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Task Extensions to iCalendar
draft-ietf-calext-ical-tasks-09

Document Type Active Internet-Draft (calext WG)
Authors Adrian Apthorp , Michael Douglass
Last updated 2024-04-18
Replaces draft-apthorp-ical-tasks
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Formats
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Stream WG state In WG Last Call
Associated WG milestone
Apr 2024
Submit task draft to IESG
Document shepherd Kenneth Murchison
Shepherd write-up Show Last changed 2024-05-08
IESG IESG state I-D Exists
Consensus boilerplate Yes
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Send notices to brong@fastmailteam.com, murch@fastmailteam.com
draft-ietf-calext-ical-tasks-09
calext                                                        A. Apthorp
Internet-Draft                                               DHL Express
Updates: RFC5545 (if approved)                               M. Douglass
Intended status: Standards Track            Bedework Commercial Services
Expires: 20 October 2024                                   18 April 2024

                      Task Extensions to iCalendar
                    draft-ietf-calext-ical-tasks-09

Abstract

   This document updates and defines extensions to the Internet
   Calendaring and Scheduling Core Object Specification (iCalendar)
   (RFC5545) to provide improved status tracking, scheduling and
   specification of tasks.

   It also defines how Calendaring Extensions to WebDAV (CalDAV) (RFC
   4791) servers can be extended to support certain automated task
   management behaviours.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 20 October 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components

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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terms and Definitions . . . . . . . . . . . . . . . . . .   3
   2.  Task Architecture . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Task Architecture Elements  . . . . . . . . . . . . . . . . .   7
   4.  Architecture Foundations  . . . . . . . . . . . . . . . . . .   8
   5.  Task Extensions . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Task Specification  . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Task type . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Task Context and Relationships  . . . . . . . . . . . . .  10
     6.3.  Task Specific Data  . . . . . . . . . . . . . . . . . . .  10
   7.  Task Deadlines, Milestones and Time Planning  . . . . . . . .  10
     7.1.  Deadlines . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.2.  Milestones  . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Task Scheduling and Assignment  . . . . . . . . . . . . . . .  11
   9.  Status Reporting  . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Improved granularity in status reporting information  . .  11
     9.2.  Relating reason and comments to ATTENDEE status
           changes.  . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.3.  Comments associated to reasons and status changes . . . .  12
     9.4.  Task Alerts and Notifications . . . . . . . . . . . . . .  13
     9.5.  Automated Status Changes  . . . . . . . . . . . . . . . .  14
   10. Modifications to Calendar Components  . . . . . . . . . . . .  14
   11. New Parameter Values  . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Redefined VTODO Participant Status . . . . . . . . . . .  15
   12. New Properties  . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Estimated Duration . . . . . . . . . . . . . . . . . . .  16
     12.2.  Reason . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.3.  Sub-State  . . . . . . . . . . . . . . . . . . . . . . .  17
     12.4.  Task Mode  . . . . . . . . . . . . . . . . . . . . . . .  18
   13. Property Extensions and Clarifications  . . . . . . . . . . .  20
     13.1.  Updated DURATION Property definition . . . . . . . . . .  20
     13.2.  Redefined STATUS Property  . . . . . . . . . . . . . . .  21
   14. New Components  . . . . . . . . . . . . . . . . . . . . . . .  21
     14.1.  Status Component . . . . . . . . . . . . . . . . . . . .  22
   15. CalDAV Support for Task Mode  . . . . . . . . . . . . . . . .  23
     15.1.  CALDAV:supported-task-mode-set Property  . . . . . . . .  23
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     17.1.  New and updated iCalendar Elements Registration  . . . .  24
       17.1.1.  Initialization of the Status registry  . . . . . . .  24
       17.1.2.  Update of the Status registry  . . . . . . . . . . .  25
       17.1.3.  Sub-State value registry . . . . . . . . . . . . . .  25

