MIT
G. Babics
Steltor
A. Taler
June 2002
Guide to Internet Calendaring
Status of this Memo
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This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright Notice
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Copyright © The Internet Society (2002). All Rights Reserved.
Abstract
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This document describes the various Internet calendaring and scheduling standards and works in progress, and the relationships between them. Its intent is to provide a context for these documents, assist in their understanding, and potentially aid in the design of standards-based calendaring and scheduling systems. The standards addressed are RFC 2445 (iCalendar), RFC 2446 (iTIP), and RFC 2447 (iMIP). The work in progress addressed is "Calendar Access Protocol" (CAP). This document also describes issues and problems that are not solved by these protocols, and that could be targets for future work.
Table of Contents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Concepts and Relationships . . . . . . . . . . . . . . . . . 4 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Fundamental Needs . . . . . . . . . . . . . . . . . . . . . 4 2.2 Protocol Requirements . . . . . . . . . . . . . . . . . . . 5 3. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2.1 Standalone Single-user System . . . . . . . . . . . . . . . 8 3.2.2 Single-user Systems Communicating . . . . . . . . . . . . . 8 3.2.3 Single-user with Multiple CUAs . . . . . . . . . . . . . . . 9 3.2.4 Single-user with Multiple Calendars . . . . . . . . . . . . 9 3.2.5 Users Communicating on a Multi-user System . . . . . . . . . 10 3.2.6 Users Communicating through Different Multi-user Systems . . 10 4. Important Aspects . . . . . . . . . . . . . . . . . . . . . 10 4.1 Timezones . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.2 Choice of Transport . . . . . . . . . . . . . . . . . . . . 11 4.3 Security . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.4 Amount of data . . . . . . . . . . . . . . . . . . . . . . . 11 4.5 Recurring Components . . . . . . . . . . . . . . . . . . . . 11 5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 11 5.1 Scheduling People, not Calendars . . . . . . . . . . . . . . 12 5.2 Administration . . . . . . . . . . . . . . . . . . . . . . . 12 5.3 Notification . . . . . . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . 12 6.1 Access Control . . . . . . . . . . . . . . . . . . . . . . . 12 6.2 Authentication . . . . . . . . . . . . . . . . . . . . . . . 12 6.3 Using E-mail . . . . . . . . . . . . . . . . . . . . . . . . 13 6.4 Other Issues . . . . . . . . . . . . . . . . . . . . . . . . 13 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 13 References . . . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15 Full Copyright Statement . . . . . . . . . . . . . . . . . . 16
1. Introduction
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Calendaring and scheduling protocols are intended to aid individuals in obtaining calendaring information and scheduling meetings across the Internet, to aid organizations in providing calendaring information on the Internet, and to provide for organizations looking for a calendaring and scheduling solution to deploy internally.
It is the intent of this document to provide a context for these documents, assist in their understanding, and potentially help in the design of standards-based calendaring and scheduling systems.
Problems not solved by these protocols, as well as security issues to be kept in mind, are discussed at the end of the document.
1.1 Terminology
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This memo uses much of the same terminology as iCalendar [RFC-2445], iTIP [RFC-2446], iMIP [RFC-2447], and [CAP]. The following definitions are provided as an introduction; the definitions in the protocol specifications themselves should be considered canonical.
Calendar
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A collection of events, to-dos, journal entries, etc. A calendar could be the content of a person or resource's agenda; it could also be a collection of data serving a more specialized need. Calendars are the basic storage containers for calendaring information.
Calendar Access Rights
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A set of rules defining who may perform what operations, such as reading or writing information, on a given calendar.
Calendar Service
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A running server application that provides access to a number of calendar stores.
Calendar Store (CS)
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A data store of a calendar service. A calendar service may have several calendar stores, and each store may contain several calendars, as well as properties and components outside of those calendars.
Calendar User (CU)
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An entity (often a human) that accesses calendar information.
Calendar User Agent (CUA)
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Software with which the calendar user communicates with a calendar service or local calendar store to access calendar information.
