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rfc:rfc2446

Network Working Group S. Silverberg Request for Comments: 2446 Microsoft Category: Standards Track S. Mansour

                                                             Netscape
                                                            F. Dawson
                                                                Lotus
                                                            R. Hopson
                                                      ON Technologies
                                                        November 1998
     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 (C) 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)".

Silverberg, et. al. Standards Track [Page 1] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 2] RFC 2446 iTIP November 1998

    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

Silverberg, et. al. Standards Track [Page 3] RFC 2446 iTIP November 1998

    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

Silverberg, et. al. Standards Track [Page 4] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 5] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 6] RFC 2446 iTIP November 1998

 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.                               |
 +================+==================================================+

Silverberg, et. al. Standards Track [Page 7] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 8] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 9] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 10] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 11] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 12] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 13] RFC 2446 iTIP November 1998

        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.

Silverberg, et. al. Standards Track [Page 14] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 15] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 16] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 17] RFC 2446 iTIP November 1998

 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+

Silverberg, et. al. Standards Track [Page 18] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 19] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 20] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 21] RFC 2446 iTIP November 1998

 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+

Silverberg, et. al. Standards Track [Page 22] RFC 2446 iTIP November 1998

  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:

Silverberg, et. al. Standards Track [Page 23] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 24] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 25] RFC 2446 iTIP November 1998

  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.

Silverberg, et. al. Standards Track [Page 26] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 27] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 28] RFC 2446 iTIP November 1998

  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:

Silverberg, et. al. Standards Track [Page 29] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 30] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 31] RFC 2446 iTIP November 1998

 |================|==================================================|
 | 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+

Silverberg, et. al. Standards Track [Page 32] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 33] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 34] RFC 2446 iTIP November 1998

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"

Silverberg, et. al. Standards Track [Page 35] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 36] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 37] RFC 2446 iTIP November 1998

 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+

Silverberg, et. al. Standards Track [Page 38] RFC 2446 iTIP November 1998

  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

Silverberg, et. al. Standards Track [Page 39] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 40] RFC 2446 iTIP November 1998

 "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

Silverberg, et. al. Standards Track [Page 41] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 42] RFC 2446 iTIP November 1998

  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+

Silverberg, et. al. Standards Track [Page 43] RFC 2446 iTIP November 1998

  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.

Silverberg, et. al. Standards Track [Page 44] RFC 2446 iTIP November 1998

 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+

Silverberg, et. al. Standards Track [Page 45] RFC 2446 iTIP November 1998

  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:

Silverberg, et. al. Standards Track [Page 46] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 47] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 48] RFC 2446 iTIP November 1998

  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

Silverberg, et. al. Standards Track [Page 49] RFC 2446 iTIP November 1998

  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.

Silverberg, et. al. Standards Track [Page 50] RFC 2446 iTIP November 1998

 +================+==================================================+
 | 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

Silverberg, et. al. Standards Track [Page 51] RFC 2446 iTIP November 1998

                           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+

Silverberg, et. al. Standards Track [Page 52] RFC 2446 iTIP November 1998

  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:

Silverberg, et. al. Standards Track [Page 53] RFC 2446 iTIP November 1998

 (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+

Silverberg, et. al. Standards Track [Page 54] RFC 2446 iTIP November 1998

  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

Silverberg, et. al. Standards Track [Page 55] RFC 2446 iTIP November 1998

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.
==============+============================+=========================

Silverberg, et. al. Standards Track [Page 56] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 57] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 58] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 59] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 60] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 61] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 62] RFC 2446 iTIP November 1998

 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".

Silverberg, et. al. Standards Track [Page 63] RFC 2446 iTIP November 1998

+———————————————————————+

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

Silverberg, et. al. Standards Track [Page 64] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 65] RFC 2446 iTIP November 1998

 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".

Silverberg, et. al. Standards Track [Page 66] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 67] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 68] RFC 2446 iTIP November 1998

+———————————————————————+

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

+———————————————————————+

Silverberg, et. al. Standards Track [Page 69] RFC 2446 iTIP November 1998

 "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

Silverberg, et. al. Standards Track [Page 70] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 71] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 72] RFC 2446 iTIP November 1998

 +--------------------------------------------------------------------+
 | 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

Silverberg, et. al. Standards Track [Page 73] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 74] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 75] RFC 2446 iTIP November 1998

 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.

Silverberg, et. al. Standards Track [Page 76] RFC 2446 iTIP November 1998

+——————————————————————–+

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

Silverberg, et. al. Standards Track [Page 77] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 78] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 79] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 80] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 81] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 82] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 83] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 84] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 85] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 86] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 87] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 88] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 89] RFC 2446 iTIP November 1998

+——————————————————————–+

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

Silverberg, et. al. Standards Track [Page 90] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 91] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 92] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 93] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 94] RFC 2446 iTIP November 1998

 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".

Silverberg, et. al. Standards Track [Page 95] RFC 2446 iTIP November 1998

+——————————————————————–+

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

Silverberg, et. al. Standards Track [Page 96] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 97] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 98] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 99] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 100] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 101] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 102] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 103] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 104] RFC 2446 iTIP November 1998

 [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.

Silverberg, et. al. Standards Track [Page 105] RFC 2446 iTIP November 1998

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.

Silverberg, et. al. Standards Track [Page 106] RFC 2446 iTIP November 1998

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

Silverberg, et. al. Standards Track [Page 107] RFC 2446 iTIP November 1998

 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

Silverberg, et. al. Standards Track [Page 108] RFC 2446 iTIP November 1998

10. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Silverberg, et. al. Standards Track [Page 109]

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