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       17.1.4.  Task Mode value registry . . . . . . . . . . . . . .  26
       17.1.5.  Participation Statuses registry  . . . . . . . . . .  26
       17.1.6.  Components Registry  . . . . . . . . . . . . . . . .  27
       17.1.7.  Properties registry  . . . . . . . . . . . . . . . .  27
   18. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   19. Normative References  . . . . . . . . . . . . . . . . . . . .  27
   20. Informative References  . . . . . . . . . . . . . . . . . . .  28
   Appendix A.  Examples of Task State Lifecycle . . . . . . . . . .  29
     A.1.  Simple Case Status Change . . . . . . . . . . . . . . . .  29
     A.2.  Example for multiple Attendees  . . . . . . . . . . . . .  29
     A.3.  Example of Failure  . . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   This document specifies extensions to the existing Internet
   Calendaring and Scheduling Core Object Specification (iCalendar)
   [RFC5545], and associated protocols, in order to enhance the
   structured communication and execution of tasks.  The enhancements
   allow for the communication, time planning and scheduling of tasks by
   and between automated systems (e.g. in smart power grids, business
   process management systems) as well as for human centered tasks.

   A "task" is a representation of an item of work assigned to an
   individual or organization.  In the iCalendar Object Model [RFC5545]
   the representation of tasks is by "VTODO" calendar components.  Tasks
   can be identified in a number of situations, either informally as ad-
   hoc tasks in personal "to-do" lists or more formally in:

   *  Business processes - ranging from repetitive workflows to adaptive
      cases and trouble ticketing

   *  Project Management - whether for large scale construction projects
      or collaborative software development

   The extensions specified here are defined in the context of an
   overall architecture for task calendaring and scheduling.

1.1.  Terms and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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   Calendaring and scheduling roles are referred to in quoted-strings of
   text with the first character of each word in upper case.  For
   example, "Organizer" refers to a role of a "Calendar User" (CU)
   within the scheduling protocol.

   Calendar components defined by [RFC5545] are referred to with
   capitalized, quoted-strings of text.  All calendar components start
   with the letter "V".  For example, "VEVENT" refers to the event
   calendar component, "VTODO" refers to the to-do calendar component,
   and "VJOURNAL" refers to the daily journal calendar component.

   Scheduling methods are referred to with capitalized, quoted-strings
   of text.  For example, "REQUEST" refers to the method for requesting
   a scheduling calendar component be created or modified; "REPLY"
   refers to the method a recipient of a request uses to update their
   status with the "Organizer" of the calendar component.

   Properties defined by [RFC5545] are referred to with capitalized,
   quoted-strings of text, followed by the word "property".  For
   example, "ATTENDEE" property refers to the iCalendar property used to
   convey the calendar address of a "Calendar User".

   Property parameters defined by this specification are referred to
   with capitalized, quoted-strings of text, followed by the word
   "parameter".  For example, "VALUE" parameter refers to the iCalendar
   property parameter used to override the default data type for a
   property value.

   Enumerated values defined by this specification are referred to with
   capitalized text, either alone or followed by the word "value".

   In tables, the quoted-string text is specified without quotes in
   order to minimize the table length.

   Terms defined and used in this specification include:

   Assignee  A calendar user assigned to perform a given task.  An
      assignee is equivalent to an attendee of an event.

   BPMS  Business Process Management Software

   Calendar User (CU)  A person or software system that accesses or
      modifies calendar information.

   Calendar User Agent (CUA)  This may be

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      1.  Software with which the calendar user communicates with a
          calendar service or local calendar store to access calendar
          information.

      2.  Software that gathers calendar data on the Calendar User's
          behalf.

   Candidate  A calendar user who might be able to perform a given task,
      prior to actually being assigned the task, e.g., a dispatcher has
      a list of taxi drivers (candidates) from which one will be
      selected to pick-up a passenger.