Component
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A piece of calendar data such as an event, a to-do or an alarm. Information about components is stored as properties of those components.
Delegator
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A calendar user who has assigned his or her participation in a scheduled calendar component (e.g. a VEVENT) to another calendar user (sometimes called the delegate or delegatee). An example of a delegator is a busy executive sending an employee to a meeting in his or her place.
Delegate
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A calendar user (sometimes called the delegatee) who has been assigned to participate in a scheduled calendar component (e.g. a VEVENT) in place of one of the attendees in that component (sometimes called the delegator). An example of a delegate is a team member sent to a particular meeting.
Designate
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A calendar user authorized to act on behalf of another calendar user. An example of a designate is an assistant scheduling meetings for his or her superior.
Local Store
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A CS that is on the same device as the CUA.
Property
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A description of some element of a component, such as a start time, title or location.
Remote Store
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A CS that is not on the same device as the CUA.
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1.2 Concepts and Relationships
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iCalendar is the language used to describe calendar objects. iTIP describes a way to use the iCalendar language to do scheduling. iMIP describes how to do iTIP scheduling via e-mail. CAP describes a way to use the iCalendar language to access a calendar store in real- time.
The relationship between calendaring protocols is similar to that between e-mail protocols. In those terms, iCalendar is analogous to RFC 2822, iTIP and iMIP are analogous to the Simple Mail Transfer Protocol (SMTP), and CAP is analogous to the Post Office Protocol (POP) or Internet Message Access Protocol (IMAP).
2. Requirements
2.1 Fundamental Needs
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The following scenarios illustrate people and organizations' basic calendaring and scheduling needs:
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a] A doctor wishes to keep track of all her appointments.
Need:
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To read and manipulate one's own calendar with only one CUA.
b] A busy musician wants to maintain her schedule with multiple devices, such as through an Internet-based agenda and with a PDA.
Need: To read and manipulate one's own calendar, possibly with solutions from different vendors.
c] A software development team wishes to more effectively schedule their time through viewing each other's calendar information.
Need: To share calendar information between users of the same calendar service.
d] A teacher wants his students to schedule appointments during his office hours.
Need: To schedule calendar events, to-dos and journals with other users of the same calendar service.
e] A movie theater wants to publish its schedule for prospective customers.
Need: To share calendar information with users of other calendar services, possibly from a number of different vendors.
f] A social club wants to schedule calendar entries effectively with its members.
Need: To schedule calendar events and to-dos with users of other calendar services, possibly from a number of different vendors.
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2.2 Protocol Requirements
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Some of these needs can be met by proprietary solutions (a, c, d), but others can not (b, e, f). These latter scenarios show that standard protocols are required for accessing information in a calendar store and scheduling calendar entries. In addition, these protocols require a common data format for representing calendar information.
These requirements are met by the following protocol specifications.
- Data format: iCalendar [RFC-2445]
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iCalendar [RFC-2445] provides a data format for representing calendar information, to be used and exchanged by other protocols. iCalendar [RFC-2445] can also be used in other contexts, such as a drag-and-drop interface, or an export/import feature. All the other calendaring protocols depend on iCalendar [RFC-2445], so all elements of a standards-based calendaring and scheduling systems will have to be able to interpret iCalendar [RFC-2445].
- Scheduling protocol: iTIP [RFC-2446]
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iTIP [RFC-2446] describes the messages used to schedule calendar events. Within iTIP messages, events are represented in iCalendar [RFC-2445] format, and have semantics that identify the message as being an invitation to a meeting, an acceptance of an invitation, or the assignment of a task.
iTIP [RFC-2446] messages are used in the scheduling workflow, where users exchange messages in order to organize things such as events and to-dos. CUAs generate and interpret iTIP [RFC-2446] messages at the direction of the calendar user. With iTIP [RFC- 2446] users can create, modify, delete, reply to, counter, and decline counters to the various iCalendar [RFC-2445] components. Furthermore, users can also request the free/busy time of other people.
iTIP [RFC-2446] is transport-independent, and has one specified transport binding: iMIP [RFC-2447] binds iTIP to e-mail. In addition [CAP] will provide a real-time binding of iTIP [RFC- 2446], allowing CUAs to perform calendar management and scheduling over a single connection.
