP:19970613T190000Z
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

4.2.4.  Countering an Event Proposal

   "A" sends a "REQUEST" to "B" and "C".  "B" makes a counter-proposal
   to "A" to change the time and location.

   "A" sends the following "REQUEST":

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:b@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:c@example.com
      DTSTART:19970701T190000Z
      DTEND:19970701T200000Z
      SUMMARY:Discuss the Merits of the election results
      LOCATION:Green Conference Room
      UID:calsrv.example.com-873970198738777a@example.com
      SEQUENCE:0
      DTSTAMP:19970611T190000Z
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   "B" sends "COUNTER" to "A", requesting changes to time and place.
   "B" uses the "COMMENT" property to communicate a rationale for the
   change.  Note that the "SEQUENCE" property is NOT incremented on a
   "COUNTER".

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:COUNTER
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:b@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:c@example.com
      DTSTART:19970701T160000Z
      DTEND:19970701T190000Z
      DTSTAMP:19970612T190000Z
      SUMMARY:Discuss the Merits of the election results
      LOCATION:Green Conference Room
      COMMENT:This time works much better and I think the big conference
       room is too big
      UID:calsrv.example.com-873970198738777a@example.com
      SEQUENCE:0
      DTSTAMP:19970611T190000Z
      END:VEVENT
      END:VCALENDAR
   "A" accepts the changes from "B".  To accept a counter-proposal, the
   "Organizer" sends a new event "REQUEST" with an incremented sequence
   number.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:b@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:c@example.com
      DTSTAMP:19970613T190000Z
      DTSTART:19970701T160000Z
      DTEND:19970701T190000Z
      SUMMARY:Discuss the Merits of the election results - changed to
       meet B's schedule
      LOCATION:Green Conference Room
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:1
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   Instead, "A" rejects "B's" counter proposal

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:DECLINECOUNTER
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:b@example.com
      COMMENT:Sorry, I cannot change this meeting time
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      DTSTAMP:19970614T190000Z
      END:VEVENT
      END:VCALENDAR

4.2.5.  Delegating an Event

   When delegating an event request to another "Calendar User", the
   "Delegator" must both update the "Organizer" with a "REPLY" and send
   a request to the "Delegate".  There is currently no protocol
   limitation to delegation depth.  It is possible for the original
   delegate to delegate the meeting to someone else, and so on.  When a
   request is delegated from one CUA to another there are a number of
   responsibilities required of the "Delegator".  The "Delegator" MUST:

   o  Send a "REPLY" to the "Organizer" with the following updates:

   o  The "Delegator's" "ATTENDEE" property "PARTSTAT" parameter set to
      "DELEGATED" and the "DELEGATED-TO" parameter is set to the address
      of the "Delegate"

   o  Add an additional "ATTENDEE" property for the "Delegate" with the
      "DELEGATED-FROM" property parameter set to the "Delegator"

   o  Indicate whether they want to continue to receive updates when the
      "Organizer" sends out updated versions of the event.  Setting the
      "RSVP" property parameter to "TRUE" will cause the updates to be
      sent, setting it to "FALSE" causes no further updates to be sent.
      Note that in either case, if the "Delegate" declines the
      invitation the "Delegator" will be notified.

   o  The "Delegator" MUST also send a copy of the original "REQUEST"
      method to the "Delegate".

   It is not required that the "Delegate" include the "Delegator" in
   their "REPLY" method.  However, it is strongly advised since this
   will inform the "Delegator" whether the "Delegate" plans to attend
   the meeting.  [Editors note: How so?]  If the "Delegate" declines the
   meeting, the "Delegator" may elect to delegate the "REQUEST" to
   another CUA.  The process is the same.

   +-----------------+----------------+--------------------------------+
   | Action          | "Organizer"    | Attendee                       |
   +-----------------+----------------+--------------------------------+
   | Initiate a      | "A" sends a    |                                |
   | meeting request | REQUEST        |                                |
   |                 | message to "B" |                                |
   |                 | and "C"        |                                |
   |                 |                |                                |
   | Delegate: "C"   |                | "C" sends a REPLY to "A" with  |
   | delegates to    |                | the ATTENDEE "PARTSTAT"        |
   | "E"             |                | parameter set to "DELEGATED"   |
   |                 |                | and with a new "ATTENDEE"      |
   |                 |                | property for "E".  "E's"       |
   |                 |                | ATTENDEE "DELEGATED-FROM"      |
   |                 |                | parameter is set to "C".       |
   |                 |                | "C's" ATTENDEE "DELEGATED-TO"  |
   |                 |                | parameter is set to "E".  "C"  |
   |                 |                | sends REQUEST message to "E"   |
   |                 |                | with the original meeting      |
   |                 |                | request information.  The      |
   |                 |                | "PARTSTAT" property parameter  |
   |                 |                | for "C" is set to "DELEGATED"  |
   |                 |                | and the "DELEGATED-TO"         |
   |                 |                | parameter is set to the        |
   |                 |                | address of "E".  An "ATTENDEE" |
   |                 |                | property is added for "E" and  |
   |                 |                | the "DELEGATED-FROM" parameter |
   |                 |                | is set to the address of "C".  |
   |                 |                |                                |
   | Confirm meeting |                | "E" sends REPLY message to "A" |
   | attendance      |                | and optionally "C" with its    |
   |                 |                | "PARTSTAT" property parameter  |
   |                 |                | set to "ACCEPTED"              |
   |                 |                |                                |
   | Optional:       | "A" sends      |                                |
   | Redistribute    | REQUEST        |                                |
   | meeting to      | message to     |                                |
   | attendees       | "B", "C" and   |                                |
   |                 | "E".           |                                |
   +-----------------+----------------+--------------------------------+

   "C" responds to the "Organizer".

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;PARTSTAT=DELEGATED;DELEGATED-
       TO="mailto:e@example.com":mailto:c@example.com
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      REQUEST-STATUS:2.0;Success
      DTSTAMP:19970611T190000Z
      END:VEVENT
      END:VCALENDAR

   Attendee "C" delegates presence at the meeting to "E".

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;PARTSTAT=DELEGATED;DELEGATED-
       TO="mailto:e@example.com":mailto:c@example.com
      ATTENDEE;RSVP=TRUE;
       DELEGATED-FROM="mailto:c@example.com":mailto:e@example.com
      DTSTART:19970701T180000Z
      DTEND:19970701T200000Z
      SUMMARY:Phone Conference
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      STATUS:CONFIRMED
      DTSTAMP:19970611T190000Z
      END:VEVENT
      END:VCALENDAR

4.2.6.  Delegate Accepts the Meeting

   To accept a delegated meeting, the delegate, "E", sends the following
   message to "A" and "C":

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;PARTSTAT=ACCEPTED;DELEGATED-
       FROM="mailto:c@example.com":mailto:e@example.com
      ATTENDEE;PARTSTAT=DELEGATED;
       DELEGATED-TO="mailto:e@example.com":mailto:c@example.com
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      REQUEST-STATUS:2.0;Success
      DTSTAMP:19970614T190000Z
      END:VEVENT
      END:VCALENDAR

4.2.7.  Delegate Declines the Meeting

   In this example the "Delegate" declines the meeting request and sets
   the "ATTENDEE" property "PARTSTAT" parameter to "DECLINED".  The
   "Organizer" SHOULD resend the "REQUEST" to "C" with the "PARTSTAT"
   parameter of the "Delegate" set to "DECLINED".  This lets the
   "Delegator" know that the "Delegate" has declined and provides an
   opportunity to the "Delegator" to either accept the request or
   delegate it to another CU.

   Response from "E" to "A" and "C".  Note the use of the "COMMENT"
   property "E" uses to indicate why the delegation was declined.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;PARTSTAT=DELEGATED;
       DELEGATED-TO="mailto:e@example.com":mailto:c@example.com
      ATTENDEE;PARTSTAT=DECLINED;
       DELEGATED-FROM="mailto:c@example.com":mailto:e@example.com
      COMMENT:Sorry, I will be out of town at that time.
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      REQUEST-STATUS:2.0;Success
      DTSTAMP:19970614T190000Z
      END:VEVENT
      END:VCALENDAR

   "A" resends the "REQUEST" method to "C".  "A" may also wish to
   express the fact that the item was delegated in the "COMMENT"
   property.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;PARTSTAT=DECLINED;
       DELEGATED-FROM="mailto:c@example.com":mailto:e@example.com
      ATTENDEE;RSVP=TRUE:mailto:c@example.com
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      SUMMARY:Phone Conference
      DTSTART:19970701T180000Z
      DTEND:19970701T200000Z
      DTSTAMP:19970614T200000Z
      COMMENT:DELEGATE (ATTENDEE mailto:e@example.com) DECLINED YOUR
       INVITATION
      END:VEVENT
      END:VCALENDAR

4.2.8.  Forwarding an Event Request

   The protocol does not prevent an "Attendee" from "forwarding" an
   "VEVENT" calendar component to other "Calendar Users".  Forwarding
   differs from delegation in that the forwarded "Calendar User" (often
   referred to as a "Party Crasher") does not replace the forwarding
   "Calendar User".  Implementations are not required to add the "Party
   Crasher" to the "Attendee" list and hence there is no guarantee that
   a "Party Crasher" will receive additional updates to the Event.  The
   forwarding "Calendar User" SHOULD NOT add the "Party Crasher" to the
   attendee list.  The "Organizer" MAY add the forwarded "Calendar User"
   to the attendee list.

