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Versions: 00 01 02                                                      
Network Working Group
Guide to Implementors                                Bob Mahoney/MIT
Internet-Draft                                       Alexander Taler/CS&T
<draft-ietf-calsch-imp-guide-01.txt>                 George Babics/CS&T
July 14, 2000
Expires: January 14, 2001

                     Implementors' Guide to Internet Calendaring

Status of this Memo

        This document is an Internet-Draft and is in full conformance
        with all provisions of Section 10 of RFC2026.

        Internet-Drafts are working documents of the Internet Engineering
        Task Force (IETF), its areas, and its working groups. Note that
        other groups may also distribute working documents as

        Internet-Drafts are draft documents valid for a maximum of six
        months and may be updated, replaced, or obsoleted by other
        documents at any time. It is inappropriate to use Internet-
        Drafts as reference material or to cite them other than as
        "work in progress."

      The list of current Internet-Drafts can be accessed at

      The list of Internet-Draft Shadow Directories can be accessed at

      Copyright (C) The Internet Society 2000.  All Rights Reserved.


        This document describes the relationship between the various internet
        calendaring and scheduling protocols defined by RFC 2445 (iCalendar),
        RFC 2446 (iTIP), and RFC 2447 (iMIP), as well as the works in
        progress,"iCalendar Real-time Interoperability Protocol" (iRIP),
        and "Calendar Access Protocol" (CAP). It's intention is to provide
        a context for these protocols, assist in their understanding, and
        ultimately help implementors in the design of their internet
        calendaring and scheduling systems.

        [Note: in the past there has been some discussion as to whether iRIP
        was a live effort, given that interest has waned and some functionality
        has been moved to CAP.  What's the status?]

        This document also describes issues and problems which are not solved
        by these protocols, and could be targets for future work.

Status of this Memo
1. Introduction
2. Requirements
   Fundamental Need
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   Protocol Requirements
3. Standards Solution

   Standalone single-user system
   Single-user systems communicating
4. Open Issues
   Choice of Transport
   Scheduling People, not calendars
5. Security Considerations
   Access Control
   Using Email
   Other issues
6. Acknowledgements
7. Bibliography
8. Author's Addresses
9. Full Copyright Statement

1. Introduction

        The calendaring and scheduling protocols are intended to provide for
        the needs of individuals attempting to obtain information and
        schedule meetings across the internet, organizations attempting to
        provide information on the internet, as well as organizations looking
        for a calendaring and scheduling solution to deploy internally.

        It is the intent of this document to provide guidance for
        implementors of calendaring and scheduling products in determining
        which of the various existing protocol documents are applicable to
        their work, as well as providing some background information and
        pointers to the less obvious implications of the available choices.

        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

        This memo uses much of the same terminology as [ICAL], [ITIP],
        [IMIP], [IRIP] and [CAP]. The following definitions are provided as
        introductory, the definitions in the protocol specifications are the
        canonical ones.

        A collection of events, todos, journal entries, etc. A calendar
        could be the content of a person's or a resource's agenda; it
        could also be a collection of data serving a more specialized
        need. Calendars are the basic storage containers for calendaring

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Calendar Access Rights
        A set of rules for a calendar describing who may perform which
        operations on that calendar, such as reading and writing

Calendar Service
        A running server application which provides access to a
        collection of calendars.

Calendar Store
        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 the

Calendar User
        An entity (often a human) which accesses calendar information.

Calendar User Agent (CUA)
        Software used by the calendar user which communicates with
        calendar services to provide the user access to calendar

        A piece of calendar data such as an event, a todo or an alarm.
        Information about components is stored as properties of those

        A property of a component, such as a description or a start time.

1.2 Concepts and Relationships

        [A rough outline, based on some comments from Bruce.  We have some
        basic problems throughout this doc with exactly where our real-time
        efforts are concentrated, iRIP or CAP.  We do not currently have an
        unexpired iRIP document.  There has been some discussion in the past
        noting that the iRIP intent has largely been rolled into CAP.  We need
        to settle on an approach.  iRIP has been left in this draft for the
        moment, but overlap is apparent]

        iCalendar is the Language to be used in calendar events.
        iTIP is how you use the language.
        iMIP is further definition for use over email.
        CAP/iRIP is the Language used over a real-time transport.

