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Versions: 00 01 02                                                      
Guide to Implementors
Network Working Group                                    Bob Mahoney/MIT
Internet-Draft                                      Alexander Taler/CS&T
Expires: <date + 6 months>

              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


   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.

   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
        Protocol Requirements
3.  Standards Solution
        Standalone single-user system
        Single-user systems communicating
4.  Open Issues
        Scheduling People, not calendars
5.  Security Considerations
        Access Control
        Using Email
        Other issues
6. Acknowldegements
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

     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.

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 organise 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.

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

       [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.

                             |    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.

     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.

3.2 Systems

   The following diagrams illustrate possible example systems and usage
   of the protocols. [ed. More coming]

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.

      -----------     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
       / \    -----------                    -----------    / \

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 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

4.2 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.

4.3 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

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.  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.

6. Acknowledgements

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

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

7. Bibliography

   [ICAL] [RFC-2445] Calendaring and Scheduling Core Object Specification
   [ITIP] [RFC-2446] iCalendar Transport-Independent Interoperability
   [IMIP] [RFC-2447] iCalendar Message-Based Interoperability Protocol
   [IRIP] draft-ietf-calsch-irip iCalendar Real-time Interoperability
   [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: alext@cst.ca

   Bob Mahoney
   77 Massachusetts Avenue
   Cambridge, MA 02139
   Tel: (617) 253-0774
   Email: bobmah@mit.edu

9. Full Copyright Statement