Network Working Group                             Bob Mahoney/MIT
Internet Draft                              George Babics/Steltor
February 20 2001                                         B. Mahoney
Internet-Draft                                                       MIT
Expires: August 20 January 16, 2002                                      G. Babics
                                                                A. Taler
                                                           July 18, 2001

                     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-

   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 Internet-Draft will expire on January 16, 2002.

Copyright Notice

   Copyright (C) The Internet Society 2000. (2001).  All Rights Reserved.


   This document describes the various internet Internet calendaring and
   scheduling standards and drafts works in progress and the relationships
   between them.  It's intention is to provide a context for these
   documents, assist in their understanding, and potentially help implementors
   implementers in the design of their internet standards based calendaring and
   scheduling systems.  The standards addressed are RFC 2445
   (iCalendar), RFC 2446 (iTIP), and RFC 2447 (iMIP).  The draft work in
   progress 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.  Its status will be discussed
   further.]  This
   document also describes issues and problems that are not solved by
   these protocols, and could be targets for future work.


Table of this Memo Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2   Concepts and Relationships . . . . . . . . . . . . . . . . .  5
   2.    Requirements . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.1   Fundamental Need Needs  . . . . . . . . . . . . . . . . . . . . .  6
   2.2   Protocol Requirements  . . . . . . . . . . . . . . . . . . .  6
   3.  Standards Solution    Solutions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.1   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.2   Systems
       Standalone single-user system
       Single-user systems communicating
   4.  Important Aspects

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       Choice of Transport  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.2.1 Standalone single-user system  . . . . . . . . . . . . . . .  9
   3.2.2 Single-user systems communicating  . . . . . . . . . . . . .  9
   3.2.3 Single-user with multiple CUA  . . . . . . . . . . . . . . . 10
   3.2.4 Single-user with multiple calendars  . . . . . . . . . . . . 10
   3.2.5 Users communicating on a multi-user system . . . . . . . . . 11
   3.2.6 Users communicating through different multi-user systems . . 11
   4.    Important Aspects  . . . . . . . . . . . . . . . . . . . . . 12
   4.1   Timezones  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.2   Choice of Transport  . . . . . . . . . . . . . . . . . . . . 12
   4.3   Security . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.4   Amount of data . . . . . . . . . . . . . . . . . . . . . . . 12
   4.5   Recurring Components . . . . . . . . . . . . . . . . . . . . 12
   5.    Open Issues
       Choice of Transport  . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.1   Scheduling People, people, not calendars . . . . . . . . . . . . . . 14
   5.2   Administration . . . . . . . . . . . . . . . . . . . . . . . 14
   5.3   Notification . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.    Security Considerations considerations  . . . . . . . . . . . . . . . . . . 15
   6.1   Access Control . . . . . . . . . . . . . . . . . . . . . . . 15
   6.2   Authentication . . . . . . . . . . . . . . . . . . . . . . . 15
   6.3   Using Email email  . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.4   Other issues
   7.  Acknowledgements
   8.  Bibliography
   9.  Author's . . . . . . . . . . . . . . . . . . . . . . . . 16
         Authors' Addresses
   10. . . . . . . . . . . . . . . . . . . . . . 16
   A.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 17
   B.    Bibliography . . . . . . . . . . . . . . . . . . . . . . . . 18
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 19

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, Internet, organizations attempting
   to provide calendaring information on the internet, 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 a context for the
   calendar standard standards and draft documents, works in progress, assist in their
   understanding, and potentially help implementors implementers in the design of
   their internet 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] iCalendar [RFC-2445],
   iTIP [RFC-2446], iMIP [RFC-2447], and [CAP].  The following
   definitions are provided as introductory, the definitions in the
   protocol specifications are the canonical ones.


      A collection of events, todos, to-dos, journal entries, etc.  A calendar
      could be the content of a 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

   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 (CS)

      A data store of a calendar service.  A calendar service may

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      several calendar stores, and each store may contain several
      calendars, as well as properties and components outside of the

   Calendar User (CU)

      An entity (often a human) that accesses calendar information.

   Calendar User Agent (CUA)

      Software used by the calendar user that communicates with calendar
      services to provide the user access to calendar information.


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


      Is a calendar user (sometimes called the delegatee) who has
      assigned his or her participation in a scheduled calendar
      component (e.g., VEVENT) to another calendar user (sometimes
      called the delegate or delegatee).


      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 assistant.

   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 time.

