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Versions: 00 01 02 03                                                   
Network Working Group                                      J. Noerenberg
draft-ietf-calsch-mod-00.txt                               Qualcomm, Inc
Category: INTERNET DRAFT                               Expires: Dec 1997

             Internet Calendar Model Specification

Status of this Memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months.
Internet-Drafts may be updated, replaced, or made obsolete by other
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To learn the current status of any Internet-Draft, please check the
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Distribution of this document is unlimited.


Internet Calendaring and Scheduling protocols require the definition
of objects to encapsulate the notion of an event to be scheduled, a
calendar on which the event will be stored, and means for exchanging
these objects across the Internet between calendars on behalf of
people for whom the calendars are meaningful. This document gives an
abstract model of the objects and the protocols necessary to
accomplish this kind of information exchange.  It will establish the
context in which other Calendaring and Scheduling RFCs can be
interpreted.  Included are brief introductions to the other components
of Internet calendar protocols and definitions of nomenclature common
to all documents defining these protocols. Reading this document will
enable implementors and users of Internet Calendaring and Scheduling
protocols to understand the goals and constraints chosen for related

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Table of Contents

1. Document framework                                        3
1.1 Model Specification                                      3
1.2 iCalendar: Core Object Specification                     3
1.3 iTIP: Transport Independent Interoperability Protocol    3
1.4 CAP: Calendar Access Protocol Specification              4
2. Calendar protocol nomenclature                            4
2.1 Calendaring lexicon                                      4
3. Model Components                                          7
3.1 Calendar User                                            7
3.2 Calendar                                                 8
3.2.1 Collection of objects                                  8
3.2.2 Properties                                             8
3.2.3 Components                                             8
3.3 Calendar User Agent (CUA)                                9
3.4 Calendar Service                                         9
3.5 Calendar Domain                                         10
3.6 Calendar Access Protocol (CAP)                          10
3.7 Transport Independent Interoperability Protocol (iTIP)      10
4. Calendar System Model                                    10
4.1 Model Component Relationship                            10
4.2 Calendar Transport Model                                12
4.2.1 Direct Access                                         13
4.2.2 Calendar Service Mediation                            13
4.2.3 Interdomain Exchange                                  13
5. Security considerations                                  14
6. References                                               15
7. Acknowledgments                                          15
8. Author's address                                         16

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The Internet Calendar Model specification provides a framework for the
set of protocols that define Calendaring and scheduling for the
Internet.  The protocols specify the contents of calendars, and how
the objects stored on calendars are represented during transmission
across the Internet.  These protocols also define the interaction
between a calendar user agent and a calendar store, as well as the
actions performed by calendar transfer agents that facilitate
communication between calendar stores.  These terms will be defined
more precisely in the section on nomenclature below.  The protocols
are the:

"Core Object Specification" [CoreObjSpec]
"Calendar Interoperability Protocol" [CalIntProt]
"Calendar Access Protocol" [CalAccProt]

1. Document framework
Calendar and Scheduling Protocols are contained in a series of related
documents. This section describes the relationship between the

1.1 Model Specification
This document - see abstract and introduction above.

1.2 iCalendar: Core Object Specification
The Core Object Specification is the authoritative definition of all
properties that may be used in the Internet Calendar and Scheduling
Protocols as well as the rules for encoding and representing the
objects that are constructed from those properties. iCalendar also
specifies the method to be used to define new attributes.

1.3 iTIP: Transport Independent Interoperability Protocol
This document specifies how calendaring systems use iCalendar objects
to interoperate with other calendar systems.  It does so in a general
way so as to allow multiple methods of communication between systems.

Binding of iTIP to a real-time protocol
This document specifies a session-layer iTIP protocol.  Multiple
bindings are possible. This WG will specify and foster implementation
of at least one binding.

Binding of iTIP to E-mail
This document specifies an iTIP protocol over Internet e-mail using
MIME. Internet e-mail protocols are given by RFCs 821, 822, 2045-2049
[RFC-821] [RFC-822] [RFC- 2045] [RFC-2046] [RFC-2047]. [RFC-2048]
[RFC-2049].  See the references for details for constructing Internet
e-mail messages.

