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Calendar Server Extension                                       C. Daboo
                                                              Apple Inc.
                                                          March 19, 2012

CalDAV: Calendar User Notifications

Abstract

This specification defines an extension to CalDAV that allows the
   server to provide notifications to calendar users.

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Table of Contents

1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   4.  Notifications  . . . . . . . . . . . . . . . . . . . . . . . .  3
     4.1.  Additional Principal Properties  . . . . . . . . . . . . .  4
       4.1.1.  CS:notification-URL Property . . . . . . . . . . . . .  5
     4.2.  Properties on Notification Resources . . . . . . . . . . .  5
       4.2.1.  CS:notificationtype Property . . . . . . . . . . . . .  5
     4.3.  XML Element Definitions  . . . . . . . . . . . . . . . . .  6
       4.3.1.  CS:notifications . . . . . . . . . . . . . . . . . . .  6
       4.3.2.  CS:notification  . . . . . . . . . . . . . . . . . . .  6
       4.3.3.  CS:dtstamp . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Notification Definitions . . . . . . . . . . . . . . . . . . .  7
     5.1.  System Status Notification . . . . . . . . . . . . . . . .  7
       5.1.1.  CS:systemstatus Element Definition . . . . . . . . . .  8
     5.2.  Quota Notification . . . . . . . . . . . . . . . . . . . .  8
       5.2.1.  CS:quotastatus Element Definition  . . . . . . . . . .  9
     5.3.  Resource Changes Notification  . . . . . . . . . . . . . . 10
       5.3.1.  CS:resource-change Element Definition  . . . . . . . . 11
       5.3.2.  CS:calendar-changes Element Definition . . . . . . . . 15
         5.3.2.1.  Handling Recurrences in CS:calendar-changes  . . . 17
       5.3.3.  CS:deleted-details Element Definition  . . . . . . . . 18
       5.3.4.  CS:notify-changes Property . . . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 21
     A.1.  Resource Created . . . . . . . . . . . . . . . . . . . . . 21
     A.2.  Resource Updated - Property Change . . . . . . . . . . . . 22
     A.3.  Resource Updated - Parameter Change  . . . . . . . . . . . 23
     A.4.  Resource Updated - Multiple Instances Change . . . . . . . 23
     A.5.  Resource Updated - Multiple User Change  . . . . . . . . . 24
     A.6.  Resource Deleted . . . . . . . . . . . . . . . . . . . . . 25
     A.7.  Collection Created . . . . . . . . . . . . . . . . . . . . 26
     A.8.  Collection Updated . . . . . . . . . . . . . . . . . . . . 26
     A.9.  Collection Deleted . . . . . . . . . . . . . . . . . . . . 27
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27

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1.  Introduction

CalDAV [RFC4791] provides a way for calendar users to store calendar
   data and exchange this data via scheduling operations.  Based on the
   WebDAV [RFC4918] protocol, it also includes the ability to manage
   access to calendar data via the WebDAV ACL [RFC3744] extension.

It is often useful for servers to communicate arbitrary information
   to calendar users, e.g., system status, message of the day, quota
   warnings, changes to shared resources made by others etc.  This
   specification defines a generic "notification" mechanism that allows
   a server to do that.  Whilst primarily aimed at CalDAV [RFC4791],
   this mechanism has been designed to be adaptable to WebDAV [RFC4918].

2.  Open Issues

1.  Define specific child elements for system status notification,
       e.g. "server-maintenance-period", "server-read-only-period",
       "client-upgrade-required".

3.  Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

When XML element types in the namespaces "DAV:" and
   "urn:ietf:params:xml:ns:caldav" are referenced in this document
   outside of the context of an XML fragment, the string "DAV:" and
   "CALDAV:" will be prefixed to the element type names respectively.

The namespace "http://calendarserver.org/ns/" is used for XML
   elements defined in this specification.  When XML element types in
   that namespace are referenced in this document outside of the context
   of an XML fragment, the string "CS:" will be prefixed to the element
   type names.

4.  Notifications

When this feature is available, a CS:notification-URL (Section 4.1.1)
   property appears on principal resources for those principals who are
   able to receive notifications.  That property specifies a single DAV:
   href element whose content refers to a WebDAV collection resource.
   Notification "messages" are deposited into this collection and can be
   retrieved by clients and acted on accordingly.