4.2.9.  Cancel A Group Event

   Individual "A" requests a meeting between individuals "A", "B", "C",
   and "D".  Individual "B" declines attendance to the meeting.
   Individual "A" decides to cancel the meeting.  The following table
   illustrates the sequence of messages that would be exchanged between
   these individuals.

   Messages related to a previously canceled event ("SEQUENCE" property
   value is less than the "SEQUENCE" property value of the "CANCEL"
   message) MUST be ignored.

   +-------------+--------------------+--------------------------------+
   | Action      | "Organizer"        | "Attendee"                     |
   +-------------+--------------------+--------------------------------+
   | Initiate a  | "A" sends a        |                                |
   | meeting     | REQUEST message to |                                |
   | request     | "B", "C", and "D"  |                                |
   |             |                    |                                |
   | Decline the |                    | "B" sends a "REPLY" message to |
   | meeting     |                    | "A" with its "PARTSTAT"        |
   | request     |                    | parameter set to "DECLINED".   |
   |             |                    |                                |
   | Cancel the  | "A" sends a CANCEL |                                |
   | meeting     | message to "B",    |                                |
   |             | "C" and "D"        |                                |
   +-------------+--------------------+--------------------------------+

   The example shows how "A" cancels the event.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:CANCEL
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;CUTYPE=INDIVIDUAL;mailto:a@example.com
      ATTENDEE;CUTYPE=INDIVIDUAL:mailto:b@example.com
      ATTENDEE;CUTYPE=INDIVIDUAL:mailto:c@example.com
      ATTENDEE;CUTYPE=INDIVIDUAL:mailto:d@example.com
      COMMENT:Mr. B cannot attend. It's raining. Lets cancel.
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:1
      STATUS:CANCELLED
      DTSTAMP:19970613T190000Z
      END:VEVENT
      END:VCALENDAR

4.2.10.  Removing Attendees

   "A" wants to remove "B" from a meeting.  This is done by sending a
   "CANCEL" to "B" and removing "B" from the attendee list in the master
   copy of the event.

   +---------------------+--------------------------------+------------+
   | Action              | "Organizer"                    | "Attendee" |
   +---------------------+--------------------------------+------------+
   | Remove an "B" as an | "A" sends a CANCEL message to  |            |
   | "Attendee"          | "B"                            |            |
   |                     |                                |            |
   | Update the master   | "A" sends the updated event to |            |
   | copy of the event   | the remaining "Attendees"      |            |
   +---------------------+--------------------------------+------------+

   The original meeting includes "A", "B", "C", and "D".  The example
   below shows the "CANCEL" that "A" sends to "B".  Note that in the
   example below the "STATUS" property is omitted.  This is used when
   the meeting itself is cancelled and not when the intent is to remove
   an "Attendee" from the Event.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:CANCEL
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      COMMENT:You're off the hook for this meeting
      UID:calsrv.example.com-873970198738777@example.com
      DTSTAMP:19970613T193000Z
      SEQUENCE:1
      END:VEVENT
      END:VCALENDAR

   The updated master copy of the event is shown below.  The "Organizer"
   MAY resend the updated event to the remaining "Attendees".  Note that
   "B" has been removed.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;CUTYPE=INDIVIDUAL:mailto:c@example.com
      ATTENDEE;CUTYPE=INDIVIDUAL:mailto:d@example.com
      ATTENDEE;CUTYPE=ROOM:mailto:cr_big@example.com
      ATTENDEE;ROLE=NON-PARTICIPANT;
       RSVP=FALSE:mailto:e@example.com
      DTSTAMP:19970611T190000Z
      DTSTART:19970701T200000Z
      DTEND:19970701T203000Z
      SUMMARY:Phone Conference
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:2
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

4.2.11.  Replacing the Organizer

   The scenario for this example begins with "A" as the "Organizer" for
   a recurring meeting with "B", "C", and "D".  "A" receives a new job
   offer in another country and drops out of touch.  "A" left no
   forwarding address or way to be reached.  Using out-of-band
   communication, the other "Attendees" eventually learn what has
   happened and reach an agreement that "B" should become the new
   "Organizer" for the meeting.  To do this, "B" sends out a new version
   of the event and the other "Attendees" agree to accept "B" as the new
   "Organizer".  "B" also removes "A" from the event.

   When the "Organizer" is replaced, the "SEQUENCE" property value MUST
   be incremented.

   This is the message "B" sends to "C" and "D"
      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:b@example.com
      ATTENDEE;ROLE=CHAIR;STATUS=ACCEPTED:mailto:b@example.com
      ATTENDEE;CUTYPE=INDIVIDUAL:mailto:c@example.com
      ATTENDEE;CUTYPE=INDIVIDUAL:mailto:d@example.com
      DTSTAMP:19970611T190000Z
      DTSTART:19970701T200000Z
      DTEND:19970701T203000Z
      RRULE:FREQ=WEEKLY
      SUMMARY:Phone Conference
      UID:123456@example.com
      SEQUENCE:1
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

4.3.  Busy Time Examples

   Busy time objects can be used in several ways.  First, a CU may
   request busy time from another CU for a specific range of time.  That
   request can be answered with a busy time Reply.  Additionally, a CU
   may simply publish their busy time for a given interval and point
   other CUs to the published location.  The following examples outline
   both scenarios.

   Individual "A" publishes busy time for one week.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      METHOD:PUBLISH
      BEGIN:VFREEBUSY
      DTSTAMP:19980101T124100Z
      ORGANIZER:mailto:a@example.com
      DTSTART:19980101T124200Z
      DTEND:19980107T124200Z
      FREEBUSY:19980101T180000Z/19980101T190000Z
      FREEBUSY:19980103T020000Z/19980103T050000Z
      FREEBUSY:19980107T020000Z/19980107T050000Z
      FREEBUSY:19980113T000000Z/19980113T010000Z
      FREEBUSY:19980115T190000Z/19980115T200000Z
      FREEBUSY:19980115T220000Z/19980115T230000Z
      FREEBUSY:19980116T013000Z/19980116T043000Z
      END:VFREEBUSY
      END:VCALENDAR

   Individual "A" requests busy time from individuals "B", "C".
   Individual "B" and "C" replies with busy time data to individual "A".
   The following table illustrates the sequence of messages that would
   be exchanged between these individuals.

   +----------------------+--------------------+-----------------------+
   | Action               | "Organizer"        | Attendee              |
   +----------------------+--------------------+-----------------------+
   | Initiate a busy time | "A" sends          |                       |
   | request              | "REQUEST" message  |                       |
   |                      | to "B" and "C"     |                       |
   |                      |                    |                       |
   | Reply to the "BUSY"  |                    | "B" sends a "REPLY"   |
   | request with "BUSY"  |                    | message to "A" with   |
   | time data            |                    | busy time data        |
   +----------------------+--------------------+-----------------------+

4.3.1.  Request Busy Time

   "A" sends a "BUSY-REQUEST" to "B" and "C" for busy time
      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VFREEBUSY
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      ATTENDEE:mailto:c@example.com
      DTSTAMP:19970613T190000Z
      DTSTART:19970701T080000Z
      DTEND:19970701T200000
      UID:calsrv.example.com-873970198738777@example.com
      END:VFREEBUSY
      END:VCALENDAR

4.3.2.  Reply To A Busy Time Request

   "B" sends a "REPLY" method type of a "VFREEBUSY" calendar component
   to "A"

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VFREEBUSY
      ORGANIZER:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      DTSTART:19970701T080000Z
      DTEND:19970701T200000Z
      UID:calsrv.example.com-873970198738777@example.com
      FREEBUSY:19970701T090000Z/PT1H,19970701T140000Z/PT30M
      DTSTAMP:19970613T190030Z
      END:VFREEBUSY
      END:VCALENDAR

   "B" is busy from 09:00 to 10:00 and from 14:00 to 14:30.