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1.4 CAP: Calendar Access Protocol Specification
This document specifies how a Calendar User Agent (CUA) will interact
with a Calendar Service using iCalendar objects.

A graphical representation of the relationship between the documents
is shown below:

                                           | Model Document   |
                                           |    iCalendar     |
                          |                                   |
      ------------------                 ------------------
         |      iTIP        |               |        CAP       |
      ------------------                 ------------------
          |               |
 ----------      -----------
| session  |    |   email   |
|   iTIP   |    |    iTIP   |
 ----------      -----------

2. Calendar protocol nomenclature
Calendaring and Scheduling uses a rich lexicon of terms that are
specific to the problems of scheduling events and reconciling
conflicting requests for time and resources.  This document will
identify the major components of these systems, and show component
relationships.  However, for the sake of completeness and to serve as
an introduction to the protocols in general, a more extensive list of
terms, and brief definitions are included here. Essential parts of the
system model have expanded definitions in this document where the
components of the model are introduced.

2.1 Calendaring lexicon

Alarm, Reminder
An asynchronous mechanism providing feedback for a pending or past
event or to-do.

Busy Time
A period of time that is not available for scheduling.

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A particular collection of calendar components.

Calendar Component
The objects that can be found in a calendar.  The components are
events, to-dos, journals, free/busies, time zones and alarms.
Components also include properties and may include other components.
A calendar component is identified by unique delimiters.

Calendar Date
A day identified by its position within the calendar year.

Calendar Scale
A particular type of calendar year, for example, Gregorian, Buddhist
Era, Japanese Emperor Era, Chinese Lunar, Islamic, and Jewish

Calendar Service
A Calendar Service (CS) stores a collection of calendars and interacts
with the Calendar User Agent (CUA) via the Calendar Access Protocol

Calendar Transport Agent (CTA)
A CTA mediates the exchange of calendar objects between calendars.  It
is generally responsible for interpreting the Transport Independent
Interoperability Protocol (iTIP) used to exchange calendar objects
either within a domain or across calendar domains.  It can be
implemented as a daemon that operates automatically without continuous
human intervention.

Calendar User Agent (CUA)
A CUA mediates the interactions between a calendar user and his
calendar.  It represents the information stored in the calendar to the
user, and enables the user to manipulate it. This is a particular
instance of the interactive process used by a calendar user.

Coordinate Universal Time (UTC)
The time scale maintained by the Bureau International de l'Heure
(International Time Bureau). UTC is often incorrectly referred to as

Daylight Saving Time (DST)
An adjustment to local time to accommodate annual changes in the
number of daylight hours. DST is also known as Advanced Time, Summer
Time, or Legal Time. Daylight Saving Time adjustments in the Southern
Hemisphere are opposite to those in the Northern Hemisphere.

A calendar component that defines a scheduled activity, minimally
specified by a start and end calendar date and time of day and a

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Free Time
A period of time available for scheduling on a calendar.

FreeBusy components describe blocks of allocated and unallocated time
on a calendar. They do not contain a description why the time is

Gregorian Calendar
A calendar scale in general use beginning in 1582.  The Gregorian
Calendar scale is based on a solar calendar consisting of common years
made up of 365 days and leap years made up of 366 days; both divided
into 12 sequential months.

A calendar component that defines a collection of information intended
for human presentation and is minimally specified by a calendar date
and one or more descriptions.

Local Time
The clock time in public use in a locale. Local time is often
referenced by the customary name for the time zone in which it is
located. The relationship between local time and UTC is based on the
offset(s) that are in use for a particular time zone. In general, the
formula is as follows:

local time = UTC + (offset)

A duration of time, specified as either a defined length of time or by
its beginning and end points.

Attributes that apply to calendar components or calendars. A calendar
component is a named set of properties.  Properties can also be used
to produce variants to suit a particular purpose.

Recurrence Rule
A notation used to represent repeating occurrences, or the exceptions
to such a repetition of an event or a to-do. The recurrence rule can
also be used in the specification of a time zone description.

