Network Working Group
Guide to Implementors                             Bob Mahoney/MIT
Internet Draft                                    Alexander Taler/CS&T
<draft-ietf-calsch-imp-guide-01.txt> Taler
<draft-ietf-calsch-imp-guide-02.txt>              George Babics/CS&T
July 14, Babics/Steltor
November 24 2000
Expires: January 14, May 24 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.

   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 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), standards and RFC 2447 (iMIP), as well as the works in
	progress,"iCalendar Real-time Interoperability Protocol" (iRIP), drafts and "Calendar Access Protocol" (CAP). the relationships between them.
   It's intention is to provide a context for these protocols, documents, assist in
   their understanding, and
	ultimately potentially help implementors in the design
   of their internet calendaring and scheduling systems. The standards
   addressed are RFC 2445 (iCalendar), RFC 2446 (iTIP), and RFC 2447
   (iMIP). The draft addressed is "Calendar Access Protocol" (CAP).

   [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?]  Its status will be discussed

   This document also describes issues and problems which that 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.  Important Aspects
       Choice of Transport
       Amount of data
       Recurring Components
   5.  Open Issues
       Choice of Transport
       Scheduling People, not calendars
   6.  Security Considerations
       Access Control
       Using Email
       Other issues
6. Acknowledgements
   7. Bibliography  Acknowledgements
   8.  Bibliography
   9.  Author's Addresses
   10. Full Copyright Statement

1. Introduction

   The calendaring and scheduling protocols are intended to provide for
   the needs of individuals attempting to obtain calendaring information
   and schedule meetings across the internet, organizations attempting
   to provide calendaring 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 is to provide guidance a context for
	implementors of calendaring the
   calendar standard and scheduling products draft documents, assist in determining
	which of the various existing protocol documents are applicable to their work, as well as providing some background information understanding,
	pointers to potentially help implementors in the less obvious implications design of the available choices. their internet
   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

   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 person 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 information.

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

   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 calendars.

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

   Calendar User Agent (CUA)
      Software used by the calendar user which that 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 components.

      Is a calendar user (sometimes called the delegatee) who has
      been assigned participation in a scheduled calendar
      component (e.g., VEVENT) by one of the attendees in the
      scheduled calendar component (sometimes called the
      delegator). An example of a delegate is a team member told
      to go to a particular meeting.

      Is a calendar user who is authorized to act on behalf of
      another calendar user. An example of a designate is an

   Local Store
      A CS which is on the same platform as the CUA.

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

   Remote Store
      A CS which is not on the same platform as the CUA.

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.
      iRIP is the Language language used over a real-time transport.

	Another way to transport
      CAP is how to use the language, in real-time, to access a
      calendar server

      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"

      A comparison with email:
      RFC822 in email:  iRIP in Calendaring (scheduling not
      POP/IMAP in email: CAP in calendaring
      iMIP uses RFC822
      RFC822 is a wrapper for email:  iTIP is a wrapper for
      calendaring objects

2. Requirements

2.1 Fundamental Needs

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

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

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

   b] 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.


      c] 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.


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

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


      e] 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.


      f] 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 four 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 format.

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

            For example the following describes an event:

            PRODID:-//ABC Corporation//NONSGML My Product//EN

         - [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.

         With [ITIP] one can create, modify, delete, reply to,
         counter, and decline counters to, the various [ICAL]
         components. Furthermore, one can also request the
         freebusy time of other people.

         For example, to invite a user to the above event, one can
         send a message like this one:

            PRODID:-//ABC Corporation//NONSGML My Product//EN

            The user, John Smith, can send a reply using the
            REPLY method.

         [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.

         For example, sending the above request using iMIP would
         look like:

            Subject: Lunch
            Mime-Version: 1.0
            Content-Type:text/calendar; method=REQUEST;charset=US-ASCII
            Content-Transfer-Encoding: 7bit

            PRODID:-//ABC Corporation//NONSGML My Product//EN

         Both CUAs and calendar services may have [ITIP]

         - [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] describes the messages used to manage calendars on a
         calendar store.

         These messages are represented in [ICAL]. With these messages
         one can do the operations in [ITIP] and other operations
         relating to a calendar store. These operations include, search,
         creating calendars, specifying calendar properties, and being
         able to specify access rights to one's calendars.

