]> Apple Inc.

1 Infinite Loop Cupertino CA 95014 USA cyrus@daboo.name http://www.apple.com/

Oracle Corporation

600 Blvd. de Maisonneuve West Suite 1900 Montreal QC H3A 3J2 CANADA bernard.desruisseaux@oracle.com http://www.oracle.com/

Applications calsify calsched calsch caldav calendar calendaring scheduling webcal webdav ical icalendar ischedule itip imip text/calendar http This document defines extensions to the CalDAV "calendar-access" feature to specify a standard way of performing scheduling transactions with iCalendar-based calendar components. This document defines the "calendar-auto-schedule" feature of CalDAV.

This document specifies extensions to the CalDAV "calendar-access" feature to enable scheduling of iCalendar-based calendar components between Calendar Users. This extension leverages the scheduling methods defined in the iCalendar Transport-independent Interoperability Protocol (iTIP) to permit Calendar Users to perform scheduling transactions such as schedule, reschedule, respond to scheduling request or cancel calendar components, as well as search for busy time information. Discussion of this Internet-Draft is taking place on the mailing list <https://www.ietf.org/mailman/listinfo/caldav>.

This specification uses much of the same terminology as iCalendar, iTIP, WebDAV, and CalDAV. The following definitions are provided to aid the reader in understanding this specification. An entity (often a human) that accesses calendar information . A resource that acts as a container of references to child calendar object resources . A resource representing a calendar object (event, to-do, journal entry, or other calendar components) . A calendar object resource contained in a calendar collection for which the server will take care of sending scheduling messages on behalf of the owner of the calendar collection. A scheduling object resource owned by an Organizer. A scheduling object resource owned by an Attendee. Add, change or remove operations on a scheduling object resource for which the server will deliver scheduling messages to other Calendar Users. A calendar object that describes a scheduling transaction such as schedule, reschedule, reply, or cancel. A resource at which busy time information requests are targeted. A collection in which incoming scheduling messages are delivered.

iTIP outlines a model where Calendar Users exchange scheduling messages with one another. Often times, clients are made responsible for generating and sending scheduling messages as well as processing incoming scheduling messages. This approach yields a number of problems, including: For most updates to a calendar component, clients are responsible for sending appropriate scheduling messages to the Organizer or the Attendees. The handling of incoming scheduling messages and the updates to calendars impacted by those messages only occurs when clients are active. Due to the update latency, it is possible for calendars of different Calendar Users to reflect different, inaccurate states. This specification uses an alternative approach where the server is made responsible for sending scheduling messages and processing incoming scheduling messages. This approach frees the clients from the submission and processing of scheduling messages and ensures better consistency of calendar data across users' calendars. The operation of creating, modifying or deleting a calendar component in a calendar is enough to trigger the server to deliver the necessary scheduling messages to the appropriate Calendar Users.

While the scheduling features described in this specification are based on iTIP, some of its more advanced features have deliberately been left out in order to keep this specification simple. In particular, the following iTIP features are not covered: publishing, countering, delegating, refreshing and forwarding calendar components, as well as replacing the Organizer of a calendar component. The goal of this specification is to provide the essential scheduling features needed. It is expected that future extensions will be developed to address the more advanced features.

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 . The Augmented BNF (ABNF) syntax used by this document to specify the format definition of new iCalendar elements is defined in . The Augmented BNF (ABNF) syntax used by this document to specify the format definition of new message header fields to be used with the HTTP/1.1 protocol is described in Section 2.1 of . Since this Augmented BNF uses the basic production rules provided in Section 2.2 of , these rules apply to this document as well. The term "protected" is used in the Conformance field of WebDAV property definitions as defined in Section 15 of .

This document uses XML DTD fragments (, Section 3.2) as a purely notational convention. WebDAV request and response bodies cannot be validated by a DTD due to the specific extensibility rules defined in Section 17 of [RFC4918] and due to the fact that all XML elements defined by that specification use the XML namespace name "DAV:". In particular: element names use the "DAV:" namespace, element ordering is irrelevant unless explicitly stated, extension elements (elements not already defined as valid child elements) may be added anywhere, except when explicitly stated otherwise, extension attributes (attributes not already defined as valid for this element) may be added anywhere, except when explicitly stated otherwise. The XML elements specified in this document are defined in the "urn:ietf:params:xml:ns:caldav" XML namespace registered by CalDAV. 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 strings "DAV:" and "CALDAV:" will be prefixed to the element types, respectively. This document inherits, and sometimes extends, DTD productions from Section 14 of . Also note that some CalDAV XML element names are identical to WebDAV XML element names, though their namespace differs. Care must be taken not to confuse the two sets of names.

The process of scheduling an event between different parties often involves a series of steps with different actors playing particular roles during the whole process. Typically there is an event "Organizer" whose role is to schedule an event between one or more "Attendees", and this is done by sending out invitations and handling responses from each Attendee. This process can typically be broken down into two phases. In the first phase, the Organizer will query the busy time information of each Attendee to determine the most appropriate time for the event. This request is sometimes called a "freebusy" lookup. In the second phase, the Organizer sends out invitations to each Attendee using the time previously determined from the freebusy lookup. There then follows exchanges between Organizer and Attendees regarding the invitation. Some Attendees may choose to attend at the time proposed by the Organizer, others may decline to attend. The Organizer needs to process each of the replies from the Attendees and take appropriate action to confirm the event, reschedule it or perhaps cancel it. The user expectation as to how a calendaring and scheduling system should respond in each of these two phases is somewhat different. In the case of a freebusy lookup, users expect to get back results immediately so that they can then move on to the invitation phase as quickly as possible. In the case of invitations, it is expected that each Attendee will reply with their participation status in their own time, so delays in receiving replies are anticipated. Thus calendaring and scheduling systems should treat these two operational phases in different ways to accommodate the user expectations, which is what this specification does. While the scenario described above only covers the case of scheduling events between Calendar Users, and requesting busy time information, this specification also provides support for the scheduling of to-dos between Calendar Users. For the majority of the following discussion, scheduling of events and freebusy lookups will be discussed, as these are the more common operations.

A server that supports the features described in this document MUST include "calendar-auto-schedule" as a field in the DAV response header from an OPTIONS request on any resource that supports any scheduling actions, properties, privileges or methods. To advertise support for the CalDAV "calendar-auto-schedule" feature a server is REQUIRED to support and advertise support for the CalDAV "calendar-access" feature.

In this example, the OPTIONS response indicates that the server supports the "calendar-access" and "calendar-auto-schedule" features and that the resource "/home/cyrus/calendars/inbox/" supports the scheduling actions, properties, privileges and methods defined in this specification.

>> Request <<

>> Response <<

This specification introduces new collection resource types that are used to manage scheduling object resources, and scheduling privileges, as well as provide scheduling functionality. It is the server's responsibility to create these collection resources, and clients have no way to create or delete them.

A scheduling Outbox collection is used as the target for busy time information requests, and to manage privileges that apply to outgoing scheduling requests. A scheduling Outbox collection MUST report the DAV:collection and CALDAV:schedule-outbox XML elements in the value of the DAV:resourcetype property. The element type declaration for CALDAV:schedule-outbox is:

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New WebDAV ACL privileges can be set on the scheduling Outbox collection to control who is allowed to send scheduling messages on behalf of the Calendar User associated with the scheduling Outbox collection. See for more details. A scheduling Outbox collection MUST NOT be a child (at any depth) of a calendar collection resource. The following WebDAV properties specified in CalDAV "calendar-access" MAY also be defined on scheduling Outbox collections: CALDAV:supported-calendar-component-set - when present this indicates the allowed calendar component types for scheduling messages submitted to the scheduling Outbox collection with the POST method. CALDAV:supported-calendar-data - when present this indicates the allowed media types for scheduling messages submitted to the scheduling Outbox collection with the POST method. CALDAV:max-resource-size - when present this indicates the maximum size in octets of a resource that the server is willing to accept for scheduling messages submitted to the scheduling Outbox collection with the POST method. CALDAV:min-date-time - when present this indicates the earliest date and time (in UTC) that the server is willing to accept for any DATE or DATE-TIME value in scheduling messages submitted to the scheduling Outbox collection with the POST method. CALDAV:max-date-time - when present this indicates the latest date and time (in UTC) that the server is willing to accept for any DATE or DATE-TIME value in scheduling messages submitted to the scheduling Outbox collection with the POST method. CALDAV:max-attendees-per-instance - when present this indicates the maximum number of ATTENDEE properties in any instance of scheduling messages submitted to the scheduling Outbox collection with the POST method. Specifically, this limits the total number of Attendees whose freebusy information can be queried in a single request. The use of child resources in a scheduling Outbox collection is reserved for future revisions or extensions of this specification.