Repeating Event or To-do
An event or to-do that repeats for one or more additional occurrences.
The recurrence may be defined with discrete dates and times and/or
with a recurrence rule.

Standard Time
Introduced by Sir Sanford Fleming and others around 1870, standard
time is a scheme for dividing the world into zones where the same time
would be kept. The original proposal was to divide the world into 24
zones, each zone having a width of 15 degrees of longitude. The center

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zone was originally the meridian passing through Greenwich, England,
called Greenwich Mean Time (GMT). The time in the zones was
decremented by one hour per zone going westwards and was incremented
by one hour per zone going eastwards from GMT. Changes have been made
to the original proposal to accommodate political boundaries. In
addition, some countries and regions specify 30 or 45 minute offsets,
rather than the full 60 minute offset. Standard time is also known as
Winter Time in some regions.

GMT and UTC are generally equivalent. However, by international
agreement, the GMT term is discouraged in favor of the term UTC for
all general time keeping.

Time Zone
A geographic region to which a specified offset from UTC applies.
While offsets can frequently be calculated by knowing the longitudinal
distance from Greenwich, England, local conventions frequently alter
the calculation, complicating the determination of local time.  Local
convention may also assign a label to identify the time zone.  There
is no world-wide standard for labels.

A calendar component that defines an action item and is minimally
specified by an effective calendar date and time of day, a due
calendar date and time of day, a priority and a description.

3. Model Components
There are several principal components in a Calendaring and Scheduling
system. Their relationship can be seen in Figure 2 below.  This
section identifies some of the salient features of the components.
The syntax and semantics for creating and transmitting these objects
are completely specified in [CoreObjSpec], [CalAccProt], and

3.1 Calendar User
A calendar user is the entity that interprets objects on a calendar,
creates them, and exchanges them with other calendar users.  A
calendar user may be a person (Ken Caminiti), a group of people (the
games of the San Diego Padres baseball club), or a resource (Padre's
home games at the "Q").  From the point of view of other calendar
users, groups and resources appear as a single entity to other
calendar users, regardless of their composition in the physical world.
Calendar users that are resources generally contain properties that
identify them as inanimate objects -- anything from a fruit bowl to
rubber bats to settle disputes during a meeting.

A calendar user owns his own calendar, and can manipulate objects
stored there via a CUA.  Access control attributes condition access to
calendars and their components and properties.

A calendar user can also manipulate the contents of other calendar
users' calendars by sending messages containing calendar objects to
them.  For example, The San Diego Padres sends calendar events for the

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1997 season to Ken Caminiti, so he knows when to show up at the
ballpark.  The Padres sends calendar events for games to be played at
home to the Qualcomm Stadium calendar so the concessionaires can order
hot dogs.

3.2 Calendar

3.2.1 Collection of objects
Calendar users own calendars.  This is a one to many relationship.  A
single calendar user may have multiple calendars.  A calendar is an
information store containing information about events,to-dos, alarms,
journals and free time, the components stored in it.  Also stored in a
calendar are properties that are global to all of the objects in the
calendar.  An example of a global property is the GEO property that
identifies the physical location where the calendar user can be found.
 Global properties such as this establish the context used to
interpret the objects stored in the calendar.  The principal
structural features of a calendar are described below in section
3.2.3.  When components or properties of a calendar are exchanged
between actors in a calendaring and scheduling network (Calendar User
Agents and Calendar Services), they are expressed in the form defined
in [CoreObjSpec].  Figure 1 below is a schematic representation of a

3.2.2 Properties
Properties are attributes of a component or a calendar.  They consist
of a name and a value.  Properties are strongly typed.  Some
properties are multivalued.  A property may have parameters that
distinguish between related properties.  Some properties may occur
multiple times in the same component instance, and may be gathered
into a logical group. Some properties may be unique to a particular
calendar or component.