   Organizer  A calendar user who creates a calendar item, requests
      free/busy information, or published free/busy information.  It is
      an Organizer who invites Attendees [RFC5545].

   Observer  A calendar user interested in a calendar component, e.g., a
      manager may have interest in all tasks that have not been
      completed.  Often represented as an attendee with ROLE=NON-
      PARTICIPANT.

   Resource  A resource in the scheduling context is any shared entity
      that can be scheduled by a calendar user, but does not control its
      own attendance status.  Resources can be of "Location",
      "Equipment", or "Role" type.

   Task  A representation of an item of work that can be assigned to one
      or more task actor assignees.  In [RFC5545], these are "VTODO"
      calendar components, which are groupings of component properties
      and possibly "VALARM" calendar components that represent an
      action-item or assignment.

2.  Task Architecture

   A reference architecture for task calendaring and scheduling is
   defined in order to identify the key logical elements involved in
   task management and the interfaces between them to enable
   interoperability.  The logical elements identified here establish an
   appropriate separation of concerns and clarify the responsibilities
   of different elements.  However, the architecture does not prescribe
   a binding or packaging of elements, i.e., software systems may be
   developed where some elements are tightly bound and the interfaces
   between bound elements are not exposed.  The task architecture is
   also described in [TARCH].

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        Task        +-------+
        Trigger             |
      +---------------------V---------------------+    +-----------+
      |           Task Generating System          |    |           |
      |        +-------------------------+        |    |           |
      |        |            O            |        |    |           |
      |        |           /|\           |        |    |           |
      |        |           / \           |        |    |           |
      |        |      Task Organizer     |        <---->           |
      |        +-^--------^--------------+        |    |           |
      |          |        |                       |    |           |
      | +--------V-+ +----V-----+    +----------+ |    |           |
      | |   Task   | | Process  |    |   Task   | |    |           |
      | |Assignment| |  Logic   <---->  Domain  | |    |           |
      | |  Rules   | |          |    |   Data   | |    |           |
      | +----------+ +----------+    +----------+ |    |           |
      |                                           |    |           |
      +------^----------+-----^-------------------+    |           |
             |          |     |                        |           |
        Availability  Task   Task                      |           |
             |          |   Status                     |           |
             |          |     |                        |           |
      +------v----------v-----+-------------------+    |           |
      |      Calendar and Scheduling System       |    | Directory |
      | +---------+  +---------+                  |    |  Service  |
      | |         |  |  Task   |                  <---->           |
      | |Schedule |  |  Lists  |                  |    |           |
      | |         |  |         |                  |    |           |
      | +---------+  +---------+      Server      |    |           |
      +-------------------------------------------+    |           |
      |                               Client      |    |           |
      | +----------------------+    +-----------+ |    |           |
      | |       Calendar       |    |   Task    | |    |           |
      | |      User Agent      +----> Specific  | <---->           |
      | |                      |    |Application| |    |           |
      | +----------------------+    +-----------+ |    |           |
      |                                           |    |           |
      +-----^---------^--------+---------+--------+    |           |
            |         |        |         |             |           |
      +-----V---------V--------V---------V--------+    |           |
      |                Task Actors                |    |           |
      |     O         O        O         O        |    |           |
      |    /|\       /|\      /|\       /|\       +---->           |
      |    / \       / \      / \       / \       |    |           |
      |          Candidate(s)        Observer(s)  |    |           |
      | Assignee(s)       Resource(s)             |    |           |
      +-------------------------------------------+    +-----------+

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                    Figure 1: Task architecture diagram

3.  Task Architecture Elements

   The following logical elements form the task architecture that this
   specification is based on:

   Task Actors  Various calendar users that may be involved in the
      monitoring or performing of a task.  The set of actors includes:
      Organizers, Observers, Resources, Assignees, and Candidates.

   Task Organizer  The Organizer of a task.

   Task Domain Data  This is any domain specific data that may be acted
      on or provides context to it in performing a task.

