Guide to Internet Calendaring
Map of the Internet calendaring standards and how they work together.
Mahoney, B., Babics, G., and A. Taler, "Guide to Internet Calendaring", RFC 3283, DOI 10.17487/RFC3283, July 2002, <https://www.rfc-editor.org/info/rfc3283>.
@misc{rfc3283,
series = {Request for Comments},
number = 3283,
howpublished = {RFC 3283},
publisher = {RFC Editor},
doi = {10.17487/RFC3283},
url = {https://www.rfc-editor.org/info/rfc3283},
author = {B. Mahoney and G. Babics and A. Taler},
title = {{Guide to Internet Calendaring}},
pagetotal = 16,
year = 2002,
month = jul,
} RFC text
Contents
Network Working Group B. Mahoney
Request for Comments: 3283 MIT
Category: Informational G. Babics
Steltor
A. Taler
June 2002
Guide to Internet Calendaring
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes the various Internet calendaring and
scheduling standards and works in progress, and the relationships
between them. Its intent is to provide a context for these
documents, assist in their understanding, and potentially aid in the
design of 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 these protocols, and that could be targets for
future work.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Concepts and Relationships . . . . . . . . . . . . . . . . . 4
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Fundamental Needs . . . . . . . . . . . . . . . . . . . . . 4
2.2 Protocol Requirements . . . . . . . . . . . . . . . . . . . 5
3. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1 Standalone Single-user System . . . . . . . . . . . . . . . 8
3.2.2 Single-user Systems Communicating . . . . . . . . . . . . . 8
3.2.3 Single-user with Multiple CUAs . . . . . . . . . . . . . . . 9
3.2.4 Single-user with Multiple Calendars . . . . . . . . . . . . 9
3.2.5 Users Communicating on a Multi-user System . . . . . . . . . 10
3.2.6 Users Communicating through Different Multi-user Systems . . 10
4. Important Aspects . . . . . . . . . . . . . . . . . . . . . 10
4.1 Timezones . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Choice of Transport . . . . . . . . . . . . . . . . . . . . 11
4.3 Security . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4 Amount of data . . . . . . . . . . . . . . . . . . . . . . . 11
4.5 Recurring Components . . . . . . . . . . . . . . . . . . . . 11
5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1 Scheduling People, not Calendars . . . . . . . . . . . . . . 12
5.2 Administration . . . . . . . . . . . . . . . . . . . . . . . 12
5.3 Notification . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . 12
6.1 Access Control . . . . . . . . . . . . . . . . . . . . . . . 12
6.2 Authentication . . . . . . . . . . . . . . . . . . . . . . . 12
6.3 Using E-mail . . . . . . . . . . . . . . . . . . . . . . . . 13
6.4 Other Issues . . . . . . . . . . . . . . . . . . . . . . . . 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . 16
1. Introduction
Calendaring and scheduling protocols are intended to aid individuals
in obtaining calendaring information and scheduling meetings across
the Internet, to aid organizations in providing calendaring
information on the Internet, and to provide for organizations looking
for a calendaring and scheduling solution to deploy internally.
It is the intent of this document to provide a context for these
documents, assist in their understanding, and potentially help in the
design of standards-based 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 an introduction; the definitions in the
protocol specifications themselves should be considered canonical.
Calendar
A collection of events, to-dos, journal entries, etc. A calendar
could be the content of a person or 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 defining who may perform what operations, such as
reading or writing information, on a given calendar.
Calendar Service
A running server application that provides access to a number of
calendar stores.
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 those
calendars.
Calendar User (CU)
An entity (often a human) that accesses calendar information.
Calendar User Agent (CUA)
Software with which the calendar user communicates with a calendar
service or local calendar store to access 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
A calendar user who has assigned his or her participation in a
scheduled calendar component (e.g. a VEVENT) to another calendar
user (sometimes called the delegate or delegatee). An example of
a delegator is a busy executive sending an employee to a meeting
in his or her place.
Delegate
A calendar user (sometimes called the delegatee) who has been
assigned to participate in a scheduled calendar component (e.g. a
VEVENT) in place of one of the attendees in that component
(sometimes called the delegator). An example of a delegate is a
team member sent to a particular meeting.
Designate
A calendar user authorized to act on behalf of another calendar
user. An example of a designate is an assistant scheduling
meetings for his or her superior.
Local Store
A CS that is on the same device as the CUA.
Property
A description of some element of a component, such as a start
time, title or location.
Remote Store
A CS that is not on the same device as the CUA.
1.2 Concepts and Relationships
iCalendar is the language used to describe calendar objects. iTIP
describes a way to use the iCalendar language to do scheduling. iMIP
describes how to do iTIP scheduling via e-mail. CAP describes a way
to use the iCalendar language to access a calendar store in real-
time.
