iCalendar Transport-Independent Interoperability Protocol

(iTIP)

Scheduling Events, BusyTime, To-dos and Journal Entries

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (1998). All Rights Reserved.

Abstract

This document specifies how calendaring systems use iCalendar objects to interoperate with other calendar systems. It does so in a general way so as to allow multiple methods of communication between systems. Subsequent documents specify interoperable methods of communications between systems that use this protocol.

The document outlines a model for calendar exchange that defines both static and dynamic event, to-do, journal and free/busy objects. Static objects are used to transmit information from one entity to another without the expectation of continuity or referential integrity with the original item. Dynamic objects are a superset of static objects and will gracefully degrade to their static counterparts for clients that only support static objects.

This document specifies an Internet protocol based on the iCalendar object specification that provides scheduling interoperability between different calendar systems. The Internet protocol is called the "iCalendar Transport-Independent Interoperability Protocol (iTIP)".

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 iCalendar-based calendar components.

iTIP is defined independent of the particular transport used to transmit the scheduling information. Companion memos to iTIP provide bindings of the interoperability protocol to a number of Internet protocols.

Table of Contents

   1 INTRODUCTION...................................................5
    1.1 FORMATTING CONVENTIONS .....................................5
    1.2 RELATED DOCUMENTS ..........................................6
    1.3 ITIP ROLES AND TRANSACTIONS ................................6
   2 INTEROPERABILITY MODELS........................................8
    2.1 APPLICATION PROTOCOL .......................................9
      2.1.1 Calendar Entry State ...................................9
      2.1.2 Delegation .............................................9
      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
    3.2 METHODS FOR VEVENT CALENDAR COMPONENTS ....................14
      3.2.1 PUBLISH ...............................................15
      3.2.2 REQUEST ...............................................17
        3.2.2.1 Rescheduling an Event..............................19
        3.2.2.2 Updating or Reconfirmation of an Event.............19
        3.2.2.3 Delegating an Event to another CU..................19
        3.2.2.4 Changing the Organizer.............................20
        3.2.2.5 Sending on Behalf of the Organizer.................20
        3.2.2.6 Forwarding to An Uninvited CU......................20
        3.2.2.7 Updating Attendee Status...........................21
      3.2.3 REPLY .................................................21
      3.2.4 ADD ...................................................23
      3.2.5 CANCEL ................................................25
      3.2.6 REFRESH ...............................................26
      3.2.7 COUNTER ...............................................28
      3.2.8 DECLINECOUNTER ........................................29
    3.3 METHODS FOR VFREEBUSY COMPONENTS ..........................31
      3.3.1 PUBLISH ...............................................32
      3.3.2 REQUEST ...............................................33
      3.3.3 REPLY .................................................34
    3.4 METHODS FOR VTODO COMPONENTS ..............................35
   