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The notification collection referenced by the CS:notification-URL
   (Section 4.1.1) property MUST have a DAV:resourcetype property with
   DAV:collection and CS:notifications (Section 4.3.1) child elements.

Notification "messages" are XML documents stored as resources in the
   notification collection.  Each XML document contains a CS:
   notification (Section 4.3.2) element as its root.  The root element
   contains a CS:dtstamp element, and one additional element which
   represents the type of notification being conveyed in the message.
   That child element will typically contain additional content that
   describes the notification.

Each notification resource has a CS:notificationtype (Section 4.2.1)
   property which contains as its single child element an empty element
   that matches the child element of the notification resource XML
   document root.  Any attributes on the child element in the XML
   document are also present in the property child element.

Notifications are automatically generated by the server (perhaps in
   response to a action) with an appropriate resource stored in the
   notifications collection of the user to whom the notification is
   targeted.  Clients SHOULD monitor the notification collection looking
   for new notification resources.  When doing so, clients SHOULD look
   at the CS:notificationtype (Section 4.2.1) property to ensure that
   the notification is of a type that the client can handle.  Once a
   client has handled the notification in whatever way is appropriate it
   SHOULD delete the notification resource.  Clients SHOULD remove
   notifications being displayed to a user when the notification
   resource is removed from the notification collection, to enable the
   user to dismiss a notification on one device and have it
   automatically removed from others.  Clients MUST ignore all
   notifications for types they do not recognize.  Servers MAY delete
   notification resources on their own if they determine that the
   notifications are no longer relevant or valid.  Servers MAY coalesce
   notifications as appropriate.

Servers MUST prevent clients from adding resources in the
   notification collection.

4.1.  Additional Principal Properties

This section defines new properties for WebDAV principal resources as
   defined in RFC3744 [RFC3744].  These properties are likely to be
   protected but the server MAY allow them to be written by appropriate
   users.

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4.1.1.  CS:notification-URL Property

Name:  notification-URL

Namespace:  http://calendarserver.org/ns/

Purpose:  Identify the URL of the notification collection owned by
      the associated principal resource.

Protected:  This property SHOULD be protected.

PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND allprop request (as defined in Section 14.2 of
      [RFC4918]).

COPY/MOVE behavior:  This property value SHOULD be preserved in COPY
      and MOVE operations.

Description:  This property is needed for a client to determine where
      the notification collection of the current user is located so that
      processing of notification messages can occur.  If not present,
      then the associated calendar user is not enabled for notification
      messages on the server.

Definition:

<!ELEMENT notification-URL (DAV:href)>

4.2.  Properties on Notification Resources

The following new WebDAV properties are defined for notification
   resources.

4.2.1.  CS:notificationtype Property

Name:  notificationtype

Namespace:  http://calendarserver.org/ns/

Purpose:  Identify the type of notification of the corresponding
      resource.

Protected:  This property MUST be protected.

PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND allprop request (as defined in Section 14.2 of
      [RFC4918]).

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COPY/MOVE behavior:  This property value MUST be preserved in COPY
      and MOVE operations.

Description:  This property allows a client, via a PROPFIND Depth:1
      request, to quickly find notification messages that the client can
      handle in a notification collection.  The single child element is
      the notification resource root element's child defining the
      notification itself.  This element MUST be empty, though any
      attributes on the element in the notification resource MUST be
      present in the property element.

Definition:

<!ELEMENT notificationtype ANY>

4.3.  XML Element Definitions

4.3.1.  CS:notifications

Name:  notifications

Namespace:  http://calendarserver.org/ns/

Purpose:  Indicates a notification collection.

Description:  This XML element is used in a DAV:resourcetype element
      to indicate that the corresponding resource is a notification
      collection.

Definition:

<!ELEMENT notifications EMPTY>

4.3.2.  CS:notification

Name:  notification

Namespace:  http://calendarserver.org/ns/

Purpose:  Notification message root element.

Description:  The root element used in notification resources.

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Definition:

<!ELEMENT notification (dtstamp, XXX) >

4.3.3.  CS:dtstamp

Name:  dtstamp

Namespace:  http://calendarserver.org/ns/

Purpose:  Date-time stamp.

Description:  Contains the date-time stamp corresponding to the
      creation of a notification message, using the format defined in
      [RFC3339], or the "compact" format without "-" and ":" characters
      between date and time elements, respectively.