A scheduling Inbox collection contains copies of incoming scheduling messages. These may be requests sent by an Organizer, or replies sent by an Attendee in response to a request. The scheduling Inbox collection is also used to manage scheduling privileges. A scheduling Inbox collection MUST report the DAV:collection and CALDAV:schedule-inbox XML elements in the value of the DAV:resourcetype property. The element type declaration for CALDAV:schedule-inbox is:

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Scheduling Inbox collections MUST only contain calendar object resources that obey the restrictions specified in iTIP. Consequently, scheduling Inbox collections MUST NOT contain any types of collection resources. Restrictions defined in Section 4.1 of CalDAV "calendar-access" on calendar object resources contained in calendar collections (e.g., "UID" uniqueness) do not apply to calendar object resources contained in a scheduling Inbox collection. Thus, multiple calendar object resources contained in a scheduling Inbox collection can have the same "UID" property value (i.e., multiple scheduling messages for the same calendar component). New WebDAV ACL privileges can be set on the scheduling Inbox collection to control from whom the Calendar User associated with the scheduling Inbox collection will accept scheduling messages from. See for more details. A scheduling Inbox collection MUST NOT be a child (at any depth) of a calendar collection resource. The following WebDAV properties specified in CalDAV "calendar-access" MAY also be defined on scheduling Inbox collections: CALDAV:calendar-timezone - when present this contains a time zone that the server can use when calendar date-time operations are carried out, for example when a time-range CALDAV:calendar-query REPORT is targeted at a scheduling Inbox collection. CALDAV:supported-calendar-component-set - when present this indicates the allowed calendar component types for scheduling messages delivered to the scheduling Inbox collection. CALDAV:supported-calendar-data - when present this indicates the allowed media types for scheduling messages delivered to the scheduling Inbox collection. CALDAV:max-resource-size - when present this indicates the maximum size in octets of a resource that the server is willing to accept for scheduling messages delivered to the scheduling Inbox collection. CALDAV:min-date-time - when present this indicates the earliest date and time (in UTC) that the server is willing to accept for any DATE or DATE-TIME value in scheduling messages delivered to the scheduling Inbox collection. CALDAV:max-date-time - when present this indicates the latest date and time (in UTC) that the server is willing to accept for any DATE or DATE-TIME value in scheduling messages delivered to the scheduling Inbox collection. CALDAV:max-instances - when present this indicates the maximum number of recurrence instances in scheduling messages delivered to the scheduling Inbox collection. CALDAV:max-attendees-per-instance - when present this indicates the maximum number of ATTENDEE properties in any instance of scheduling messages delivered to the scheduling Inbox collection.

This specification extends the CALDAV:calendar-query and CALDAV:calendar-multiget REPORTs to return results for calendar object resources in scheduling Inbox collections. When a CALDAV:calendar-query REPORT includes a time-range query and targets a scheduling Inbox collection, if any calendar object resources contain "VEVENT" calendar components that do not include a "DTSTART" iCalendar property (as allowed by iTIP) then such components MUST always match the time-range query test. Note that the CALDAV:free-busy-query REPORT is not supported on scheduling Inbox collections.

When a calendar object resource is created, modified or removed from a calendar collection, the server examines the calendar data and checks to see whether the data represents a scheduling object resource. If it does, the server will automatically attempt to deliver a scheduling message to the appropriate Calendar Users. Several types of scheduling operations can occur in this case, equivalent to iTIP "REQUEST", "REPLY", "CANCEL", and "ADD" operations.

Calendar object resources on which the server performs automatic scheduling transactions are referred to as scheduling object resources. There are two types of scheduling object resources: organizer scheduling object resources, and attendee scheduling object resources. A calendar object resource is considered to be a valid organizer scheduling object resource if the "ORGANIZER" iCalendar property is present and set in all the calendar components to a value that matches one of the calendar user addresses of the owner of the calendar collection. A calendar object resource is considered to be a valid attendee scheduling object resource if the "ORGANIZER" iCalendar property is present and set in all the calendar components to the same value and doesn't match one of the calendar user addresses of the owner of the calendar collection, and at least one of the "ATTENDEE" iCalendar property values matches one of the calendar user addresses of the owner of the calendar collection. The creation of attendee scheduling object resources is typically done by the server, with the resource being created in an appropriate calendar collection (see ).

The server's behavior when processing a scheduling object resource depends on whether it is owned by the Organizer or an Attendee specified in the calendar data.

An Organizer can create, modify or remove a scheduling object resource. The create, modify and remove behaviors for the server are each described next, and the way these are invoked via HTTP requests is described in . The Organizer of a calendar component may also be an Attendee of that calendar component. In such cases the server MUST NOT send a scheduling message to the Attendee that matches the Organizer.

When a scheduling object resource is created by the Organizer, the server will inspect each "ATTENDEE" property to determine if a scheduling message should be delivered to this Attendee according to the value of the "SCHEDULE-AGENT" property parameter (see ) as described in the table below:

The attempt to deliver the scheduling message will either succeed or fail. In all cases, the server MUST add a "SCHEDULE-STATUS" iCalendar property parameter (see ) to the "ATTENDEE" iCalendar property in the scheduling object resource being created, and set its value as described in . This will result in the created calendar object resource differing from the calendar data sent in the HTTP request. As a result clients MAY reload the calendar data from the server in order to update to the new server generated state information. Servers MUST NOT set the "SCHEDULE-STATUS" property parameter on the "ATTENDEE" property of Attendees for which it did not attempt to deliver a scheduling message. Restrictions: The server SHOULD reject any attempt to set the "PARTSTAT" iCalendar property parameter value of the "ATTENDEE" iCalendar property of other users in the calendar object resource to a value other than "NEEDS-ACTION" if the "SCHEDULE-AGENT" property parameter value is not present or set to the value "SERVER". The server MAY reject attempts to create a scheduling object resource that specifies a "UID" property value already specified in a scheduling object resource contained in another calendar collection of the Organizer. The server MUST take into account scheduling privileges as described in when handling the creation of a scheduling object resource. Restrictions on calendar object resources defined in Section 4.1 of MUST also be enforced. The server MUST return an error with the CALDAV:allowed-organizer-scheduling-object-change precondition code () when the Organizer attempts to change the iCalendar data in a manner that is forbidden.

