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RFC 6638

Scheduling Extensions to CalDAV

Pages: 78
Proposed Standard
Updates:  47915546
Updated by:  7953
Part 3 of 4 – Pages 34 to 54
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Top   ToC   RFC6638 - Page 34   prevText

4. Processing Incoming Scheduling Messages

Scheduling operations can cause the delivery of a scheduling message into an "Organizer's" or "Attendee's" scheduling Inbox collection. Servers MUST automatically process incoming scheduling messages using the rules defined by [RFC5546], by creating or updating the corresponding scheduling object resources on calendars owned by the owner of the scheduling Inbox collection. In addition, the scheduling message is stored in the scheduling Inbox collection as an indicator to the client that a scheduling operation has taken place. Scheduling messages are typically removed from the scheduling Inbox collection by the client once the calendar user has acknowledged the change. The server MUST take into account privileges on the scheduling Inbox collection when processing incoming scheduling messages, to determine whether delivery of the scheduling message is allowed. Privileges on calendars containing any matching scheduling object resource are not considered in this case (i.e., a schedule message from another user can cause modifications to resources in calendar collections that the other user would not normally have read or write access to). Additionally, servers MUST take into account any scheduling Inbox collection preconditions (see Section 2.2) when delivering the scheduling message, and MUST take into account the similar preconditions on any calendar collection that contains, or would contain, the corresponding scheduling object resource.

4.1. Processing "Organizer" Requests, Additions, and Cancellations

For a scheduling message sent by an "Organizer", the server first tries to locate a corresponding scheduling object resource belonging to the "Attendee". If no matching scheduling object resource exists, the server treats the scheduling message as a new message; otherwise, it is treated as an update. In the case of a new message, the server processes the scheduling message and creates a new scheduling object resource as per Section 4.3. In the case of an update, the server processes the scheduling message and updates the matching scheduling object resource belonging to the "Attendee" to reflect the changes sent by the "Organizer". In each case, the scheduling message MUST only appear in the "Attendee's" scheduling Inbox collection once all automatic processing has been done.
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4.2. Processing "Attendee" Replies

For a scheduling message reply sent by an "Attendee", the server first locates the corresponding scheduling object resource belonging to the "Organizer". If the corresponding scheduling object resource cannot be found, the server SHOULD ignore the scheduling message. The server MUST then update the "PARTSTAT" iCalendar property parameter value of each "ATTENDEE" iCalendar property in the scheduling object resource to match the changes indicated in the reply (taking into account the fact that an "Attendee" could have created a new overridden iCalendar component to indicate different participation status on one or more instances of a recurring event). The server MUST also update or add the "SCHEDULE-STATUS" property parameter on each matching "ATTENDEE" iCalendar property and set its value to that of the "REQUEST-STATUS" property in the reply, or to "2.0" if "REQUEST-STATUS" is not present (also taking into account recurrence instances). If there are multiple "REQUEST-STATUS" properties in the reply, the "SCHEDULE-STATUS" property parameter value is set to a comma-separated list of status codes, one from each "REQUEST-STATUS" property. The server SHOULD send scheduling messages to all the other "Attendees" indicating the change in participation status of the "Attendee" replying, subject to the recurrence requirements of Section 3.2.6. The scheduling message MUST only appear in the "Organizer's" scheduling Inbox collection once all automatic processing has been done.

4.3. Default Calendar Collection

The server processes scheduling messages received for an "Attendee" by creating a new scheduling object resource in a calendar collection belonging to the "Attendee", when one does not already exist. A calendar user that is an "Attendee" in a scheduling operation MUST have at least one valid calendar collection available. If there is no valid calendar collection, then the server MUST reject the attempt to deliver the scheduling message to the "Attendee". Servers MAY provide support for a default calendar collection -- that is, the calendar collection in which new scheduling object resources will be created. The CALDAV:schedule-default-calendar-URL WebDAV property, which can be present on the scheduling Inbox collection of a calendar user, specifies whether this calendar user has a default calendar collection. See Section 9.2.
Top   ToC   RFC6638 - Page 36
   Servers SHOULD create new scheduling object resources in the default
   calendar collection, if the CALDAV:schedule-default-calendar-URL
   WebDAV property is set.