Definition:

<!ELEMENT dtstamp (#PCDATA)>

5.  Notification Definitions

This section defines a set of common notification types.

5.1.  System Status Notification

The system status notification is used to convey a URI and/or textual
   description to the user.  The assumption is that the URI points to a
   webpage where current system status is described in detail, with the
   provided description being a summary of that.  A "type" attribute on
   the element is used to indicate the importance of the current status
   notification, and has the values "low", "medium" and "high",
   representing the increasing level of importance of the message
   respectively.

Servers might have knowledge of specific calendar user language
   preferences, in which case it MAY localise the CS:description value
   as appropriate based on the calendar user accessing the notification,
   but if it does, it SHOULD include an xml:lang attribute on the CS:
   description element to indicate what language is being used.

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5.1.1.  CS:systemstatus Element Definition

Name:  systemstatus

Namespace:  http://calendarserver.org/ns/

Purpose:  Indicates a system status notification.

Description:  This XML element is used in a CS:notification element
      to describe a system status notification.

Definition:

<!ELEMENT systemstatus (DAV:href?, CS:description?)>
   <!ATTLIST systemstatus type (low | medium | high) "low">

<!ELEMENT description CDATA>

Example:  This is an example of the body of a notification resource
      for an emergency system outage:

<?xml version="1.0" encoding="UTF-8"?>
   <CS:notification xmlns:D="DAV:"
                    xmlns:CS="http://calendarserver.org/ns/">
     <CS:dtstamp>2011-12-09T11:12:53-05:00</CS:dtstamp>
     <CS:systemstatus type="high">
       <D:href>http://example.com/emergency_shutdown.html</D:href>
       <CS:description xml:lang='en_US'
       >Emergency shutdown now</CS:description>
     </CS:systemstatus>
   </CS:notification>

Example:  This is an example of the WebDAV property on the example
      notification resource above:

<?xml version="1.0" encoding="UTF-8"?>
   <CS:notificationtype xmlns:D="DAV:"
                        xmlns:CS="http://calendarserver.org/ns/">
     <CS:systemstatus type="high" />
   </CS:notificationtype>

5.2.  Quota Notification

The quota notification is used to convey information about the status
   of one or more quotas for the user.  The notification contains
   elements for different types of quota being reported to the user.  In

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some cases these may be warnings (e.g., a user getting to 80% of
   their quota limit), or in other cases errors (e.g., a user exceeding
   their quota).

5.2.1.  CS:quotastatus Element Definition

Name:  quotastatus

Namespace:  http://calendarserver.org/ns/

Purpose:  Indicates a quota status notification.

Description:  This XML element is used in a CS:notification element
      to describe a quota status notification.  The CS:quota-percent-
      used element contains an integer greater than or equal to zero.
      If the value is greater than or equal to 100, then the user's
      quota has been reached or exceeded.  The DAV:href element contains
      a URI for a webpage where the user can go to get further
      information about their quota status or take corrective action.

Definition:

<!ELEMENT quota-status (quota+)>

<!ELEMENT quota (quota-type, quota-percent-used?,
                      quota-count?, DAV:href?)>
     <!ATTLIST quota type (warning | exceeded) "exceeded">

<!ELEMENT quota-type ANY>

<!ELEMENT quota-percent-used CDATA>

<!ELEMENT quota-count CDATA>

Example:  This is an example of the body of a notification resource
      for a quota warning:

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Example:  This is an example of the body of a notification resource
      for a quota that has been exceeded, and a count-based limit that
      is shown as a warning:

<?xml version="1.0" encoding="UTF-8"?>
   <CS:notification xmlns:D="DAV:"
                    xmlns:CS="http://calendarserver.org/ns/">
     <CS:dtstamp>2011-12-09T11:12:53-05:00</CS:dtstamp>
     <CS:quota-status>
       <CS:quota type="exceeded">
         <CS:quota-type><CS:attachments /></CS:quota-type>
         <CS:quota-percent-used>102</CS:quota-percent-used>
         <D:href>https://example.com/fix-account.html</D:href>
       </CS:quota>
       <CS:quota type="warning">
         <CS:quota-type><CS:events /></CS:quota-type>
         <CS:quota-percent-used>82</CS:quota-percent-used>
         <CS:quota-count>4980</CS:quota-count>
         <D:href>https://example.com/buy-more-space.html</D:href>
       </CS:quota>
     </CS:quota-status>
   </CS:notification>

5.3.  Resource Changes Notification

The resource change notification is used to inform the user of new,
   updated or deleted resources caused by changes made by someone else
   (note: servers MUST NOT generate notifications to users for changes
   they themselves make, though the possibility of an automated process
   acting on behalf of a user needs to be considered).  This
   notification can be used by clients to show changes that a user can
   acknowledge in their own time.  When the notification is present, it
   can be displayed on all devices a user is accessing their data from.
   When the user acknowledges and dismisses the notification on one
   device, other devices SHOULD also remove the notification when they

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next synchronize the notification collection.