3.2.3 Components
Components are collections of property values.  A particular set of
values for the properties of a component define a particular
component.  Some components may contain certain other components.  The
set of components in a calendar are identified below. Events
Event components are defined for specific starting date-time, have a
duration on a calendar, and a description.  Other properties of an
event may specify a location or other attributes that define the
event, such as resources that are part of the event.  Events may also
contain an Alarm component. To-do
While like an event, a To-do doesn't reserve a specific block of time
on a calendar.  A To- do component must have a starting date-time that
identifies its first appearance on the calendar.  It must also have a
date-time that specifies when the To-do expires.  A To-do must have a
description.  It may also contain an alarm component.

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An alarm component may occur in an Event or To-do.  It contains a
date-time. When present, and the date-time is passed, it will cause a
CUA or CS to notify the user the date-time has passed. Journal
A journal component is a textual item that is associated with a
particular date. As its name suggests, its purpose is to record
information meaningful about the date, but not necessarily tied to
other calendar components on a calendar. Timezone
Timezone components encapsulate rules for calculating local time from
UTC. Including this component in an event component enables a receiver
to calculate the universal time value for time values expressed in the
sender's local time. This component is especially useful for
expressing recurring events whose instances span a change in the time
reference such as the transition between Standard time and Daylight
Savings time.  Time values expressed in local time are always
interpreted in the receiver's local time.  The sender must provide
another context using UTC values and Timezone components if this is
not the interpretation intended by the sender.

  |         |            |           |            |           |
to-do     event       journal    freebusy     timezone    property...
    |              |
property...      alarm

Figure 1: Calendar Object Model

3.3 Calendar User Agent (CUA)
A CUA mediates the interactions between a calendar user and his
calendar.  It represents the information stored in the calendar to the
user, and enables the user to manipulate it. This is a particular
instance of the interactive process used by a calendar user.

3.4 Calendar Service
A Calendar Service (CS) stores a collection of calendars and interacts
with the Calendar User Agent (CUA) via the Calendar Access Protocol

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3.5 Calendar Domain
A collection of calendars that can be grouped together constitutes a
Calendar Domain. The relation used to bound the group is arbitrary.
Frequently membership in an organization will be used to define the
domain, but it could be a shared Internet address domain, as well.  A
Calendar Domain provides a contiguous address space for all the
calendars, CTAs and CUAs contained in the domain.  It must be possible
for any Calendar User (via the facilities of a CUA and/or CTA) to
determine whether they are members of a particular domain, or if other
Calendar Users are members.  CTAs and CUAs can take advantage of
domain information when routing event messages.

3.6 Calendar Access Protocol (CAP)
When calendar users need to manipulate calendars that are not stored
on the same computer where the CUA executes, the CUA will use the
Calendar Access Protocol to exchange components with the Calendar
Service (CS). CAP specifies the beginning and ending of the session
between the CUA and the CS.  Using CAP, the CUA will mediate
authentication of the user to the service.  CAP requires calendar
components and calendar properties to be expressed in the on-the-wire
data format defined in [CalObjSpec]. A CUA must not be required to
maintain a connection to a CS via CAP in order to display a Calendar
for a Calendar User or accept commands from a user to change a
Calendar's content.  By using CAP a CUA need not have specific
information on how a particular CS stores a Calendar and vice versa.
This enables specification and exchange of components and properties
independently of Calendar storage models adopted by particular CUAs or

3.7 Transport Independent Interoperability Protocol (iTIP)
CSs in a domain or across domains exchange components and properties
using iTIP. Like CAP, iTIP uses iCalendar formats to represent
components and properties. iTIP defines the beginning and ending of
the exchange session, as well the users for whom the messages are
intended.  iTIP permits unauthenticated delivery of components and
properties to a CS.  A CS may accept or reject delivery without
interaction with a user.  But a CS may require further confirmation of
receipt of a component or property before it is acted upon by the CS.

4. Calendar System Model
This section presents the Calender and Scheduling system model.  It
describes how the elements identified and described in the section 3
relate to each other. This is done in two parts. Table 1 in section
4.1 below summarizes the relationships between pairs of elements.
Section 4.2, Calendar Transport Model, identifies and describes the
different transport modes that are supported by Calendaring and
Scheduling protocols.