   Task Specific Application  A task specific application renders the
      data concerning the task (including task domain data) for
      presentation and manipulation by a task actor.

   Process Logic  Determines under what conditions a task (or tasks) is
      generated and the actions to take on completion, or some other
      status event occurring (or not) on the task.

   Task Trigger  This is some event that gives rise to the generation of
      a task according to Process Logic.  Task triggers can come from
      many sources including, for example; a task being requested
      through the calendaring system, a status change in the progression
      of a business process being managed by a business process
      management or Enterprise resource planning (ERP) system.

   Task Assignment Rules  Govern how actors are assigned to a task.  A
      range of different assignment patterns [WfRP] may be considered,
      including the two general cases:

      a.  Delegation to a named actor or group of actors

      b.  Advertising to a pool of actors for self-selection

      In either case the assignment may be made based on a variety of
      criteria including, name, availability, skills, capacity, etc.

   Task Generating System  A system that creates and assigns tasks in
      response to some initiating event (task trigger).  Task creation
      is according to Process Logic with task assignment determined by
      Task Assignment Rules.  This system also tracks the status of
      tasks and will initiate further actions based upon the status.  A
      task generating system can take many forms, for example; Business

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      Process Management System, Project Management System, Bug Tracking
      System, Building Control System.  A Task Generating System may
      also be a human.  In iCalendar terms the Task Generating System is
      the organizer.

   Human Task Generation  Task creation, assignment and tracking
      coordinated by a human organizer is a special case of a task
      generating system.  In this case Task Assignment Rules and Process
      Logic may be either explicit or tacit.

   Directory Service  A software system that stores and provides access
      to information providing details of task actors that may
      participate or be interested in a task.

   Calendar and Scheduling System  A software system that stores,
      publishes and synchronizes calendar data such as events, tasks and
      journal entries for actors.  In the context of tasks this includes
      schedules (i.e. allocated time and availability to perform tasks)
      and task lists.  A calendar and scheduling system typically
      consists of server and client software components.

   It is not within the scope of this document to specify how Process
   Logic or Task Assignment Rules are codified.  Such logic and rules
   may be codified in a variety of ways, including traditional
   programming languages (e.g.  C++, Java) or process modelling
   languages (e.g.  BPMN [BPMN]).

4.  Architecture Foundations

   The key standards that enable interoperability between the logical
   elements of the architecture are the Internet Calendaring and
   Scheduling Core Object Specification (iCalendar) [RFC5545] and
   associated protocols.  Task and task status are represented by the
   iCalendar "VTODO" component.  Protocols include, in particular, the
   iCalendar Transport-Independent Interoperability Protocol (iTIP)
   [RFC5546] for task assignment and scheduling, and Calendaring
   Extensions to WebDAV (CalDAV) [RFC4791] for client server
   communication.

   Additionally, this specification uses definitions from Support for
   iCalendar Relationships [RFC9253].  The LINK, REFID, RELATED-TO and
   CONCEPT properties enable context and a rich set of relationships
   between tasks and other iCalendar components to be specified.

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5.  Task Extensions

   In order to support the task architecture described in Section 2,
   this document defines a number of extensions to the current iCalendar
   standards in the areas of:

   Task Specification  improved ability to specify domain specific tasks

   Task Deadlines, Milestones and Time Planning  clarification of
      deadlines and extension for task duration to support task time
      planning

   Task Scheduling and Assignment  ensure support for common pattens of
      scheduling and assigning tasks

   Task Status Tracking  improved granularity in status tracking
      information and alerting task actors to pending or actual task
      status changes

   These extensions are supported mainly by additions to the properties
   and parameters used within the "VTODO" component.

6.  Task Specification

   The specification of tasks must be semantically explicit in order for
   them to be managed within the context of a business process or
   project, and be understood by both humans and IT systems.  The
   current VTODO component only provides for simple ad-hoc tasks or 'to
   do' lists, and is therefore extended by this specification as
   follows:

   Task type  explicitly what type of task is to be performed is
      identified.