   Servers MAY allow clients to change the default calendar collection
   by changing the value of the CALDAV:schedule-default-calendar-URL
   WebDAV property on the scheduling Inbox collection.  However, the
   server MUST ensure that any new value for that property refers to a
   valid calendar collection belonging to the owner of the scheduling
   Inbox collection.

   Servers MUST reject any attempt to delete the default calendar
   collection.

4.3.1. Additional Method Preconditions

This specification defines additional method preconditions (see Section 16 of WebDAV [RFC4918]) to provide machine-parseable information in error responses.
4.3.1.1. CALDAV:default-calendar-needed Precondition
Name: default-calendar-needed Namespace: urn:ietf:params:xml:ns:caldav Apply to: DELETE Use with: 403 Forbidden Purpose: (precondition) -- The client attempted to delete the calendar collection currently referenced by the CALDAV:schedule- default-calendar-URL property, or attempted to remove the CALDAV: schedule-default-calendar-URL property on the scheduling Inbox collection on a server that doesn't allow such operations. Definition: <!ELEMENT default-calendar-needed EMPTY>
4.3.1.2. CALDAV:valid-schedule-default-calendar-URL Precondition
Name: valid-schedule-default-calendar-URL Namespace: urn:ietf:params:xml:ns:caldav Apply to: PROPPATCH
Top   ToC   RFC6638 - Page 37
   Use with:  403 Forbidden

   Purpose:  (precondition) -- The client attempted to set the CALDAV:
      schedule-default-calendar-URL property to a DAV:href element that
      doesn't reference a valid calendar collection.  Note: Servers that
      do not allow clients to change the CALDAV:schedule-default-
      calendar-URL property would simply return the DAV:cannot-modify-
      protected-property precondition defined in Section 16 of WebDAV
      [RFC4918].

   Definition:

     <!ELEMENT valid-schedule-default-calendar-URL EMPTY>

5. Request for Busy Time Information

Busy time information of one or more calendar users can be determined by submitting a POST request targeted at the scheduling Outbox collection of the calendar user requesting the information (the "Organizer"). To accomplish this, the request body MUST contain a "VFREEBUSY" calendar component with the "METHOD" iCalendar property set to the value "REQUEST" as specified in Section 3.3.2 of iTIP [RFC5546]. The resource identified by the Request-URI MUST be a resource collection of type CALDAV:schedule-outbox (Section 2.1). The "ORGANIZER" property value in the "VFREEBUSY" component MUST match one of the calendar user addresses of the owner of the Outbox collection. A response to a busy time request that indicates status for one or more calendar users MUST be an XML document with a CALDAV:schedule- response XML element as its root element. This element MUST contain one CALDAV:response element for each calendar user, with each such element in turn containing elements that indicate which calendar user they correspond to, the scheduling status for that calendar user, any error codes, and an optional description. For a successful busy time request, a CALDAV:calendar-data element is also present for each calendar user, containing the actual busy time information (i.e., an iCalendar "VFREEBUSY" component). See Section 10 for details on the child elements. See Appendix B.5 for an example busy time request and response.
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5.1. Status Codes

The list below summarizes the most common status codes used for this method. However, clients need to be prepared to handle other 2/3/4/5xx series status codes as well. 200 (OK) - The command succeeded. 204 (No Content) - The command succeeded. 400 (Bad Request) - The client has provided an invalid scheduling message. 403 (Forbidden) - The client cannot submit a scheduling message to the specified Request-URI. 404 (Not Found) - The URL in the Request-URI was not present. 423 (Locked) - The specified resource is locked, and the client either is not a lock owner or the lock type requires a lock token to be submitted and the client did not submit it.

5.2. Additional Method Preconditions

The following are existing preconditions that are reused for the POST method on an Outbox collection. o DAV:need-privileges [RFC3744] o CALDAV:supported-calendar-data [RFC4791] o CALDAV:valid-calendar-data [RFC4791] o CALDAV:max-resource-size [RFC4791] The following are new method preconditions for the POST method on an Outbox collection.

