RFC 2446
iCalendar Transport-Independent Interoperability Protocol (iTIP) Scheduling Events, BusyTime, To-dos and Journal Entries
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)".
iTIP complements the iCalendar object specification by adding semantics for group scheduling methods commonly available in current calendar systems. These scheduling methods permit two or more calendar systems to perform transactions such as publish, schedule, reschedule, respond to scheduling requests, negotiation of changes or cancel iCalendar-based calendar components. iTIP is defined independent of the particular transport used to transmit the scheduling information. Companion memos to iTIP provide bindings of the interoperability protocol to a number of Internet protocols. Table of Contents 1 INTRODUCTION...................................................5 1.1 FORMATTING CONVENTIONS .....................................5 1.2 RELATED DOCUMENTS ..........................................6 1.3 ITIP ROLES AND TRANSACTIONS ................................6 2 INTEROPERABILITY MODELS........................................8 2.1 APPLICATION PROTOCOL .......................................9 2.1.1 Calendar Entry State ...................................9 2.1.2 Delegation .............................................9 2.1.3 Acting on Behalf of other Calendar Users ..............10 2.1.4 Component Revisions ...................................10 2.1.5 Message Sequencing ....................................11 3 APPLICATION PROTOCOL ELEMENTS.................................12 3.1 COMMON COMPONENT RESTRICTION TABLES .......................13 3.2 METHODS FOR VEVENT CALENDAR COMPONENTS ....................14 3.2.1 PUBLISH ...............................................15 3.2.2 REQUEST ...............................................17 3.2.2.1 Rescheduling an Event..............................19 3.2.2.2 Updating or Reconfirmation of an Event.............19 3.2.2.3 Delegating an Event to another CU..................19 3.2.2.4 Changing the Organizer.............................20 3.2.2.5 Sending on Behalf of the Organizer.................20 3.2.2.6 Forwarding to An Uninvited CU......................20 3.2.2.7 Updating Attendee Status...........................21 3.2.3 REPLY .................................................21 3.2.4 ADD ...................................................23 3.2.5 CANCEL ................................................25 3.2.6 REFRESH ...............................................26 3.2.7 COUNTER ...............................................28 3.2.8 DECLINECOUNTER ........................................29 3.3 METHODS FOR VFREEBUSY COMPONENTS ..........................31 3.3.1 PUBLISH ...............................................32 3.3.2 REQUEST ...............................................33 3.3.3 REPLY .................................................34 3.4 METHODS FOR VTODO COMPONENTS ..............................35
3.4.1 PUBLISH ...............................................35
3.4.2 REQUEST ...............................................37
3.4.2.1 REQUEST for Rescheduling a VTODO...................39
3.4.2.2 REQUEST for Update or Reconfirmation of a VTODO....39
3.4.2.3 REQUEST for Delegating a VTODO.....................40
3.4.2.4 REQUEST Forwarded To An Uninvited Calendar User....40
3.4.2.5 REQUEST Updated Attendee Status....................41
3.4.3 REPLY .................................................41
3.4.4 ADD ...................................................43
3.4.5 CANCEL ................................................44
3.4.6 REFRESH ...............................................46
3.4.7 COUNTER ...............................................48
3.4.8 DECLINECOUNTER ........................................49
3.5 METHODS FOR VJOURNAL COMPONENTS ...........................50
3.5.1 PUBLISH ...............................................51
3.5.2 ADD ...................................................52
3.5.3 CANCEL ................................................53
3.6 STATUS REPLIES ............................................55
3.7 IMPLEMENTATION CONSIDERATIONS .............................57
3.7.1 Working With Recurrence Instances .....................57
3.7.2 Attendee Property Considerations ......................58
3.7.3 X-Tokens ..............................................59
4 EXAMPLES......................................................59