When a scheduling object resource is modified by the Organizer, the server will inspect each "ATTENDEE" property in the new calendar data to determine which ones have the "SCHEDULE-AGENT" iCalendar property parameter. It will then need to compare this with the "ATTENDEE" properties in the existing calendar object resource that is being modified. For each Attendee in the old and new calendar data on a per-instance basis, and taking into account the addition or removal of Attendees, the server will determine whether to deliver a scheduling message to the Attendee. The following table determines whether the server needs to deliver a scheduling message, and if so which iTIP scheduling method to use. The values "SERVER", "CLIENT", and "NONE" in the top and left titles of the table refer to the "SCHEDULE-AGENT" parameter value of the "ATTENDEE" property, and the values "<Absent>" and "<Removed>" are used to cover the cases where the "ATTENDEE" property is not present (Old) or is being removed (New).

| SERVER | CLIENT | NONE | | | | (default) | | | +===+===========+===========+===========+===========+===========+ | | | -- | REQUEST / | -- | -- | | | | | ADD | | | | +-----------+-----------+-----------+-----------+-----------+ | | SERVER | CANCEL | REQUEST | CANCEL | CANCEL | | O | (default) | | | | | | l +-----------+-----------+-----------+-----------+-----------+ | d | CLIENT | -- | REQUEST / | -- | -- | | | | | ADD | | | | +-----------+-----------+-----------+-----------+-----------+ | | NONE | -- | REQUEST / | -- | -- | | | | | ADD | | | +---+-----------+-----------+-----------+-----------+-----------+ ]]>

The attempt to deliver the scheduling message will either succeed or fail. In all cases, the server MUST add a "SCHEDULE-STATUS" iCalendar property parameter to the "ATTENDEE" iCalendar property in the scheduling object resource being modified, and set its value as described in . This will result in the created calendar object resource differing from the calendar data sent in the HTTP request. As a result clients MAY reload the calendar data from the server in order to update to the new server generated state information. Restrictions: The server MAY reject any attempt to set the "PARTSTAT" iCalendar property parameter value of the "ATTENDEE" iCalendar property of other users in the calendar object resource to a value other than "NEEDS-ACTION" if the "SCHEDULE-AGENT" property parameter value is not present or set to the value "SERVER". The server MUST take into account scheduling privileges as described in when handling the modification of a scheduling object resource. Restrictions on calendar object resources defined in Section 4.1 of MUST also be enforced. The server MUST return an error with the CALDAV:allowed-organizer-scheduling-object-change precondition code () when the Organizer attempts to change the iCalendar data in a manner that is forbidden.

When a scheduling object resource is removed by the Organizer, the server will inspect each "ATTENDEE" property in the scheduling object resource being removed to determine which ones have the "SCHEDULE-AGENT" iCalendar property parameter. For each Attendee the server will determine whether to attempt to deliver a scheduling message into the Attendee's scheduling Inbox collection, based on the table below:

Restrictions: The server MUST take into account scheduling privileges as described in when handling the deletion of a scheduling object resource.

An Attendee can create, modify or remove a scheduling object resource by issuing HTTP requests with an appropriate method. The create, modify and remove behaviors for the server are each described next, and the way these are invoked via HTTP requests is described in .

Attendees are allowed to make some changes to a scheduling object resource, though key properties such as start time, end time, location, and summary are typically under the control of the Organizer. The server MUST allow Attendees to: change their own "PARTSTAT" iCalendar property parameter value. add, modify or remove any "TRANSP" iCalendar properties. add, modify or remove any "PERCENT-COMPLETE" iCalendar properties. add, modify or remove any "COMPLETED" iCalendar properties. add, modify or remove any "VALARM" iCalendar components. add, modify or remove the "CALSCALE" iCalendar property within the top-level "VCALENDAR" component. modify the "PRODID" iCalendar property within the top-level "VCALENDAR" component. add "EXDATE" iCalendar properties and possibly remove components for overridden recurrence instances. add, modify or remove any "CREATED", "DTSTAMP" and "LAST-MODIFIED" iCalendar properties. add, modify or remove "SCHEDULE-STATUS" iCalendar property parameters on "ATTENDEE" properties that have a "SCHEDULE-AGENT" parameter set to "CLIENT". add new components to represent overridden recurrence instances, provided the only changes to the recurrence instance follow the rules above. The server MUST return an error with the CALDAV:allowed-attendee-scheduling-object-change precondition code () when the Attendee attempts to change the iCalendar data in a manner forbidden by the server.

Typically an Attendee does not create scheduling object resources, as scheduling messages delivered to them on the server are automatically processed by the server and placed on one of their calendars (see ). However, in some cases a scheduling message may get delivered directly to the client, and the Attendee may wish to store that on the server. In that case the client creates a scheduling object resource in a suitable calendar belonging to the Attendee. It can then set the "SCHEDULE-AGENT" iCalendar property parameter on all "ORGANIZER" iCalendar properties in the resource to determine how the server treats the resource. The value of the "SCHEDULE-AGENT" iCalendar property parameter on all "ORGANIZER" iCalendar properties MUST be the same. SCHEDULE-AGENT Action SERVER (default) The server will attempt to process changes to the resource using the normal rules for attendee scheduling object resources. CLIENT The server does no special processing of the resource. The client is assumed to be handling Attendee replies etc. NONE The server does no special processing of the resource. In some cases a server may not be able to process an Attendee scheduling object resource that originated from another system (i.e., where the server is unable to deliver scheduling messages to the Organizer). In such cases the server MUST add a "SCHEDULE-STATUS" iCalendar property parameter to all "ORGANIZER" iCalendar properties in the resource with a suitable value indicating a error.

When a scheduling object resource is modified by an Attendee, the server behavior depends on the value of the "SCHEDULE-AGENT" iCalendar property parameter on the "ORGANIZER" iCalendar properties: SCHEDULE-AGENT Action SERVER (default) The server will attempt to process the removal using the behavior listed below. CLIENT The server does no special processing of the resource. The client is assumed to be handling any Attendee replies etc. NONE The server does no special processing of the resource. The server will inspect the changes by comparing the new scheduling object resource with the existing scheduling object resource. If the Attendee changes one or more "PARTSTAT" iCalendar property values on any component, or adds an overridden component with a changed "PARTSTAT" property, then the server MUST deliver an iTIP "REPLY" scheduling message to the Organizer to indicate the new participation status of the Attendee. If the Attendee adds an "EXDATE" property value to effectively remove a recurrence instance, the server MUST deliver an iTIP "REPLY" scheduling message to the Organizer to indicate that the Attendee has declined the instance (i.e., the Attendee's "PARTSTAT" iCalendar property parameter value is set to "DECLINED"). The attempt to deliver the scheduling message will either succeed or fail. In all cases, the server MUST add a "SCHEDULE-STATUS" iCalendar property parameter to the "ORGANIZER" iCalendar property in the scheduling object resource being created, and set its value as described in . This will result in the created calendar object resource differing from the calendar data sent in the HTTP request. As a result clients MAY reload the calendar data from the server in order to update to the new server generated state information.

When a scheduling object resource is removed by an Attendee, the server behavior depends on the value of the "SCHEDULE-AGENT" iCalendar property parameter on the "ORGANIZER" iCalendar properties: SCHEDULE-AGENT Action SERVER (default) The server will attempt to process the removal using either behaviors (1) or (2) listed below. CLIENT The server does no special processing of the resource. The client is assumed to be handling any Attendee replies etc. NONE The server does no special processing of the resource. If the HTTP request contains a "Schedule-Reply" request header set to the value "T" or there is no "Schedule-Reply" request header, then the server MUST attempt to deliver a scheduling message to the Organizer indicating that the Attendee has a "PARTSTAT" iCalendar property parameter value set to "DECLINED". That is, the Attendee has chosen not to attend any instances. If the server is unable to deliver the scheduling message, the remove action MUST fail, and an appropriate "SCHEDULE-STATUS" iCalendar property parameter set on the "ORGANIZER" property in the scheduling object resource stored by the server. If the HTTP request contains a "Schedule-Reply" request header set to the value "F", the server MUST NOT attempt to deliver a scheduling message. The resource is simply removed. This provides the client a way to silently remove unwanted scheduling messages.

This section describes how use of various HTTP methods on a scheduling object resource will cause a create, modify or remove action on that resource as described above. The use of these methods is subject to the restrictions in , in addition to what is described below.