5.2.1. CALDAV:valid-scheduling-message Precondition

Name: valid-scheduling-message Namespace: urn:ietf:params:xml:ns:caldav Apply to: POST Use with: 400 Bad Request
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   Purpose:  (precondition) -- The resource submitted in the POST
      request MUST obey all the restrictions specified in Section 3.3.2
      of iTIP [RFC5546].

   Definition:

     <!ELEMENT valid-scheduling-message EMPTY>

5.2.2. CALDAV:valid-organizer Precondition

Name: valid-organizer Namespace: urn:ietf:params:xml:ns:caldav Apply to: POST Use with: 403 Forbidden Purpose: (precondition) -- The "ORGANIZER" property value in the POST request's scheduling message MUST match one of the calendar user addresses of the owner of the scheduling Outbox collection being targeted by the request. Definition: <!ELEMENT valid-organizer EMPTY>

6. Scheduling Privileges

New scheduling privileges are defined in this section. All the scheduling privileges MUST be non-abstract and MUST appear in the DAV:supported-privilege-set property of scheduling Outbox and Inbox collections on which they are defined. The tables specified in Appendix A clarify which scheduling methods (e.g., "REQUEST", "REPLY", etc.) are controlled by each scheduling privilege defined in this section.

6.1. Privileges on Scheduling Inbox Collections

This section defines new WebDAV Access Control List (ACL) [RFC3744] privileges that are defined for use on scheduling Inbox collections. These privileges determine whether delivery of scheduling messages from a calendar user is allowed by the calendar user who "owns" the scheduling Inbox collection. This allows calendar users to choose which other calendar users can schedule with them.
Top   ToC   RFC6638 - Page 40
   Note that when a scheduling message is delivered to a calendar user,
   in addition to a scheduling object resource being created in the
   calendar user's scheduling Inbox collection, a new scheduling object
   resource might be created or an existing one updated in a calendar
   belonging to the calendar user.  In that case, the ability to create
   or update the scheduling object resource in the calendar is
   controlled by the privileges assigned to the scheduling Inbox
   collection.

   The privileges defined in this section are ignored if applied to a
   resource other than a scheduling Inbox collection.

6.1.1. CALDAV:schedule-deliver Privilege

CALDAV:schedule-deliver is an aggregate privilege as per Section 6.3. <!ELEMENT schedule-deliver EMPTY>

6.1.2. CALDAV:schedule-deliver-invite Privilege

The CALDAV:schedule-deliver-invite privilege controls the processing and delivery of scheduling messages coming from an "Organizer". <!ELEMENT schedule-deliver-invite EMPTY>

6.1.3. CALDAV:schedule-deliver-reply Privilege

The CALDAV:schedule-deliver-reply privilege controls the processing and delivery of scheduling messages coming from an "Attendee". <!ELEMENT schedule-deliver-reply EMPTY>

6.1.4. CALDAV:schedule-query-freebusy Privilege

The CALDAV:schedule-query-freebusy privilege controls freebusy requests targeted at the owner of the scheduling Inbox collection. <!ELEMENT schedule-query-freebusy EMPTY>

6.2. Privileges on Scheduling Outbox Collections

This section defines new WebDAV ACL [RFC3744] privileges that are defined for use on scheduling Outbox collections. These privileges determine which calendar users are allowed to send scheduling messages on behalf of the calendar user who "owns" the scheduling Outbox collection. This allows calendar users to choose other calendar users who can act on their behalf (e.g., assistants working on behalf of their boss).
Top   ToC   RFC6638 - Page 41
   The privileges defined in this section are ignored if applied to a
   resource other than a scheduling Outbox collection.