        Another way to put it is as follows:
        iCalendar are the words
        iTIP is the grammar book or the "Rosetta Stone".
        iMIP is "expressing it in email terminology" an EMAIL dictionary
        CAP/iRIP is "expressing it for use in a Real Time transport"

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2. Requirements

2.1 Fundamental Needs

        The following examples illustrate people's basic calendaring and
        scheduling needs:

        a] A busy musician wants to maintain her schedule on an
           internet-based agenda which she can access from anywhere.

        Need: Read and manipulate one's own calendar.

        b] A software development team wishes to share agenda information
           by using a group scheduling product in order to more effectively
           schedule their time.

        Need: Share calendar information with users using the same
        calendar service.

        c] A teacher wants his students to be able to book time slots
           during his office hours.

        Need: Schedule calendar events and todos with users using the
        same calendar service.

        d] A movie theatre wants to publish its schedule so that
           prospective customers can easily access it.

        Need: Share calendar information with users using other calendar
        services, possibly from different vendors.

        e] A social club wants to be able to organize events more
           effectively by booking time with its members.

        Need: Schedule calendar events and todos with users using other
        calendar services, possibly from different vendors.

        f]   A doctor wishes to keep track of all his appointments.

        Need: Read and manipulate one's own calendar with only one CUA.

2.2 Protocol requirements

        The first three needs can be satisfied through proprietary solutions,
        but the last two cannot. From these needs we can establish that
        protocols are required for accessing information in a calendar store,
        and for scheduling events and todos. In addition these protocols
        require a data format for representing calendar information.

        These roles are filled by the following protocol requirements.

                - [ICAL] is the data format

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                [ICAL] provides data format for representing calendar information
                which the other protocols can use. [ICAL] can also be used in
                other contexts such as a drag and drop format or an export/import

                All the other protocols depend on [ICAL], so all elements of a
                standards-based calendaring and scheduling systems will have to
                interpret [ICAL].

                - [ITIP] is the scheduling protocol

                [ITIP] describes the messages used to schedule calendar events.
                These messages are represented in [ICAL], and have semantics that
                include such things as being an invitation to a meeting, an
                acceptance of an invitation or the assignation of a task.

                [ITIP] messages are used in the scheduling work flow, where users
                exchange messages in order to organize things such as events and
                todos. CUAs generate and interpret [ITIP] messages at the
                direction of the calendar user.

                [ITIP] is transport-independent, but has two specified transport
                bindings, [IMIP] is a binding to email and [IRIP] is a real-time
                binding. In addition [CAP] will provide a second real-time
                binding of [ITIP], allowing CUAs to perform calendar management
                as well as scheduling over a single connection.

                Both CUAs and calendar services may have [ITIP] interpreters.

                - [CAP] is the calendar management protocol

                [CAP] describes the messages used to manage calendars. These
                messages are represented in [ICAL], and have semantics such as
                being a search for data, being data in response to a search or
                the being the creation of a meeting.

                [CAP] also provides a real-time binding for the calendar
                management messages. Although other bindings, such as an email
                binding, could be defined, this is not done because it is
                inappropriate for this protocol.

        The following diagram describes the implementation dependencies
        between the protocols. A calendar system using these standards
        will implement at least one of the leaves of the tree. The
        calendar management message and transport protocol parts of CAP are
        separated in the diagram to highlight its relationship to ITIP.