When a PUT method request is received, the server will execute the following actions, provided all appropriate preconditions are met: Existing Destination Resource Resulting Destination Resource Server Action None Calendar object resource None None Scheduling object resource Create Calendar object resource Calendar object resource None Calendar object resource Scheduling object resource Create Scheduling object resource Calendar object resource Remove Scheduling object resource Scheduling object resource Modify

When a COPY method request is received, the server will execute the following actions based on the source and destination collections in the request: Source Collection Destination Collection Server Action Non-calendar collection Non-calendar collection None Non-calendar collection Calendar collection (1) Calendar collection Non-calendar collection None Calendar collection Calendar collection (2) Note 1. The same rules as used for PUT above are applied for the destination of the COPY request. Note 2. The server MAY reject this as per , otherwise None. The behavior of a COPY method request on a calendar collection is undefined.

When a MOVE method request is received, the server will execute the following actions based on the source and destination collections in the request: Source Collection Destination Collection Server Action Non-calendar collection Non-calendar collection None Non-calendar collection Calendar collection (1) Calendar collection Non-calendar collection (2) Calendar collection Calendar collection None Note 1. The same rules as used for PUT above are applied for the destination of the MOVE request. Note 2. The same rules as used for DELETE below are applied for the source of the MOVE request. The behavior of a MOVE method request on a calendar collection is undefined.

When a DELETE method is targeted at a scheduling object resource the server will execute the Remove action. When a DELETE method is targeted at a calendar collection the server will execute the Remove action on all scheduling object resources contained in the calendar collection.

This specification defines method preconditions (see Section 16 of WebDAV), in addition to the ones in , to provide machine-parsable information in error responses.

unique-scheduling-object-resource urn:ietf:params:xml:ns:caldav PUT, COPY, and MOVE 403 Forbidden (precondition) -- Servers MAY reject requests to create a scheduling object resource with an iCalendar "UID" property value already in use by another scheduling object resource owned by the same user in other calendar collections. Servers SHOULD report the URL of the scheduling object resource that is already making use of the same "UID" property value in the DAV:href element.

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same-organizer-in-all-components urn:ietf:params:xml:ns:caldav PUT, COPY, and MOVE 403 Forbidden (precondition) -- All the calendar components in a scheduling object resource MUST contain the same "ORGANIZER" property value when present.

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allowed-organizer-scheduling-object-change urn:ietf:params:xml:ns:caldav PUT, COPY, and MOVE 403 Forbidden (precondition) -- Servers MAY impose restrictions on modifications allowed by an Organizer. For instance, servers MAY prevent the Organizer setting the "PARTSTAT" property parameter to a value other than "NEEDS-ACTION" if the corresponding "ATTENDEE" property has the "SCHEDULE-AGENT" property parameter set to "SERVER", or has no "SCHEDULE-AGENT" property parameter. See .

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allowed-attendee-scheduling-object-change urn:ietf:params:xml:ns:caldav PUT, COPY, and MOVE 403 Forbidden (precondition) -- Servers MAY impose restrictions on modifications allowed by an Attendee. Attendee modifications that servers MUST allow are specified in .

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Whenever the server generates a scheduling message for delivery to a Calendar User, it MUST ensure that a "DTSTAMP" iCalendar property is present and MUST set the value to the UTC time that the scheduling message was generated (as required by iCalendar). iTIP places certain requirements on how the "SEQUENCE" iCalendar property value in scheduling messages changes. The server MUST ensure that for each type of scheduling operation, the "SEQUENCE" iCalendar property value is appropriately updated. If the client does not update the "SEQUENCE" iCalendar property itself when that is required, the server MUST update the property.

When delivering scheduling messages for recurring calendar components to Attendees, servers MUST ensure that Attendees only get information about recurrence instances that explicitly include them as an Attendee. For example, if an Attendee is invited to a single recurrence instance of a recurring event, and no others, the scheduling object resource contained in the Organizer's calendar collection will contain an overridden instance in the form of a separate calendar component. That separate calendar component will include the "ATTENDEE" property referencing the "one-off" Attendee. That Attendee will not be listed in any other calendar components in the scheduling object resource. Any scheduling messages delivered to the Attendee will only contain information about this overridden instance. As another example, an Attendee could be excluded from one instance of a recurring event. In that case the scheduling object resource contained in the calendar collection of the Organizer will include an overridden instance with an "ATTENDEE" list that does not include the Attendee being excluded. The scheduling message that will be delivered to the Attendee will not specify the overridden instance but rather include an "EXDATE" property in the master recurring component defining the recurrence set.

The iCalendar property parameter "SCHEDULE-FORCE-SEND" defined in can be used by a Calendar User to force the server to send a scheduling message to an Attendee or the Organizer in a situation where the server would not normally send a scheduling message. For instance, an Organizer could use this property parameter to request an Attendee, that previously declined an invitation, to reconsider their participation status without being forced to modify the event.

Scheduling operations can cause the delivery of a scheduling message into an Organizer's or Attendee's scheduling Inbox collection. In the former case the scheduling messages are replies from Attendees, in the latter case the scheduling messages are requests, cancellations or additions from the Organizer. Servers MUST automatically process incoming scheduling messages using the rules defined by , 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. 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 ) when delivering the scheduling message, and it MUST take into account the similar preconditions on any calendar collection which contains, or would contain, the corresponding scheduling object resource.

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 MUST process the scheduling message and create a new scheduling object resource in an appropriate calendar collection for the Attendee. In the case of an update, the server MUST process the scheduling message and update 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.

For a scheduling message reply sent by an Attendee, the server first locates the corresponding scheduling object resource belonging to the Organizer. 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 recurrence 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 . The scheduling message MUST only appear in the Organizer's scheduling Inbox collection once all automatic processing has been done.

Once the processing of an incoming scheduling message is completed by the server, the message is made available as a child resource in the scheduling Inbox collection of the Calendar User that received the message, to serve as a notification that a change has been made to the corresponding scheduling object resource. Scheduling messages are typically removed from the scheduling Inbox collection by the client once the calendar user has acknowledged the change.

The server is REQUIRED to process 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 if this Calendar User has a default calendar collection. See . 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 servers 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.

This specification defines additional method preconditions (see Section 16 of WebDAV) to provide machine-parsable information in error responses.

default-calendar-needed urn:ietf:params:xml:ns:caldav DELETE 403 Forbidden (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.

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valid-schedule-default-calendar-URL urn:ietf:params:xml:ns:caldav PROPPATCH 403 Forbidden (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.

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The POST method is used to request busy time information of one or more Calendar Users by submitting a request at the scheduling Outbox collection of the Calendar User requesting the information (the Organizer). To accomplish this, the request body of a POST method 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. The resource identified by the Request-URI MUST be a resource collection of type CALDAV:schedule-outbox. The "ORGANIZER" property in the "VFREEBUSY" component MUST match that of the Calendar User who "owns" the Outbox collection.

The following are examples of response codes one would expect to be used for this method. However, unless explicitly prohibited, any 2/3/4/5xx series response code can be used in a response. 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.

This specification defines additional method preconditions for the POST method. Preconditions defined in WebDAV ACL and CalDAV that applies to the POST method are also listed here for completeness.

need-privileges DAV: POST 403 Forbidden (precondition) -- The currently authenticated user MUST be granted the CALDAV:schedule-send-freebusy privilege on the scheduling Outbox collection being targeted by the request.

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supported-collection urn:ietf:params:xml:ns:caldav POST 400 Bad Request (precondition) -- The Request-URI MUST identify the location of a scheduling Outbox collection.

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supported-calendar-data urn:ietf:params:xml:ns:caldav POST 400 Bad Request (precondition) -- The resource body submitted in the POST request MUST be a supported media type (e.g., text/calendar).