6.2.1. CALDAV:schedule-send Privilege

CALDAV:schedule-send is an aggregate privilege as per Section 6.3. <!ELEMENT schedule-send EMPTY>

6.2.2. CALDAV:schedule-send-invite Privilege

The CALDAV:schedule-send-invite privilege controls the sending of scheduling messages by "Organizers". Users granted the DAV:bind privilege on a calendar collection, or the DAV:write privilege on scheduling object resources, will also need the CALDAV:schedule-send-invite privilege granted on the scheduling Outbox collection of the owner of the calendar collection or scheduling object resource in order to be allowed to create, modify, or delete scheduling object resources in a way that will trigger the CalDAV server to deliver scheduling messages to "Attendees". <!ELEMENT schedule-send-invite EMPTY>

6.2.3. CALDAV:schedule-send-reply Privilege

The CALDAV:schedule-send-reply privilege controls the sending of scheduling messages by "Attendees". Users granted the DAV:bind privilege on a calendar collection, or the DAV:write privilege on scheduling object resources, will also need the CALDAV:schedule-send-reply privilege granted on the scheduling Outbox collection of the owner of the calendar collection or scheduling object resource in order to be allowed to create, modify, or delete scheduling object resources in a way that will trigger the CalDAV server to deliver scheduling message replies to the "Organizer". <!ELEMENT schedule-send-reply EMPTY>

6.2.4. CALDAV:schedule-send-freebusy Privilege

The CALDAV:schedule-send-freebusy privilege controls the use of the POST method to submit scheduling messages that specify the scheduling method "REQUEST" with a "VFREEBUSY" calendar component. <!ELEMENT schedule-send-freebusy EMPTY>
Top   ToC   RFC6638 - Page 42

6.3. Aggregation of Scheduling Privileges

Server implementations MUST aggregate the scheduling privileges as follows: DAV:all contains CALDAV:schedule-deliver and CALDAV:schedule-send; CALDAV:schedule-deliver contains CALDAV:schedule-deliver-invite, CALDAV:schedule-deliver-reply, and CALDAV:schedule-query-freebusy; CALDAV:schedule-send contains CALDAV:schedule-send-invite, CALDAV: schedule-send-reply, and CALDAV:schedule-send-freebusy. The following diagram illustrates how scheduling privileges are aggregated according to the above requirements. [DAV:all] (aggregate) | +-- [CALDAV:schedule-deliver] (aggregate) | | | +-- [CALDAV:schedule-deliver-invite] | +-- [CALDAV:schedule-deliver-reply] | +-- [CALDAV:schedule-query-freebusy] | +-- [CALDAV:schedule-send] (aggregate) | +-- [CALDAV:schedule-send-invite] +-- [CALDAV:schedule-send-reply] +-- [CALDAV:schedule-send-freebusy]

7. Additional iCalendar Property Parameters

This specification defines additional iCalendar property parameters to support the CalDAV scheduling extensions.

7.1. Schedule Agent Parameter

Parameter Name: SCHEDULE-AGENT Purpose: To specify the agent expected to deliver scheduling messages to the corresponding "Organizer" or "Attendee".
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   Format Definition:  This property parameter is defined by the
      following notation:

     scheduleagentparam = "SCHEDULE-AGENT" "="
                      ("SERVER"      ; The server handles scheduling
                     / "CLIENT"      ; The client handles scheduling
                     / "NONE"        ; No scheduling
                     / x-name        ; Experimental type
                     / iana-token)   ; Other IANA-registered type
     ;
     ; If the parameter is not present, its value defaults to SERVER.
     ; "x-name" and "iana-token" are defined in Section 3.1 of
     ; [RFC5545].

   Description:  This property parameter MAY be specified on "ORGANIZER"
      or "ATTENDEE" iCalendar properties.  In the absence of this
      parameter, the value "SERVER" MUST be used for the default
      behavior.  The value determines whether or not a scheduling
      operation on a server will cause a scheduling message to be sent
      to the corresponding calendar user identified by the "ORGANIZER"
      or "ATTENDEE" property value.  When the value "SERVER" is
      specified, or the parameter is absent, then it is the server's
      responsibility to send a scheduling message as part of a
      scheduling operation.  When the value "CLIENT" is specified, that
      indicates that the client is handling scheduling messages with the
      calendar user itself.  When "NONE" is specified, no scheduling
      messages are being sent to the calendar user.

      Servers MUST NOT include this parameter in any scheduling messages
      sent as the result of a scheduling operation.

      Clients MUST NOT include this parameter in any scheduling messages
      that they themselves send.