4.1 Model Component Relationship
The table below identifies the defined relationships between pairs of
elements. If a cell contains a number that means that the elements
specified in the row and column headings have a relationship with each

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other.  If a cell is blank then there is no relationship between the
corresponding pair of model elements. Following the table the numbered
relationships are described in detail.

Table 1: Component Relationships

         Calendar  Calendar     Calendar   Calendar   Calendar    iTIP   CAP
           User               User      Service    Domain
Calendar              1       2

Calendar                      3           4          5

User                          6           7          8         6,8    7

Calendar                                  9          9         9

Calendar                                             10        10



1. The relationship between a Calendar and a Calendar User is defined
in the Model Components section 3.

2. A Calendar User interacts with the system through a Calendar User
Agent.  The Calendar User Agent is typically (but not necessarily) an
interactive program that the Calendar User depends on to maintain
their own calendar and to interact with other users' calendars, for
example to invite them to meetings.

3. A Calendar User Agent interacts with a Calendar typically on behalf
of a Calendar User. However, a Calendar User Agent may be a program
without a user interface, for example a program that scans multiple
calendars for vacation entries and maintains a summary in a separate

4. A Calendar Service may store a Calendar although this model does
not require that.  In fact, this model does not define how a Calendar
is stored, leaving it open to the implementation to determine that.
The calendar may be stored in a file system, in memory, in the message
store of a messaging system etc.

5. The relationship between a Calendar and a Calendar Domain is
defined in the Model Components, section 3.

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6. A Calendar User Agent may interact directly with another Calendar
User Agent by using iTIP.

7. A Calendar User Agent may interact with a Calendar Service using
the Calendar Access Protocol.

8. A Calendar User Agent may interact with a Calendar Domain via iTIP.

9. A Calendar Service may interact with another Calendar Service or a
Calendar Domain using iTIP.

10. Calendar Domains may interact with each other using iTIP.

4.2 Calendar Transport Model
There are several transport modes in this architecture.  The figures
below illustrate the different modes that are allowed.  Three modes
are required to handled the different kinds of calendar exchanges
across the Internet, person to person, group interactions local to a
particular network, and exchanges across networks.

 ------------                  ------------
| CUA  | rcvr| -----      ----|rcvr|  CUA  |
|       -----|      \   /     |----        |
|            |       \ /      |            |
|            |        X       |            |
|            |       / \      |            |
|       -----|      /   \     |----        |
|      | sndr| -----      ----|sndr|       |
 ------------       \    /     ------------

Figure 2: Direct Access

 ------------                  ------------
| CUA        |                |       CUA  |
|            |                |            |
|            |                |            |
 ------------                  ------------
        \                          /
         \   ------- CAP -------  /
          \                      /
           \   --------------   /
            \ |      CS      | /
              |              |
              |              |

Figure 3: Calendar Service Mediation

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 -----------------                     ------------------
|                 |                   |                  |
| Calendar Domain |                   | Calendar Domain  |
|                 |                   |                  |
|                 |                   |                  |
|  -------        |                   |                  |
| |  CUA  |       |                   |                  |
|  -------        |                   |                  |
|     |           |                   |                  |
|     | -- CAP    |                   |                  |
|     |           |                   |                  |
|  ------------   |                   |---------         |
| | CS   | rcvr|----------     -------|rcvr|    |        |
| |       -----|  |       \   /       |----     |        |
| |            |  |        \ /        | Gateway |        |
| |            |  |         X         |         |        |
| |            |  |        / \        |         |        |
| |       -----|  |       /   \       |----     |        |
| |      | sndr| ---------      ------|sndr|    |        |
|  ------------   |       \    /      |---------         |
|                 |        iTIP       |                  |
|                 |                   |                  |
 -----------------                     ------------------

 Figure 4: Interdomain Exchange

4.2.1 Direct Access
For direct access, two calendar user agents (CUA) exchange calendar
components by using iTIP.  Each CUA provides an iTIP sender and
receiver.  As is generally the case, the methods used by the CUA to
store calendar data locally are not relevant to the transport model.
For this mode, calendar users must be uniquely identifiable across the
Internet, and access to CUAs must conform with privacy and security