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valid-calendar-data urn:ietf:params:xml:ns:caldav POST 400 Bad Request (precondition) -- The resource submitted in the POST request MUST be valid data for the media type being specified (e.g., a valid iCalendar object).

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valid-scheduling-message urn:ietf:params:xml:ns:caldav POST 400 Bad Request (precondition) -- The resource submitted in the POST request MUST obey all restrictions specified for the POST request (e.g., the scheduling message follow the restrictions of iTIP).

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valid-organizer urn:ietf:params:xml:ns:caldav POST 403 Forbidden (precondition) -- The Calendar User identified by the "ORGANIZER" property in the POST request's scheduling message MUST be the Calendar User (or one of the Calendar Users) associated with the scheduling Outbox collection being targeted by the request;

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max-resource-size urn:ietf:params:xml:ns:caldav POST 403 Forbidden (precondition) -- The resource submitted in the POST request MUST have a size in octets less than or equal to the value of the CALDAV:max-resource-size property (defined in Section 5.2.5 of ) specified on the scheduling Outbox collection targeted by the request.

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A POST request can return freebusy information for one or more Calendar Users. Thus the response needs to contain separate status information for each recipient. This specification defines a new XML response body to convey multiple recipient status. A response to a POST method that indicates status for one or more recipients 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 recipient, with each of those containing elements that indicate which recipient they correspond to, the scheduling status for that recipient, any error codes and an optional description. See for the detail on the child elements. In the case of a successful freebusy request, the CALDAV:response elements can also contain CALDAV:calendar-data elements which contain freebusy information (e.g., an iCalendar VFREEBUSY component) indicating the busy state of the corresponding recipient. See for an example freebusy request and response.

Because replies from Attendees and updates from Organizers are automatically processed by the server, clients might be in a situation where their copy of a calendar resource is different from the one currently on the server. When an Attendee or Organizer makes a change to the client's copy of the calendar resource, if the client writes the data to the server it could overwrite the changes already made there. Typically, clients use the ETag value and If-Match request headers to avoid the "lost update problem". Clients can also use ETag and If-Match to avoid this problem. However, when doing so the client will likely have to resolve the differences between the new resource and the original one, and the changes made by the Attendee or Organizer in the client. This can be a complicated comparison particularly when recurring components are present. Additionally, the data on the server may change frequently as Attendees change their participation status, triggering updates to the Organizer, and consequently other Attendees' copies of the scheduling object resource. If the ETag/If-Match behavior were used, clients would be forced to reconcile their cached copy of a scheduling object resource with the updated one on the server in order to attempt to write the user's changes back. This could lead to a race condition that can effectively result in a temporary denial of service when, for example, there is an event with a large Attendee list. A "storm" of updates will occur if Attendees all start responding at the same time, and this would prevent Attendees and the Organizer from being able to update their own copies of the scheduling object resource as the server copy is changing frequently. A solution is to have the server determine the best way to merge changes made on the server with changes being made by the client. For example, if an Attendee changes their participation status and triggers an update to the Organizer's copy of the event, but the Organizer also updates their cached copy of the event and attempts to write it back, rather than failing on a conditional If-Match when the Organizer writes their data, the server would instead take the changes made by the Organizer and apply the Attendee changes and store the result. Thus a form of "weak" ETag matching behavior is needed such that scheduling changes made automatically on the server do not invalidate the tag, so that when clients store data conditionally based on the tag value, the server knows it can apply the merge behavior. In order to do that, this specification introduces a new WebDAV resource property CALDAV:schedule-tag with a corresponding response header "Schedule-Tag", and a new "If-Schedule-Tag-Match" request header to allow client changes to be appropriately merged with server changes in the case where the changes on the server were the result of an "inconsequential" scheduling message update. An "inconsequential" scheduling message is one which simply updates the status information of Attendees due to a reply from an Attendee. Servers MUST support requests targeted at scheduling object resources using the "If-Schedule-Tag-Match" request header. Consequently, the server MUST support the "Schedule-Tag" response header and CALDAV:schedule-tag property for scheduling object resources. Servers MUST automatically resolve conflicts with "inconsequential" changes done to scheduling object resources when the "If-Schedule-Tag-Match" request header is specified. The If-Schedule-Tag-Match request header applies only to the Request-URI, and not to the Destination of a COPY or MOVE in the same way as the If-Match request header. Clients SHOULD use the If-Schedule-Tag-Match header on requests that update scheduling object resources. A response to any successful GET or PUT request targeting a scheduling object resource MUST include a Schedule-Tag response header with the value set to the same value as the CALDAV:schedule-tag WebDAV property of the resource. A response to any successful COPY or MOVE request that specifies a Destination request header targeting a scheduling object resource MUST include a Schedule-Tag response header with the value set to the same value as the CALDAV:schedule-tag WebDAV property of the resource identified in the Request-URI. The Schedule-Tag feature is designed to be used to address the problem of "inconsequential" changes on the server only. Normal ETag operations are used in all other cases, e.g., for synchronization. The value of the CALDAV:schedule-tag property changes according to these rules: For an Organizer's copy of a scheduling object resource: The server MUST NOT change the CALDAV:schedule-tag property value when the scheduling object resource is updated as the result of automatically processing a scheduling message reply from an Attendee. For instance, when an Attendee replies to the Organizer, the CALDAV:schedule-tag property is unchanged after the Organizer's scheduling object resource has been automatically updated by the server with the Attendee's new participation status. The server MUST change CALDAV:schedule-tag property value when the scheduling object resource is changed directly via an HTTP request (e.g., PUT, COPY or MOVE). For an Attendee's copy of a scheduling object resource: The server MUST change the CALDAV:schedule-tag property value when the scheduling object resource is changed as the result of processing a scheduling message update from an Organizer that contains changes other than just the participation status of Attendees. The server MUST NOT change the CALDAV:schedule-tag property value when the scheduling object resource is changed as the result of processing a scheduling message update from an Organizer that only specify changes in the participation status of Attendees. For instance, when Attendee "A" replies to Organizer "O", and Attendee "B" receives a scheduling message update from Organizer "O" with the new participation status of Attendee "A", the CALDAV:schedule-tag property of Attendee "B"s scheduling object resource MUST NOT be changed. The server MUST change the CALDAV:schedule-tag property value when the scheduling object resource is changed directly via an HTTP request (e.g., PUT, COPY or MOVE).

Clients can use the If-Schedule-Tag-Match request header to do a PUT request that ensures that "inconsequential" changes on the server do not result in a precondition error. The value of the request header is set to the last Schedule-Tag value received for the resource being modified. If the value of the If-Schedule-Tag-Match header matches the current value of the CALDAV:schedule-tag property the server MUST take any "ATTENDEE" property changes for all Attendees other than the owner of the scheduling object resource and apply those to the new resource being stored. Otherwise, the server MUST fail the request with a 412 Precondition Failed status code.

Clients can use the If-Schedule-Tag-Match request header to do a DELETE, COPY or MOVE request that ensures that "inconsequential" changes on the server do not result in a precondition error. The value of the request header is set to the last Schedule-Tag value received for the resource being deleted. If the value of the If-Schedule-Tag-Match header matches the current value of the CALDAV:schedule-tag property the server performs the normal DELETE, COPY or MOVE request processing for the resource. Otherwise, the server MUST fail the request with a 412 Precondition Failed status code.