      The parameter value MUST be the same on every "ORGANIZER" property
      in a scheduling object resource.

      The parameter value MUST be the same on each "ATTENDEE" property
      whose values match in a scheduling object resource.

      Servers and clients MUST treat x-name and iana-token values they
      do not recognize the same way as they would the "NONE" value.

   Example:

     ORGANIZER;SCHEDULE-AGENT=SERVER:mailto:bernard@example.com
     ATTENDEE;SCHEDULE-AGENT=NONE:mailto:cyrus@example.com
Top   ToC   RFC6638 - Page 44

7.2. Schedule Force Send Parameter

Parameter Name: SCHEDULE-FORCE-SEND Purpose: To force a scheduling message to be sent to the calendar user specified by the property. Format Definition: This property parameter is defined by the following notation: scheduleforcesendparam = "SCHEDULE-FORCE-SEND" "=" ("REQUEST" ; Force a "REQUEST" / "REPLY" ; Force a "REPLY" / iana-token) ; ; "iana-token" is defined in Section 3.1 of [RFC5545]. Its value ; MUST be an IANA-registered iCalendar "METHOD" property value. Description: This property parameter MAY be specified on "ATTENDEE" and "ORGANIZER" properties on which the "SCHEDULE-AGENT" property parameter is set to the value "SERVER" or is not specified. This property parameter is used to force a server to send a scheduling message to a specific calendar user in situations where the server would not send a scheduling message otherwise (e.g., when no change that warrants the delivery of a new scheduling message was performed on the scheduling object resource). An "Organizer" MAY specify this parameter on an "ATTENDEE" property with the value "REQUEST" to force a "REQUEST" scheduling message to be sent to this "Attendee". An "Attendee" MAY specify this parameter on the "ORGANIZER" with the value "REPLY" to force a "REPLY" scheduling message to be sent to the "Organizer". Servers MUST NOT preserve this property parameter in scheduling object resources, nor include it in any scheduling messages sent as the result of a scheduling operation. Clients MUST NOT include this parameter in any scheduling messages that they themselves send. Servers MUST set the "SCHEDULE-STATUS" parameter of the "ATTENDEE" or "ORGANIZER" to 2.3 (i.e., "Success; invalid property parameter ignored"; see Section 3.6 of [RFC5546]) when the "SCHEDULE-FORCE- SEND" parameter is set to an iana-token value they do not recognize.
Top   ToC   RFC6638 - Page 45
   Example:

     ORGANIZER;SCHEDULE-FORCE-SEND=REPLY:mailto:cyrus@example.com
     ATTENDEE;SCHEDULE-FORCE-SEND=REQUEST:mailto:bernard@example.com

7.3. Schedule Status Parameter

Parameter Name: SCHEDULE-STATUS Purpose: To specify the status codes returned from processing of the most recent scheduling message sent to the corresponding "Attendee", or received from the corresponding "Organizer". Format Definition: This property parameter is defined by the following notation: schedulestatusparam = "SCHEDULE-STATUS" "=" ( statcode / DQUOTE statcode *("," statcode) DQUOTE) ; ; "statcode" is defined in Section 3.8.8.3 of [RFC5545]. The ; value is a single "statcode" or a comma-separated list of ; "statcode" values. Description: This property parameter MAY be specified on the "ATTENDEE" and "ORGANIZER" properties. Servers MUST only add or change this property parameter on any "ATTENDEE" properties corresponding to calendar users who were sent a scheduling message via a scheduling operation. Clients SHOULD NOT change or remove this parameter if it was provided by the server. In the case where the client is handling the scheduling, the client MAY add, change, or remove this parameter to indicate the last scheduling message status it received. Servers MUST add this parameter to any "ORGANIZER" properties corresponding to calendar users who were sent a scheduling message reply by an "Attendee" via a scheduling operation. Clients SHOULD NOT change or remove this parameter if it was provided by the server. In the case where the client is handling the scheduling, the client MAY add, change, or remove this parameter to indicate the last scheduling message status it received. Servers MUST NOT include this parameter in any scheduling messages sent as the result of a scheduling operation. Clients MUST NOT include this parameter in any scheduling messages that they themselves send.
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      Values for this property parameter are described in Section 3.2.9.