A new WebDAV property CS:notify-changes (Section 5.3.4) is defined
   for calendar collections.  This allows users to enable or disable the
   sending of resource change notifications for the calendar and its
   child resources.  Servers MUST allow users to set this property on a
   per-user basis on any calendars accessible to them.  Servers MUST
   honor the chosen setting to enable or disable change notifications.

Servers can send notifications for calendar object resources, and
   ones for calendar collections.  Servers SHOULD coalesce notifications
   that refer to the same resource into a single notification resource,
   containing multiple CS:created, CS:updated or CS:deleted elements all
   with the same DAV:href child element value.  Servers MAY coalesce
   changes to multiple resources into a change notification for the
   parent collection of those resources and use a CS:collection-changes
   element to indicate the number of individual resources that changed.

5.3.1.  CS:resource-change Element Definition

Name:  resource-change

Namespace:  http://calendarserver.org/ns/

Purpose:  Indicates that resources have been created, updated or
      deleted.

Description:  This XML element is used in a CS:notification element
      to describe a resource change notification.  It can describe a
      change directly to a calendar object resource or to a calendar
      collection.

When used for a calendar object resource change, it can contain
      one of the CS:created, or CS:deleted elements, or multiple CS:
      updated elements, which indicate a created, deleted or updated
      resource, respectively.  The DAV:href element within those
      elements, contains the URI of the changed resource, optional
      information about who changed the resource and when that change
      was made (the CS:changed-by element), and information specific to
      the nature of the change.  Servers SHOULD coalesce resource change
      notifications for the same resource into a single notification
      resource where possible.  The CS:updated element optionally
      contains CS:content and/or DAV:prop elements to indicate a change
      to the body of the resource or resource WebDAV properties,
      respectively.  The DAV:prop element MAY contain a list of property
      elements to indicate which properties changed.  The CS:updated
      element can also contain zero or more CS:calendar-changes elements
      to list details of the changes.  If no CS:calendar-changes element

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is present, the specific details are not provided, and clients
      will need to assume that some set of changes occurred, but the
      server is unwilling to disclose the full details.  The CS:deleted
      element can also contain zero or more CS:deleted-details elements
      to list details of the deleted resource.

When used for a calendar collection change, it can contain a CS:
      collection-changes element.  The DAV:href element within that
      element, contains the URI of the changed calendar collection.  The
      DAV:prop element indicates a change to WebDAV properties on the
      calendar collection resource.  The CS:child-created, CS:child-
      updated, and CS:child-deleted elements each contain a positive
      integer value indicating how many child resources were added,
      updated or deleted in the collection, respectively.

Definition:

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<!ELEMENT resource-change (created | updated+ | deleted |
                              collection-changes)>
     <!ELEMENT created (DAV:href, changed-by?, ANY)>
     <!ELEMENT updated (DAV:href, changed-by?, content?,
                        DAV:prop?, calendar-changes*)>
       <!ELEMENT content EMPTY>
     <!ELEMENT deleted (DAV:href, changed-by?, deleted-details)>

<!ELEMENT changed-by (common-name | (first-name, last-name),
                           dtstamp?, DAV:href)>
       <!ELEMENT common-name CDATA>
       <!ELEMENT first-name CDATA>
       <!ELEMENT last-name CDATA>

<!ELEMENT collection-changes (DAV:href, changed-by*, DAV:prop?,
                                   child-created?, child-updated?,
                                   child-deleted?>

<!ELEMENT child-created CDATA>
       <!ELEMENT child-updated CDATA>
       <!ELEMENT child-deleted CDATA>

Example:  This is an example of the body of a notification resource
      for changes where one resource has been created:

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Example:  This is an example of the body of a notification resource
      for changes where a resource has been updated twice.  One of the
      updated resources elements contains additional information
      indicating which recurrence instances in the iCalendar data were
      changed:

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Example:  This is an example of the body of a notification resource
      for changes where one resource has been deleted:

Example:  This example is the same as the previous three, except that
      all the individual resource changes have been coalesced into a
      single notification about changes to the parent calendar
      collection:

5.3.2.  CS:calendar-changes Element Definition

Name:  calendar-changes

Namespace:  http://calendarserver.org/ns/

Purpose:  Indicates which portions of an calendar object resource
      have changed, or provides details of deleted calendar object
      resources.