      3.4.1 PUBLISH ...............................................35
      3.4.2 REQUEST ...............................................37
        3.4.2.1 REQUEST for Rescheduling a VTODO...................39
        3.4.2.2 REQUEST for Update or Reconfirmation of a VTODO....39
        3.4.2.3 REQUEST for Delegating a VTODO.....................40
        3.4.2.4 REQUEST Forwarded To An Uninvited Calendar User....40
        3.4.2.5 REQUEST Updated Attendee Status....................41
      3.4.3 REPLY .................................................41
      3.4.4 ADD ...................................................43
      3.4.5 CANCEL ................................................44
      3.4.6 REFRESH ...............................................46
      3.4.7 COUNTER ...............................................48
      3.4.8 DECLINECOUNTER ........................................49
    3.5 METHODS FOR VJOURNAL COMPONENTS ...........................50
      3.5.1 PUBLISH ...............................................51
      3.5.2 ADD ...................................................52
      3.5.3 CANCEL ................................................53
    3.6 STATUS REPLIES ............................................55
    3.7 IMPLEMENTATION CONSIDERATIONS .............................57
      3.7.1 Working With Recurrence Instances .....................57
      3.7.2 Attendee Property Considerations ......................58
      3.7.3 X-Tokens ..............................................59
   4 EXAMPLES......................................................59
    4.1 PUBLISHED EVENT EXAMPLES ..................................59
      4.1.1 A Minimal Published Event .............................60
      4.1.2 Changing A Published Event ............................60
      4.1.3 Canceling A Published Event ...........................61
      4.1.4 A Rich Published Event ................................62
      4.1.5 Anniversaries or Events attached to entire days .......63
    4.2 GROUP EVENT EXAMPLES ......................................63
      4.2.1 A Group Event Request .................................64
      4.2.2 Reply To A Group Event Request ........................65
      4.2.3 Update An Event .......................................65
      4.2.4 Countering an Event Proposal ..........................66
      4.2.5 Delegating an Event ...................................68
      4.2.6 Delegate Accepts the Meeting ..........................70
      4.2.7 Delegate Declines the Meeting .........................71
      4.2.8 Forwarding an Event Request ...........................72
      4.2.9 Cancel A Group Event ..................................72
      4.2.10 Removing Attendees ...................................74
      4.2.11 Replacing the Organizer ..............................75
    4.3 BUSY TIME EXAMPLES ........................................76
      4.3.1 Request Busy Time .....................................77
      4.3.2 Reply To A Busy Time Request ..........................77
    4.4 RECURRING EVENT AND TIME ZONE EXAMPLES ....................78
      4.4.1 A Recurring Event Spanning Time Zones .................78
      4.4.2 Modify A Recurring Instance ...........................79
      4.4.3 Cancel an Instance ....................................81
      4.4.4 Cancel Recurring Event ................................81
      4.4.5 Change All Future Instances ...........................82
      4.4.6 Add A New Instance To A Recurring Event ...............82
      4.4.7 Add A New Series of Instances To A Recurring Event ....83
      4.4.8 Counter An Instance Of A Recurring Event ..............87
      4.4.9 Error Reply To A Request ..............................88
    4.5 GROUP TO-DO EXAMPLES ......................................89
      4.5.1 A VTODO Request .......................................90
      4.5.2 A VTODO Reply .........................................90
      4.5.3 A VTODO Request for Updated Status ....................91
      4.5.4 A Reply: Percent-Complete .............................91
      4.5.5 A Reply: Completed ....................................92
      4.5.6 An Updated VTODO Request ..............................92
      4.5.7 Recurring VTODOs ......................................92
        4.5.7.1 Request for a Recurring VTODO......................93
        4.5.7.2 Calculating due dates in recurring VTODOs..........93
        4.5.7.3 Replying to an instance of a recurring VTODO.......93
    4.6 JOURNAL EXAMPLES ..........................................94
    4.7 OTHER EXAMPLES ............................................94
      4.7.1 Event Refresh .........................................94
      4.7.2 Bad RECURRENCE-ID .....................................95
   5 APPLICATION PROTOCOL FALLBACKS................................97
    5.1 PARTIAL IMPLEMENTATION ....................................97
      5.1.1 Event-Related Fallbacks ...............................97
      5.1.2 Free/Busy-Related Fallbacks ...........................99
      5.1.3 To-Do-Related Fallbacks ...............................99
      5.1.4 Journal-Related Fallbacks ............................101
    5.2 LATENCY ISSUES ...........................................102
      5.2.1 Cancellation of an Unknown Calendar Component. .......102
      5.2.2 Unexpected Reply from an Unknown Delegate ............103
    5.3 SEQUENCE NUMBER ..........................................103
   6 SECURITY CONSIDERATIONS......................................103
    6.1 SECURITY THREATS .........................................103
      6.1.1 Spoofing the "Organizer" .............................103
      6.1.2 Spoofing the "Attendee" ..............................103
      6.1.3 Unauthorized Replacement of the Organizer ............104
      6.1.4 Eavesdropping ........................................104
      6.1.5 Flooding a Calendar ..................................104
      6.1.6 Procedural Alarms ....................................104
      6.1.7 Unauthorized REFRESH Requests ........................104
    6.2 RECOMMENDATIONS ..........................................104
      6.2.1 Use of [RFC-1847] to secure iTIP transactions ........105
      6.2.2 Implementation Controls ..............................105
   7 ACKNOWLEDGMENTS..............................................106
   8 BIBLIOGRAPHY.................................................106
   9 AUTHORS' ADDRESSES...........................................107
   10 FULL COPYRIGHT STATEMENT....................................109

1 Introduction

This document specifies how calendaring systems use iCalendar 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 so as to allow multiple methods of communication between systems. 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

In order to refer to elements of the calendaring and scheduling model, core object or interoperability protocol defined in [iCAL] and [iTIP] several formatting conventions have been utilized.

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 [RFC-2119].

   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 defined by [iTIP]. Calendar components
   defined by [iCAL] 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 defined by [iTIP] 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 [iCAL] 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 memo are referred to with lower case, 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 memo 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 memos that, along with this one, describe the Internet calendaring and scheduling standards. This document, [iTIP], specifies an interoperability protocol for scheduling between different implementations. The related documents are:

[iCAL] - specifies the objects, data types, properties and

property parameters used in the protocols, along with the

methods for representing and encoding them;

[iMIP] specifies an Internet email binding for [iTIP].

This memo does not attempt to repeat the specification of concepts or definitions from these other memos. Where possible, references are made to the memo that provides for the specification of these concepts or definitions.