   Remote Store

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

1.2 Concepts and Relationships

   iCalendar is the Language to be language used in to describe calendar events. objects.  iTIP is how you
   a way to use the language. language to do scheduling.  iMIP is further definition for use over how to do iTIP
   with email.
      iRIP is the language used over a real-time transport  CAP is how a way to use the language, in real-time, to access a calendar server

      Another way to put it is as follows:
      iCalendar are
   store in real-time.

   The relationship between the words
      iTIP calendaring protocols is similar to that
   between 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

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      RFC822 protocols.  In those terms iCalendar is a wrapper for email: like RFC
   822, iTIP and iMIP are like SMTP, and CAP is a wrapper for
      calendaring objects like POP or IMAP.

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 CUA.

      b] A busy musician wants to maintain her schedule on with different
      devices, such as with an
      internet-based Internet-based agenda which she can access from anywhere. or with a PDA.

      Need: Read and manipulate one's own calendar. calendar, possibly with
      solutions from different vendors.

      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
         slot schedule calendar
      entries during his office hours.

      Need: Schedule calendar events events, to-dos and todos journals with users
      using the same calendar service.

      e] A movie theatre theater 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 schedule calendar entries
      effectively by booking time with its members.

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

2.2 Protocol requirements

   The first four Requirements

   Some of the needs can be satisfied through met with proprietary solutions, solutions (a, c, d),
   but the last two cannot. others can not (b, e, f).  From these needs we can establish that
   protocols are required for accessing information in a calendar store,
   and for scheduling events and todos. calendar entries.  In addition these protocols
   require a data format for representing calendar information.

   These roles are filled by the following protocol requirements. specifications.

      - [ICAL] iCalendar [RFC-2445] is the data format


      iCalendar [RFC-2445] provides data format for representing
      calendar information which the other protocols can use. [ICAL]  iCalendar
      [RFC-2445] can

Mahoney/Babics                        4             Expires August 2001 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], iCalendar [RFC-2445], so all elements of a standards-based 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
      iCalendar [RFC-2445].

      - [ITIP] iTIP [RFC-2446] is the scheduling protocol


      iTIP [RFC-2446] describes the messages used to schedule calendar
      events.  These messages are represented in [ICAL], iCalendar [RFC-2445],
      and have semantics that include such things as being an invitation
      to a meeting, an acceptance of an invitation or the assignation assignment of
      a task.


      iTIP [RFC-2446] messages are used in the scheduling work flow, workflow,
      where users exchange messages in order to organize things such as
      events and todos. to-dos.  CUAs generate and interpret
         [ITIP] iTIP [RFC-2446]
      messages at the direction of the calendar user.  With [ITIP] iTIP [RFC-
      2446] one can create, modify, delete, reply to, counter, and
      decline counters to, to the various [ICAL] iCalendar [RFC-2445] components. Furthermore, one can also request the
         freebusy time of other people.

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

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

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

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         [ITIP] free/busy time of other

      iTIP [RFC-2446] is transport-independent, but and has two one specified
      transport bindings, [IMIP] bindings: iMIP [RFC-2447] is a binding to email and
         [IRIP] is a real-time binding. email.  In
      addition [CAP] will provide a second real-time binding of [ITIP], iTIP [RFC-
      2446], 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.

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

            C:content-transfer-encoding: 7bit

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            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 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
      event representations or operation requests as " calendar object

      There are two main methodologies for communicating use 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 the first methodology only. The second one
      is dependent on iTIP. It requires understanding of iTIP and the
      ability to communicate with other CAP servers using iTIP. Since,
      currently, iMIP is the only binding of iTIP, the second method

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      is also dependent on iMIP.

      Additionally, [RFC-2445] to
      describe various components such as events and to-dos.  With these
      messages one can do the operations in iTIP specification describes [RFC-2446] and other
      operations relating to a
      transport-independent grammar for communicating between systems.
      The iMIP specification utilizes iTIP calendar store, such as search, creating
      calendars, specifying calendar properties, and being able to express iCalendar
      specify access rights to one's calendars.