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Description:  This XML element is used in a CS:updated element to
      describe how a calendar object resource changed, or in a CS:
      deleted element to provide details of a deleted resource.  It can
      identify the master instance, or individual recurrence instances,
      and for each indicate which iCalendar properties and parameters
      changed during the update for which the notification was
      generated.  For details of handling recurrences please see
      Section 5.3.2.1.

Definition:

<!ELEMENT calendar-changes (recurrence+) >

<!ELEMENT recurrence
         ((master | recurrenceid), added?, removed?, changes?)>

<!ELEMENT master EMPTY>

<!ELEMENT recurrenceid CDATA>

<!ELEMENT added EMPTY>

<!ELEMENT removed EMPTY>

<!ELEMENT changes changed-property*>

<!ELEMENT changed-property changed-parameter*>
         <!ATTLIST changed-property name PCDATA>

<!ELEMENT changed-parameter EMPTY>
           <!ATTLIST changed-parameter name PCDATA>

Example:  This example indicates that a non-recurring component, or
      the master component in a recurring component, was changed and
      that the change was to the "SUMMARY" iCalendar property.

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<CS:calendar-changes xmlns:CS="http://calendarserver.org/ns/">
     <CS:recurrence>
       <CS:master/>
       <CS:changes>
         <CS:changed-property name="SUMMARY"/>
       </CS:changes>
     </CS:recurrence>
   </CS:calendar-changes>

Example:  This example indicates that an instance of a recurring
      component was changed and that the change was to the "DTSTART"
      iCalendar property.

<CS:calendar-changes xmlns:CS="http://calendarserver.org/ns/">
     <CS:recurrence>
       <CS:recurrenceid>20111215T160000Z</CS:recurrenceid>
       <CS:changes>
         <CS:changed-property name="DTSTART"/>
       </CS:changes>
     </CS:recurrence>
   </CS:calendar-changes>

5.3.2.1.  Handling Recurrences in CS:calendar-changes

Changes to recurring components can be complex.  This section
   describes the possible set of changes that could occur, and what the
   CS:calendar-changes element will contain as a result.

Master exists, unchanged override added  In this case, a CS:
      recurrence element will be present, containing a CS:recurrence-id
      element with a value equal to the RECURRENCE-ID property value (in
      UTC) of the added component.  A CS:added element will be present.
      There will not be any CS:removed or CS:changes elements.

Master exists, changed override added  In this case, a CS:recurrence
      element will be present, containing a CS:recurrence-id element
      with a value equal to the RECURRENCE-ID property value (in UTC) of
      the added component.  Both CS:added and CS:changes elements will
      be present.  There will not be a CS:removed element.

Master exists, override changed  In this case, a CS:recurrence
      element will be present, containing a CS:recurrence-id element
      with a value equal to the RECURRENCE-ID property value (in UTC) of
      the added component.  A CS:changes element will be present.  There
      will not be any CS:added or CS:removed elements.

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Master exists, override removed  In this case, a CS:recurrence
      element will be present, containing a CS:recurrence-id element
      with a value equal to the RECURRENCE-ID property value (in UTC) of
      the added component.  A CS:removed element will be present.  There
      will not be a CS:added element.  A CS:changes element will only be
      present if the removed component differs from the "derived" master
      instance.

Master exists, override cancelled  In this case, a CS:recurrence
      element will be present, containing a CS:recurrence-id element
      with a value equal to the RECURRENCE-ID property value (in UTC) of
      the added component.  A CS:removed element will be present.  There
      will not be any CS:added or CS:changes element.  There will also
      be a CS:master element present, with an appropriate CS:changes
      element, likely covering a change to "RRULE" or addition of
      "EXDATE" properties.

Master does not exist, override added  In this case, a CS:recurrence
      element will be present, containing a CS:recurrence-id element
      with a value equal to the RECURRENCE-ID property value (in UTC) of
      the added component.  A CS:added element will be present.  There
      will not be a CS:removed or CS:changes element.