4.4.  Recurring Event and Time Zone Examples

4.4.1.  A Recurring Event Spanning Time Zones

   This event describes a weekly phone conference.  The "Attendees" are
   each in a different time zone.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VTIMEZONE
      TZID:America-SanJose
      TZURL:http://zones.stds_r_us.net/tz/America-SanJose
      TZURL:http://example.com/tz/America-SanJose
      BEGIN:STANDARD
      DTSTART:19671029T020000
      RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
      TZOFFSETFROM:-0700
      TZOFFSETTO:-0800
      TZNAME:PST
      END:STANDARD
      BEGIN:DAYLIGHT
      DTSTART:19870405T020000
      RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4
      TZOFFSETFROM:-0800
      TZOFFSETTO:-0700
      TZNAME:PDT
      END:DAYLIGHT
      END:VTIMEZONE
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED;
       CUTYPE=INDIVIDUAL:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:b@example.fr
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:c@example.jp
      DTSTAMP:19970613T190030Z
      DTSTART;TZID=America-SanJose:19970701T140000
      DTEND;TZID=America-SanJose:19970701T150000
      RRULE:FREQ=WEEKLY;INTERVAL=20;WKST=SU;BYDAY=TU
      RDATE;TZID=America-SanJose:19970910T140000
      EXDATE;TZID=America-SanJose:19970909T140000
      EXDATE;TZID=America-SanJose:19971028T140000
      SUMMARY:Weekly Phone Conference
      UID:calsrv.example.com-873970198738777@example.com
      SEQUENCE:0
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   The first two components of this iCalendar object are the time zone
   components.  The "DTSTART" date coincides with the first instance of
   the RRULE property.

   The recurring meeting is defined in a particular time zone,
   presumably that of the originator.  The client for each "Attendee"
   has the responsibility of determining the recurrence time in the
   "Attendee's" time zone.

   The repeating event starts on Tuesday, July 1, 1997 at 2:00pm PDT.
   "Attendee" b@example.fr is in France where the local time on this
   date is 9 hours ahead of PDT or 23:00.  "Attendee" c@example.jp is in
   Japan where local time is 8 hours ahead of UTC or Wednesday, July 2
   at 06:00.  The event repeats weekly on Tuesdays (in PST/PDT).  The
   "RRULE" property results in 20 instances.  The last instance falls on
   Tuesday, November 11, 1997 2:00pm PDT.  The "RDATE" property adds
   another instance: WED, 10-SEP-1997 2:00 PM PST.

   There are two exceptions to this recurring appointment.  The first
   one is:

   o  TUE, 09-SEP-1997 23:00 GMT
   o  TUE, 09-SEP-1997 14:00 PDT
   o  WED, 10-SEP-1997 06:00 JST

   and the second is:

   o  TUE, 28-OCT-1997 23:00 GMT
   o  TUE, 28-OCT-1997 14:00 PST
   o  WED, 29-OCT-1997 06:00 JST

4.4.2.  Modify A Recurring Instance

   In this example the "Organizer" issues a recurring meeting.  Later
   the "Organizer" changes an instance of the event by changing the
   "DTSTART" property.  Note the use of "RECURRENCE-ID" property and
   "SEQUENCE" property in the second request.

   Original Request:

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:guid-1@host1.com
      UID:guid-1@example.com
      SEQUENCE:0
      RRULE:FREQ=MONTHLY;BYMONTHDAY=1;UNTIL=19980901T210000Z
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      ATTENDEE:mailto:c@example.com
      ATTENDEE:mailto:d@example.com
      DESCRIPTION:IETF-C&S Conference Call
      CLASS:PUBLIC
      SUMMARY:IETF Calendaring Working Group Meeting
      DTSTART:19970601T210000Z
      DTEND:19970601T220000Z
      LOCATION:Conference Call
      DTSTAMP:19970526T083000Z
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   The event request below is to change the time of a specific instance.
   This changes the July 1st instance to July 3rd.

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:guid-1@host1com
      UID:guid-1@example.com
      RECURRENCE-ID:19970701T210000Z
      SEQUENCE:1
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      ATTENDEE:mailto:c@example.com
      ATTENDEE:mailto:d@example.com
      DESCRIPTION:IETF-C&S Conference Call
      CLASS:PUBLIC
      SUMMARY:IETF Calendaring Working Group Meeting
      DTSTART:19970703T210000Z
      DTEND:19970703T220000Z
      LOCATION:Conference Call
      DTSTAMP:19970626T093000Z
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

4.4.3.  Cancel an Instance

   In this example the "Organizer" of a recurring event deletes the
   August 1st instance.

      BEGIN:VCALENDAR
      METHOD:CANCEL
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:guid-1@host1.com
      UID:guid-1@example.com
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      ATTENDEE:mailto:c@example.com
      ATTENDEE:mailto:d@example.com
      RECURRENCE-ID:19970801T210000Z
      SEQUENCE:2
      STATUS:CANCELLED
      DTSTAMP:19970721T093000Z
      END:VEVENT
      END:VCALENDAR

4.4.4.  Cancel Recurring Event

   In this example the "Organizer" wishes to cancel the entire recurring
   event and any exceptions.

      BEGIN:VCALENDAR
      METHOD:CANCEL
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:guid-1@host1.com
      UID:guid-1@example.com
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      ATTENDEE:mailto:c@example.com
      ATTENDEE:mailto:d@example.com
      DTSTAMP:19970721T103000Z
      STATUS:CANCELLED
      SEQUENCE:3
      END:VEVENT
      END:VCALENDAR

4.4.5.  Change All Future Instances

   This example changes the meeting location from a conference call to
   Seattle starting September 1 and extends to all future instances.

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:guid-1@host1.com
      UID:guid-1@example.com
      RECURRENCE-ID;THISANDFUTURE:19970901T210000Z
      SEQUENCE:3
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      ATTENDEE;RSVP=TRUE:mailto:c@example.com
      ATTENDEE;RSVP=TRUE:mailto:d@example.com
      DESCRIPTION:IETF-C&S Discussion
      CLASS:PUBLIC
      SUMMARY:IETF Calendaring Working Group Meeting
      DTSTART:19970901T210000Z
      DTEND:19970901T220000Z
      LOCATION:Building 32, Microsoft, Seattle, WA
      DTSTAMP:19970526T083000Z
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

4.4.6.  Add A New Instance To A Recurring Event

   This example adds a one-time additional instance to the recurring
   event.  "Organizer" adds a second July meeting on the 15th.

      BEGIN:VCALENDAR
      METHOD:ADD
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:123456789@host1.com
      UID:123456789@example.com
      SEQUENCE:4
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      ATTENDEE;RSVP=TRUE:mailto:c@example.com
      ATTENDEE;RSVP=TRUE:mailto:d@example.com
      DESCRIPTION:IETF-C&S Conference Call
      CLASS:PUBLIC
      SUMMARY:IETF Calendaring Working Group Meeting
      DTSTART:19970715T210000Z
      DTEND:19970715T220000Z
      LOCATION:Conference Call
      DTSTAMP:19970629T093000Z
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

4.4.7.  Add A New Series of Instances To A Recurring Event

   The scenario for this example involves an ongoing meeting, originally
   set up to occur every Tuesday.  The "Organizer" later decides that
   the meetings need to be on Tuesdays and Thursdays, but does not want
   to reschedule the entire meeting or lose any of the per-instance
   information already collected.

   The original event:

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:123456789@host1.com
      UID:123456789@example.com
      SEQUENCE:0
      RRULE:WKST=SU;BYDAY=TU;FREQ=WEEKLY
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      SUMMARY:Review Accounts
      DTSTART:19980303T210000Z
      DTEND:19980303T220000Z
      LOCATION:The White Room
      DTSTAMP:19980301T093000Z
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   Assume that many other updates happen to this event and that a lot of
   instance-specific information exists in the recurring series.  The
   "SEQUENCE" property value is 7 for the next update.  Now the
   "Organizer" wants to add Thursdays to the event:

      BEGIN:VCALENDAR
      METHOD:ADD
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:123456789@host1.com
      UID:123456789@example.com
      SEQUENCE:7
      RRULE:WKST=SU;BYDAY=TH;FREQ=WEEKLY
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      SUMMARY:Review Accounts
      DTSTART:19980303T210000Z
      DTEND:19980303T220000Z
      DTSTAMP:19980303T193000Z
      LOCATION:The Usual conference room
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR
   Alternatively, if the "Organizer" is not concerned with per-instance
   updates, the entire event can be rescheduled using a "REQUEST".  This
   is done by using the "UID" of the event to reschedule and including
   the modified "RRULE".  Note, that since this is an entire
   rescheduling of the event, any instance-specific information will be
   lost.