   Task context and relationships  how a specific task relates to other
      tasks and other objects that need to be understood for the
      effective execution of a task.

   Task specific data  the form and content of domain data provided as
      input to a task and/or that may be output from a task.

   Organizer and attendee  recognizes that a task organizer or attendee
      can be an automated system.

6.1.  Task type

   The [RFC9253] CONCEPT property is used to identify the type of task,
   for example;

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   CONCEPT:http://example.com/task/delivery

6.2.  Task Context and Relationships

   The [RFC9253] LINK property specifies a link to external information,
   which may be context to the task.  For example:

   LINK;LINKREL=SOURCE:http://example.com/package/1234567890

   LINK;LINKREL=describedby:mid:752142.141482.307E5@mx123.example.com

   The external information may be data to be manipulated in performing
   the task.  See Section 6.3.

   The [RFC9253] REFID property is used to identify a key used to group
   tasks by that key.

   REFID:Manhattan

   REFID:1234567890

   Extensions to the RELATED-TO property defined in [RFC9253] allow
   temporal relationships between tasks as found in project management
   to be specified as well as parent/child relationships and
   dependencies (DEPENDS-ON).  Tasks (VTODOs) may also be related to
   other calendar components; for example to a VEVENT to block time to
   perform a task.

6.3.  Task Specific Data

   The LINK property can be used to relate a domain specific service to
   the task.  For example, it might be a URI pointing to a web page
   where the status of the task can be directly manipulated.

   LINK;LINKREL="vacation-system";VALUE=URI:
    http://example.com/vacation-approval?id=1234

   Additionally, it might be used to link data specific to the task, for
   example an electronic copy of a signature taken to confirm delivery
   of a package.

   LINK;LINKREL="electronic-signature";VALUE=URI:
    http://example.com/delivery/sig1234.jpg

7.  Task Deadlines, Milestones and Time Planning

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7.1.  Deadlines

   Deadlines for starting and finishing a task are defined by the
   DTSTART, DUE and DURATION properties.  DTSTART represents the
   earliest start time for beginning work on a task.  DUE, or DTSTART +
   DURATION represent the latest finish time for a task.  Thus, these
   properties define a "window" within which a task has to be performed.
   However, there is currently no way to indicate how long the task is
   expected to take.  This document defines a new property, in
   Section 12.1 ESTIMATED-DURATION, to allow the estimated time that a
   task should take to be specified separately from the deadlines for
   starting and finishing a task.  This supports time planning by
   enabling calendar user agents to display when tasks should occur and
   therefore allow calendar users to visualize when tasks should be
   performed and allocate time to them.

7.2.  Milestones

   A task that has intermediary deadlines (i.e., milestones) SHOULD be
   expressed by child VTODO components (i.e., sub-tasks associated with
   each of the milestones) in conjunction with the RELATED-TO property
   to relate the parent and child tasks.

8.  Task Scheduling and Assignment

   Tasks are assigned to actors using one or more [RFC5545] ATTENDEE
   properties and/or one or more [RFC9073] PARTICIPANT components.

   Communication of task assignment or delegation to one or more actors
   who are allocated to a task by the organizer is directly supported by
   iTIP, i.e., all included ATTENDEES in an iTIP REQUEST are expected to
   perform the task.

   The offering or advertising of a task to one or more (potential)
   actors where only one or a subset of the candidates may accept the
   task will be addressed by a later specification.

9.  Status Reporting

9.1.  Improved granularity in status reporting information

   This document defines a new "VSTATUS" component (see section
   Section 14.1) that can be used to group related information about the
   status of the task.  This might include information on why (REASON)
   and when (DTSTAMP) a status has changed.  In addition, new status
   values are specified to provide for task suspension, failure and
   preparation.