The relationship between calendaring protocols is similar to that
between e-mail protocols. In those terms, iCalendar is analogous to
RFC 2822, iTIP and iMIP are analogous to the Simple Mail Transfer
Protocol (SMTP), and CAP is analogous to the Post Office Protocol
(POP) or Internet Message Access Protocol (IMAP).
2. Requirements
2.1 Fundamental Needs
The following scenarios illustrate people and organizations' basic
calendaring and scheduling needs:
a] A doctor wishes to keep track of all her appointments.
Need: To read and manipulate one's own calendar with only one CUA.
b] A busy musician wants to maintain her schedule with multiple
devices, such as through an Internet-based agenda and with a PDA.
Need: To read and manipulate one's own calendar, possibly with
solutions from different vendors.
c] A software development team wishes to more effectively schedule
their time through viewing each other's calendar information.
Need: To share calendar information between users of the same
calendar service.
d] A teacher wants his students to schedule appointments during
his office hours.
Need: To schedule calendar events, to-dos and journals with other
users of the same calendar service.
e] A movie theater wants to publish its schedule for prospective
customers.
Need: To share calendar information with users of other calendar
services, possibly from a number of different vendors.
f] A social club wants to schedule calendar entries effectively
with its members.
Need: To schedule calendar events and to-dos with users of other
calendar services, possibly from a number of different vendors.
2.2 Protocol Requirements
Some of these needs can be met by proprietary solutions (a, c, d),
but others can not (b, e, f). These latter scenarios show that
standard protocols are required for accessing information in a
calendar store and scheduling calendar entries. In addition, these
protocols require a common data format for representing calendar
information.
These requirements are met by the following protocol specifications.
- Data format: iCalendar [RFC-2445]
iCalendar [RFC-2445] provides a data format for representing
calendar information, to be used and exchanged by other protocols.
iCalendar [RFC-2445] can also be used in other contexts, such as a
drag-and-drop interface, or an export/import feature. All the
other calendaring protocols depend on iCalendar [RFC-2445], so all
elements of a standards-based calendaring and scheduling systems
will have to be able to interpret iCalendar [RFC-2445].
- Scheduling protocol: iTIP [RFC-2446]
iTIP [RFC-2446] describes the messages used to schedule calendar
events. Within iTIP messages, events are represented in iCalendar
[RFC-2445] format, and have semantics that identify the message 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] users can create, modify, delete, reply to, counter, and
decline counters to the various iCalendar [RFC-2445] components.
Furthermore, users can also request the free/busy time of other
people.
iTIP [RFC-2446] is transport-independent, and has one specified
transport binding: iMIP [RFC-2447] binds iTIP to e-mail. In
addition [CAP] will provide a real-time binding of iTIP [RFC-
2446], allowing CUAs to perform calendar management and scheduling
over a single connection.
- Calendar management protocol: [CAP]
[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. These
messages make it possible to perform iTIP [RFC-2446] operations,
as well as other operations relating to a calendar store such as
searching, creating calendars, specifying calendar properties, and
specifying calendar access rights.
3. Solutions
3.1 Examples
Returning to the scenarios presented in section 2.1, the calendaring
protocols can be used 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 her to easily import her data 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 over the Internet.
She can then use any available [CAP] clients to access the data.
A proprietary system that provides access through a Web-based
interface could also be employed, but the use of [CAP] would be
superior in that it would allow the use of third party
applications, such as PDA synchronization tools.
c] The development team can use a calendar service which supports
[CAP], and 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 e-mail. This solution has
the drawback that it is difficult to examine other users' agendas,
making the organization of meetings more difficult.
Proprietary solutions are also available, but they require that
all members 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 e-mail 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
iCalendar [RFC-2445], HTML, or XML 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] [CAP] would be preferred for publishing a movie theater's
schedule, since it provides advanced access and search
capabilities. It also allows easy integration with customers'
calendar systems.
Non-standard methods such as serving data over HTTP could also be
employed, but would be harder to integrate with customers'
systems.
Using a completely proprietary solution would be very difficult,
if not impossible, since it would require every user to install
and use the proprietary software.
f] The social club could distribute meeting information in the
form of iTIP [RFC-2446] messages, sent via e-mail using iMIP
[RFC-2447]. The club could distribute meeting invitations, as
well as a full published agenda.
Alternatively, the 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 systems and their usage of
the various 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 others
using iTIP [RFC-2446]. The easiest binding of iTIP [RFC-2446] to use
would be iMIP [RFC-2447], since messages can be held in the users'
mail queues, which we assume to already exist. [CAP] could also be
used.