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] iCalendar [RFC-2445] as a storage mechanism.  This
      would allow the doctor to easily import his store into another
      application that supports [ICAL]. iCalendar [RFC-2445].

      b] The musician who wishes to access her agenda from anywhere can
      use a [CAP] enabled calendar service accessible through the internet.
      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

      Alternatively, each member could use an [IMIP]-enabled iMIP [RFC-2447]-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] iTIP [RFC-2446] bindings.  [CAP] or [IRIP] provide
      provides real-time access, but could require additional
      configuration. [IMIP]  iMIP [RFC-2447] 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 iTIP [RFC-2446] free/busy requests.  Non-
      standard methods could also be employed, such as serving up ICAL,
      HTML, XML through 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] For publishing a movie theatre's theater's schedule [CAP] provides the
      most advanced access and search capabilities.  It also allows easy
      integration with its customer's calendar

Mahoney/Babics                        8             Expires August 2001 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

      f] The social club could distribute meeting information in the
      form of [ITIP] iTIP [RFC-2446] messages.  This could be done over email
      using [IMIP], or [IRIP] depending on the recipient. iMIP [RFC-2447].  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.

3.2.1 Standalone single-user system

   A single user system that does not communicate with other systems
   need not employ any of the protocols.  However, it may use [ICAL] iCalendar
   [RFC-2445] 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]. iTIP [RFC-2446].  The easiest binding of [ITIP] iTIP [RFC-2446] to use
   is [IMIP], iMIP [RFC-2447], since
   it since the 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. could also be

          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

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                   |   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].

Mahoney/Babics                        10             Expires August
2001 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

   There are a number of important aspects of these calendaring
   documents that of which people, especially implementors, implementers, should be aware of. aware.

4.1 Timezones

   The dates and times in components can refer to timezones. time zones.  These
   time zones 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 timezones time zones and timezone time zone differences.  New time
   zones may be added, and others removed.  Two different vendors may
   describe the same time zone differently (such as by using a different

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 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 6) 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] iMIP [RFC-2447] it can be controlled, by restricting the maximum
   size of the email that the application can download.

4.5 Recurring Components

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   In [iCAL] iCAL [RFC-2445] 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

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,
   requesters is released, 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 database access can be
   performed, and checking access rights and authentication credentials
   before releasing information.  [CAP] uses SASL for this
   authentication.  In [IMIP], iMIP [RFC-2447], this may present some
   challenges, as authentication is  often not a consideration in store-and-forward store-
   and-forward protocols.

   Authentication is also important for scheduling, in that receivers of
   scheduling messages should be able to validate the apparent

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2001 sender.
   Since scheduling messages are wrapped in MIME, MIME [RFC-2045], 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], 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 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 implementers consider requiring authentication as a default,
   using mechanisms such as are described in Section 2 3 of [IMIP]. iMIP [RFC-
   2447].  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 [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.

7. Acknowledgements

Authors' Addresses

   Bob Mahoney
   77 Massachusetts Avenue
   Cambridge, MA  02139

   Phone: (617) 253-0774

   George Babics
   2000 Peel Street
   Montreal, Quebec  H3A 2W5

   Phone: (514) 733-8500 x4201

   Alex Taler


Appendix A. Acknowledgments

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

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


Appendix B. Bibliography


      [RFC-2445] Dawson, F.  and D.  Stenerson, "Internet Calendaring
      and Scheduling Core Object Specification
   [ITIP] - iCalendar", RFC 2445,
      November 1998.

      [RFC-2446] iCalendar 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] iCalendar Dawson, F., Mansour, S.  and S.  Silverberg, "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 - iMIP", RFC 2447,
      November 1998.

      [RFC-2045] MIME Freed, N.  and N.  Borenstein, "Multipurpose Internet
      Mail Extensions (MIME) - Part 1: One: Format of Internet Message Bodies
   [RFC-2046] MIME Part 2: Media Types
   [RFC 2047] MIME Part 3: Message Header Extensions for Non-ASCII Text

Mahoney/Babics                        13             Expires August
   [RFC-2048] MIME Part 4: Registration Procedures
   [RFC-2049] MIME Part 5: Conformance Criteria
      Bodies", RFC 2045, November 1996.

      [CAP] Mansour, S., Royer, D., Babics, G., and Examples

9. Author's Addresses

   Bob Mahoney
   77 Massachusetts Avenue
   Cambridge, MA 02139
   Tel: (617) 253-0774

   George Babics
   Steltor (formerly CS&T/Lexacom)
   2000 Peel Street
   Montreal, Quebec, Canada
   H3A 2W5
   Tel: (514) 733-8500 x4201
   Fax: (514) 733-8878
   mailto:                                                             m

10. Hill, P.  "Calendar
      Access Protocol (CAP)" draft-ietf-calsch-cap-04.txt

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Mahoney/Babics                        14             Expires August


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