This section specifies additional requirements on the handling of the "PARTSTAT" property parameter when the "SCHEDULE-AGENT" property parameter on the corresponding "ATTENDEE" property is set to the value "SERVER" or is not present. Clients SHOULD, and servers MUST reset the "PARTSTAT" property parameter value of all "ATTENDEE" properties, except the one that corresponds to the Organizer, to "NEEDS-ACTION" when the Organizer reschedules an event. A reschedule of an event occurs when any "DTSTART", "DTEND", "DURATION", "DUE", "RRULE", "RDATE", or "EXDATE" property changes in a calendar component such that existing recurrence instances are impacted by the changes, as shown in the table below. Property Server Action DTSTART, DTEND, DURATION, DUE Any change to these properties MUST result in "PARTSTAT" being set to "NEEDS-ACTION"    RRULE A change to or addition of this property that results in the addition of new recurring instances or a change in time for existing recurring instances MUST result in "PARTSTAT" being reset to "NEEDS-ACTION" on each affected component.    RDATE A change to or addition of this property that results in the addition of new recurring instances or a change in time for existing recurring instances MUST result in "PARTSTAT" being reset to "NEEDS-ACTION" on each affected component.    EXDATE A change to or removal of this property that results in the re-instatement of recurring instances MUST result in "PARTSTAT" being set to "NEEDS-ACTION" on each affected component. The server MAY allow the Organizer's client to change an Attendee's "PARTSTAT" property parameter value to "NEEDS-ACTION" at any other time (e.g., when the "LOCATION" property value changes, an Organizer might wish to re-invite Attendees who may be impacted by the change).

When scheduling with an Attendee there are two types of status information that can be returned during the transaction. The first type of status information is a "delivery" status that indicates whether the scheduling message from the Organizer to the Attendee was delivered or not, or what the current status of delivery is. The second type of status information is a "reply" status corresponding to the Attendee's own "REQUEST-STATUS" information from the scheduling message reply that is sent back to the Organizer. Similarly, when an Attendee sends a reply back to the Organizer, there will be "delivery" status information for the scheduling message sent to the Organizer. However, there is no "REQUEST-STATUS" sent back by the Organizer, so there is no equivalent of the "reply" status as per scheduling messages to Attendees. The "delivery" status information on an "ORGANIZER" or "ATTENDEE" iCalendar property is conveyed in the "SCHEDULE-STATUS" property parameter value (). The status code value for "delivery" status can be one of the following: Delivery Status Code Description 1.0 The scheduling message is pending. i.e. the server is still in the process of sending the message. The status code value can be expected to change once the server has completed its sending and delivery attempts.     1.1 The scheduling message has been successfully sent. However, the server does not have explicit information about whether the scheduling message was successfully delivered to the recipient. This state can occur with "store and forward" style scheduling protocols such as iMIP (iTIP using email).     1.2 The scheduling message has been successfully delivered.     3.7 The scheduling message was not delivered because the server did not recognize the calendar user address as a valid calendar user. Note that this code applies to both Organizer and Attendee calendar user addresses.     3.8 The scheduling message was not delivered due to insufficient privileges. Note that this code applies to both privileges granted by both the Organizer and Attendee calendar users.     5.1 The scheduling message was not delivered because the server could not complete delivery of the message. This is likely due to a temporary failure, and the originator can try to send the message again at a later time.     5.2 The scheduling message was not delivered because the server was not able to find a suitable way to deliver the message. This is likely a permanent failure, and the originator should not try to send the message again, at least without verifying/correcting the calendar user address of the recipient.     5.3 The scheduling message was not delivered and was rejected because scheduling with that recipient is not allowed. This is likely a permanent failure, and the originator should not try to send the message again. The status code for "reply" status can be any of the valid iTIP "REQUEST-STATUS" values. The 1.xx "REQUEST-STATUS" codes are new. This specification modifies item (2) of Section 3.6 of by adding the following restriction: For a 1.xx code, all components MUST have exactly the same code. Definition of the new 1.xx codes is as follows:

1.0 Pending. None. Delivery of the iTIP message is pending.

1.1 Sent. None. The iTIP message has been sent, though no information about successful delivery is known.

1.2 Delivered. None. The iTIP message has been sent and delivered.

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

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

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 an automatic scheduling transaction 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 an automatic scheduling transaction. 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 an automatic scheduling transaction. 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.

SCHEDULE-FORCE-SEND To force a scheduling message to be sent to the Calendar User specified by the property. This property parameter is defined by the following notation:

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 an automatic scheduling transaction. 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 ) when the "SCHEDULE-FORCE-SEND" parameter is set to a x-name or iana-token value they do not recognize.

SCHEDULE-STATUS 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. This property parameter is defined by the following notation:

This property parameter MAY be specified on the "ATTENDEE" and "ORGANIZER" properties. Servers MUST add this property parameter to any "ATTENDEE" properties corresponding to Calendar Users who were sent a scheduling message via an automatic scheduling transaction. 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 an automatic scheduling transaction. 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 an automatic scheduling transaction. Clients MUST NOT include this parameter in any scheduling messages that they themselves send. Suitable values for this property parameter are described in .

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

When an Attendee removes a scheduling object resource, and the Schedule-Reply header is not present, or present and set to the value "T", 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 automatic scheduling transaction processing. When the Schedule-Reply header is set to the value "F", the server MUST NOT send a scheduling message as part of its normal automatic scheduling transaction processing. The Schedule-Reply request header is used by a client to indicate to a server whether or not an automatic scheduling transaction should 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. All scheduling object resources MUST support the Schedule-Reply request 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 . All scheduling object resources MUST support the Schedule-Tag 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.

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

schedule-calendar-transp urn:ietf:params:xml:ns:caldav Determines whether the calendar object resources in a calendar collection will affect the owner's freebusy. This property MAY be protected and SHOULD NOT be returned by a PROPFIND allprop request (as defined in Section 14.2 of ). This property value SHOULD be kept during a MOVE operation, and SHOULD be copied and preserved in a COPY. 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 should contribute to the owner's freebusy or not. When the CALDAV:opaque element is used, all calendar object resources in the corresponding calendar collection MUST contribute to freebusy, assuming 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 freebusy. If this property is not present on a calendar collection, then the default value CALDAV:opaque MUST be assumed.

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schedule-default-calendar-URL urn:ietf:params:xml:ns:caldav Specifies a default calendar for an Attendee where new scheduling object resources are created. 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. This property is only defined on a scheduling Inbox collection which cannot be moved or copied. 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 .

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schedule-tag urn:ietf:params:xml:ns:caldav Indicates whether a scheduling object resource has had a "consequential" change made to it. opaque-tag (defined in Section 3.11 of ) This property MUST be protected as only the server can update the value. This property is only defined on scheduling object resources. It MUST be preserved when a scheduling object resource is copied or moved and the resulting resource is also a scheduling object resource. If the source resource is not a scheduling object resource but the destination resource is, this property MUST be added to the destination resource. The CALDAV:schedule-tag property MUST be defined on all scheduling object resources. This property is described in .

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CalDAV servers MUST support and adhere to the requirements of WebDAV ACL. Furthermore, CalDAV servers that advertise support for the "calendar-auto-schedule" feature MUST also support the scheduling privileges 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 clarify which scheduling methods (e.g., "REQUEST", "REPLY", etc.) are controlled by each scheduling privilege defined in this section.

This section defines new WebDAV ACL privileges that are 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. 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.

CALDAV:schedule-deliver is an aggregate privilege that contains all the scheduling privileges that control the processing and delivery of incoming scheduling messages, that is, CALDAV:schedule-deliver-invite and CALDAV:schedule-deliver-reply, as well as freebusy requests targeted at the owner of the scheduling Inbox collection, that is, CALDAV:schedule-query-freebusy. ]]>

The CALDAV:schedule-deliver-invite privilege controls the processing and delivery of scheduling messages coming from an Organizer. ]]>

The CALDAV:schedule-deliver-reply privilege controls the processing and delivery of scheduling messages coming from an Attendee. ]]>

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

This section defines new WebDAV ACL 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 to send schedule messages to other calendar users (e.g. assistants working on behalf of their boss). The privileges defined in this section are ignored if applied to a resource other than a scheduling Outbox collection.