4.1 PUBLISHED EVENT EXAMPLES ..................................59
4.1.1 A Minimal Published Event .............................60
4.1.2 Changing A Published Event ............................60
4.1.3 Canceling A Published Event ...........................61
4.1.4 A Rich Published Event ................................62
4.1.5 Anniversaries or Events attached to entire days .......63
4.2 GROUP EVENT EXAMPLES ......................................63
4.2.1 A Group Event Request .................................64
4.2.2 Reply To A Group Event Request ........................65
4.2.3 Update An Event .......................................65
4.2.4 Countering an Event Proposal ..........................66
4.2.5 Delegating an Event ...................................68
4.2.6 Delegate Accepts the Meeting ..........................70
4.2.7 Delegate Declines the Meeting .........................71
4.2.8 Forwarding an Event Request ...........................72
4.2.9 Cancel A Group Event ..................................72
4.2.10 Removing Attendees ...................................74
4.2.11 Replacing the Organizer ..............................75
4.3 BUSY TIME EXAMPLES ........................................76
4.3.1 Request Busy Time .....................................77
4.3.2 Reply To A Busy Time Request ..........................77
4.4 RECURRING EVENT AND TIME ZONE EXAMPLES ....................78
4.4.1 A Recurring Event Spanning Time Zones .................78
4.4.2 Modify A Recurring Instance ...........................79
4.4.3 Cancel an Instance ....................................81
4.4.4 Cancel Recurring Event ................................81
4.4.5 Change All Future Instances ...........................82
4.4.6 Add A New Instance To A Recurring Event ...............82
4.4.7 Add A New Series of Instances To A Recurring Event ....83
4.4.8 Counter An Instance Of A Recurring Event ..............87
4.4.9 Error Reply To A Request ..............................88
4.5 GROUP TO-DO EXAMPLES ......................................89
4.5.1 A VTODO Request .......................................90
4.5.2 A VTODO Reply .........................................90
4.5.3 A VTODO Request for Updated Status ....................91
4.5.4 A Reply: Percent-Complete .............................91
4.5.5 A Reply: Completed ....................................92
4.5.6 An Updated VTODO Request ..............................92
4.5.7 Recurring VTODOs ......................................92
4.5.7.1 Request for a Recurring VTODO......................93
4.5.7.2 Calculating due dates in recurring VTODOs..........93
4.5.7.3 Replying to an instance of a recurring VTODO.......93
4.6 JOURNAL EXAMPLES ..........................................94
4.7 OTHER EXAMPLES ............................................94
4.7.1 Event Refresh .........................................94
4.7.2 Bad RECURRENCE-ID .....................................95
5 APPLICATION PROTOCOL FALLBACKS................................97
5.1 PARTIAL IMPLEMENTATION ....................................97
5.1.1 Event-Related Fallbacks ...............................97
5.1.2 Free/Busy-Related Fallbacks ...........................99
5.1.3 To-Do-Related Fallbacks ...............................99
5.1.4 Journal-Related Fallbacks ............................101
5.2 LATENCY ISSUES ...........................................102
5.2.1 Cancellation of an Unknown Calendar Component. .......102
5.2.2 Unexpected Reply from an Unknown Delegate ............103
5.3 SEQUENCE NUMBER ..........................................103
6 SECURITY CONSIDERATIONS......................................103
6.1 SECURITY THREATS .........................................103
6.1.1 Spoofing the "Organizer" .............................103
6.1.2 Spoofing the "Attendee" ..............................103
6.1.3 Unauthorized Replacement of the Organizer ............104
6.1.4 Eavesdropping ........................................104
6.1.5 Flooding a Calendar ..................................104
6.1.6 Procedural Alarms ....................................104
6.1.7 Unauthorized REFRESH Requests ........................104
6.2 RECOMMENDATIONS ..........................................104
6.2.1 Use of [RFC-1847] to secure iTIP transactions ........105
6.2.2 Implementation Controls ..............................105
7 ACKNOWLEDGMENTS..............................................106
8 BIBLIOGRAPHY.................................................106