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   Note that while VSTATUS is intended to allow multiple date-stamped
   status changes to be stored it is not intended to be used as a
   history of changes to a tasks properties.

9.2.  Relating reason and comments to ATTENDEE status changes.

   The [RFC9073] PARTICIPANT component can be used to provide additional
   information about why an ATTENDEE participation status has changed.
   The COMMENT property can also be used to include additional human-
   readable information about why the associated STATUS or ATTENDEE
   property changed.  For example, if a driver failed to deliver a
   package because of a puncture it might be expressed as

   BEGIN:VSTATUS
   STATUS:FAILED
   REASON:https://example.com/reason/delivery-failed
   SUBSTATE:ERROR
   DTSTAMP:20130212T120000Z
   COMMENT:Breakdown
   END:VSTATUS

   ATTENDEE;PARTSTAT=FAILED:mailto:xxx@example.com
   ...
   BEGIN:PARTICIPANT
   CALENDAR-ADDRESS:mailto:xxx@example.com
   DTSTAMP:20130226T1104510Z
   REASON:https://example.com/reason/van-break-down
   COMMENT:Puncture
   END:PARTICIPANT

9.3.  Comments associated to reasons and status changes

   Multiple comments and reasons may have the same status.  As
   situations change further VSTATUS components can be added to provide
   additional information..

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   CONCEPT:https://example.com/task/delivery
   BEGIN:VSTATUS
   STATUS:FAILED
   SUBSTATE:ERROR
   DTSTAMP:20220212T104900Z
   COMMENT:Out of time
   END:VSTATUS
   BEGIN:VSTATUS
   STATUS:FAILED
   COMMENT:Traffic Accident on E44
   REASON:https://example.com/reason/traffic
   DTSTAMP:20220212T110451Z
   END:VSTATUS
   BEGIN:VSTATUS
   STATUS:FAILED
   COMMENT:Arrived after office hours
   REASON:https://example.com/reason/closed
   DTSTAMP:20220212T180451Z
   END:VSTATUS

9.4.  Task Alerts and Notifications

   Different needs to alert or notify task actors of pending or actual
   task status changes are recognized:

   Alarms  Alarms (VALARM components) operate in the calendar user agent
      space to notify the task actor of a pending task state for a task
      they are assigned to or are interested in.

   Current standards (see [RFC9074]) indicate VALARMs SHOULD be removed
   from incoming data and many systems in fact do so.  In a task
   assignment scenario it may be appropriate for the organizer to be
   able to set alarms for the participants.  A system implementing these
   standards may choose to preserve VALARMs but sending a task via some
   external service may result in them being removed.  This issue is not
   addressed by this specification.

   Escalations  An escalation or notification to the ATTENDEE,
      ORGANIZER, or other task actor may be required if a deadline
      associated with a task is exceeded or for some other reason.
      Process Logic identifying when and who to propagate escalations to
      is the responsibility of the Task Generating System, e.g., a BPMS.

   Notifications  Task actors (observers) not directly involved in

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      performing a task but with a known interest in a given task's
      status can be identified by the PARTICIPANT component [RFC9073]
      against certain components e.g.  ALARM, to identify which task
      events the stakeholder/party is interested in.  Notifications on
      shared calendars will allow task actors to register an interest in
      changes to tasks within a calendar (see Appendix A).

9.5.  Automated Status Changes

   A new property, TASK-MODE, is introduced to instruct servers to apply
   automated operations for changing the status of a task.

10.  Modifications to Calendar Components

   The following changes to the syntax defined in iCalendar [RFC5545]
   are made here.  New elements are defined in subsequent sections.