1.3 ITIP Roles and Transactions

ITIP defines methods for exchanging [iCAL] objects for the purposes of group calendaring and scheduling between "Calendar Users" (CUs). CUs take on one of two roles in iTIP. The CU who initiates an exchange takes on the role of "Organizer". For example, the CU who proposes a group meeting is the "Organizer". 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 _ATTENDEE_ property. Its use is to convey descriptive context to an "Attendee" such as "chair", "req-participant" or "non-participant" and has nothing to do with the calendaring workflow.

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 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.                                           |
   |                |                                                  |
   | DECLINE-       | Used by the "Organizer" to decline the proposed  |
   | COUNTER        | counter-proprosal.                               |
   +================+==================================================+

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 [iMIP] 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                   |
   +------------------------------------------+
   |Real-time | Store-and-Fwd | Other         |
   |Transport | Transport     | Transports... |
   +------------------------------------------+

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 [iCAL]. For clarity, these rules are paraphrased here in terms of how they are applied in [iTIP]. For a given "UID" in a calendar component:

. For the "PUBLISH" and "REQUEST" methods, the "SEQUENCE" property

value is incremented according to the rules defined in [iCAL].

. The "SEQUENCE" property value MUST be incremented each time the

"Organizer" uses the "ADD" or "CANCEL" methods.

. 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.

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 [iCAL] 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 memo 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      |
   |Decline- |        |       |          |           |
   |Counter  |  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 contains "MUST" if the component or property must be present, "MAY" if the component or property is optional, and "NOT" if the component or property must not be present. Entries in the second column sometimes contain comments for further clarification.

3.1 Common Component Restriction Tables

The restriction table below applies to properties of the iCalendar object. That is, the properties at the outermost scope. 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 [iCAL].
   
   Component/Property  Presence
   ------------------- ----------------------------------------------
   CALSCALE            0 or 1
   PRODID              1
   VERSION             1       Value MUST be "2.0"
   X-PROPERTY          0+

DateTime values MAY refer to a "VTIMEZONE" component. The property restrictions in the table below apply to any "VTIMEZONE" component in an ITIP message.

   Component/Property  Presence
   ------------------- ----------------------------------------------
   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 or 1
          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 or 1
          TZOFFSET     1
          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 or 1
          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 or 1
          TZOFFSETFROM 1
          TZOFFSETTO   1
          X-PROPERTY   0+
       TZID            1
       TZURL           0 or 1
       X-PROPERTY      0+

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

   Component/Property  Presence
   ------------------- ----------------------------------------------
   VALARM              0+
       ACTION          1
       ATTACH          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
------------------- ----------------------------------------------
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 or 1  This property may contain a list of
                            values
     CLASS          0 or 1
     COMMENT        0 or 1
     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+
     EXRULE         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 or 1 This property MAY contain a list of values
     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:

     .  Invite "Attendees" to an event;
     .  Reschedule an existing event;
     .  Response to a REFRESH request;
     .  Update the details of an existing event, without rescheduling it;
     .  Update the status of "Attendees" of an existing event, without
        rescheduling it;
     .  Reconfirm an existing event, without rescheduling it;
     .  Forward a "VEVENT" to another uninvited CU.
     .  For an existing "VEVENT" calendar component, delegate the role of
        "Attendee" to another CU;
     .  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
-----------------------------------------------------------------
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 or 1  This property may contain a list of values
    CLASS           0 or 1
    COMMENT         0 or 1
    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+
    EXRULE          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 or 1  This property MAY contain a list of values
    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
------------------- ----------------------------------------------
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 or 1  This property may contain a list of values
    CLASS           0 or 1
    COMMENT         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+
    EXRULE          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 or 1  This property MAY contain a list of values
    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 in a "VEVENT" calendar component is used to add one or more instances to an existing "VEVENT". Unlike the "REQUEST" method, when using issuing an "ADD" method, the "Organizer" does not send the full "VEVENT" description; only the new instance(s). The "ADD" method is especially useful if there are instance-specific properties to be preserved in a recurring "VEVENT". For instance, an "Organizer" may have originally scheduled a weekly Thursday meeting. At some point, several instances changed. Location or start time may have changed, or some instances may have unique "DESCRIPTION" properties. The "ADD" method allows the "Organizer" to add new instances to an existing event using a single ITIP message without redefining the entire recurring pattern.