4.2.2 Calendar Service Mediation
Using Calendar Service Mediation a CUA interoperates with a Calendar
Service (CS) using CAP to exchange calendar components.  The CS takes
responsibility for mediating receipt and delivery of components with
other collaborating CUAs.  The principal difference between this mode
and Interdomain Exchange is that CSs do not need to use iTIP to
facilitate exchange of components.  A consequence of this mode is that
CUAs and CSs need not use addresses that are unique across the
Internet.  However, consistency with other modes makes a consistent
address model an obvious simplification for implementors.

4.2.3 Interdomain Exchange
With Interdomain Exchange a Calendar Service (CS) supporting a group
of users in one domain can exchange calendar components with a
different calendar domain. Domains may or may not be within the same
Internet network domain.  Like Direct Access, iTIP is the vehicle
which permits component exchange.  In the figure, one domain is

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illustrated with a Calendar Service providing iTIP service.  The 2nd
domain in this figure has a distinct iTIP sender and receiver.  In
order to provide end-to-end privacy components must be wrapped in a
cryptographically secure wrapper to insure only the intended
corespondents can interpret the components.  This wrapper is not
required unless privacy must be assured.  In order to provide backward
compatibility with existing calendar and scheduling systems, a privacy
wrapper cannot be a required aspect of the component exchange.

5. Security considerations
The Core Object Specification must provides the means to classify the
intended sensitivity level of an event, to-do or journal calendar
component (i.e., PUBLIC, PRIVATE, or CONFIDENTIAL).  It must also
provide a means to wrap all components in an exchange in a
cryptographically secure envelope so that only the intended
correspondents can decode the message.

The Transport Independent Interoperability Protocol must provides a
description of the authentication step that must be defined in any of
the iTIP bindings.

The Calendar Access Protocol must provide a description of the
elements of the calendaring system security model and the protocol
elements for creating and manipulating the access control to a

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6. References
[RFC-821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, USC/Information Sciences Institute, August 1982.

[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.

[RFC-2045] Borenstein, N. & Freed, N., "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies", RFC
2045, Nov 1996

[RFC-2046] Borenstein, N. & Freed, N., "Multipurpose Internet Mail
Extensions MIME) Part Two: Media Types", RFC 2046, Nov 1996

[RFC-2047] Borenstein, N. & Freed, N., "MIME (Multipurpose Internet
Mail Extensions) Part Three: Message Header Extensions for Non-ASCII
Text", RFC 2047, Nov 1996

[RFC-2048] Borenstein, N. & Freed, N., "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures", RFC 2048, Nov

[RFC-2049] Borenstein, N. & Freed, N., "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC
2049, Nov 1996

[CoreObjSpec] "Core Object Specification"

[iTIP] "Calendar Interoperability Protocol"

[CalAccProt] "Calendar Access Protocol"

7. Acknowledgments
The author is extremely grateful for the assistance, patience and
tolerance of the members of the CalSch working group.  Their ideas and
suggestions are crucial to making this a useful document.  In
particular, the author would like to thank
Anik Ganguly
Derik Stenerson
Frank Dawson
Their comments and ideas were particularly important for the original
formulation of this draft.  I would also like to thank Qualcomm,
Incorporated for allowing the time necessary to bring this effort to

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8. Author's address

For more information, please contact the author via Internet Mail.

John W. Noerenberg, II
Qualcomm, Incorporated
6455 Lusk Blvd
San Diego, CA 92131

email: jwn2@qualcomm.com
ph: +1 619 658 3510

The "Internet Calendar Model Specification" is the work of the Internet
Engineering Task Force Working Group on Calendaring and Scheduling.
The chairman of that group, Anik Ganguly, may be reached at:

Anik Ganguly
Campbell Services, Inc.
21700 Northwestern Highway
10th Floor
Southfield, MI 48075
email: anik@ontime.com

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