CALDAV:schedule-send is an aggregate privilege that contains all the scheduling privileges that control the use of methods that will cause scheduling messages to be delivered to other users, that is, CALDAV:schedule-send-invite and CALDAV:schedule-send-reply, as well as freebusy requests to be targeted at other users, that is, CALDAV:schedule-send-freebusy. ]]>

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 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 organizer scheduling messages to other calendar users. ]]>

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 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 attendee scheduling messages to other calendar users. ]]>

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. ]]>

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

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

schedule-inbox-URL urn:ietf:params:xml:ns:caldav Identify the URL of the scheduling Inbox collection owned by the associated principal resource. This property MAY be protected. This property SHOULD NOT be returned by a PROPFIND allprop request (as defined in Section 14.2 of ). This property value SHOULD be preserved in COPY and MOVE operations. This property is needed for a client to determine where the scheduling Inbox collection of the current user is located so that processing of scheduling messages can occur. If not present, then the associated calendar user is not enabled for reception of scheduling messages on the server.

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schedule-outbox-URL urn:ietf:params:xml:ns:caldav Identify the URL of the scheduling Outbox collection owned by the associated principal resource. This property MAY be protected. This property SHOULD NOT be returned by a PROPFIND allprop request (as defined in Section 14.2 of ). This property value SHOULD be preserved in COPY and MOVE operations. This property is needed for a client to determine where the scheduling Outbox collection of the current user is located so that sending of scheduling messages can occur. If not present, then the associated calendar user is not enabled for the sending of scheduling messages on the server.

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calendar-user-address-set urn:ietf:params:xml:ns:caldav Identify the calendar addresses of the associated principal resource. This property MAY be protected. This property SHOULD NOT be returned by a PROPFIND allprop request (as defined in Section 14.2 of ). This property value SHOULD be preserved in COPY and MOVE operations. Support for this property is REQUIRED. This property is needed to map calendar user addresses in iCalendar data to principal resources and their associated scheduling Inbox and Outbox collections. In the event that a user has no well defined identifier for their calendar user address, the URI of their principal resource can be used. This property SHOULD be searchable using the DAV:principal-property-search REPORT. The DAV:principal-search-property-set REPORT SHOULD identify this property as such. If not present, then the associated calendar user is not enabled for scheduling on the server.

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mailto:bernard@example.com mailto:bernard.desruisseaux@example.com ]]>

calendar-user-type urn:ietf:params:xml:ns:caldav Identifies the calendar user type of the associated principal resource. Same values allowed for the iCalendar "CUTYPE" property parameter defined in Section 3.2.3 of . This property MAY be protected. This property SHOULD NOT be returned by a PROPFIND allprop request (as defined in Section 14.2 of ). This property value SHOULD be preserved in COPY and MOVE operations. Clients can query principal resources in order to lookup attendees available on the server. When doing this, it is useful to know, or restrict the query to, certain types of calendar user (e.g., only search for "people", or only search for "rooms"). This property MAY be defined on principal resources to indicate the type of calendar user associated with the principal resource. Its value is the same as the iCalendar "CUTYPE" property parameter that can be used on "ATTENDEE" properties. This property SHOULD be searchable using the DAV:principal-property-search REPORT. The DAV:principal-search-property-set REPORT SHOULD identify this property as such.

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INDIVIDUAL< /C:calendar-user-type> ]]>

schedule-response urn:ietf:params:xml:ns:caldav Contains the set of responses for a POST method request. See .

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response urn:ietf:params:xml:ns:caldav Contains a single response for a POST method request. See .

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

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request-status urn:ietf:params:xml:ns:caldav The iTIP "REQUEST-STATUS" property value for this response. See .

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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 TLS as defined in for all scheduling transactions.

When handling a scheduling transaction: 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. Servers MUST verify that the currently authenticated user has the CALDAV:schedule-send privilege, or a suitable 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. Servers MUST only deliver scheduling messages to recipients when the CALDAV:schedule-deliver privilege, or a suitable 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. 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. 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.

When handling a POST request on a scheduling Outbox collection: 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. 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. 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.

As noted in , Attendees can use the Schedule-Reply request header 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.

The message header fields below should be added to the Permanent Message Header Field Registry (see ).

Header field name: Schedule-Reply Applicable protocol: http Status: standard Author/Change controller: IETF Specification document(s): this specification () Related information: none

Header field name: Schedule-Tag Applicable protocol: http Status: standard Author/Change controller: IETF Specification document(s): this specification () Related information: none

Header field name: If-Schedule-Tag-Match Applicable protocol: http Status: standard Author/Change controller: IETF Specification document(s): this specification () Related information: none

The following iCalendar property parameters should be added to the iCalendar Property Parameter Registry defined in Section 8.3.3 of . Parameter Status Reference SCHEDULE-AGENT Current RFCXXXX, SCHEDULE-STATUS Current RFCXXXX, SCHEDULE-FORCE-SEND Current RFCXXXX,

The following iCalendar "REQUEST-STATUS" values should be added to the iCalendar REQUEST-STATUS Value Registry defined in Section 7.3 of . Status Code Status Reference 1.0 Current RFC XXXX, 1.1 Current RFC XXXX, 1.2 Current RFC XXXX,

This specification adds two new IANA registries for iCalendar elements. Additional codes MAY be used, provided the process described in Section 8.2.1 of is used to register them.

The following table has been used to initialize the schedule agent values registry. Schedule Agent Status Reference SERVER Current RFC XXXX, CLIENT Current RFC XXXX, NONE Current RFC XXXX,

The following table has been used to initialize the schedule send values registry. Schedule Force Send Status Reference REQUEST Current RFC XXXX, REPLY Current RFC XXXX,

The authors would like to thank the following individuals for contributing their ideas and support for writing this specification: Mike Douglass, Lisa Dusseault, Helge Hess, Arnaud Quillaud, Julian F. Reschke, Wilfredo Sanchez Vega, Simon Vaillancourt, and Jim Whitehead. The authors would also like to thank the Calendaring and Scheduling Consortium for advice with this specification, and for organizing interoperability testing events to help refine it.

&rfc2119; &rfc2616; &rfc2818; &rfc3744; &rfc3864; &rfc4791; &rfc4918; &rfc5234; &rfc5545; &rfc5546; &W3C.REC-xml-20081126; &rfc6047; &rfc3283;

The following tables specify which scheduling privileges grant the right to a calendar user to deliver a scheduling message to the scheduling Inbox collection of another calendar user. The appropriate behavior depends on the calendar component type as well as the scheduling "METHOD" specified in the scheduling message.

The following tables specify which scheduling privileges grant the right to a Calendar User to perform busy time information requests and to submit scheduling messages to other Calendar Users as the result of a scheduling transaction. The appropriate behavior depends on the calendar component type as well as the scheduling "METHOD" specified in the scheduling message.

This section describes some example scheduling transactions that give a general idea of how scheduling is carried out between CalDAV clients and servers from the perspective of meeting Organizers and Attendees. In the following examples the requests and responses are incomplete and are only for illustrative purposes. In particular, HTTP authentication headers and behaviors are not shown, even though they are required in normal operation.

In the following example, Cyrus invites Wilfredo, Bernard and Mike to a single instance event by simply creating a new scheduling object resource in one of his calendar collection by using the PUT method.

>> Request <<

>> Response <<

Once the event creation has been completed, Cyrus's client will retrieve the event back from the server to get the schedule status of each Attendee. In this example, the server reports that a scheduling message was delivered to Wilfredo, a scheduling message is still pending for Bernard, and the server was unable to deliver a scheduling message to Mike.

>> Request <<

>> Response <<

In the following example, Wilfredo's client retrieves and deletes the new scheduling message that appeared in his scheduling Inbox collection after the server automatically processed it and created a new scheduling object resource in his default calendar collection.

>> Request <<

>> Response <<

>> Request <<

>> Response <<

In the following example, Wilfredo's accepts Cyrus's invitation and sets a reminder on the event.