9 AUTHORS' ADDRESSES...........................................107
10 FULL COPYRIGHT STATEMENT....................................109
1 Introduction This document specifies how calendaring systems use iCalendar objects to interoperate with other calendar systems. In particular, it specifies how to schedule events, to-dos, or daily journal entries. It further specifies how to search for available busy time information. It does so in a general way so as to allow multiple methods of communication between systems. Subsequent documents specify transport bindings between systems that use this protocol. This protocol is based on messages sent from an originator to one or more recipients. For certain types of messages, a recipient may reply, in order to update their status and may also return transaction/request status information. The protocol supports the ability for the message originator to modify or cancel the original message. The protocol also supports the ability for recipients to suggest changes to the originator of a message. The elements of the protocol also define the user roles for its transactions. 1.1 Formatting Conventions In order to refer to elements of the calendaring and scheduling model, core object or interoperability protocol defined in [iCAL] and [iTIP] several formatting conventions have been utilized. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119]. Calendaring and scheduling roles are referred to in quoted-strings of text with the first character of each word in upper case. For example, "Organizer" refers to a role of a "Calendar User" (CU) within the scheduling protocol defined by [iTIP]. Calendar components defined by [iCAL] are referred to with capitalized, quoted-strings of text. All calendar components start with the letter "V". For example, "VEVENT" refers to the event calendar component, "VTODO" refers to the to-do calendar component and "VJOURNAL" refers to the daily journal calendar component. Scheduling methods defined by [iTIP] are referred to with capitalized, quoted-strings of text. For example, "REQUEST" refers to the method for requesting a scheduling calendar component be created or modified, "REPLY" refers to the method a recipient of a request uses to update their status with the "Organizer" of the calendar component. Properties defined by [iCAL] are referred to with capitalized, quoted-strings of text, followed by the word "property". For example, "ATTENDEE" property refers to the iCalendar property used to convey the calendar address of a "Calendar User". Property parameters
defined by this memo are referred to with lower case, quoted-strings of text, followed by the word "parameter". For example, "value" parameter refers to the iCalendar property parameter used to override the default data type for a property value. Enumerated values defined by this memo are referred to with capitalized text, either alone or followed by the word "value". In tables, the quoted-string text is specified without quotes in order to minimize the table length. 1.2 Related Documents Implementers will need to be familiar with several other memos that, along with this one, describe the Internet calendaring and scheduling standards. This document, [iTIP], specifies an interoperability protocol for scheduling between different implementations. The related documents are: [iCAL] - specifies the objects, data types, properties and property parameters used in the protocols, along with the methods for representing and encoding them; [iMIP] specifies an Internet email binding for [iTIP]. This memo does not attempt to repeat the specification of concepts or definitions from these other memos. Where possible, references are made to the memo that provides for the specification of these concepts or definitions. 1.3 ITIP Roles and Transactions ITIP defines methods for exchanging [iCAL] objects for the purposes of group calendaring and scheduling between "Calendar Users" (CUs). CUs take on one of two roles in iTIP. The CU who initiates an exchange takes on the role of "Organizer". For example, the CU who proposes a group meeting is the "Organizer". The CUs asked to participate in the group meeting by the "Organizer" take on the role of "Attendee". Note that "role" is also a descriptive parameter to the _ATTENDEE_ property. Its use is to convey descriptive context to an "Attendee" such as "chair", "req-participant" or "non-participant" and has nothing to do with the calendaring workflow.