Master does not exist, override changed  In this case, a CS:
      recurrence element will be present, containing a CS:recurrence-id
      element with a value equal to the RECURRENCE-ID property value (in
      UTC) of the added component.  A CS:changes element will be
      present.  There will not be any CS:added or CS:removed elements.

Master does not exist, override removed  In this case, a CS:
      recurrence element will be present, containing a CS:recurrence-id
      element with a value equal to the RECURRENCE-ID property value (in
      UTC) of the added component.  A CS:removed element will be
      present.  There will not be any CS:added or CS:changes element.

5.3.3.  CS:deleted-details Element Definition

Name:  deleted-details

Namespace:  http://calendarserver.org/ns/

Purpose:  Provides summary information about a deleted resource or
      collection.

Description:  This XML element is used in a CS:deleted element to
      describe useful information about a deleted resource or
      collection, so clients can provide a meaningful notification
      message to users.  This element has two variants: one for deletion

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of a calendar object resource, the other for deletion of a
      calendar collection.

Definition:

<!ELEMENT deleted-details ((deleted-component,
                               deleted-summary,
                               deleted-next-instance?,
                               deleted-had-more-instances?) |
                              deleted-displayname)>

<!ELEMENT deleted-component CDATA>

<!ELEMENT deleted-summary CDATA>

<!ELEMENT deleted-next-instance CDATA>
     <!ATTLIST deleted-next-instance tzid PCDATA>

<!ELEMENT deleted-had-more-instances EMPTY>

<!ELEMENT deleted-displayname CDATA>

Example:  This example indicates deletion of a non-recurring event
      that was yet to occur at the time of deletion.

<CS:deleted-details xmlns:CS="http://calendarserver.org/ns/">
     <CS:deleted-component>VEVENT</CS:deleted-component>
     <CS:deleted-summary>Birthday Party</CS:deleted-summary>
     <CS:deleted-next-instance tzid="America/New_York
     >20120505T120000</CS:deleted-next-instance>
   </CS:deleted-details>

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Example:  This example indicates deletion of a calendar.

<CS:deleted-details xmlns:CS="http://calendarserver.org/ns/">
     <CS:deleted-displayname>Holidays</CS:deleted-displayname>
   </CS:deleted-details>

5.3.4.  CS:notify-changes Property

Name:  notify-changes

Namespace:  http://calendarserver.org/ns/

Purpose:  Allows a user to specify whether resource change
      notifications are generated by the server.

Protected:  This property MUST NOT be protected.

PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND allprop request (as defined in Section 14.2 of
      [RFC4918]).

COPY/MOVE behavior:  This property value MUST be preserved in COPY
      and MOVE operations.

Description:  This property allows a user to enable or disable the
      server generation of resource change notifications for the
      calendar collection, and all its child resources, on which the
      property resides.  If the property is not present on a calendar
      collection, the client and server MUST assume that resource change
      notifications are enabled.

Definition:

<!ELEMENT notify-changes (true|false)>
     <!ELEMENT true EMPTY>
     <!ELEMENT false EMPTY>

6.  Security Considerations

Some notification mechanisms might allow a user to trigger a
   notification to be delivered to other users (e.g., an invitation to
   share a calendar).  In such cases servers MUST ensure that suitable
   limits are placed on the number and frequency of such user generated
   notifications.

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TBD: More?

7.  IANA Considerations

This document does not require any actions on the part of IANA.

8.  Acknowledgments

This specification is the result of discussions between the various
   Apple calendar server and client teams.

9.  References

9.1.  Normative References

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", RFC 3339, July 2002.

[RFC4918]  Dusseault, L., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

9.2.  Informative References

[RFC3744]  Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web
              Distributed Authoring and Versioning (WebDAV)
              Access Control Protocol", RFC 3744, May 2004.

[RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              March 2007.

Appendix A.  Examples

This section provides more detailed examples of resource change
   notifications for illustrative purposes only.

A.1.  Resource Created

This is an example of the body of a notification resource where one
   resource has been created.

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A.2.  Resource Updated - Property Change

This is an example of the body of a notification resource where one
   non-recurring event has had its "DTSTART" and "SUMMARY" iCalendar
   property values changed.