   Example:

     ATTENDEE;SCHEDULE-STATUS="2.0":mailto:bernard@example.com
     ATTENDEE;SCHEDULE-STATUS="2.0,2.4":mailto:cyrus@example.com

8. Additional Message Header Fields

This specification defines additional HTTP request and response headers for use with CalDAV.

8.1. Schedule-Reply Request Header

Schedule-Reply = "Schedule-Reply" ":" ("T" | "F") Example: Schedule-Reply: F When an "Attendee" removes a scheduling object resource as per Section 3.2.2.4, and the Schedule-Reply header is set to the value "T" (true) or is not present, the server MUST send an appropriate reply scheduling message with the "Attendee's" "PARTSTAT" iCalendar property parameter value set to "DECLINED" as part of its normal scheduling operation processing. When the Schedule-Reply header is set to the value "F" (false), the server MUST NOT send a scheduling message as part of its normal scheduling operation processing. The Schedule-Reply request header is used by a client to indicate to a server whether or not a scheduling operation ought to occur when an "Attendee" deletes a scheduling object resource. In particular, it controls whether a reply scheduling message is sent to the "Organizer" as a result of the removal. There are situations in which unsolicited scheduling messages need to be silently removed (or ignored) for security or privacy reasons. This request header allows the scheduling object resource to be removed if such a need arises.

8.2. Schedule-Tag Response Header

The Schedule-Tag response header provides the current value of the CALDAV:schedule-tag property value. The behavior of this response header is described in Section 3.2.10. All scheduling object resources MUST support the Schedule-Tag header.
Top   ToC   RFC6638 - Page 47
     Schedule-Tag = "Schedule-Tag" ":" opaque-tag
     ; "opaque-tag" is defined in Section 3.11 of [RFC2616].

   Example:

     Schedule-Tag: "12ab34-cd56ef"

8.3. If-Schedule-Tag-Match Request Header

The If-Schedule-Tag-Match request header field is used with a method to make it conditional. Clients can set this header to the value returned in the Schedule-Tag response header, or the CALDAV:schedule- tag property, of a scheduling object resource previously retrieved from the server to avoid overwriting "consequential" changes to the scheduling object resource. All scheduling object resources MUST support the If-Schedule-Tag- Match header. If-Schedule-Tag-Match = "If-Schedule-Tag-Match" ":" opaque-tag ; "opaque-tag" is defined in Section 3.11 of [RFC2616]. Example: If-Schedule-Tag-Match: "12ab34-cd56ef"

9. Additional WebDAV Properties

This specification defines the following new WebDAV properties for use with CalDAV.

9.1. CALDAV:schedule-calendar-transp Property

Name: schedule-calendar-transp Namespace: urn:ietf:params:xml:ns:caldav Purpose: Determines whether the calendar object resources in a calendar collection will affect the owner's busy time information. Protected: This property MAY be protected and SHOULD NOT be returned by a PROPFIND DAV:allprop request (as defined in Section 14.2 of [RFC4918]). COPY/MOVE behavior: This property value SHOULD be kept during a MOVE operation, and SHOULD be copied and preserved in a COPY.
Top   ToC   RFC6638 - Page 48
   Description:  This property SHOULD be defined on all calendar
      collections.  If present, it contains one of two XML elements that
      indicate whether the calendar object resources in the calendar
      collection ought to contribute to the owner's busy time.  When the
      CALDAV:opaque element is used, all calendar object resources in
      the corresponding calendar collection MUST contribute to busy
      time, assuming that access privileges and other iCalendar
      properties allow it to.  When the CALDAV:transparent XML element
      is used, the calendar object resources in the corresponding
      calendar collection MUST NOT contribute to busy time.

      If this property is not present on a calendar collection, then the
      default value CALDAV:opaque MUST be assumed.