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
------------------- ----------------------------------------------
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 or 1 This property MAY contain a list of values
    CLASS           0 or 1
    COMMENT         0 or 1
    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+
    EXRULE          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 or 1  This property MAY contain a list of values
    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) to indicate cancellation of the specified "VEVENT" calendar component and all instances before (or after); or

(b) individual recurrence instances may be cancelled by specifying

multiple "RECURRENCE-ID" properties corresponding to 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
------------------- ----------------------------------------------
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 or 1
    ATTACH          0+
    CATEGORIES      0 or 1  This property may contain a list of values
    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+
    EXRULE          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 or 1
    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
------------------- ----------------------------------------------
METHOD              1      MUST be "REFRESH"

VEVENT              1
    ATTENDEE        1      MUST be the address of requestor
    DTSTAMP         1
    ORGANIZER       1
    UID             1      MUST be the UID associated with original
                           REQUEST
    COMMENT         0 or 1
    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
    EXRULE          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
------------------- ----------------------------------------------
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 or 1  This property may contain a list of values
    CLASS           0 or 1
    COMMENT         0 or 1
    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+
    EXRULE          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 or 1  This property may contain a list of values
    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
------------------- ----------------------------------------------
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 or 1
    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
    EXRULE          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 "A4.34.3".

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" 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
------------------- ----------------------------------------------
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        1+      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.

    COMMENT         0 or 1
    CONTACT         0+
    X-PROPERTY      0+
    URL             0 or 1  Specifies busy time URL

    ATTENDEE        0
    DURATION        0
    REQUEST-STATUS  0
    UID             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
------------------- ----------------------------------------------
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 or 1
    CONTACT         0+
    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
------------------- ----------------------------------------------
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        1+      (values MUST all be of the same data
                            type. Multiple instances are allowed.
                            Multiple instances MUST be sorted in
                            ascending order. Values MAY NOT overlap)
    ORGANIZER       1       MUST be the request originator's address
    UID             1

    COMMENT         0 or 1
    CONTACT         0+
    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
------------------- ----------------------------------------------
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 or 1  This property may contain a list of values
    CLASS            0 or 1
    COMMENT          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+
    EXRULE           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 or 1  This property may contain a list of values
    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:

     .  Assign a to-do to one or more "Calendar Users";
     .  Reschedule an existing to-do;
     .  Update the details of an existing to-do, without rescheduling
        it;
     .  Update the completion status of "Attendees" of an existing
        to-do, without rescheduling it;
     .  Reconfirm an existing to-do, without rescheduling it;
     .  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
------------------- ----------------------------------------------
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 or 1   This property may contain a list of
                               values
    CLASS             0 or 1
    COMMENT           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+
    EXRULE            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 or 1  This property may contain a list of
                              values
    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
-------------------  ---------------------------------------------
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 or 1 This property may contain a list of values
    CLASS            0 or 1
    COMMENT          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+
    EXRULE           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 or 1  This property may contain a list of values
    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
------------------- ----------------------------------------------
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 or 1  This property may contain a list of
                              values
    CLASS             0 or 1
    COMMENT           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+
    EXRULE            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 or 1  This property may contain a list of
                              values
    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) to indicate cancellation of the specified "VTODO" calendar component and all instances before (or after); or

(b) individual recurrence instances may be cancelled by specifying

multiple "RECURRENCE-ID" properties corresponding to 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
-------------------  ---------------------------------------------
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 or 1 This property MAY contain a list of values
    CLASS            0 or 1
    COMMENT          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+
    EXRULE           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 or 1 This property MAY contain a list of values
    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
-------------------  ---------------------------------------------
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
    CONTACT          0
    CREATED          0
    DESCRIPTION      0
    DTSTART          0
    DUE              0
    DURATION         0
    EXDATE           0
    EXRULE           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
------------------- ----------------------------------------------
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 or 1 This property MAY contain a list of values
    CLASS            0 or 1
    COMMENT          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+
    EXRULE           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 or 1 This property MAY contain a list of values
    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
-------------------  ---------------------------------------------
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 or 1  This property may contain a list of values
    CLASS            0 or 1
    COMMENT          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+
    EXRULE           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 or 1  This property MAY contain a list of values
    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
------------------- ----------------------------------------------
METHOD               1       MUST be "PUBLISH"
VJOURNAL             1+
    DESCRIPTION      1       Can be null.
    DTSTAMP          1
    DTSTART          1
    ORGANIZER        1
    UID              1

    ATTACH           0+
    CATEGORIES       0 or 1  This property MAY contain a list of values
    CLASS            0 or 1
    COMMENT          0 or 1
    CONTACT          0+
    CREATED          0 or 1
    EXDATE           0+
    EXRULE           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

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
------------------- ----------------------------------------------
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 or 1 This property MAY contain a list of values
    CLASS            0 or 1
    COMMENT          0 or 1
    CONTACT          0+
    CREATED          0 or 1
    EXDATE           0+
    EXRULE           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

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) to indicate cancellation of the specified "VTODO" calendar component and all instances before (or after); or

(b) individual recurrence instances may be cancelled by specifying

multiple "RECURRENCE-ID" properties corresponding to 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
-------------------  ---------------------------------------------
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 or 1  This property MAY contain a list of values
    CLASS            0 or 1
    COMMENT          0 or 1
    CONTACT          0+
    CREATED          0 or 1
    DESCRIPTION      0 or 1
    DTSTART          0 or 1
    EXDATE           0+
    EXRULE           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 the following values:

