Network Working Group B. Mahoney
Internet-Draft MIT
Expires: January 16, 2002 G. Babics
Steltor
A. Taler
July 18, 2001
Guide to Internet Calendaring
draft-ietf-calsch-inetcal-guide-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes the various Internet calendaring and
scheduling standards and 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
implementers in the design of their standards based calendaring and
scheduling systems. The standards addressed are RFC 2445
(iCalendar), RFC 2446 (iTIP), and RFC 2447 (iMIP). The work in
progress addressed is "Calendar Access Protocol" (CAP). This
document also describes issues and problems that are not solved by
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these protocols, and could be targets for future work.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Concepts and Relationships . . . . . . . . . . . . . . . . . 5
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Fundamental Needs . . . . . . . . . . . . . . . . . . . . . 6
2.2 Protocol Requirements . . . . . . . . . . . . . . . . . . . 6
3. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1 Scheduling people, not calendars . . . . . . . . . . . . . . 14
5.2 Administration . . . . . . . . . . . . . . . . . . . . . . . 14
5.3 Notification . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Security considerations . . . . . . . . . . . . . . . . . . 15
6.1 Access Control . . . . . . . . . . . . . . . . . . . . . . . 15
6.2 Authentication . . . . . . . . . . . . . . . . . . . . . . . 15
6.3 Using email . . . . . . . . . . . . . . . . . . . . . . . . 15
6.4 Other issues . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 17
B. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . 18
Full Copyright Statement . . . . . . . . . . . . . . . . . . 19
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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 to provide a context for the
calendar standards and works in progress, assist in their
understanding, and potentially help implementers in the design of
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 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.
Calendar
A collection of events, 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
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 (CS)
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
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calendars.
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.
Component
A piece of calendar data such as an event, a to-do or an alarm.
Information about components is stored as properties of those
components.
Delegator
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).
Delegate
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.
Designate
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.
Property
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.
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1.2 Concepts and Relationships
iCalendar is the language used to describe calendar objects. iTIP is
a way to use the language to do scheduling. iMIP is how to do iTIP
with email. CAP is a way to use the language, to access a calendar
store in real-time.
The relationship between the calendaring protocols is similar to that
between the email protocols. In those terms iCalendar is like RFC
822, iTIP and iMIP are like SMTP, and CAP is like POP or IMAP.
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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 with different
devices, such as with an Internet-based agenda or with a PDA.
Need: Read and manipulate one's own 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 schedule calendar
entries during his office hours.
Need: Schedule calendar events, to-dos and journals with users
using the same calendar service.
e] A movie 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 schedule calendar entries
effectively with its members.
Need: Schedule calendar events and to-dos with users using other
calendar services, possibly from different vendors.
2.2 Protocol Requirements
Some of the needs can be met with proprietary solutions (a, c, d),
but others can not (b, e, f). From these needs we can establish that
protocols are required for accessing information in a calendar store,
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and for scheduling calendar entries. In addition these protocols
require a data format for representing calendar information.
These roles are filled by the following protocol specifications.
- iCalendar [RFC-2445] is the data format
iCalendar [RFC-2445] provides data format for representing
calendar information which the other protocols can use. iCalendar
[RFC-2445] 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 iCalendar [RFC-2445], so all elements of a standards-
based calendaring and scheduling systems will have to interpret
iCalendar [RFC-2445].
- iTIP [RFC-2446] is the scheduling protocol
iTIP [RFC-2446] describes the messages used to schedule calendar
events. These messages are represented in iCalendar [RFC-2445],
and have semantics that include such things as being an invitation
to a meeting, an acceptance of an invitation or the assignment of
a task.
iTIP [RFC-2446] messages are used in the scheduling workflow,
where users exchange messages in order to organize things such as
events and to-dos. CUAs generate and interpret iTIP [RFC-2446]
messages at the direction of the calendar user. With iTIP [RFC-
2446] one can create, modify, delete, reply to, counter, and
decline counters to the various iCalendar [RFC-2445] components.
Furthermore, one can also request the free/busy time of other
people.
iTIP [RFC-2446] is transport-independent, and has one specified
transport bindings: iMIP [RFC-2447] is a binding to email. In
addition [CAP] will provide a real-time binding of iTIP [RFC-
2446], allowing CUAs to perform calendar management as well as
scheduling over a single connection.