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:123456789@host1.com
      UID:123456789@example.com
      SEQUENCE:7
      RRULE:WKST=SU;BYDAY=TU,TH;FREQ=WEEKLY
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      SUMMARY:Review Accounts
      DTSTART:19980303T210000Z
      DTEND:19980303T220000Z
      DTSTAMP:19980303T193000Z
      LOCATION:The White Room
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   The next series of examples illustrate how an "Organizer" would
   respond to a "REFRESH" submitted by an "Attendee" after a series of
   instance-specific modifications.  To convey all instance-specific
   changes, the "Organizer" must provide the latest event description
   and the relevant instances.  The first three examples show the
   history including the initial "VEVENT" request and subsequent
   instance changes and finally the "REFRESH".

   Original Request:

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:123456789@host1.com
      UID:123456789@example.com
      SEQUENCE:0
      RDATE:19980304T180000Z
      RDATE:19980311T180000Z
      RDATE:19980318T180000Z
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      SUMMARY:Review Accounts
      DTSTART:19980304T180000Z
      DTEND:19980304T200000Z
      DTSTAMP:19980303T193000Z
      LOCATION:Conference Room A
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   Organizer changes 2nd instance Location and Time:

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:123456789@host1.com
      UID:123456789@example.com
      SEQUENCE:1
      RECURRENCE-ID:19980311T180000Z
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      SUMMARY:Review Accounts
      DTSTART:19980311T160000Z
      DTEND:19980311T180000Z
      DTSTAMP:19980306T193000Z
      LOCATION:The Small conference room
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   Organizer adds a 4th instance of the meeting using the "ADD" method
      BEGIN:VCALENDAR
      METHOD:ADD
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:123456789@host1.com
      UID:123456789@example.com
      SEQUENCE:2
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      SUMMARY:Review Accounts
      DTSTART:19980315T180000Z
      DTEND:19980315T200000Z
      DTSTAMP:19980307T193000Z
      LOCATION:Conference Room A
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   If "B" requests a "REFRESH", "A" responds with the following to
   capture all instance-specific data.  In this case both the initial
   request and an additional "VEVENT" that specifies the instance-
   specific data are included.  Because these are both of the same type
   (they are both "VEVENTS"), they can be conveyed in the same iCalendar
   object.

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:123456789@host1.com
      UID:123456789@example.com
      SEQUENCE:2
      RDATE:19980304T180000Z
      RDATE:19980311T160000Z
      RDATE:19980315T180000Z
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      SUMMARY:Review Accounts
      DTSTART:19980304T180000Z
      DTEND:19980304T200000Z
      DTSTAMP:19980303T193000Z
      LOCATION:Conference Room A
      STATUS:CONFIRMED
      END:VEVENT
      BEGIN:VEVENT
      SEQUENCE:2
      RECURRENCE-ID:19980311T160000Z
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      SUMMARY:Review Accounts
      DTSTART:19980311T160000Z
      DTEND:19980304T180000Z
      DTSTAMP:19980306T193000Z
      LOCATION:The Small conference room
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

4.4.8.  Counter An Instance Of A Recurring Event

   In this example one of the "Attendees" counters the "DTSTART"
   property of the proposed second July meeting.

      BEGIN:VCALENDAR
      METHOD:COUNTER
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:guid-1@host1.com
      UID:guid-1@example.com
      RECURRENCE-ID:19970715T210000Z
      SEQUENCE:4
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;RSVP=TRUE:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      ATTENDEE;RSVP=TRUE:mailto:c@example.com
      ATTENDEE;RSVP=TRUE:mailto:d@example.com
      DESCRIPTION:IETF-C&S Conference Call
      CLASS:PUBLIC
      SUMMARY:IETF Calendaring Working Group Meeting
      DTSTART:19970715T220000Z
      DTEND:19970715T230000Z
      LOCATION:Conference Call
      COMMENT:May we bump this by an hour? I have a conflict
      DTSTAMP:19970629T094000Z
      END:VEVENT
      END:VCALENDAR

4.4.9.  Error Reply To A Request

   The following example illustrates a scenario where a meeting is
   proposed containing an unsupported property and a bad property.

   Original Request
      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:guid-1@host1.com
      UID:guid-1@example.com
      SEQUENCE:0
      RRULE:FREQ=MONTHLY;BYMONTHDAY=1
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      ATTENDEE;RSVP=TRUE:mailto:c@example.com
      ATTENDEE;RSVP=TRUE:mailto:d@example.com
      DESCRIPTION:IETF-C&S Conference Call
      CLASS:PUBLIC
      SUMMARY:IETF Calendaring Working Group Meeting
      DTSTART:19970601T210000Z
      DTEND:19970601T220000Z
      DTSTAMP:19970602T094000Z
      LOCATION:Conference Call
      STATUS:CONFIRMED
      FOO:BAR
      END:VEVENT
      END:VCALENDAR

   Response from "B" to indicate that RRULE is not supported and an
   unrecognized property was encountered

      BEGIN:VCALENDAR
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      REQUEST-STATUS:2.8;Repeating event ignored. Scheduled as a single
       event;RRULE
      REQUEST-STATUS:3.0;Invalid Property Name;FOO
      UID:guid-1@host1.com
      UID:guid-1@example.com
      SEQUENCE:0
      DTSTAMP:19970603T094000Z
      END:VEVENT
      END:VCALENDAR

4.5.  Group To-do Examples

   Individual "A" creates a group task in which individuals "A", "B",
   "C" and "D" will participate.  Individual "B" confirms acceptance of
   the task.  Individual "C" declines the task.  Individual "D"
   tentatively accepts the task.  The following table illustrates the
   sequence of messages that would be exchanged between these
   individuals.  Individual "A" then issues a "REQUEST" method to obtain
   the status of the to-do from each participant.  The response
   indicates the individual "Attendee's" completion status.  The table
   below illustrates the message flow.

   +--------------+------------------------+---------------------------+
   | Action       | "Organizer"            | Attendee                  |
   +--------------+------------------------+---------------------------+
   | Initiate a   | "A" sends a REQUEST    |                           |
   | to-do        | message to "B", "C",   |                           |
   | request      | and "D"                |                           |
   |              |                        |                           |
   | Accept the   |                        | "B" sends a "REPLY"       |
   | to-do        |                        | message to "A" with its   |
   | request      |                        | "PARTSTAT" parameter set  |
   |              |                        | to "ACCEPTED".            |
   |              |                        |                           |
   | Decline the  |                        | "C" sends a REPLY message |
   | to-do        |                        | to "A" with its           |
   | request      |                        | "PARTSTAT" parameter set  |
   |              |                        | to "DECLINED".            |
   |              |                        |                           |
   | Tentatively  |                        | "D" sends a REPLY message |
   | accept the   |                        | to "A" with its           |
   | to-do        |                        | "PARTSTAT" parameter set  |
   | request      |                        | to "TENTATIVE".           |
   |              |                        |                           |
   | Check        | "A" sends a REQUEST    |                           |
   | attendee     | message to "B" and "D" |                           |
   | completion   | with current           |                           |
   | status       | information.           |                           |
   |              |                        |                           |
   | Attendee     |                        | "B" sends a "REPLY"       |
   | indicates    |                        | message indicating        |
   | percent      |                        | percent complete          |
   | complete     |                        |                           |
   |              |                        |                           |
   | Attendee     |                        | "D" sends a "REPLY"       |
   | indicates    |                        | message indicating        |
   | completion   |                        | completion                |
   +--------------+------------------------+---------------------------+

4.5.1.  A VTODO Request

   A sample "REQUEST" for a "VTODO" calendar component that "A" sends to
   "B", "C", and "D".