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A.3.  Resource Updated - Parameter Change

This is an example of the body of a notification resource where one
   non-recurring event has had the "PARTSTAT" iCalendar property
   parameter on an "ATTENDEE" property changed, and a "TRANSP" property
   added.

A.4.  Resource Updated - Multiple Instances Change

This is an example of the body of a notification resource where two
   instances of a recurring event have their "DTSTART" and "SUMMARY"
   iCalendar property values changed.

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<?xml version="1.0" encoding="UTF-8"?>
   <CS:notification xmlns:D="DAV:"
                    xmlns:CS="http://calendarserver.org/ns/">
     <CS:dtstamp>2011-12-09T11:51:14-05:00</CS:dtstamp>
     <CS:resource-change>
       <CS:updated>
         <D:href>http://example.com/cyrus/calendar/new.ics</D:href>
         <CS:changed-by>
           <CS:first-name>Cyrus</CS:first-name>
           <CS:last-name>Daboo</CS:last-name>
           <D:href>/principals/cyrusdaboo</D:href>
         </CS:changed-by>
         <CS:calendar-changes>
           <CS:recurrence>
             <CS:recurrenceid>20120209T170000Z</CS:recurrenceid>
             <CS:changes>
               <CS:changed-property name="DTSTART"/>
               <CS:changed-property name="SUMMARY"/>
             </CS:changes>
           </CS:recurrence>
           <CS:recurrence>
             <CS:recurrenceid>20120210T170000Z</CS:recurrenceid>
             <CS:changes>
               <CS:changed-property name="DTSTART"/>
               <CS:changed-property name="SUMMARY"/>
             </CS:changes>
           </CS:recurrence>
         </CS:calendar-changes>
       </CS:updated>
     </CS:resource-change>
   </CS:notification>

A.5.  Resource Updated - Multiple User Change

This is an example of the body of a notification resource where two
   instances of a recurring event have their "DTSTART" and "SUMMARY"
   iCalendar property values changed.  Each instance was changed by a
   different user.

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<?xml version="1.0" encoding="UTF-8"?>
   <CS:notification xmlns:D="DAV:"
                    xmlns:CS="http://calendarserver.org/ns/">
     <CS:dtstamp>2011-12-09T11:51:14-05:00</CS:dtstamp>
     <CS:resource-change>
       <CS:updated>
         <D:href>http://example.com/cyrus/calendar/new.ics</D:href>
         <CS:changed-by>
           <CS:first-name>Cyrus</CS:first-name>
           <CS:last-name>Daboo</CS:last-name>
           <D:href>/principals/cyrusdaboo</D:href>
         </CS:changed-by>
         <CS:calendar-changes>
           <CS:recurrence>
             <CS:recurrenceid>20120209T170000Z</CS:recurrenceid>
             <CS:changes>
               <CS:changed-property name="DTSTART"/>
               <CS:changed-property name="SUMMARY"/>
             </CS:changes>
           </CS:recurrence>
         </CS:calendar-changes>
       </CS:updated>
       <CS:updated>
         <D:href>http://example.com/cyrus/calendar/new.ics</D:href>
         <CS:changed-by>
           <CS:first-name>Eric</CS:first-name>
           <CS:last-name>York</CS:last-name>
           <D:href>/principals/ericyork</D:href>
         </CS:changed-by>
         <CS:calendar-changes>
           <CS:recurrence>
             <CS:recurrenceid>20120210T170000Z</CS:recurrenceid>
             <CS:changes>
               <CS:changed-property name="DTSTART"/>
               <CS:changed-property name="SUMMARY"/>
             </CS:changes>
           </CS:recurrence>
         </CS:calendar-changes>
       </CS:updated>
     </CS:resource-change>
   </CS:notification>

A.6.  Resource Deleted

This is an example of the body of a notification resource where one
   resource has been deleted.  The resource was a VEVENT whose next
   occurrence was in the future on 20120210T170000Z.

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A.7.  Collection Created

This is an example of the body of a notification resource where a
   calendar collection has been created.

A.8.  Collection Updated

This is an example of the body of a notification resource where
   coalesced changes in a calendar collection are shown.  In this case 1
   child resource was created, 2 updated, and 1 deleted.

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A.9.  Collection Deleted

This is an example of the body of a notification resource where a
   calendar collection has been deleted.

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Author's Address

Cyrus Daboo
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

Email: cyrus@daboo.name
   URI:   http://www.apple.com/

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