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   ; Addition of VSTATUS as a valid component for VEVENT
   eventc     = "BEGIN" ":" "VEVENT" CRLF
                eventprop *alarmc *participantc *locationc *resourcec *statusc
                "END" ":" "VEVENT" CRLF

   ; Addition of VSTATUS as a valid component for VTODO
   todoc      = "BEGIN" ":" "VTODO" CRLF
                todoprop *alarmc *participantc *locationc *resourcec *statusc
                "END" ":" "VTODO" CRLF

   ; Addition of properties ESTIMATED-DURATION and TASK-MODE to VTODO
   todoprop =/ est-duration /
               task-mode

   ; Addition of VSTATUS as a valid component for VJOURNAL
   journalc   = "BEGIN" ":" "VJOURNAL" CRLF
                 jourprop *statusc
                 "END" ":" "VJOURNAL" CRLF

   ; Addition of VSTATUS as a valid component for VFREEBUSY
   freebusyc  = "BEGIN" ":" "VFREEBUSY" CRLF
                fbprop *participantc *locationc *resourcec *statusc
                "END" ":" "VFREEBUSY" CRLF

   ; Addition of VSTATUS as a valid component for PARTICIPANT
   participantc = "BEGIN" ":" "PARTICIPANT" CRLF
                  partprop *locationc *resourcec
                  *statusc
                  "END" ":" "PARTICIPANT" CRLF

   ; Addition of propertY REASON to PARTICIPANT
   partprop =/ reason

11.  New Parameter Values

11.1.  Redefined VTODO Participant Status

   Participant status parameter type values are defined in
   Section 3.2.12 of [RFC5545].  This specification redefines that type
   to include the new value FAILED for VTODO iCalendar components.

   Format Definition  This property parameter is extended by the
      following notation:

   partstat-todo    =/ *("FAILED")  ; To-do cannot be completed

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   Example

   ATTENDEE;REASON="https://example.com/reason/not-enough-time";
    PARTSTAT=FAILED:mailto:jsmith@example.com

12.  New Properties

12.1.  Estimated Duration

   Property Name  ESTIMATED-DURATION

   Purpose  This property specifies the estimated positive duration of
      time the corresponding task will take to complete.

   Value Type  DURATION

   Property Parameters  IANA and non-standard property parameters can be
      specified on this property.

   Conformance  This property can be specified in "VTODO" calendar
      components.

   Format Definition  This property is defined by the following
      notation:

   est-duration  = "ESTIMATED-DURATION" durparam ":" dur-value CRLF
                   ;consisting of a positive duration of time.

   durparam      = *(";" other-param)

   Description  In a "VTODO" calendar component the property MAY be used
      to specify the estimated duration for the to-do, with or without
      an explicit time window in which the event should be started and
      completed.  When present, DTSTART and DUE/DURATION represent the
      window in which the task can be performed.  ESTIMATED-DURATION
      SHOULD be passed from ORGANIZER to ATTENDEE in iTIP [RFC5546]
      messages.

   Example  The following is an example of this property that estimates
      the duration of a task to be one hour:

   ESTIMATED-DURATION:PT1H

12.2.  Reason

   Property name  REASON

   Purpose  To indicate the reason for a status change or change of

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      attendee participation status.

   Value Type  URI

   Property Parameters  IANA and non-standard property parameters can be
      specified on this property.

   Conformance  This property can be specified in "VSTATUS" and
      PARTICIPANT calendar components.

   Format Definition  This property is defined by the following
      notation:

   reason      = "REASON" reasonparam ":" uri CRLF

   reasonparam = *(";" other-param)

   Description  This property allows the change in status of a task or
      participant status to be qualified by the reason for the change
      with a codified reason.  Typically, reasons are defined within the
      context of the task type and therefore SHOULD include the name-
      space of the authority defining the task.

   Example

   REASON:https://example.com/reason/delivered-on-time

12.3.  Sub-State

   Property name  SUBSTATE

   Purpose  To provide additional granularity of task status for e.g.
      IN-PROCESS.

   Value Type  TEXT

   Property Parameters  IANA and non-standard property parameters can be
      specified on this property.

   Conformance  This property can be specified in a "VSTATUS" calendar
      component.