         [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

         For example, one can schedule the implementation above meeting using CAP:

            C:CONTENT-TYPE:text/calendar; method=CREATE;charset=US-ASCII
            C:content-transfer-encoding: 7bit
            C:PRODID:-//ABC Corporation//NONSGML My Product//EN
            C: .
            S: 2.0
            S: Content-Type:text/calendar; method=RESPONSE;
            S: BEGIN:VCALENDAR
            S: VERSION:2.1
            S: METHOD:RESPONSE
            S: BEGIN:VEVENT
            S: TARGET::cap://
            S: REQUEST-STATUS:2.0
            S: END:VEVENT
            S: END:VCALENDAR
            S: Content-Type:text/calendar; method=RESPONSE;
            S: BEGIN:VCALENDAR
            S: VERSION:2.1
            S: METHOD:RESPONSE
            S: BEGIN:VEVENT
            S: TARGET::cap://
            S: REQUEST-STATUS:2.0
            S: END:VEVENT
            S: END:VCALENDAR
            S: .

         Note that "C" indicates the data sent by the client,
         and "S" the data sent by the server. Furthermore, CAP
         is still a draft, thus the details of one can create
         such an event may change.

      The dependencies between the protocols. A calendar system different protocols are as follows:

      iCalendar is the language set used to describe/specify calendaring
      events or operations.

      When specified using correct iCalendar grammar, we refer to these standards
      event representations or operation requests as " calendar object

      There are two main methodologies for communicating iCalendar

      1) Via a store-and-forward mechanism (usually email), using the
         iMIP specification.

      2) Via an on-the-wire mechanism (a directly connected state,
         however briefly), using the CAP specification.

      A system may implement at least the first methodology only. The second one
      is dependent on iTIP. It requires understanding of iTIP and the leaves of
      ability to communicate with other CAP servers using iTIP. Since,
      currently, iMIP is the tree. The
	calendar management message and transport protocol parts only binding of CAP are
	separated in iTIP, the second method
      is also dependent on iMIP.

      Additionally, the diagram to highlight its relationship to ITIP.

                             |    iCalendar     |
                    |                                    |
            ------------------                           |
           | iTIP        |                          |
            ------------------                           |
                    |                                    |
                    |                          ----------|-------
                    |                         |  CAP     |       |
                    |                         |        message   |
            ----------------------------------------   format    |
            |               |                 |    |     |       |
       ----------      -----------            |    |     |       |
      | Session  |    |   E-mail  |           |   transport      |
      |   iRIP   |    | specification describes a
      transport-independent grammar for communicating between systems.
      The iMIP   |           |   protocol       |
       ----------      -----------             ------------------ specification utilizes iTIP to express iCalendar objects.

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 doctor can use a proprietary CUA with a local store,
            and perhaps use [ICAL] as a storage mechanism.  This would
            allow the doctor to easily import his store into another
            application that supports [ICAL].

         b] 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.


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

            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.


         d] 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 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


         e] 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 systems.

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


         f] 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

            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

	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 that 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
      / \  -----------                    -----------  / \

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

                |   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.

                      +------------[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] or [ITIP].

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

4. Open Issues

	Many issues Important Aspects

   There are not currently resolved by a number of important aspects of these protocols, calendaring
   documents that people, especially implementors, should be aware of.

4.1 Timezones

   The dates and many
	desirable features are not yet provided. Some times in components can refer to timezones. These
   timezones can be defined in some central store, or they may be
   defined by a user to fit his or her needs. Any user and application
   should be aware of the more prominent
	ones follow.

4.1 timezones and timezone differences.

4.2 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

   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.


4.3 Security

   See the "Security Considerations" section below.

4.4 Amount of data

   In some cases a component may be very large. For instance, some
   attachments may be very large. Some applications may be low-bandwidth
   or be limited in the amount of data they can store. The size of the
   data may be controlled in [CAP], by specifying maximums. In [iMIP] it
   can be controlled, by restricting the maximum size of the email that
   the application can download.

4.5 Recurring Components

   In [iCAL] one can specify complex recurrence rules for VEVENTs,
   VTODOs, and VJOURNALs. There is the danger that applications
   interpret these rules differently. Thus, one must make sure that one
   is careful with recurrence rules.

5. 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.

5.1 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 place.


5.2 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.


5.3 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.


6. Security considerations


6.1 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].


6.2 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

   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.


6.3 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.


6.4 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.

   With both [iMIP] [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.


7. Acknowledgements

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

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


8. Bibliography

   [ICAL] [RFC-2445] Calendaring and Scheduling Core Object
   [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


9. Author's Addresses

   Alexander Taler
3333 Graham Boulevard, 5th Floor
Montreal, QC H3R 3L5
Tel: (514) 733-8500
   Bob Mahoney
   77 Massachusetts Avenue
   Cambridge, MA 02139
   Tel: (617) 253-0774

   George Babics
Research & Development
Corporate Software & Technologies
3333 Graham Boulevard, 5th floor
Montr─al, Qu─bec,
   Steltor (formerly CS&T/Lexacom)
   2000 Peel Street
   Montreal, Quebec, Canada
H3R 3L5
   H3A 2W5
   Tel: (514) 733-8500 x303 x4201
   Fax: (514) 733-8878


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