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VTODO
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR:mailto:a@example.com
      ATTENDEE;RSVP=TRUE:mailto:b@example.com
      ATTENDEE;RSVP=TRUE:mailto:c@example.com
      ATTENDEE;RSVP=TRUE:mailto:d@example.com
      DTSTART:19970701T170000Z
      DUE:19970722T170000Z
      PRIORITY:1
      SUMMARY:Create the requirements document
      UID:calsrv.example.com-873970198738777-00@example.com
      SEQUENCE:0
      DTSTAMP:19970717T200000Z
      STATUS:Needs Action
      END:VTODO
      END:VCALENDAR

4.5.2.  A VTODO Reply

   "B" accepts the to-do.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VTODO
      ORGANIZER:mailto:a@example.com
      ATTENDEE;PARTSTAT=ACCEPTED:mailto:b@example.com
      UID:calsrv.example.com-873970198738777-00@example.com
      COMMENT:I'll send you my input by e-mail
      SEQUENCE:0
      DTSTAMP:19970717T203000Z
      REQUEST-STATUS:2.0;Success
      END:VTODO
      END:VCALENDAR
   "B" could have declined the TODO or indicated tentative acceptance by
   setting the "PARTSTAT" property parameter sequence to "DECLINED" or
   "TENTATIVE", respectively.

4.5.3.  A VTODO Request for Updated Status

   "A" requests status from all "Attendees".

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VTODO
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR:mailto:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:b@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:d@example.com
      UID:calsrv.example.com-873970198738777-00@example.com
      SUMMARY:Create the requirements document
      PRIORITY:1
      SEQUENCE:0
      STATUS:IN-PROCESS
      DTSTART:19970701T170000Z
      DTSTAMP:19970717T230000Z
      END:VTODO
      END:VCALENDAR

4.5.4.  A Reply: Percent-Complete

   A reply indicating the task being worked on and that "B" is 75%
   complete with "B's" part of the assignment.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VTODO
      ORGANIZER:mailto:a@example.com
      ATTENDEE;PARTSTAT=IN-PROCESS:mailto:b@example.com
      PERCENT-COMPLETE:75
      UID:calsrv.example.com-873970198738777-00@example.com
      DTSTAMP:19970717T233000Z
      SEQUENCE:0
      END:VTODO
      END:VCALENDAR

4.5.5.  A Reply: Completed

   A reply indicating that "D" completed "D's" part of the assignment.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VTODO
      ORGANIZER:mailto:a@example.com
      ATTENDEE;PARTSTAT=COMPLETED:mailto:d@example.com
      UID:calsrv.example.com-873970198738777-00@example.com
      DTSTAMP:19970717T233000Z
      SEQUENCE:0
      END:VTODO
      END:VCALENDAR

4.5.6.  An Updated VTODO Request

   Organizer "A" resends the "VTODO" calendar component.  "A" sets the
   overall completion for the to-do at 40%.

     BEGIN:VCALENDAR
     PRODID:-//ACME/DesktopCalendar//EN
     PRODID:-//Example/ExampleCalendarClient//EN
     METHOD:REQUEST
     VERSION:2.0
     BEGIN:VTODO
     ORGANIZER:mailto:a@example.com
     ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
     ATTENDEE;PARTSTAT=ACCEPTED;CUTYPE=INDIVIDUAL:mailto:b@example.com
     ATTENDEE;PARTSTAT=IN-PROCESS;CUTYPE=INDIVIDUAL:mailto:d@example.com
     DTSTART:19970701T170000Z
     DUE:19970722T170000Z
     PRIORITY:1
     SUMMARY:Create the requirements document
     UID:calsrv.example.com-873970198738777-00@example.com
     SEQUENCE:1
     DTSTAMP:19970718T100000Z
     STATUS:IN-PROGRESS
     PERCENT-COMPLETE:40
     END:VTODO
     END:VCALENDAR

4.5.7.  Recurring VTODOs

   The following examples relate to recurring "VTODO" calendar
   components.

4.5.7.1.  Request for a Recurring VTODO

   In this example "A" sends a recurring "VTODO" calendar component to
   "B" and "D".

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REQUEST
      VERSION:2.0
      BEGIN:VTODO
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR:mailto:a@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:b@example.com
      ATTENDEE;RSVP=TRUE;CUTYPE=INDIVIDUAL:mailto:d@example.com
      RRULE:FREQ=MONTHLY;COUNT=10;BYDAY=1FR
      DTSTART:19980101T100000-0700
      DUE:19980103T100000-0700
      SUMMARY:Send Status Reports to Area Managers
      UID:calsrv.example.com-873970198738777-00@example.com
      SEQUENCE:0
      DTSTAMP:19970717T200000Z
      STATUS:NEEDS ACTION
      PRIORITY:1
      END:VTODO
      END:VCALENDAR

4.5.7.2.  Calculating due dates in recurring VTODOs

   The due date in a recurring "VTODO" calendar component is either a
   fixed interval specified in the "REQUEST" method or specified using
   the "RECURRENCE-ID" property.  The former is calculated by applying
   the difference between "DTSTART" and "DUE" properties and applying it
   to each of the start of each recurring instance.  Hence, if the
   initial "VTODO" calendar component specifies a "DTSTART" property
   value of "19970701T190000Z" and a "DUE" property value of
   "19970801T190000Z" the interval of one day which is applied to each
   recurring instance of the "VTODO" calendar component to determine the
   "DUE" date of the instance.

4.5.7.3.  Replying to an instance of a recurring VTODO

   In this example "B" updates "A" on a single instance of the "VTODO"
   calendar component.

      BEGIN:VCALENDAR
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REPLY
      VERSION:2.0
      BEGIN:VTODO
      ATTENDEE;PARTSTAT=IN-PROCESS:mailto:b@example.com
      PERCENT-COMPLETE:75
      UID:calsrv.example.com-873970198738777-00@example.com
      DTSTAMP:19970717T233000Z
      RECURRENCE-ID:19980101T170000Z
      SEQUENCE:1
      END:VTODO
      END:VCALENDAR

4.6.  Journal Examples

   The iCalendar object below describes a single journal entry for
   October 2, 1997.  The "RELATED-TO" property references the phone
   conference event for which minutes were taken.

      BEGIN:VCALENDAR
      METHOD:PUBLISH
      PRODID:-//ACME/DesktopCalendar//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VJOURNAL
      DTSTART:19971002T200000Z
      ORGANIZER:mailto:a@example.com
      SUMMARY:Phone conference minutes
      DESCRIPTION:The editors meeting was held on October 1, 1997.
       Details are in the attached document.
      UID:0981234-1234234-2410@example.com
      RELATED-TO:0981234-1234234-2402-35@example.com
      ATTACH:ftp://ftp.example.com/pub/ed/minutes100197.txt
      END:VJOURNAL
      END:VCALENDAR

4.7.  Other Examples

4.7.1.  Event Refresh

   Refresh the event with "UID" property value of "guid-1-
   12345@host1.com":
   12345@example.com":

      BEGIN:VCALENDAR
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REFRESH
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      ATTENDEE:mailto:c@example.com
      ATTENDEE:mailto:d@example.com
      UID: guid-1-12345@host1.com guid-1-12345@example.com
      DTSTAMP:19970603T094000
      END:VEVENT
      END:VCALENDAR

4.7.2.  Bad RECURRENCE-ID

   Component instances are identified by the combination of "UID",
   "RECURRENCE-ID", and "SEQUENCE".  When an "Organizer" sends a request
   to an "Attendee", there are three cases in which an instance cannot
   be found.  They are:

   1.  The component with the referenced "UID" and "RECURRENCE-ID" has
       been found but the "SEQUENCE" number in the calendar store does
       not match that of the ITIP message.

   2.  The component with the referenced "UID" has been found, the
       "SEQUENCE" numbers match, but the "RECURRENCE-ID" cannot be
       found.

   3.  The "UID" and "SEQUENCE" numbers are found but the CUA does not
       support recurrences.

   In case (1), two things can happen.  If the "SEQUENCE" number of the
   "Attendee's" instance is larger than that in the "Organizer's"
   message then the "Attendee" is receiving an out-of-sequence message
   and MUST ignore it.  If the "SEQUENCE" number of the "Attendee's"
   instance is smaller, then the "Organizer" is sending out a newer
   version of the component and the "Attendee's" version needs to be
   updated.  Since one or more updates have been missed, the "Attendee"
   SHOULD send a "REFRESH" message to the "Organizer" to get an updated
   version of the event.

   In case (2), something has gone wrong.  Both the "Organizer" and the
   "Attendee" should have the same instances, but the "Attendee" does
   not have the referenced instance.  In this case the "Attendee" SHOULD
   send a "REFRESH" to the "Organizer" to get an updated version of the
   event.

   In case (3), the limitations of the "Attendee's" CUA makes it
   impossible to match an instance other than the single instance
   scheduled.  In this case, the "Attendee" need not send a "REFRESH" to
   the "Organizer".