- [CAP] is the calendar management protocol
[CAP] describes the messages used to manage calendars on a
calendar store. These messages use iCalendar [RFC-2445] to
describe various components such as events and to-dos. With these
messages one can do the operations in iTIP [RFC-2446] and other
operations relating to a calendar store, such as search, creating
calendars, specifying calendar properties, and being able to
specify access rights to one's calendars.
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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 iCalendar [RFC-2445] as a storage mechanism. This
would allow the doctor to easily import his store into another
application that supports iCalendar [RFC-2445].
b] The musician who wishes to access her agenda from anywhere can
use a [CAP] enabled calendar service accessible 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 [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 [RFC-2446] bindings. [CAP]
provides real-time access, but could require additional
configuration. iMIP [RFC-2447] would be the easiest to configure,
but may require more 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 [RFC-2446] free/busy requests. Non-
standard methods could also be employed, such as serving up ICAL,
HTML, XML over HTTP.
A proprietary system could also be used, but would require that
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all students be able to use software from a specific vendor.
e] For publishing a movie theater'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 [RFC-2446] messages. This could be done over email
using 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 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 [RFC-2446]. The easiest binding of iTIP [RFC-2446] to use
is iMIP [RFC-2447], since since the messages can be held in their
mail queue, which we assume to already exist. [CAP] could also be
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used.
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.
----------
+------------[CAP]------ | CS |
| | |
O ----------- ----------
-+- | CUA |
A | |
/ \ -----------
| ----------
+------------[CAP]------ | CS |
| |
----------
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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 iMIP [RFC-2447].
O ----------- ----------
-+- | CUA w | -----[CAP]-------| CS |
A |local store| | |
/ \ ----------- ----------
|
[CAP] or [iMIP]
|
O ----------- ----------
-+- | CUA w/o | -----[CAP]-------| CS |
A |local store| | |
/ \ ----------- ----------
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4. Important Aspects
There are a number of important aspects of these calendaring
documents of which people, especially implementers, should be aware.
4.1 Timezones
The dates and times in components can refer to 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 time zones and 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
name).
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
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.
4.3 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 [RFC-2447] it can be controlled, by restricting the maximum
size of the email that the application can download.
4.5 Recurring Components
In iCAL [RFC-2445] one can specify complex recurrence rules for
VEVENTs, VTODOs, and VJOURNALs. There is the danger that
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applications interpret these rules differently. Thus, one must make
sure that one is careful with recurrence rules.
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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.
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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
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 database access can be
performed, and checking access rights and authentication credentials
before releasing information. [CAP] uses SASL for this
authentication. In iMIP [RFC-2447], this may present some
challenges, as authentication is often not a consideration in store-
and-forward protocols.
Authentication is also important for scheduling, in that receivers of
scheduling messages should be able to validate the apparent sender.
Since scheduling messages are wrapped in MIME [RFC-2045], signing and
encryption 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
[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 implementers consider requiring authentication as a default,
using mechanisms such as are described in Section 3 of 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.
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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 [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.
Authors' Addresses
Bob Mahoney
MIT
E40-327
77 Massachusetts Avenue
Cambridge, MA 02139
US
Phone: (617) 253-0774
EMail: bobmah@mit.edu
George Babics
Steltor
2000 Peel Street
Montreal, Quebec H3A 2W5
CA
Phone: (514) 733-8500 x4201
EMail: georgeb@steltor.com
Alex Taler
EMail: dissent@elea.dhs.org
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Appendix A. Acknowledgments
Thanks to the following who have participated in the development of
this document:
Eric Busboom, Pat Egen, David Madeo, Shawn Packwood, Bruce Kahn.
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Appendix B. Bibliography
[RFC-2445] Dawson, F. and D. Stenerson, "Internet Calendaring
and Scheduling Core Object Specification - iCalendar", RFC 2445,
November 1998.
[RFC-2446] Silverberg, S., Mansour, S., Dawson, F. and R.
Hopson, "iCalendar Transport-Independent Interoperability Protocol
(iTIP): Scheduling Events, Busy Time, To-dos and Journal Entries",
RFC 2446, November 1998.
[RFC-2447] Dawson, F., Mansour, S. and S. Silverberg, "iCalendar
Message-Based Interoperability Protocol - iMIP", RFC 2447,
November 1998.
[RFC-2045] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) - Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[CAP] Mansour, S., Royer, D., Babics, G., and Hill, P. "Calendar
Access Protocol (CAP)" draft-ietf-calsch-cap-04.txt
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