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                             |    iCalendar     |
                    |                                    |
            ------------------                           |
           |      iTIP        |                          |
            ------------------                           |
                    |                                    |
                    |                          ----------|-------
                    |                         |  CAP     |       |
                    |                         |        message   |
            ----------------------------------------   format    |
            |               |                 |    |     |       |
       ----------      -----------            |    |     |       |
      | Session  |    |   E-mail  |           |   transport      |
      |   iRIP   |    |    iMIP   |           |   protocol       |
       ----------      -----------             ------------------

3. Solutions

3.1 Examples

        Returning to the examples of section 2.1, they can be solved using
        the protocols in the following ways:

        a] The musician who wishes to access her agenda from anywhere can
                use a [CAP] enabled calendar service accessible through the
                internet. She can then use whichever [CAP] clients are
                available to access the data.

                A proprietary system could also be employed which provides
                access through a web-based interface, but the use of [CAP] would
                be superior in that it would allow the use of third party tools,
                such as PDA synchronization tools.

        b] The development team can use a calendar service which supports
                [CAP] and then each member can use a [CAP]-enabled CUA of their

                Alternatively, each member could use an [IMIP]-enabled CUA, and
                they could book meetings over email. This solution has the
                drawback that it is difficult to examine the other agendas,
                making organizing meetings more difficult.

                Proprietary solutions are also available, but they require that
                all people use clients by the same vendor, and disallow the use
                of third party applications.

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        c] The teacher can set up a calendar service, and have students
                book time through any of the [ITIP] bindings. [CAP] or [IRIP]
                provide real-time access, but could require additional
                configuration. [IMIP] would be the easiest to configure, but
                may require more email 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] free-busy requests. Non-standard
                methods could also be employed, such as serving up ICAL, HTML,
                XML through HTTP.

                A proprietary system could also be used, but would require that
                all students be able to use software from a specific vendor.

        d] For publishing a movie theatre's schedule [CAP] provides the
                most advanced access and search capabilities. It also allows
                easy integration with its customer's calendar systems.

                Non-standard methods such as serving data over HTTP could also
                be employed, but would be harder to integrate with customer's

                Using a completely proprietary solutions would be very difficult
                since it would require every user to install and use proprietary

        e] The social club could distribute meeting information in the form
                of [ITIP] messages. This could be done over email using [IMIP],
                or [IRIP] depending on the recipient. Meeting invitations, as
                well as a full published agenda could be distributed.

                Alternatively, the social 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.

        f] The doctor can use a proprietary CUA with a local store,
            and perhaps use [ICAL] as a storage mechanism.

3.2 Systems

        The following diagrams illustrate possible example systems and usage
        of the protocols.

3.2.1 Standalone single-user system

        A single user system which does not communicate with other systems
        need not employ any of the protocols. However, it may use [ICAL] as
        a data format in some places.

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         -----------       O
        | CUA w/    |     -+- user
        |local store|      A
         -----------      / \

3.2.2 Single-user systems communicating

        Users with single-user systems may schedule meetings with each other
        using [ITIP]. The easiest binding of [ITIP] to use is [IMIP], since
        it messages can be held in their mail queue, which we assume to
        already exist. [IRIP] or [CAP] would require at least one user to run
        a listening server.

         O   -----------                    -----------   O
        -+- | CUA w/    | -----[IMIP]----- | CUA w/    | -+- user
         A  |local store|     Internet     |local store|  A
        / \  -----------                    -----------  / \

3.2.3   Single-user with multiple CUA

   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 an accessibility.  Some of these clients may have
   local stores and others may not.  If they do, then they need to
   ensure that the data on the CUA is synchronized 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

   A single user may have many independent calendars.  One may be work
   related, another for personal use.  The CUA may or may not have a
   local store.  If it does, then it needs to ensure that the data on
   the CUA is synchronized with the data on both of the CS.

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                   +------------[CAP]------ |   CS     |
                   |                        |          |
        O     -----------                    ----------
       -+-   |  CUA      |
        A    |           |
       / \    -----------
                   |                         ----------
                   +------------[CAP]------ |   CS     |
                                            |          |

3.2.5   Users communicating on a multi-user system

   Users on a multi-user system may schedule meetings with each other
   using [CAP]-enabled CUA and service.  The CUA may or may not have
   a local store.  If they do, then they need to ensure that the
   data on the CUA is synchronized 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

  Users on a multi-user system may need to schedule meetings with
  user on a different multi user system.  The services can
  communicate using [CAP]

       O     -----------                    ----------
      -+-   |   CUA w   | -----[CAP]-------|   CS     |
       A    |local store|                  |          |
      / \    -----------                    ----------
       O     -----------                    ----------
      -+-   |  CUA w/o  | -----[CAP]-------|   CS     |
       A    |local store|                  |          |
      / \    -----------                    ----------

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4. Open Issues

        Many issues are not currently resolved by these protocols, and many
        desirable features are not yet provided. Some of the more prominent
        ones follow.