   The example below shows a sequence in which an "Attendee" sends a
   "REFRESH" to the "Organizer".

   +-------------------------+--------------------+--------------------+
   | Action                  | "Organizer"        | Attendee           |
   +-------------------------+--------------------+--------------------+
   | Update an instance      | "A" sends          |                    |
   | request                 | "REQUEST" message  |                    |
   |                         | to "B"             |                    |
   |                         |                    |                    |
   | Attendee requests       |                    | "B" sends a        |
   | refresh because         |                    | "REFRESH" message  |
   | "RECURRENCE-ID" was not |                    | to "A"             |
   | found                   |                    |                    |
   |                         |                    |                    |
   | Refresh the entire      | "A" sends the      |                    |
   | Event                   | latest copy of the |                    |
   |                         | Event to "B"       |                    |
   |                         |                    |                    |
   | Attendee handles the    |                    | "B" updates to the |
   | request and updates the |                    | latest copy of the |
   | instance                |                    | meeting.           |
   +-------------------------+--------------------+--------------------+

   Request from "A":

      BEGIN:VCALENDAR
      METHOD:REQUEST
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      VERSION:2.0
      BEGIN:VEVENT
      UID:acme-12345@host1.com
      UID:example-12345@example.com
      SEQUENCE:3
      RRULE:FREQ=WEEKLY
      RDATE;VALUE=PERIOD:19970819T210000Z/199700819T220000Z
      ORGANIZER:mailto:a@example.com
      ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      DESCRIPTION:IETF-C&S Conference Call
      SUMMARY:IETF Calendaring Working Group Meeting
      DTSTART:19970801T210000Z
      DTEND:19970801T220000Z
      RECURRENCE-ID:19970809T210000Z
      DTSTAMP:19970726T083000
      STATUS:CONFIRMED
      END:VEVENT
      END:VCALENDAR

   "B" has the event with "UID" property "acme-12345@host1.com" "example-12345@example.com" but
   "B's" "SEQUENCE" property value is "1" and the event does not have an
   instance at the specified recurrence time.  This means that "B" has
   missed at least one update and needs a new copy of the event.  "B"
   requests the latest copy of the event with the following refresh
   message:

      BEGIN:VCALENDAR
      PRODID:-//RDU Software//NONSGML HandCal//EN
      PRODID:-//Example/ExampleCalendarClient//EN
      METHOD:REFRESH
      VERSION:2.0
      BEGIN:VEVENT
      ORGANIZER:mailto:a@example.com
      ATTENDEE:mailto:b@example.com
      UID:acme-12345@host1.com
      UID:example-12345@example.com
      DTSTAMP:19970603T094000
      END:VEVENT
      END:VCALENDAR

5.  Application Protocol Fallbacks

5.1.  Partial Implementation

   Applications that support this specification are not required to
   support the entire protocol.  The following describes how methods and
   properties SHOULD "fallback" in applications that do not support the
   complete protocol.  If a method or property is not addressed in this
   section, it may be ignored.

5.1.1.  Event-Related Fallbacks

   +----------------+--------------------------------------------------+
   | Method         | Fallback                                         |
   +----------------+--------------------------------------------------+
   | PUBLISH        | Required                                         |
   | REQUEST        | PUBLISH                                          |
   | REPLY          | Required                                         |
   | ADD            | Required                                         |
   | CANCEL         | Required                                         |
   | REFRESH        | Required                                         |
   | COUNTER        | Reply with Not Supported                         |
   | DECLINECOUNTER | Required if EVENT-COUNTER is implemented;        |
   |                | otherwise reply with Not Supported               |
   +----------------+--------------------------------------------------+

   +-----------------+-------------------------------------------------+
   | iCalendar       | Fallback                                        |
   | Property        |                                                 |
   +-----------------+-------------------------------------------------+
   | CALSCALE        | Ignore; assume GREGORIAN                        |
   | PRODID          | Ignore                                          |
   | METHOD          | Required as described in the Method list above  |
   | VERSION         | Ignore                                          |
   +-----------------+-------------------------------------------------+

   +-----------------+-------------------------------------------------+
   | Event-Related   | Fallback                                        |
   | Components      |                                                 |
   +-----------------+-------------------------------------------------+
   | VALARM          | Reply with Not Supported                        |
   | VTIMEZONE       | Required if any DateTime value refers to a time |
   |                 | zone.                                           |
   +-----------------+-------------------------------------------------+
   +-----------------+-------------------------------------------------+
   | Component       | Fallback                                        |
   | Property        |                                                 |
   +-----------------+-------------------------------------------------+
   | ATTACH          | Ignore                                          |
   | ATTENDEE        | Required if EVENT-REQUEST is not implemented;   |
   |                 | otherwise reply with Not Supported              |
   | CATEGORIES      | Ignore                                          |
   | CLASS           | Ignore                                          |
   | COMMENT         | Ignore                                          |
   | COMPLETED       | Ignore                                          |
   | CONTACT         | Ignore                                          |
   | CREATED         | Ignore                                          |
   | DESCRIPTION     | Required                                        |
   | DURATION        | Reply with Not Supported                        |
   | DTSTAMP         | Required                                        |
   | DTSTART         | Required                                        |
   | DTEND           | Required                                        |
   | EXDATE          | Ignore                                          |
   | GEO             | Ignore                                          |
   | LAST-MODIFIED   | Ignore                                          |
   | LOCATION        | Required                                        |
   | ORGANIZER       | Ignore                                          |
   | PRIORITY        | Ignore                                          |
   | RELATED-TO      | Ignore                                          |
   | RDATE           | Ignore                                          |
   | RRULE           | Ignore.  The first instance occurs on the       |
   |                 | DTStart property.  If implemented, VTIMEZONE    |
   |                 | MUST also be implemented.                       |
   | RECURRENCE-ID   | Required if RRULE is implemented; otherwise     |
   |                 | Ignore                                          |
   | REQUEST-STATUS  | Required                                        |
   | RESOURCES       | Ignore                                          |
   | SEQUENCE        | Required                                        |
   | STATUS          | Ignore                                          |
   | SUMMARY         | Ignore                                          |
   | TRANSP          | Required if FREEBUSY is implemented; otherwise  |
   |                 | Ignore                                          |
   | URL             | Ignore                                          |
   | UID             | Required                                        |
   | X-              | Ignore                                          |
   +-----------------+-------------------------------------------------+

5.1.2.  Free/Busy-Related Fallbacks

   +---------+---------------------------------------------------------+
   | Method  | Fallback                                                |
   +---------+---------------------------------------------------------+
   | PUBLISH | Implementations MAY ignore the METHOD type.  The        |
   |         | REQUEST-STATUS "3.14;Unsupported capability" MUST be    |
   |         | returned.                                               |
   | REQUEST | Implementations MAY ignore the METHOD type.  The        |
   |         | REQUEST-STATUS "3.14;Unsupported capability" MUST be    |
   |         | returned.                                               |
   | REPLY   | Implementations MAY ignore the METHOD type.  The        |
   |         | REQUEST-STATUS "3.14;Unsupported capability" MUST be    |
   |         | returned.                                               |
   +---------+---------------------------------------------------------+

   +-----------------+-------------------------------------------------+
   | iCalendar       | Fallback                                        |
   | Property        |                                                 |
   +-----------------+-------------------------------------------------+
   | CALSCALE        | Ignore; assume GREGORIAN.                       |
   | PRODID          | Ignore                                          |
   | METHOD          | Required as described in the Method list above. |
   | VERSION         | Ignore                                          |
   +-----------------+-------------------------------------------------+

   +-----------------+-------------------------------------------------+
   | Component       | Fallback                                        |
   | Property        |                                                 |
   +-----------------+-------------------------------------------------+
   | COMMENT         | Ignore                                          |
   | CONTACT         | Ignore                                          |
   | DTEND           | Required                                        |
   | DTSTAMP         | Required                                        |
   | DTSTART         | Required                                        |
   | DURATION        | Required                                        |
   | FREEBUSY        | Required                                        |
   | ORGANIZER       | Ignore                                          |
   | REQUEST-STATUS  | Ignore                                          |
   | UID             | Required                                        |
   | URL             | Ignore                                          |
   | X-              | Ignore                                          |
   +-----------------+-------------------------------------------------+