   Definition:

     <!ELEMENT schedule-calendar-transp (opaque | transparent)>

     <!ELEMENT opaque EMPTY>
     <!-- Affects busy time searches -->

     <!ELEMENT transparent EMPTY>
     <!-- Invisible to busy time searches -->

   Example:

     <C:schedule-calendar-transp
          xmlns:C="urn:ietf:params:xml:ns:caldav">
       <C:opaque/>
     </C:schedule-calendar-transp>

9.2. CALDAV:schedule-default-calendar-URL Property

Name: schedule-default-calendar-URL Namespace: urn:ietf:params:xml:ns:caldav Purpose: Specifies a default calendar for an "Attendee" where new scheduling object resources are created. Protected: This property MAY be protected in the case where a server does not support changing the default calendar, or does not support a default calendar. COPY/MOVE behavior: This property is only defined on a scheduling Inbox collection that cannot be moved or copied.
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   Description:  This property MAY be defined on a scheduling Inbox
      collection.  If present, it contains zero or one DAV:href XML
      elements.  When a DAV:href element is present, its value indicates
      a URL to a calendar collection that is used as the default
      calendar.  When no DAV:href element is present, it indicates that
      there is no default calendar.  In the absence of this property,
      there is no default calendar.  When there is no default calendar,
      the server is free to choose the calendar in which a new
      scheduling object resource is created.  See Section 4.3.

   Definition:

     <!ELEMENT schedule-default-calendar-URL (DAV:href?)>

   Example:

     <C:schedule-default-calendar-URL xmlns:D="DAV:"
         xmlns:C="urn:ietf:params:xml:ns:caldav">
       <D:href>/home/cyrus/calendars/work/</D:href>
     </C:schedule-default-calendar-URL>

9.3. CALDAV:schedule-tag Property

Name: schedule-tag Namespace: urn:ietf:params:xml:ns:caldav Purpose: Indicates whether a scheduling object resource has had a "consequential" change made to it. Value: opaque-tag (defined in Section 3.11 of [RFC2616]) Protected: This property MUST be protected, as only the server can update the value. COPY/MOVE behavior: This property value is determined by the server and MAY be different from the value on the source resource. Description: The CALDAV:schedule-tag property MUST be defined on all scheduling object resources. This property is described in Section 3.2.10. Definition: <!ELEMENT schedule-tag (#PCDATA)>
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   Example:

     <C:schedule-tag xmlns:C="urn:ietf:params:xml:ns:caldav"
     >"12345-67890"</C:schedule-tag>

10. XML Element Definitions

10.1. CALDAV:schedule-response XML Element

Name: schedule-response Namespace: urn:ietf:params:xml:ns:caldav Purpose: Contains the set of responses for a POST method request. Description: See Section 5. Definition: <!ELEMENT schedule-response (response*)>

10.2. CALDAV:response XML Element

Name: response Namespace: urn:ietf:params:xml:ns:caldav Purpose: Contains a single response for a POST method request. Description: See Section 5. Definition: <!ELEMENT response (recipient, request-status, calendar-data?, DAV:error?, DAV:responsedescription?)> <!-- CALDAV:calendar-data is defined in Section 9.6 of RFC 4791 and when used here uses the definition with content (#PCDATA) only. -->

10.3. CALDAV:recipient XML Element

Name: recipient Namespace: urn:ietf:params:xml:ns:caldav
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   Purpose:  The calendar user address that the enclosing response for a
      POST method request is for.

   Description:  See Section 5.

   Definition:

     <!ELEMENT recipient (DAV:href)>

10.4. CALDAV:request-status XML Element

Name: request-status Namespace: urn:ietf:params:xml:ns:caldav Purpose: The iTIP "REQUEST-STATUS" property value for this response. Description: See Section 5. Definition: <!ELEMENT request-status (#PCDATA)>

11. Security Considerations

The process of scheduling involves the sending and receiving of scheduling messages. As a result, the security problems related to messaging in general are relevant here. In particular, the authenticity of the scheduling messages needs to be verified. Servers and clients MUST use an HTTP connection protected with Transport Layer Security (TLS) as defined in [RFC2818] for all scheduling operations. Clients MUST use the procedures detailed in Section 6 of [RFC6125] to verify the authenticity of the server. Servers MUST make use of HTTP authentication [RFC2617] to verify the authenticity of the calendar user for whom the client is sending requests.