- Calendar management protocol: [CAP]
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[CAP] describes the messages used to manage calendars on a calendar store. These messages use iCalendar [RFC-2445] to describe various components such as events and to-dos. These messages make it possible to perform iTIP [RFC-2446] operations, as well as other operations relating to a calendar store such as searching, creating calendars, specifying calendar properties, and specifying calendar access rights.
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3. Solutions
3.1 Examples
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Returning to the scenarios presented in section 2.1, the calendaring protocols can be used in the following ways:
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a] The doctor can use a proprietary CUA with a local store, and perhaps use iCalendar [RFC-2445] as a storage mechanism. This would allow her to easily import her data store into another application that supports iCalendar [RFC-2445].
b] The musician who wishes to access her agenda from anywhere can use a [CAP]-enabled calendar service accessible over the Internet. She can then use any available [CAP] clients to access the data.
A proprietary system that provides access through a Web-based interface could also be employed, but the use of [CAP] would be superior in that it would allow the use of third party applications, such as PDA synchronization tools.
c] The development team can use a calendar service which supports [CAP], and each member can use a [CAP]-enabled CUA of their choice.
Alternatively, each member could use an iMIP [RFC-2447]-enabled CUA, and they could book meetings over e-mail. This solution has the drawback that it is difficult to examine other users' agendas, making the organization of meetings more difficult.
Proprietary solutions are also available, but they require that all members use clients by the same vendor, and disallow the use of third party applications.
d] The teacher can set up a calendar service, and have students book time through any of the iTIP [RFC-2446] bindings. [CAP] provides real-time access, but could require additional configuration. iMIP [RFC-2447] would be the easiest to configure, but may require more e-mail processing.
If [CAP] access is provided then determining the state of the teacher's schedule is straightforward. If not, this can be determined through iTIP [RFC-2446] free/busy requests. Non- standard methods could also be employed, such as serving up iCalendar [RFC-2445], HTML, or XML over HTTP.
A proprietary system could also be used, but would require that all students be able to use software from a specific vendor.
e] [CAP] would be preferred for publishing a movie theater's schedule, since it provides advanced access and search capabilities. It also allows easy integration with customers' calendar systems.
Non-standard methods such as serving data over HTTP could also be employed, but would be harder to integrate with customers' systems.
Using a completely proprietary solution would be very difficult, if not impossible, since it would require every user to install and use the proprietary software.
f] The social club could distribute meeting information in the form of iTIP [RFC-2446] messages, sent via e-mail using iMIP [RFC-2447]. The club could distribute meeting invitations, as well as a full published agenda.
Alternatively, the club could provide access to a [CAP]-enabled calendar service. However, this solution would be more expensive since it requires the maintenance of a server.
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3.2 Systems
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The following diagrams illustrate possible systems and their usage of the various protocols.
3.2.1 Standalone Single-user System
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A single user system that does not communicate with other systems need not employ any of the protocols. However, it may use iCalendar [RFC-2445] as a data format in some places.
----------- O | CUA w/ | -+- user |local store| A ----------- / \
3.2.2 Single-user Systems Communicating
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Users with single-user systems may schedule meetings with each others using iTIP [RFC-2446]. The easiest binding of iTIP [RFC-2446] to use would be iMIP [RFC-2447], since messages can be held in the users' mail queues, which we assume to already exist. [CAP] could also be used.
O ----------- ----------- O -+- | CUA w/ | -----[iMIP]----- | CUA w/ | -+- user A |local store| Internet |local store| A / \ ----------- ----------- / \
3.2.3 Single-user with Multiple CUAs
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A single user may use more than one CUA to access his or her calendar. The user may use a PDA, a Web client, a PC, or some other device, depending on accessibility. Some of these clients may have local stores and others may not. Those with local stores need to synchronize the data on the CUA with the data on the CS.