5.1.3.  To-Do-Related Fallbacks

   +----------------+--------------------------------------------------+
   | Method         | Fallback                                         |
   +----------------+--------------------------------------------------+
   | PUBLISH        | Required                                         |
   | REQUEST        | PUBLISH                                          |
   | REPLY          | Required                                         |
   | ADD            | Required                                         |
   | CANCEL         | Required                                         |
   | REFRESH        | Required                                         |
   | COUNTER        | Reply with Not Supported                         |
   | DECLINECOUNTER | Required if VTODO - COUNTER is implemented;      |
   |                | otherwise reply with Not Supported               |
   +----------------+--------------------------------------------------+

   +-----------------+-------------------------------------------------+
   | iCalendar       | Fallback                                        |
   | Property        |                                                 |
   +-----------------+-------------------------------------------------+
   | CALSCALE        | Ignore; assume GREGORIAN.                       |
   | PRODID          | Ignore                                          |
   | METHOD          | Required as described in the Method list above. |
   | VERSION         | Ignore                                          |
   +-----------------+-------------------------------------------------+

   +-----------------+-------------------------------------------------+
   | To-Do-Related   | Fallback                                        |
   | Components      |                                                 |
   +-----------------+-------------------------------------------------+
   | VALARM          | Reply with Not Supported                        |
   | VTIMEZONE       | Required if any DateTime value refers to a time |
   |                 | zone.                                           |
   +-----------------+-------------------------------------------------+
   +------------------+------------------------------------------------+
   | Component        | Fallback                                       |
   | Property         |                                                |
   +------------------+------------------------------------------------+
   | ATTACH           | Ignore                                         |
   | ATTENDEE         | Required if REQUEST is not implemented;        |
   |                  | otherwise ignore                               |
   | CATEGORIES       | Ignore                                         |
   | CLASS            | Ignore                                         |
   | COMMENT          | Ignore                                         |
   | COMPLETED        | Required                                       |
   | CONTACT          | Ignore                                         |
   | CREATED          | Ignore                                         |
   | DESCRIPTION      | Required                                       |
   | DUE              | Required                                       |
   | DURATION         | Ignore Reply with Not Supported                |
   | DTSTAMP          | Required                                       |
   | DTSTART          | Required                                       |
   | EXDATE           | Ignore Reply with Not Supported                |
   | LAST-MODIFIED    | Ignore                                         |
   | LOCATION         | Ignore                                         |
   | ORGANIZER        | Ignore                                         |
   | PERCENT-COMPLETE | Ignore                                         |
   | PRIORITY         | Required                                       |
   | RECURRENCE-ID    | Ignore                                         |
   | RELATED-TO       | Ignore                                         |
   | REQUEST-STATUS   | Ignore                                         |
   | RDATE            | Ignore                                         |
   | RRULE            | Ignore.  The first instance occurs on the      |
   |                  | DTSTART property.  If implemented, VTIMEZONE   |
   |                  | MUST also be implemented.                      |
   | RESOURCES        | Ignore                                         |
   | SEQUENCE         | Required                                       |
   | STATUS           | Required                                       |
   | SUMMARY          | Ignore                                         |
   | URL              | Ignore                                         |
   | UID              | Required                                       |
   | X-               | Ignore                                         |
   +------------------+------------------------------------------------+

5.1.4.  Journal-Related Fallbacks

   +---------+---------------------------------------------------------+
   | Method  | Fallback                                                |
   +---------+---------------------------------------------------------+
   | PUBLISH | Implementations MAY ignore the METHOD type.  The        |
   |         | REQUEST-STATUS "3.14;Unsupported capability" MUST be    |
   |         | returned.                                               |
   | ADD     | Implementations MAY ignore the METHOD type.  The        |
   |         | REQUEST-STATUS "3.14;Unsupported capability" MUST be    |
   |         | returned.                                               |
   | CANCEL  | Implementations MAY ignore the METHOD type.  The        |
   |         | REQUEST-STATUS "3.14;Unsupported capability" MUST be    |
   |         | returned.                                               |
   +---------+---------------------------------------------------------+

   +-----------------+-------------------------------------------------+
   | iCalendar       | Fallback                                        |
   | Property        |                                                 |
   +-----------------+-------------------------------------------------+
   | CALSCALE        | Ignore; assume GREGORIAN.                       |
   | PRODID          | Ignore                                          |
   | METHOD          | Required as described in the Method list above. |
   | VERSION         | Ignore                                          |
   +-----------------+-------------------------------------------------+

   +-----------------+-------------------------------------------------+
   | Journal-Related | Fallback                                        |
   | Components      |                                                 |
   +-----------------+-------------------------------------------------+
   | VTIMEZONE       | Required if any DateTime value refers to a time |
   |                 | zone                                            |
   +-----------------+-------------------------------------------------+
   +-----------------+-------------------------------------------------+
   | Component       | Fallback                                        |
   | Property        |                                                 |
   +-----------------+-------------------------------------------------+
   | ATTACH          | Ignore                                          |
   | ATTENDEE        | Required if JOURNAL-REQUEST is implemented;     |
   |                 | otherwise ignore                                |
   | CATEGORIES      | Ignore                                          |
   | CLASS           | Ignore                                          |
   | COMMENT         | Ignore                                          |
   | CONTACT         | Ignore                                          |
   | CREATED         | Ignore                                          |
   | DESCRIPTION     | Required                                        |
   | DTSTAMP         | Required                                        |
   | DTSTART         | Required                                        |
   | EXDATE          | Ignore                                          |
   | LAST-MODIFIED   | Ignore                                          |
   | ORGANIZER       | Ignore                                          |
   | RECURRENCE-ID   | Ignore                                          |
   | RELATED-TO      | Ignore                                          |
   | RDATE           | Ignore.                                         |
   | RRULE           | Ignore.  The first instance occurs on the       |
   |                 | DTSTART property.  If implemented, VTIMEZONE    |
   |                 | MUST also be implemented.                       |
   | SEQUENCE        | Required                                        |
   | STATUS          | Ignore                                          |
   | SUMMARY         | Required                                        |
   | URL             | Ignore                                          |
   | UID             | Required                                        |
   | X-              | Ignore                                          |
   +-----------------+-------------------------------------------------+

5.2.  Latency Issues

   With a store-and-forward transport, it is possible for events to
   arrive out of sequence.  That is, a "CANCEL" method may be received
   prior to receiving the associated "REQUEST" for the calendar
   component.  This section discusses a few of these scenarios.

5.2.1.  Cancellation of an Unknown Calendar Component.

   When a "CANCEL" method is received before the original "REQUEST"
   method the calendar will be unable to correlate the "UID" property of
   the cancellation with an existing calendar component.  It is
   suggested that messages that can not be correlated that also contain
   non-zero sequence numbers be held and not discarded.  Implementations
   MAY age them out if no other messages arrive with the same "UID"
   property value and a lower sequence number.

5.2.2.  Unexpected Reply from an Unknown Delegate

   When an "Attendee" delegates an item to another CU they MUST send a
   "REPLY" method to the "Organizer" using the "ATTENDEE" properties to
   indicate that the request was delegated and to whom.  Hence, it is
   possible for an "Organizer" to receive an "REPLY" from a CU not
   listed as one of the original "Attendees".  The resolution is left to
   the implementation but it is expected that the calendaring software
   will either accept the reply or hold it until the related "REPLY"
   method is received from the "Delegator".  If the version of the
   "REPLY" method is out of date the "Organizer" SHOULD treat the
   message as a "REFRESH" message and update the delegate with the
   correct version.

5.3.  Sequence Number

   Under some conditions, a CUA may receive requests and replies with
   the same "SEQUENCE" property value.  The "DTSTAMP" property is
   utilized as a tie-breaker when two items with the same "SEQUENCE"
   property value are evaluated.

6.  Security Considerations

   ITIP is an abstract transport protocol which will be bound to a real-
   time transport, a store-and-forward transport, and perhaps other
   transports.  The transport protocol will be responsible for providing
   facilities for authentication and encryption using standard Internet
   mechanisms that are mutually understood between the sender and
   receiver.

6.1.  Security Threats

6.1.1.  Spoofing the "Organizer"

   In iTIP, the "Organizer" (or someone working on the "Organizer's"
   behalf) is the only person authorized to make changes to an existing
   "VEVENT", "VTODO", "VJOURNAL" calendar component and republish it or
   redistribute updates to the "Attendees".  An iCalendar object that
   maliciously changes or cancels an existing "VEVENT", "VTODO" or
   "VJOURNAL" calendar component may be constructed by someone other
   than the "Organizer" and republished or sent to the "Attendees".