4.1 Choice of Transport

        There are issues to be aware of in choosing a transport mechanism.
        The choices are a network protocol, such as CAP, or a store and forward
        (email) solution.

        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 innately time-sensitive
        data to unbounded latency.  Large or heavily utilized mail systems may
        experience an unacceptable delay in message receipt.

4.2 Scheduling people, not calendars

        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. These protocols do not attempt
        to provide unique access for contacting a single person. Instead,
        'calendar addresses' are booked, which may be email addresses or
        individual calendars. It is up to the users themselves to
        orchestrate mechanisms to ensure that the bookings go to the right

4.3 Administration

        These 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.

4.4 Notification

        People often wish to be notified of upcoming events, new events, or
        changes to events. These protocols do not attempt to address these
        needs in a real-time fashion. 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 will have to poll the
        data store.

5. Security considerations

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5.1 Access Control

        There has to be reasonable granularity in the configuration options
        for access to data through [CAP], so that what should be released to
        requestors is, and what shouldn't isn't. Details of handling this
        are described in [CAP].

5.2 Authentication

        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 data base access can be
        performed, and checking access rights and authentication credentials
        before releasing information. [CAP] uses SASL for this authentication.
        In [IMIP], 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, signing and encryption
        is available for free. 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], 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.

5.3 Using email

        Because scheduling information can be transmitted over mail without
        any authentication information, email spoofing is extremely easy if
        the receiver is not checking for authentication. It is suggested
        that implementors consider requiring authentication as a default,
        using mechanisms such as are described in Section 2 of [IMIP].

        The use of email, 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.

5.4 Other issues

        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 is the
        'lock' icon used in some web browsers during secure connections.

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        With both [iMIP] 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.

6. Acknowledgements

        Thanks to the following who have participated in the development of
        this document:

Eric Busboom, Pat Egen, David Madeo, Shawn Packwood, Bruce Kahn.

7. Bibliography

[ICAL] [RFC-2445] Calendaring and Scheduling Core Object Specification
[ITIP] [RFC-2446] iCalendar Transport-Independent Interoperability Protocol
[IMIP] [RFC-2447] iCalendar Message-Based Interoperability Protocol
[IRIP] draft-ietf-calsch-irip iCalendar Real-time Interoperability Protocol
[CAP] draft-ietf-calsch-cap Calendar Access Protocol

[RFC-1847] Security Multiparts for MIME
[RFC-2045] MIME Part 1: Format of Internet Message Bodies
[RFC-2046] MIME Part 2: Media Types
[RFC 2047] MIME Part 3: Message Header Extensions for Non-ASCII Text
[RFC-2048] MIME Part 4: Registration Procedures
[RFC-2049] MIME Part 5: Conformance Criteria and Examples

8. Author's Addresses

Alexander Taler
3333 Graham Boulevard, 5th Floor
Montreal, QC H3R 3L5
Tel: (514) 733-8500
Email: alex@elea.dhs.org

Bob Mahoney
77 Massachusetts Avenue
Cambridge, MA 02139
Tel: (617) 253-0774
Email: bobmah@mit.edu
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George Babics
Research & Development
Corporate Software & Technologies
3333 Graham Boulevard, 5th floor
MontrÄal, QuÄbec, Canada
H3R 3L5
Tel: (514) 733-8500 x303
Fax: (514) 733-8878
E-mail: georgeb@cst.ca

9. Full Copyright Statement

      "Copyright (C) The Internet Society 2000.  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