>> Request <<

>> Response <<

Once the event modification has been completed, Wilfredo's client will retrieve the event back from the server to get the schedule status of the Organizer.

>> Request <<

>> Response <<

On reception of Wilfredo's reply, Cyrus's server will automatically update Cyrus's scheduling object resource, make Wilfredo's scheduling message available in Cyrus's scheduling Inbox collection, and deliver an updated scheduling message to Bernard to share Wilfredo's updated participation status. In this example, Cyrus's client retrieves and deletes this scheduling message in his scheduling Inbox collection.

>> Request <<

>> Response <<

>> Request <<

>> Response <<

Cyrus's client then retrieves the event back from the server with Wilfredo's updated participation status.

>> Request <<

>> Response <<

In this example, Cyrus requests the busy time information of Wilfredo, Bernard and Mike.

>> Request <<

>> Response << mailto:wilfredo@example.com 2.0;Success BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Example Corp.//CalDAV Server//EN METHOD:REPLY BEGIN:VFREEBUSY UID:4FD3AD926350 DTSTAMP:20090602T200733Z DTSTART:20090602T000000Z DTEND:20090604T000000Z ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com ATTENDEE;CN="Wilfredo Sanchez Vega":mailto:wilfredo@example.com FREEBUSY;FBTYPE=BUSY:20090602T110000Z/20090602T120000Z FREEBUSY;FBTYPE=BUSY:20090603T170000Z/20090603T180000Z END:VFREEBUSY END:VCALENDAR mailto:bernard@example.net 2.0;Success BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Example Corp.//CalDAV Server//EN METHOD:REPLY BEGIN:VFREEBUSY UID:4FD3AD926350 DTSTAMP:20090602T200733Z DTSTART:20090602T000000Z DTEND:20090604T000000Z ORGANIZER;CN="Cyrus Daboo":mailto:cyrus@example.com ATTENDEE;CN="Bernard Desruisseaux":mailto:bernard@example.net FREEBUSY;FBTYPE=BUSY:20090602T150000Z/20090602T160000Z FREEBUSY;FBTYPE=BUSY:20090603T090000Z/20090603T100000Z FREEBUSY;FBTYPE=BUSY:20090603T180000Z/20090603T190000Z END:VFREEBUSY END:VCALENDAR mailto:mike@example.org 3.7;Invalid calendar user ]]>

In the following example, Cyrus attempts to create, on behalf of Wilfredo, an event with Bernard specified as an Attendee. The request fails since Wilfredo didn't grant Cyrus the right to invite other Calendar Users on his behalf.

>> Request <<

>> Response << /home/wilfredo/calendars/outbox/ ]]>

In the following example, Bernard declines the second recurrence instance of a daily recurring event he's been invited to by Cyrus.

>> Request <<

>> Response <<

Bernard's participation status update will cause his server to deliver a scheduling message to Cyrus. Cyrus's client will find the following reply message from Bernard in Cyrus's scheduling Inbox collection:

>> Request <<

>> Response <<

In the following example, Bernard removes from his calendar the third recurrence instance of a daily recurring event he's been invited to by Cyrus. This is accomplished by the addition of an "EXDATE" property to the scheduling object resource stored by Bernard.

>> Request <<

Bernard's deletion of a recurrence instance will cause his server to deliver a scheduling message to Cyrus. Cyrus's client will find the following reply message from Bernard in Cyrus's scheduling Inbox collection:

>> Request <<

>> Response <<

Updated to RFC 6047 reference. Various minor clarifications to behavior and terminology done. Clarified that Inbox/Outbox are the server's responsibility to create. Changed MAY to SHOULD for server rejecting organizer PARTSTAT changes of attendees. Allow COMPLETED as a valid attendee change. Allow SCHEDULE-STATUS as a valid attendee change on SCHEDULE-AGENT=CLIENT attendee properties. COPY or MOVE on a calendar collection now declared to be undefined. Changed pre-condition error codes from 409 to 403. Clarified that rules 5546 must be used when server processes incoming scheduling messages. default-calendar-delete-allowed -> default-calendar-needed. Clarified that SCHEDULE-AGENT must be the same on all matching properties. Added more text justifying the need for calendar-user-type property.

Fixed some examples. Tweaked XML conventions. Removed description in SCHEDULE-STATUS example values. Tweaked 3.7 and 3.8 SCHEDULE-STATUS description to indicate it applies to the Organizer as well as Attendee. Updated to RFC 5545 reference. AD Review: clarified text about inbox resource deletion being acknowledgment of change. AD Review: clarified description of freebusy Outbox POST. AD Review: registered new 1.xx request-status codes and added new restriction on usage as per iTIP. AD Review: changes SHOULD NOT to MUST NOT for new property parameters when clients send scheduling messages. AD Review: CALDAV:schedule-calendar-transp now preserved during COPY. AD Review: changed CALDAV- to CALDAV: in acl descriptions. AD Review: fixed various minor typos. AD Review: Added text to new principal properties to indicate that if they are not present, then the user is not enabled for the various scheduling operations. AD Review: clarified use of CALDAV:calendar-data element in CALDAV:response element. AD Review: made reference to 5545 IANA registry procedures for the two new element registries. AD Review: Fixed description of B5. example. Fixed SCHEDULE-AGENT/SCHEDULE-STATUS behavior for Attendee replies.

Added "Updates 4791". XML conventions changed to match that in CardDAV spec. Reworded child response behavior for Outbox. Reworded "octet size". If-Schedule-Match descriptions changed to remove implication that it is purely a conditional operation. Schedule-Reply header descriptions generalized to resource removal rather than just HTTP DELETE. Fixed various examples.

Restructured document. Clarified that CALDAV:schedule-calendar-transp only applies to calendar collection. Removed CALDAV:schedule-state property on scheduling messages in the scheduling Inbox collection. Added conditional requests on scheduling object resources. Added section on handling of PARTSTAT. Added SCHEDULE-FORCE-SEND iCalendar property parameter. Added clarification on child resources in scheduling Outbox collections. Clarified Attendee changes that server MUST allow, and removed restrictions on changes that Attendee MUST NOT do. Added Example Scheduling Transactions appendix. Scheduling privileges are no longer required to be non-abstract. Removed handling of REFRESH requests. Removed handling of VJOURNAL components. Completed IANA Considerations section. Added references to RFC3283 and RFC5234. Updated references to iCalendar, iTIP and iMIP.

Removed distinction between scheduling calendar collections and basic calendar collections - now just have calendar collections. Clients now "MAY" reload data rather than "SHOULD" reload data. Fixed <C:recipient> in examples. Removed CALDAV:attachments-allowed precondition on POST to Outbox as that is no longer relevant. Added CALDAV:default-calendar-delete-allowed precondition for DELETE. Relaxed MUST->MAY for Organizer setting PARTSTAT value. Tweaked restrictions on Create/Modify to emphasize that 4791 restrictions also apply. Added comment that 'opaque' is the default when the CALDAV:schedule-calendar-transp property is not present. Description of Schedule-Reply header changed to reflect that it is only relevant for Attendees. Minor typos fixed.

This draft has changed substantially since the -04 version. The primary reason for this change was implementation experience from a number of vendors who implemented products based on the earlier drafts. Experience showed that the client/server interaction was not reliable in keeping scheduling messages synchronized between organizer and attendees. In addition the latency in updates due to clients being offline proved unacceptable to users. These issues led to the redesign of this specification to support a server-based processing model that eliminates all the problems seen previously. Whilst this adds significant complexity to the server in that it needs to be a full blown iTIP processing agent, it does remove a lot of the same complexity from clients, opening up the possibility of supporting complex scheduling behaviors even with "thin" clients. In the judgement of the authors, we consider this new specification to be a substantial improvement over the old one and believe it represents a stronger protocol that will lead to better interoperability.