6.1.2.  Spoofing the "Attendee"

   In iTIP, an "Attendee" of a "VEVENT" or "VTODO" calendar component
   (or someone working on the "Attendee's" behalf) is the only person
   authorized to update any parameter associated with their "ATTENDEE"
   property and send it to the "Organizer".  An iCalendar object that
   maliciously changes the "ATTENDEE" parameters may be constructed by
   someone other than the real "Attendee" and sent to the "Organizer".

6.1.3.  Unauthorized Replacement of the Organizer

   There will be circumstances when "Attendees" of an event or to-do
   decide, using out-of-band mechanisms, that the "Organizer" must be
   replaced.  When the new "Organizer" sends out the updated "VEVENT" or
   "VTODO" the "Attendee's" CUA will detect that the "Organizer" has
   been changed, but it has no way of knowing whether or not the change
   was mutually agreed upon.

6.1.4.  Eavesdropping

   The iCalendar object is constructed with human-readable clear text.
   Any information contained in an iCalendar object may be read and/or
   changed by unauthorized persons while the object is in transit.

6.1.5.  Flooding a Calendar

   Implementations MAY provide a means to automatically incorporate
   "REQUEST" methods into a calendar.  This presents the opportunity for
   a calendar to be flooded with requests, which effectively block all
   the calendar's free time.

6.1.6.  Unauthorized REFRESH Requests

   It is possible for an "Organizer" to receive a "REFRESH" request from
   someone who is not an "Attendee" of an event or to-do.  Only
   "Attendee's" of an event or to-do are authorized to receive replies
   to "REFRESH" requests.  Replying to such requests to anyone who is
   not an "Attendee" may be a security problem.

6.2.  Recommendations

   For an application where the information is sensitive or critical and
   the network is subject is subject to a high probability of attack,
   iTIP transactions SHOULD be encrypted.  This may be accomplished
   using public key technology, specifically Security Multiparts for
   MIME [RFC1847] in the iTIP transport binding.  This helps mitigate
   the threats of spoofing, eavesdropping and malicious changes in
   transit.

6.2.1.  Use of [RFC1847] to secure iTIP transactions

   iTIP transport bindings MUST provide a mechanism based on Security
   Multiparts for MIME [RFC1847] to enable authentication of the
   sender's identity, and privacy and integrity of the data being
   transmitted.  This allows the receiver of a signed iCalendar object
   to verify the identity of the sender.  This sender may then be
   correlated to an "ATTENDEE" property in the iCalendar object.  If the
   correlation is made and the sender is authorized to make the
   requested change or update then the operation may proceed.  It also
   allows the message to be encrypted to prevent unauthorized reading of
   the message contents in transit. iTIP transport binding documents
   describe this process in detail.

   Implementations MAY provide controls for users to disable this
   capability.

6.2.2.  Implementation Controls

   The threat of unauthorized replacement of the "Organizer" SHOULD be
   mitigated by a calendar system that uses this protocol by providing
   controls or alerts that make "Calendar Users" aware of such
   "Organizer" changes and allowing them to decide whether or not the
   request should be honored.

   The threat of flooding a calendar SHOULD be mitigated by a calendar
   system that uses this protocol by providing controls that may be used
   to limit the acceptable sources for iTIP transactions, and perhaps
   the size of messages and volume of traffic, by source.

   The threat of malicious procedural alarms SHOULD be mitigated by a
   calendar system that uses this protocol by providing controls that
   may be used to disallow procedural alarms in iTIP transactions and/or
   remove all alarms from the object before delivery to the recipient.

   The threat of unauthorized "REFRESH" requests SHOULD be mitigated by
   a calendar system that uses this protocol by providing controls or
   alerts that allow "Calendar User" to decide whether or not the
   request should be honored.  An implementation MAY decide to maintain,
   for audit or historical purposes, "Calendar Users" who were part of
   an attendee list and who were subsequently uninvited.  Similar
   controls or alerts should be provided when a "REFRESH" request is
   received from these "Calendar Users" as well.

7.  IANA Consideration

   TBD.  Will be based on 2445bis changes.

8.  References

8.1.  Normative References

   [I-D.ietf-calsify-rfc2445bis]
              Desruisseaux, B., "Internet Calendaring and Scheduling
              Core Object Specification (iCalendar)",
              draft-ietf-calsify-rfc2445bis-08
              draft-ietf-calsify-rfc2445bis-07 (work in progress),
              February 2008.
              July 2007.

   [I-D.ietf-calsify-rfc2447bis]
              Melnikov, A., "iCalendar Message-Based Interoperability
              Protocol (iMIP)", draft-ietf-calsify-rfc2447bis-03 (work
              in progress), February 2007.

   [RFC1847]  Galvin, J., Murphy, S., Crocker, S., and N. Freed,
              "Security Multiparts for MIME: Multipart/Signed and
              Multipart/Encrypted", RFC 1847, October 1995.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

8.2.  Informative References

Appendix A.  Acknowledgments

   This is an update to the original iTIP document authored by
   S.Silverberg, S. Mansour, F. Dawson and R. Hopson.

   This revision is the product of the Calsify IETF Working Group, and
   several participants have made important contributions to this
   specification, including: Oliver Block, Bernard Desruisseaux, Mike
   Douglass, Tim Hare, Ciny Joy, Bruce Kahn, Reinhold Kainhofer, Eliot
   Lear, Jonathan Lennox, Andy Mabbett, Aki Niemi, John W. Noerenberg
   II, Robert Ransdell, Caleb Richardson.

Appendix B.  Change History (to be removed prior to publication as an
             RFC)

   Changes in -06:
   a.  Added text to SEQUENCE number description about detecting a
       significant change.
   b.  Added text to REQUEST-STATUS section describing the allowed
       combinations of codes.

   c.  Added text to REQUEST-STATUS section to require new codes to be
       defined in a standard or experimental RFC.
   d.  Removed THISANDFUTURE.
   e.  Cleaned up some examples.

   Changes in -05:
   a.  Changed "memo" to "specification".
   b.  Removed EXRULE as it is no longer in 2445bis.
   c.  Removed section on PROCEDURE alarms since those are no longer in
       2445bis.
   d.  Changed TYPE= to CUTYPE=.
   e.  Fixed formatting of examples.
   f.  Use lowercase mailto everywhere.

   Changes in -04:
   a.  Added empty IANA Considerations section - to be done.
   b.  Formatting fixes.
   c.  Changed VEVENT, VTODO, VJOURNAL cancel descriptions that
       incorrectly implied RECURRENCE-ID could appear multiple times in
       a single component.
   d.  [Issue91] FREEBUSY properties changed to '0+' from '1+' in
       VFREEBUSY PUBLISH and REPLY tables.
   e.  [Issue89] Description for VEVENT ADD method simplified for
       clarity.
   f.  Fixed some iCalendar property/parameter/value capitalization
       issues.
   g.  MAY NOT -> MUST NOT in VFREEBUSY FREEBUSY response type.

   Changes in -03:
   a.  Added empty IANA Considerations section - to be done.

   Changes in -02:
   a.  Tweaked abstract wording.
   b.  Fixed some improper references.
   c.  Removed bogus TZOFFSET entry from VTIMEZONE restriction table.
   d.  Changed TZNAME entry in VTIMEZONE restriction table to "0+" from
       "0 or 1" to fall in line with 2445.
   e.  Changed COMMENT entry in component restriction tables to "0+"
       from "0 or 1" to fall in line with 2445.
   f.  Added ATTENDEE entry in VALARM restriction table to match email
       alarm type in 2445.
   g.  Changed CATEGORIES entry in component restriction tables to "0+"
       from "0 or 1" to fall in line with 2445.
   h.  Changed RESOURCES entry in component restriction tables to "0+"
       from "0 or 1" to fall in line with 2445.
   i.  Changed CONTACT entry in VFREEBUSY restriction table to "0 or 1"
       from "0+" to fall in line with 2445.

   j.  Made UID required in VFREEBUSY publish.
   k.  Added COMPLETED entry in VTODO restriction tables to fall in line
       with 2445.
   l.  Added REQUEST-STATUS entry in VJOURNAL restriction tables to fall
       in line with 2445.

   Changes in -01:
   a.  Updated to 2445bis and 2447bis references.

   Changes in -00 (from RFC2446):
   a.  Updated to latest i-d boilerplate text.
   b.  Rewrote abstract and introduction.
   c.  Reformatted Formatting Conventions section.
   d.  Split ITIP Roles and Transactions into two sections
   e.  iTIP roles uses a table to describe different roles
   f.  Converted tables into xml2rfc format.

Author's Address

   Cyrus Daboo (editor)
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Email: cyrus@daboo.name
   URI:   http://www.apple.com/

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