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

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

"Attendee" initiated actions:

. "Attendee" changes status for a particular recurrence instance . "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 [RFC-822] 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 "TYPE=INDIVIDUAL"

2. Failing (1) look for attendees where "TYPE=GROUP" or 'TYPE=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"                    |
   +---------------------------------+---------------------------------+
   | Publish an event                | "A" sends or posts a PUBLISH    |
   |                                 | message                         |
   +---------------------------------+---------------------------------+
   | "B" reads a published event     |                                 |
   +---------------------------------+---------------------------------+
   | Publish an updated event        | "A" sends or posts a PUBLISH    |
   |                                 | message                         |
   +---------------------------------+---------------------------------+
   | "B" reads the updated event     |                                 |
   +---------------------------------+---------------------------------+
   | Cancel a published event        | "A" sends or posts a CANCEL     |
   |                                 | message                         |
   +---------------------------------+---------------------------------+
   | "B" reads the canceled 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
   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
   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
   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
   METHOD:PUBLISH
   SCALE:GREGORIAN
   VERSION:2.0
   BEGIN:VTIMEZONE
   TZID:America-Chicago
   TZURL:http://zones.stds_r_us.net/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/
   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/
   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
   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 meeting | "A" sends a REQUEST |                          |
| request            | message to "B", "C",|                          |
|                    | and "D"             |                          |
+---------------------------------------------------------------------+
| Accept the meeting |                     | "B" sends a REPLY        |
| request            |                     | message to "A" with its  |
|                    |                     | ATTENDEE "partstat" para-|
|                    |                     | set to "accepted"        |
+---------------------------------------------------------------------+
| Decline the meeting|                     | "C" sends a REPLY        |
| request            |                     | message to "A" with its  |
|                    |                     | ATTENDEE "partstat" para-|
|                    |                     | set to "declined"        |
+---------------------------------------------------------------------+
| Tentatively accept |                     | "D" sends a REPLY        |
| the meeting request|                     | message to "A" with its  |
|                    |                     | ATTENDEE "partstat" para-|
|                    |                     | set to "tentative"       |
+---------------------------------------------------------------------+
| Confirm meeting    | "A" sends a REQUEST |                          |
| status with        | message to "B" and  |                          |
| attendees          | "D" with updated    |                          |
|                    | 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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED;CN=BIG A:Mailto:A@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL;CN=B:Mailto:B@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL;CN=C:Mailto:C@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL;CN=Hal:Mailto:D@example.com
   ATTENDEE;RSVP=FALSE;TYPE=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
   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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:Mailto:A@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL:Mailto:B@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL:Mailto:C@example.com
   ATTENDEE;RSVP=TRUE;TYPE=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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:Mailto:A@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL:Mailto:B@example.com
   ATTENDEE;RSVP=TRUE;TYPE=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
   METHOD:COUNTER
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:Mailto:A@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL:Mailto:B@example.com
   ATTENDEE;RSVP=TRUE;TYPE=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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:Mailto:A@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL:Mailto:B@example.com
   ATTENDEE;RSVP=TRUE;TYPE=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
   METHOD:DECLINECOUNTER
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;RSVP=TRUE;TYPE=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:

     .  Send a "REPLY" to the "Organizer" with the following updates:
     .  The "Delegator's" "ATTENDEE" property "partstat" parameter set
        to "delegated" and the "delegated-to" parameter is set to the
        address of the "Delegate"
     .  Add an additional "ATTENDEE" property for the "Delegate" with
        the "delegated-from" property parameter set to the "Delegator"
     .  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.
     .  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 meeting | "A" sends a REQUEST |                          |
| request            | message to "B" and  |                          |
|                    | "C"                 |                          |
+---------------------------------------------------------------------+
| Delegate:          |                     | "C" sends a REPLY to "A" |
| "C" delegates to   |                     | with the ATTENDEE.       |
| "E"                |                     | "partstat" parameter set |
|                    |                     | to "delegated" and with a|
|                    |                     | new "ATTENDEE" property  |
|                    |                     | for "E". "E's" ATTENDEE  |
|                    |                     | "delegated-from" param   |
|                    |                     | is set to "C". "C's"     |
|                    |                     | ATTENDEE "delegated-to"  |
|                    |                     | param 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  |
| attendance         |                     | to "A" and optionally "C"|
|                    |                     | with its "partstat"      |
|                    |                     | property parameter set   |
|                    |                     | to "ACCEPTED"            |
+---------------------------------------------------------------------+
| Optional:          | "A" sends REQUEST   |                          |
| Redistribute       | message to "B", "C" |                          |
| meeting to         | and "E".            |                          |
| attendees          |                     |                          |
+---------------------------------------------------------------------+

"C" responds to the "Organizer".