The ITIP methods are listed below and their usage and semantics are defined in section 3 of this document. +================+==================================================+ | Method | Description | |================+==================================================| | PUBLISH | Used to publish a calendar entry to one or more | | | Calendar Users. There is no interactivity | | | between the publisher and any other calendar | | | user. An example might include a baseball team | | | publishing its schedule to the public. | | | | | REQUEST | Used to schedule a calendar entry with other | | | Calendar Users. Requests are interactive in that | | | they require the receiver to respond using | | | the Reply methods. Meeting Requests, Busy | | | Time requests and the assignment of VTODOs to | | | other Calendar Users are all examples. | | | Requests are also used by the "Organizer" to | | | update the status of a calendar entry. | | | | | REPLY | A Reply is used in response to a Request to | | | convey "Attendee" status to the "Organizer". | | | Replies are commonly used to respond to meeting | | | and task requests. | | | | | ADD | Add one or more instances to an existing | | | VEVENT, VTODO, or VJOURNAL. | | | | | CANCEL | Cancel one or more instances of an existing | | | VEVENT, VTODO, or VJOURNAL. | | | | | REFRESH | The Refresh method is used by an "Attendee" to | | | request the latest version of a calendar entry. | | | | | COUNTER | The Counter method is used by an "Attendee" to | | | negotiate a change in the calendar entry. | | | Examples include the request to change a | | | proposed Event time or change the due date for a | | | VTODO. | | | | | DECLINE- | Used by the "Organizer" to decline the proposed | | COUNTER | counter-proprosal. | +================+==================================================+
Group scheduling in iTIP is accomplished using the set of "request" and "response" methods described above. The following table shows the methods broken down by who can send them. +================+==================================================+ | Originator | Methods | |================+==================================================| | Organizer | PUBLISH, REQUEST, ADD, CANCEL, DECLINECOUNTER | | | | | Attendee | REPLY, REFRESH, COUNTER | | | REQUEST only when delegating | +================+==================================================+ Note that for some calendar component types, the allowable methods are a subset of the above set. 2 Interoperability Models There are two distinct protocols relevant to interoperability: an "Application Protocol" and a "Transport Protocol". The Application Protocol defines the content of the iCalendar objects sent between sender and receiver to accomplish the scheduling transactions listed above. The Transport Protocol defines how the iCalendar objects are sent between the sender and receiver. This document focuses on the Application Protocol. Binding documents such as [iMIP] focus on the Transport Protocol. The connection between Sender and Receiver in the diagram below refers to the Application Protocol. The iCalendar objects passed from the Sender to the Receiver are presented in Section 3, Application Protocol Elements. +----------+ +----------+ | | iTIP | | | Sender |<-------------------->| Receiver | | | | | +----------+ +----------+ There are several variations of this diagram in which the Sender and Receiver take on various roles of a "Calendar User Agent" (CUA) or a "Calendar Service" (CS). The architecture of iTIP is depicted in the diagram below. An application written to this specification may work with bindings for the store-and-forward transport, the real time transport, or both. Also note that iTIP could be bound to other transports.
+------------------------------------------+ | iTIP | +------------------------------------------+ |Real-time | Store-and-Fwd | Other | |Transport | Transport | Transports... | +------------------------------------------+ 2.1 Application Protocol In the iTIP model, a calendar entry is created and managed by an "Organizer". The "Organizer" interacts with other CUs by sending one or more of the iTIP messages listed above. "Attendees" use the "REPLY" method to communicate their status. "Attendees" do not make direct changes to the master calendar entry. They can, however, use the "COUNTER" method to suggest changes to the "Organizer". In any case, the "Organizer" has complete control over the master calendar entry. 2.1.1 Calendar Entry State There are two distinct states relevant to calendar entries: the overall state of the entry and the state associated with an "Attendee" to that entry. The state of an entry is defined by the "STATUS" property and is controlled by the "Organizer." There is no default value for the "STATUS" property. The "Organizer" sets the "STATUS" property to the appropriate value for each calendar entry. The state of a particular "Attendee" relative to an entry is defined by the "partstat" parameter in the "ATTENDEE" property for each "Attendee". When an "Organizer" issues the initial entry, "Attendee" status is unknown. The "Organizer" specifies this by setting the "partstat" parameter to "NEEDS-ACTION". Each "Attendee" modifies their "ATTENDEE" property "partstat" parameter to an appropriate value as part of a "REPLY" message sent back to the "Organizer". 2.1.2 Delegation Delegation is defined as the process by which an "Attendee" grants another CU (or several CUs) the right to attend on their behalf. The "Organizer" is made aware of this change because the delegating "Attendee" informs the "Organizer". These steps are detailed in the REQUEST method section.