O ----------- ----------- O
-+- | CUA w/ | -----[iMIP]----- | CUA w/ | -+- user
A |local store| Internet |local store| A
/ \ ----------- ----------- / \
3.2.3 Single-user with Multiple CUAs
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 on accessibility. Some of these clients may have
local stores and others may not. Those with local stores need to
synchronize the data on the CUA 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; for example, one
may contain work-related information and another personal
information. The CUA may or may not have a local store. If it does,
then it needs to synchronize the data of the CUA 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 CUAs and services. The CUAs may or may not have
local stores. Those with local stores need to synchronize the data
on the CUAs 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 users
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| | |
/ \ ----------- ----------
4. Important Aspects
There are a number of important aspects of these calendaring
standards of which people, especially implementers, should be aware.
4.1 Timezones
The dates and times in components can refer to a specific time zone.
Time zones can be defined in a central store, or they may be defined
by a user to fit his or her needs. All users and applications should
be aware of time zones and time zone differences. New time zones may
need to 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 between a network
protocol such as [CAP], or a store and forward protocol, such as iMIP
[RFC-2447].
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 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, a
component with a very large attachment. Some applications may be
low-bandwidth or may be limited in the amount of data they can store.
Maximum component size may be set in [CAP]. It can also be
controlled in iMIP [RFC-2447] by restricting the maximum size of the
e-mail that the application can download.
4.5 Recurring Components
In iCAL [RFC-2445], one can specify complex recurrence rules for
VEVENTs, VTODOs, and VJOURNALs. One must be careful to correctly
interpret these recurrence rules and pay extra attention to being
able to interoperate using them.
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 are outlined below.
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. The calendaring protocols do
not attempt to provide unique access for contacting a given person.
Instead, 'calendar addresses' are booked, which may be e-mail
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
The calendaring 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 existing events. The calendaring protocols do not attempt
to address these needs in a real-time system. 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 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
requesters is released, and what shouldn't is not. 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 the Simple Authentication
Security Layer (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 are freely available. 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 E-mail
Because scheduling information can be transmitted over mail without
any authentication information, e-mail 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 e-mail, 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 of
this 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.
Acknowledgments
Thanks to the following, who have participated in the development of
this document:
Eric Busboom, Pat Egen, David Madeo, Shawn Packwood, Bruce Kahn,
Alan Davies, Robb Surridge.
References
[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)", Work in Progress.
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
Alexander Taler
EMail: alex@0--0.org
Full Copyright Statement
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AI ELI5
On this page
§1Introduction and Terminology
RFC 3283 is a guide to Internet calendaring and scheduling standards, intended to help developers and architects understand how different protocols work together. It addresses three existing standards—iCalendar (RFC 2445), iTIP (RFC 2446), and iMIP (RFC 2447)—and the emerging Calendar Access Protocol (CAP).
The guide defines essential terminology for understanding calendaring systems. A calendar is a collection of events, to-dos and journal entries that serve as an agenda. A calendar service is a server application providing access to calendar stores. A calendar user (CU) is a person or entity that accesses calendar information, while a calendar user agent (CUA) is the software the user employs to access calendars. A component is a piece of calendar data such as a specific event or to-do item.
The document also covers roles in scheduling: a delegator is a user who assigns their participation in an event to another person (the delegate), while a designate is a user authorized to act on behalf of another.
Before you understand calendars on the Internet, you need to know the words. A calendar is your schedule. A user is a person. An agent is the app they use. Once you know these words, everything else makes sense.
§2How the Four Standards Relate
iCalendar is the data format—the language used to describe calendar objects. Think of it as analogous to email's RFC 2822: it defines the structure of a calendar message or file.
iTIP (iCalendar Transport-Independent Interoperability Protocol) is the scheduling protocol that uses the iCalendar language to describe scheduling transactions—creating events, accepting invitations, proposing alternative times and responding to requests for free/busy information.
iMIP (iCalendar Message-Based Interoperability Protocol) is a transport binding that carries iTIP messages via email. Analogous to SMTP in the email world, iMIP enables scheduling across the Internet using existing email infrastructure.
CAP (Calendar Access Protocol) provides real-time access to a calendar store—similar to how IMAP and POP provide email access. CAP allows a calendar user agent to create calendars, search for events, manage access rights and perform calendar operations beyond basic scheduling.