   BEGIN:VCALENDAR
   PRODID:-//ACME/DesktopCalendar//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
   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
   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
   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
   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 meeting | "A" sends a REQUEST |                         |
   | request            | message to "B", "C",|                         |
   |                    | and "D"             |                         |
   +--------------------------------------------------------------------+
   | Decline the meeting|                     | "B" sends a "REPLY"     |
   | request            |                     | message to "A" with its |
   |                    |                     | "partstat" para-         |
   |                    |                     | set to "declined".      |
   +--------------------------------------------------------------------+
   | Cancel the meeting | "A" sends a CANCEL  |                         |
   |                    | message to "B", "C" |                         |
   |                    | and "D"             |                         |
   +--------------------------------------------------------------------+

The example shows how "A" cancels the event.

   BEGIN:VCALENDAR
   PRODID:-//ACME/DesktopCalendar//EN
   METHOD:CANCEL
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;TYPE=INDIVIDUAL;Mailto:A@example.com
   ATTENDEE;TYPE=INDIVIDUAL:Mailto:B@example.com
   ATTENDEE;TYPE=INDIVIDUAL:Mailto:C@example.com
   ATTENDEE;TYPE=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"      | "A" sends a CANCEL  |                         |
   | as an "Attendee"   | message to "B"      |                         |
   +--------------------------------------------------------------------+
   | Update the master  | "A" sends the       |                         |
   | copy of the event  | updated event to    |                         |
   |                    | 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
   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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:Mailto:A@example.com
   ATTENDEE;TYPE=INDIVIDUAL:Mailto:C@example.com
   ATTENDEE;TYPE=INDIVIDUAL:Mailto:D@example.com
   ATTENDEE;TYPE=ROOM: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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:B@example.com
   ATTENDEE;ROLE=CHAIR;STATUS=ACCEPTED:Mailto:B@example.com
   ATTENDEE;TYPE=INDIVIDUAL:Mailto:C@example.com
   ATTENDEE;TYPE=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
   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    | "A" sends "REQUEST" |                         |
| time request       | message to "B" and  |                         |
|                    | 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
   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
   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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VTIMEZONE
   TZID:America-SanJose
   TZURL:http://zones.stds_r_us.net/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;TYPE=INDIVIDUAL:A@example.COM
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL:B@example.fr
   ATTENDEE;RSVP=TRUE;TYPE=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:

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

and the second is:

   TUE, 28-OCT-1997 23:00 GMT
   TUE, 28-OCT-1997 14:00 PST
   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
   VERSION:2.0
   BEGIN:VEVENT
   UID:guid-1@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:guid-1@host1com
   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
   VERSION:2.0
   BEGIN:VEVENT
   UID:guid-1@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:guid-1@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:guid-1@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:123456789@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:123456789@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:123456789@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:123456789@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:123456789@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:123456789@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:123456789@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:123456789@host1.com
   SEQUENCE:2
   RDATE:19980304T180000Z
   RDATE:19980311T160000Z
   RDATE:19980315T180000Z
   Error! Bookmark not defined.
   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
   Error! Bookmark not defined.
   SEQUENCE:2
   RECURRENCE-ID:19980311T160000Z
   Error! Bookmark not defined.
   ATTENDEE;ROLE=CHAIR;Error! Bookmark not defined.
   ATTENDEE;Error! Bookmark not defined.
   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
   VERSION:2.0
   BEGIN:VEVENT
   UID:guid-1@host1.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
   VERSION:2.0
   BEGIN:VEVENT
   UID:guid-1@host1.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
   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
   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 to-do   | "A" sends a REQUEST |                         |
| request            | message to "B", "C",|                         |
|                    | and "D"             |                         |
+--------------------------------------------------------------------+
| Accept the to-do   |                     | "B" sends a "REPLY"     |
| request            |                     | message to "A" with its |
|                    |                     | "partstat" paramater    |
|                    |                     | set to "accepted".      |
+--------------------------------------------------------------------+
| Decline the to-do  |                     | "C" sends a REPLY       |
| request            |                     | message to "A" with its |
|                    |                     | "partstat" parameter    |
|                    |                     | set to "declined".      |
+--------------------------------------------------------------------+
| Tentatively accept |                     | "D" sends a REPLY       |
| the to-do request  |                     | message to "A" with its |
|                    |                     | "partstat" parameter    |
|                    |                     | set to "tentative".     |
+--------------------------------------------------------------------+
| Check attendee     | "A" sends a REQUEST |                         |
| completion status  | message to "B" and  |                         |
|                    | "D" with current    |                         |
|                    | information.        |                         |
+--------------------------------------------------------------------+
| Attendee indicates |                     | "B" sends a "REPLY"     |
| percent complete   |                     | message indicating      |
|                    |                     | percent complete        |