2.1.3 Acting on Behalf of other Calendar Users In many organizations one user will act on behalf of another to organize and/or respond to meeting requests. ITIP provides two mechanisms that support these activities. First, the "Organizer" is treated as a special entity, separate from "Attendees". All responses from "Attendees" flow to the "Organizer", making it easy to separate a calendar user organizing a meeting from calendar users attending the meeting. Additionally, iCalendar provides descriptive roles for each "Attendee". For instance, a role of "chair" may be ascribed to one or more "Attendees". The "chair" and the "Organizer" may or may not be the same calendar user. This maps well to scenarios where an assistant may manage meeting logistics for another individual who chairs a meeting. Second, a "sent-by" parameter may be specified in either the "Organizer" or "Attendee" properties. When specified, the "sent-by" parameter indicates that the responding CU acted on behalf of the specified "Attendee" or "Organizer". 2.1.4 Component Revisions The "SEQUENCE" property is used by the "Organizer" to indicate revisions to the calendar component. The rules for incrementing the "SEQUENCE" number are defined in [iCAL]. For clarity, these rules are paraphrased here in terms of how they are applied in [iTIP]. For a given "UID" in a calendar component: . For the "PUBLISH" and "REQUEST" methods, the "SEQUENCE" property value is incremented according to the rules defined in [iCAL]. . The "SEQUENCE" property value MUST be incremented each time the "Organizer" uses the "ADD" or "CANCEL" methods. . The "SEQUENCE" property value MUST NOT be incremented when using "REPLY", "REFRESH", "COUNTER", "DECLINECOUNTER", or when sending a delegation "REQUEST". In some circumstances the "Organizer" may not have received responses to the final revision sent out. In this situation, the "Organizer" may wish to send an update "REQUEST", and set "RSVP=TRUE" for all "Attendees", so that current responses can be collected.
The value of the "SEQUENCE" property contained in a response from an "Attendee" may not always match the "Organizer's" revision. Implementations may choose to have the CUA indicate to the CU that the response is to an entry that has been revised and allow the CU to decide whether or not to accept the response. 2.1.5 Message Sequencing CUAs that handle the [iTIP] application protocol must often correlate a component in a calendar store with a component received in the [iTIP] message. For example, an event may be updated with a later revision of the same event. To accomplish this, a CUA must correlate the version of the event already in its calendar store with the version sent in the [iTIP] message. In addition to this correlation, there are several factors that can cause [iTIP] messages to arrive in an unexpected order. That is, an "Organizer" could receive a reply to an earlier revision of a component AFTER receiving a reply to a later revision. To maximize interoperability and to handle messages that arrive in an unexpected order, use the following rules: 1. The primary key for referencing a particular iCalendar component is the "UID" property value. To reference an instance of a recurring component, the primary key is composed of the "UID" and the "RECURRENCE-ID" properties. 2. The secondary key for referencing a component is the "SEQUENCE" property value. For components where the "UID" is the same, the component with the highest numeric value for the "SEQUENCE" property obsoletes all other revisions of the component with lower values. 3. "Attendees" send "REPLY" messages to the "Organizer". For replies where the "UID" property value is the same, the value of the "SEQUENCE" property indicates the revision of the component to which the "Attendee" is replying. The reply with the highest numeric value for the "SEQUENCE" property obsoletes all other replies with lower values. 4. In situations where the "UID" and "SEQUENCE" properties match, the "DTSTAMP" property is used as the tie-breaker. The component with the latest "DTSTAMP" overrides all others. Similarly, for "Attendee" responses where the "UID" property values match and the "SEQUENCE" property values match, the response with the latest "DTSTAMP" overrides all others.
Hence, CUAs must persist the following component properties: "UID", "RECURRENCE-ID", "SEQUENCE", and "DTSTAMP". Furthermore, for each "ATTENDEE" property of a component CUAs must persist the "SEQUENCE" and "DTSTAMP" property values associated with the "Attendee's" response.
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