----------- | CUA w | -----[CAP]----------+ |local store| | O ----------- ---------- -+- | CS | A | | / \ ---------- ----------- | | CUA w/o | -----[CAP]----------+ |local store| -----------
3.2.4 Single-user with Multiple Calendars
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A single user may have many independent calendars; for example, one may contain work-related information and another personal information. The CUA may or may not have a local store. If it does, then it needs to synchronize the data of the CUA with the data on both of the CS.
---------- +------------[CAP]------ | CS | | | | O ----------- ---------- -+- | CUA | A | | / \ ----------- | ---------- +------------[CAP]------ | CS | | | ----------
3.2.5 Users Communicating on a Multi-user System
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Users on a multi-user system may schedule meetings with each other using [CAP]-enabled CUAs and services. The CUAs may or may not have local stores. Those with local stores need to synchronize the data on the CUAs with the data on the CS.
O ----------- -+- | CUA w | -----[CAP]----------+ A |local store| | / \ ----------- ---------- | CS | | | ---------- O ----------- | -+- | CUA w/o | -----[CAP]----------+ A |local store| / \ -----------
3.2.6 Users Communicating through Different Multi-user Systems
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Users on a multi-user system may need to schedule meetings with users on a different multi-user system. The services can communicate using [CAP] or iMIP [RFC-2447].
O ----------- ---------- -+- | CUA w | -----[CAP]-------| CS | A |local store| | | / \ ----------- ---------- | [CAP] or [iMIP] | O ----------- ---------- -+- | CUA w/o | -----[CAP]-------| CS | A |local store| | | / \ ----------- ----------
4. Important Aspects
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There are a number of important aspects of these calendaring standards of which people, especially implementers, should be aware.
4.1 Timezones
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The dates and times in components can refer to a specific time zone. Time zones can be defined in a central store, or they may be defined by a user to fit his or her needs. All users and applications should be aware of time zones and time zone differences. New time zones may need to be added, and others removed. Two different vendors may describe the same time zone differently (such as by using a different name).
4.2 Choice of Transport
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There are issues to be aware of in choosing between a network protocol such as [CAP], or a store and forward protocol, such as iMIP [RFC-2447].
The use of a network ("on-the-wire") mechanism may require some organizations to make provisions to allow calendaring traffic to traverse a corporate firewall on the required ports. Depending on the organizational culture, this may be a challenging social exercise.
The use of an email-based mechanism exposes time-sensitive data to unbounded latency. Large or heavily utilized mail systems may experience an unacceptable delay in message receipt.
4.3 Security
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See the "Security Considerations" (Section 6) section below.
4.4 Amount of data
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In some cases, a component may be very large, for instance, a component with a very large attachment. Some applications may be low-bandwidth or may be limited in the amount of data they can store. Maximum component size may be set in [CAP]. It can also be controlled in iMIP [RFC-2447] by restricting the maximum size of the e-mail that the application can download.
4.5 Recurring Components
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In iCAL [RFC-2445], one can specify complex recurrence rules for VEVENTs, VTODOs, and VJOURNALs. One must be careful to correctly interpret these recurrence rules and pay extra attention to being able to interoperate using them.
5. Open Issues
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Many issues are not currently resolved by these protocols, and many desirable features are not yet provided. Some of the more prominent ones are outlined below.
5.1 Scheduling People, not Calendars
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Meetings are scheduled with people; however, people may have many calendars, and may store these calendars in many places. There may also be many routes to contact them. The calendaring protocols do not attempt to provide unique access for contacting a given person. Instead, 'calendar addresses' are booked, which may be e-mail addresses or individual calendars. It is up to the users themselves to orchestrate mechanisms to ensure that the bookings go to the right place.
5.2 Administration
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The calendaring protocols do not address the issues of administering users and calendars on a calendar service. This must be handled by proprietary mechanisms for each implementation.