+--------------------------------------------------------------------+
| Attendee indicates |                     | "D" sends a "REPLY"     |
| completion         |                     | message indicating      |
|                    |                     | 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
   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
   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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VTODO
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR:Mailto:A@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL:Mailto:B@example.com
   ATTENDEE;RSVP=TRUE;TYPE=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
   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
   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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VTODO
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:Mailto:A@example.com
   ATTENDEE;PARTSTAT=ACCEPTED;TYPE=INDIVIDUAL:Mailto:B@example.com
   ATTENDEE;PARTSTAT=IN-PROCESS;TYPE=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
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VTODO
   ORGANIZER:Mailto:A@example.com
   ATTENDEE;ROLE=CHAIR:Mailto:A@example.com
   ATTENDEE;RSVP=TRUE;TYPE=INDIVIDUAL:Mailto:B@example.com
   ATTENDEE;RSVP=TRUE;TYPE=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
   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
   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":

   BEGIN:VCALENDAR
   PRODID:-//RDU Software//NONSGML HandCal//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
   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"   |
| refresh because    |                     | message to "A"          |
| "RECURRENCE-ID" was|                     |                         |
| not found          |                     |                         |
+--------------------------------------------------------------------+
| Refresh the entire | "A" sends the       |                         |
| Event              | latest copy of the  |                         |
|                    | Event to "B"        |                         |
+--------------------------------------------------------------------+
| Attendee handles   |                     | "B" updates to the      |
| the request and    |                     | latest copy of the      |
| updates the        |                     | meeting.                |
| instance           |                     |                         |
+--------------------------------------------------------------------+

Request from "A":

   BEGIN:VCALENDAR
   METHOD:REQUEST
   PRODID:-//RDU Software//NONSGML HandCal//EN
   VERSION:2.0
   BEGIN:VEVENT
   UID:acme-12345@host1.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" 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
   METHOD:REFRESH
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:Mailto:A@example.com
   ATTENDEE:Mailto:B@example.com
   UID:acme-12345@host1.com
   DTSTAMP:19970603T094000
   END:VEVENT
   END:VCALENDAR

5 Application Protocol Fallbacks

5.1 Partial Implementation

Applications that support this memo 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
Property         Fallback
--------------   -----------------------------------------------------
CALSCALE         Ignore; assume GREGORIAN
PRODID           Ignore
METHOD           Required as described in the Method list above
VERSION          Ignore
Event-Related
Components       Fallback
--------------   -----------------------------------------------------
VALARM           Reply with Not Supported
VTIMEZONE        Required if any DateTime value refers to a time zone.

Component
Property         Fallback
--------------   -----------------------------------------------------
ATTACH           Ignore
ATTENDEE         Required if EVENT-REQUEST is not implemented;
                 otherwise reply with Not Supported
CATEGORIES       Ignore
CLASS            Ignore
COMMENT          Ignore
COMPLETED        Ignore
nCONTACT          Ignore
CREATED          Ignore
DESCRIPTION      Required
DURATION         Reply with Not Supported
DTSTAMP          Required
DTSTART          Required
DTEND            Required
EXDATE           Ignore
EXRULE           Ignore Reply with Not Supported. If implemented,
                 VTIMEZONE MUST also be implemented.
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
Property         Fallback
--------------   -----------------------------------------------------
CALSCALE         Ignore; assume GREGORIAN.
PRODID           Ignore
METHOD           Required as described in the Method list above.
VERSION          Ignore

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

To-Do-Related
Components       Fallback
--------------   -----------------------------------------------------
VALARM           Reply with Not Supported
VTIMEZONE        Required if any DateTime value refers to a time zone.

Component
Property         Fallback
--------------   -----------------------------------------------------
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
EXRULE           Ignore Reply with Not Supported. If implemented,
                 VTIMEZONE MUST also be implemented.
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
Property         Fallback
--------------   -----------------------------------------------------
CALSCALE         Ignore; assume GREGORIAN.
PRODID           Ignore
METHOD           Required as described in the Method list above.
VERSION          Ignore

Journal-Related
Components       Fallback
--------------   -----------------------------------------------------
VTIMEZONE        Required if any DateTime value refers to a time zone.
Component
Property         Fallback
--------------   -----------------------------------------------------
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
EXRULE           Ignore Reply with Not Supported. If implemented,
                 VTIMEZONE MUST also be implemented.
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 Procedural Alarms

The "REQUEST" methods for "VEVENT" and "VTODO" calendar components MAY contain "VALARM" calendar components. The "VALARM" calendar component may be of type "PROCEDURE" and MAY have an attachment containing an executable program. Implementations that incorporate these types of alarms are subject to any virus or malicious attack that may occur as a result of executing the attachment.