The relationship is architecturally clean: iCalendar is the data format that all others build on, iTIP is the scheduling protocol (transport-independent), iMIP binds iTIP to email, and CAP provides real-time store access.
iCalendar is the language calendars use, like English. iTIP is the rules for scheduling meetings, like how to write a polite invitation. iMIP is sending that invitation by email. CAP is a live calendar server you can connect to whenever you want.
| Standard | Purpose | Analogy |
|---|---|---|
| iCalendar | Describes calendar data: events, to-dos, alarms | Email message format (RFC 2822) |
| iTIP | Protocol for scheduling: invitations, responses, free/busy | SMTP (mail transmission) |
| iMIP | Carries iTIP messages via email | Email with special content types |
| CAP THIS RFC | Real-time calendar store access and management | IMAP/POP (mail retrieval) |
§3What Calendaring Systems Need to Do
The document identifies six usage scenarios that motivate the need for standards.
A doctor managing her own calendar needs to read and manipulate her calendar with a single application. A musician with multiple devices needs to access and update her calendar from an Internet-based client and a PDA while maintaining synchronization. A software team needs to share calendar information to schedule meetings across multiple users on the same calendar service. A teacher and students need to schedule office hours and appointments within a calendar system.
The more demanding scenarios require interoperability across system boundaries. A movie theater publishing its schedule to customers must share calendar information with users of different vendors' systems. A social club scheduling meetings with members needs to coordinate across multiple different calendar services, possibly from different vendors.
Some of these needs can be met with proprietary solutions; others cannot. Interoperability across vendors and systems requires open standards for both the data format (iCalendar) and the protocols for scheduling (iTIP) and access (CAP).
People need to keep track of their own calendars, share schedules with others, and let computers from different companies talk to each other. That's why standards are needed—so all the different calendar apps can work together.
§4Deployment Patterns
The RFC illustrates how these standards apply to different system architectures.
A single-user standalone system needs no protocol at all, though it may use iCalendar as a storage format internally for portability to other applications.
For users with single-user systems communicating, iTIP via iMIP (email-based scheduling) is natural because email is already ubiquitous. This allows users to schedule meetings by sending and receiving calendar messages through their existing email systems.
A single user with multiple devices can synchronize a calendar across a PDA, web client and PC by connecting to a CAP-enabled calendar service. Each device uses CAP to read and update the shared calendar on the server.
For multiple users on the same multi-user system, CAP enables direct access to shared calendars, making it easy to view others' schedules and organize meetings without email latency.
When users on different multi-user systems need to schedule, the systems can coordinate using either CAP or iMIP, depending on whether they choose real-time or email-based communication.
There are different ways to set up calendars: just for one person, using email to trade invitations, connecting to a server, or connecting two servers together. The standards support all of these.
§5Key Implementation Challenges
The document highlights several important aspects that implementers must consider.
Timezones: Events can be specified in particular time zones. Time zones may be defined centrally or customized per user. Vendors may describe the same time zone differently, requiring care in interoperability.
Transport choice: Implementers must decide between network protocols like CAP (which may require firewall configuration) and email-based protocols like iMIP (which expose time-sensitive data to mail system latency). Each has trade-offs.
Data size: Calendar components may be very large, particularly with attachments. Systems may limit maximum component size and the amount of data that can be downloaded.
Recurring components: The RFC warns that recurring events with complex recurrence rules require careful interpretation to ensure interoperability.
Open issues: Several problems remain unsolved. Meeting scheduling operates on calendar addresses (often email addresses), but people may have multiple calendars in multiple locations, making it difficult to route bookings reliably. Neither iTIP nor CAP addresses user and calendar administration; this remains vendor-specific. Real-time event notification is not provided; clients must poll for changes instead. These gaps could be targets for future standards work.
When you build a calendar system, you have to think about time zones, email delays, file sizes and repeating events. There are also some problems nobody has solved yet, like knowing which calendar address is really for one person.
§6Security and Privacy Concerns
The RFC emphasizes that calendar systems handle sensitive scheduling and personal information, requiring careful security design.
Access control: CAP must offer granular configuration options so that only appropriate information is released to requesters. Only authorized users should see sensitive calendar data.
Authentication: Access control must be coupled with strong authentication so the right people see the right information. CAP uses the Simple Authentication and Security Layer (SASL) for authentication. iMIP, being email-based, faces challenges since store-and-forward protocols do not typically include authentication; implementers should consider requiring it as a default, using mechanisms such as signing and encryption.
Calendar spam: Because scheduling messages can be sent via email without authentication, email spoofing is trivial and calendar spam is possible. Systems allowing automated request processing should defend against malicious message flooding.
Denial of service: Once these systems become widely deployed, adversaries are likely to exploit the ability to flood calendars with bogus requests, requiring detection and recovery methods.
User experience: The security context should be obvious to users. Because underlying mechanisms may not be clear, user interfaces should make the security state apparent, similar to how browsers display a lock icon during secure connections.
Calendars hold private information, so you have to check that the right person is asking for it, make sure the message really came from who it says it did, and watch out for fake invitations and spam.