11.1. Preventing Denial-of-Service Attacks

Servers MUST ensure that clients cannot consume excessive server resources by carrying out "large" scheduling operations. In particular, servers SHOULD enforce CALDAV:max-resource-size, CALDAV: max-instances, and CALDAV:max-attendees-per-instance preconditions as applicable for scheduling Inbox and Outbox collections.
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11.2. Verifying Scheduling Operations

When handling a scheduling operation: 1. Servers MUST verify that the principal associated with the DAV: owner of the calendar collection in which a scheduling object resource is being manipulated contains a CALDAV:schedule-outbox- URL property value. 2. Servers MUST verify that the currently authenticated user has the CALDAV:schedule-send privilege, or a sub-privilege aggregated under this privilege, on the scheduling Outbox collection of the DAV:owner of the calendar collection in which a scheduling object resource is being manipulated. 3. Servers MUST only deliver scheduling messages to recipients when the CALDAV:schedule-deliver privilege, or a sub-privilege aggregated under this privilege, is granted on the recipient's scheduling Inbox collection for the principal associated with the DAV:owner of the calendar collection in which a scheduling object resource is being manipulated. 4. To prevent impersonation of calendar users, the server MUST verify that the "ORGANIZER" property in an organizer scheduling object resource matches one of the calendar user addresses of the DAV:owner of the calendar collection in which the resource is stored. 5. To prevent spoofing of an existing scheduling object resource, servers MUST verify that the "UID" iCalendar property value in a new scheduling object resource does not match that of an existing scheduling object resource with a different "ORGANIZER" property value.

11.3. Verifying Busy Time Information Requests

When handling a POST request on a scheduling Outbox collection: 1. Servers MUST verify that the principal associated with the calendar user address specified in the "ORGANIZER" property of the scheduling message data in the request contains a CALDAV: schedule-outbox-URL property value that matches the scheduling Outbox collection targeted by the request. 2. Servers MUST verify that the currently authenticated user has the CALDAV:schedule-send privilege, or a sub-privilege aggregated under this privilege, on the scheduling Outbox collection targeted by the request.
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   3.  Servers MUST only return valid freebusy information for
       recipients when the CALDAV:schedule-deliver privilege, or a
       sub-privilege aggregated under this privilege, is granted on the
       recipient's scheduling Inbox collection for the principal
       associated with the DAV:owner of the scheduling Outbox collection
       targeted by the request.

11.4. Privacy Issues

This specification only defines how calendar users on the same server are able to schedule with each other -- unauthenticated users have no way to carry out scheduling operations. Access control privileges (as per Section 6) can control which of those users can schedule with others. Calendar users not wishing to expose their calendar information to other users can do so by denying privileges to specific users, or all users, for all scheduling operations, or perhaps only freebusy. "Attendees" can also use the Schedule-Reply request header (Section 8.1) with the value set to "F" to prevent notification to an "Organizer" that a scheduling object resource was deleted. This allows "Attendees" to remove unwanted scheduling messages without any response to the "Organizer". Servers MUST NOT expose any private iCalendar data, or WebDAV resource state information (URLs, WebDAV properties, etc.) for one calendar user to another via scheduling messages or error responses to scheduling operations. In particular, as per Section 8.1 of [RFC4918], authorization errors MUST take preference over other errors.

11.5. Mitigation of iTIP Threats

Section 6.1 of iTIP [RFC5546] defines a set of potential threats in a scheduling system, and Section 6.2 of [RFC5546] defines recommendations on how those can be addressed in protocols using iTIP. This specification addresses the iTIP threats in the following manner: Spoofing the "Organizer": Addressed by item 4 in Section 11.2. Spoofing the "Attendee": Addressed by Section 3.2.2.1 and item 2 in Section 11.2. Unauthorized Replacement of the "Organizer": Addressed by item 5 in Section 11.2. Eavesdropping and Data Integrity: Addressed by requiring TLS.
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   Flooding a Calendar:  Addressed by requirements in Section 11.1.

   Unauthorized REFRESH Requests:  This specification does not support
      the REFRESH method.



(page 54 continued on part 4)

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