6.1.7 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

[RFC-1847] in the iTIP transport binding. This helps mitigate the threats of spoofing, eavesdropping and malicious changes in transit.

6.2.1 Use of [RFC-1847] to secure iTIP transactions

iTIP transport bindings MUST provide a mechanism based on Security Multiparts for MIME [RFC-1847] 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 Acknowledgments

A hearty thanks to the following individuals who have participated in the drafting, review and discussion of this memo:

Anik Ganguly, Dan Hickman, Paul Hill, Daryl Huff, Bruce Kahn, Antoine Leca, Bob Mahoney, John Noerenberg, Leo Parker, John Rose, Doug Royer, Vinod Seraphin, Richard Shusterman, Derik Stenerson, John Sun, Alexander Taler, Kevin Tsurutome.

8 Bibliography

   [iCAL]     Dawson, F. and D. Stenerson, "Internet Calendaring and
              Scheduling Core Object Specification - iCalendar", RFC
              2445, November 1998.
   
   [iMIP]     Dawson, F., Mansour, S. and S. Silverberg, "iCalendar
              Message-Based Interoperability Protocol - iMIP", RFC 2447,
              November 1998.
   
   [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [US-ASCII] Coded Character Set--7-bit American Standard Code for
              Information Interchange, ANSI X3.4-1986.

9 Authors' Addresses

The following address information is provided in a vCard v3.0, Electronic Business Card, format.

The authors of this memo are:

   BEGIN:VCARD
   VERSION:3.0
   N:Dawson;Frank
   FN:Frank Dawson
   ORG:Lotus Development Corporation
   ADR;WORK;POSTAL;PARCEL:;;6544 Battleford Drive;Raleigh;NC;27613-
    3502;USA
   TEL;TYPE=WORK,MSG:+1-919-676-9515
   TEL;TYPE=WORK,FAX:+1-919-676-9564
   EMAIL;TYPE=PREF,INTERNET:Frank_Dawson@Lotus.com
   EMAIL;TYPE=INTERNET:fdawson@earthlink.net
   URL:http://home.earthlink.net/~fdawson
   END:VCARD
   
   BEGIN:VCARD
   VERSION:3.0
   N:Hopson;Ross
   FN:Ross Hopson
   ORG:On Technology, Inc.
   ADR;TYPE=WORK,POSTAL,PARCEL:;Suite 1600;434 Fayetteville St.
     Mall\, Two Hannover Square;Raleigh;NC;27601
   TEL;TYPE=WORK,MSG:+1-919-890-4036
   TEL;TYPE=WORK,FAX:+1-919-890-4100
   EMAIL;TYPE=INTERNET:rhopson@on.com
   END:VCARD
   
   BEGIN:VCARD
   VERSION:3.0
   N:Mansour;Steve
   FN:Steve Mansour
   ORG:Netscape Communications Corporation
   ADR;TYPE=WORK,POSTAL,PARCEL:;;501 East Middlefield Road;Mountain
     View;CA;94043;USA
   TEL;TYPE=WORK,MSG:+1-650-937-2378
   TEL;TYPE=WORK,FAX:+1-650-937-2103
   EMAIL;TYPE=INTERNET:sman@netscape.com
   END:VCARD
   BEGIN:VCARD
   VERSION:3.0
   N:Silverberg;Steve
   FN:Steve Silverberg
   ORG:Microsoft Corporation
   ADR;TYPE=WORK,POSTAL,PARCEL:;;One Microsoft Way;
   Redmond;WA;98052-6399;USA
   TEL;TYPE=WORK,MSG:+1-425-936-9277
   TEL;TYPE=WORK,FAX:+1-425-936-8019
   EMAIL;INTERNET:stevesil@Microsoft.com
   END:VCARD

The iCalendar object is a result of the work of the Internet Engineering Task Force Calendaring and scheduling Working Group. The chairman of that working group is:

   BEGIN:VCARD
   VERSION:3.0
   N:Ganguly;Anik
   FN:Anik Ganguly
   ORG:Open Text Inc.
   ADR;TYPE=WORK,POSTAL,PARCEL:;Suite 101;38777 West Six Mile Road;
    Livonia;MI;48152;USA
   TEL;TYPE=WORK,MSG:+1-734-542-5955
   EMAIL;TYPE=INTERNET:ganguly@acm.org
   END:VCARD

The co-chairman of that working group is:

   BEGIN:VCARD
   VERSION:3.0
   N:Moskowitz;Robert
   FN:Robert Moskowitz
   NICKNAME:Bob
   EMAIL; TYPE=INTERNET:rgm-ietf@htt-consult.com
   END:VCARD

10. Full Copyright Statement

Copyright © The Internet Society (1998). All Rights Reserved.

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