5.3 Notification
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People often wish to be notified of upcoming events, new events, or changes to existing events. The calendaring protocols do not attempt to address these needs in a real-time system. Instead, the ability to store alarm information on events is provided, which can be used to provide client-side notification of upcoming events. To organize notification of new or changed events, clients have to poll the data store.
6. Security Considerations
6.1 Access Control
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There has to be reasonable granularity in the configuration options for access to data through [CAP], so that what should be released to requesters is released, and what shouldn't is not. Details of handling this are described in [CAP].
6.2 Authentication
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Access control must be coupled with a good authentication system, so that the right people get the right information. For [CAP], this means requiring authentication before any database access can be performed, and checking access rights and authentication credentials before releasing information. [CAP] uses the Simple Authentication Security Layer (SASL) for this authentication. In iMIP [RFC-2447], this may present some challenges, as authentication is often not a consideration in store-and-forward protocols.
Authentication is also important for scheduling, in that receivers of scheduling messages should be able to validate the apparent sender. Since scheduling messages are wrapped in MIME [RFC-2045], signing and encryption are freely available. For messages transmitted over mail, this is the only available alternative. It is suggested that developers take care in implementing the security features in iMIP [RFC-2447], bearing in mind that the concept and need may be foreign or non-obvious to users, yet essential for the system to function as they might expect.
The real-time protocols provide for the authentication of users, and the preservation of that authentication information, allowing for validation by the receiving end-user or server.
6.3 Using E-mail
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Because scheduling information can be transmitted over mail without any authentication information, e-mail spoofing is extremely easy if the receiver is not checking for authentication. It is suggested that implementers consider requiring authentication as a default, using mechanisms such as are described in Section 3 of iMIP [RFC- 2447]. The use of e-mail, and the potential for anonymous connections, means that 'calendar spam' is possible. Developers should consider this threat when designing systems, particularly those that allow for automated request processing.
6.4 Other Issues
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The current security context should be obvious to users. Because the underlying mechanisms may not be clear to users, efforts to make clear the current state in the UI should be made. One example of this is the 'lock' icon used in some Web browsers during secure connections.
With both iMIP [RFC-2447] and [CAP], the possibilities of Denial of Service attacks must be considered. The ability to flood a calendar system with bogus requests is likely to be exploited once these systems become widely deployed, and detection and recovery methods will need to be considered.
Acknowledgments
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Thanks to the following, who have participated in the development of this document:
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Eric Busboom, Pat Egen, David Madeo, Shawn Packwood, Bruce Kahn, Alan Davies, Robb Surridge.
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References
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[RFC-2445] Dawson, F. and D. Stenerson, "Internet Calendaring and Scheduling Core Object Specification - iCalendar", RFC 2445, November 1998. [RFC-2446] Silverberg, S., Mansour, S., Dawson, F. and R. Hopson, "iCalendar Transport-Independent Interoperability Protocol (iTIP): Scheduling Events, Busy Time, To-dos and Journal Entries", RFC 2446, November 1998. [RFC-2447] Dawson, F., Mansour, S. and S. Silverberg, "iCalendar Message-Based Interoperability Protocol - iMIP", RFC 2447, November 1998. [RFC-2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) - Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [CAP] Mansour, S., Royer, D., Babics, G., and Hill, P., "Calendar Access Protocol (CAP)", Work in Progress.
Authors' Addresses
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Bob Mahoney MIT E40-327 77 Massachusetts Avenue Cambridge, MA 02139 US Phone: (617) 253-0774 EMail: bobmah@mit.edu George Babics Steltor 2000 Peel Street Montreal, Quebec H3A 2W5 CA Phone: (514) 733-8500 x4201 EMail: georgeb@steltor.com Alexander Taler EMail: alex@0--0.org
Full Copyright Statement
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Copyright © The Internet Society (2002). All Rights Reserved.
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Acknowledgement
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Funding for the RFC Editor function is currently provided by the Internet Society.