   Format Definition  This property is defined by the following
      notation:

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   substate          = "SUBSTATE" substateparam ":" substatevalue CRLF

   substateparam     = *(";" other-param)

   substatevalue       = ("OK"        ; everything is fine(the default)
                        / "ERROR"     ; something is wrong (the REASON
                        ;               code explains why)
                        / "SUSPENDED" ; waiting on some other task to
                        ;               complete or availability of a
                        ;               resource (REASON code explains
                        ;               why)
                        / iana-token) ; Other IANA-registered type

   Description  The sub-state property allows additional qualification
      and granularity of states to be recorded, in particular for the
      IN-PROCESS state.  It allows individual sub-states to be recorded
      without the need to define and publish a sub-task associated with
      a parent task purely to track that a particular state has been
      reached.  This property also allows parallel states to be
      expressed e.g. that a task has been suspended at whatever state it
      has reached.

   Example

   BEGIN:VSTATUS
   STATUS:FAILED
   REASON:https://example.com/reason/no-one-home
   SUBSTATE:ERROR
   END:VSTATUS

   BEGIN:VSTATUS
   STATUS:IN-PROCESS
   REASON:https://example.com/reason/paint-drying
   SUBSTATE:SUSPENDED
   END:VSTATUS

12.4.  Task Mode

   Property Name  TASK-MODE

   Purpose  This property specifies automatic operations that servers
      acting on behalf of the organizer apply to tasks based on changes
      in attendee status (PARTSTAT).

   Value Type  TEXT

   Property Parameters  IANA and non-standard property parameters can be
      specified on this property.

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   Conformance  This property can be specified zero or more times in a
      "VTODO" calendar component.

   Format Definition  This property is defined by the following
      notation:

   task-mode   = "TASK-MODE taskmodeparam ":" taskvalue
                 *("," taskvalue) CRLF

   taskvalue   = "AUTOMATIC-COMPLETION" ; set STATUS completed
                   ;if all attendees have completed
                   / "AUTOMATIC-FAILURE"
                   / "SERVER"
                   / "CLIENT"
                   / iana-token
                   / x-name

   taskmodeparam      = *(";" other-param)

   Description  In a "VTODO" calendar component this property MAY be
      used to indicate to servers how they can automatically change the
      state of the task based on iTIP replies from Attendees.  For
      example, the server can automatically set the overall task status
      to COMPLETED when every attendee has marked their own status
      (PARTSTAT) as COMPLETED, or the server could mark the task as
      FAILED if its DUE date passes without it being completed.  TASK-
      MODE processing is performed on the organizer's copy of the task.

   To set the status, add a VSTATUS component as specified in
   Section 14.1.

   The property value is a list of one or more IANA registered tokens
   that defines modes to be used for the task.  This specification
   defines three modes which are described in the following subsections.

   Examples

   TASK-MODE:AUTOMATIC-COMPLETION,AUTOMATIC-FAILURE
   TASK-MODE:SERVER
   TASK-MODE:AUTOMATIC-FAILURE

   AUTOMATIC-COMPLETION Task Mode  The task mode value "AUTOMATIC-
      COMPLETION" indicates to the server that it can change the "VTODO"
      component's status to "COMPLETED" as soon as all ATTENDEEs in the
      task have replied with a "PARTSTAT" parameter set to "COMPLETED".

   AUTOMATIC-FAILURE Task Mode  The task mode value "AUTOMATIC-FAILURE"

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      indicates to the server that it SHOULD change the "VTODO"
      component's status to "FAILED" if either:

      1.  the PARTSTAT of one ATTENDEE is set to FAILED; or

      2.  the current time is past the effective due date of the
          component and the task has not yet been completed.

      |  NOTE: The effective due date is either the "DUE" property value
      |  or the combination of the "DTSTART" and "DURATION" property
      |  values.

   CLIENT Task Mode  The task mode value "CLIENT" is an instruction to
      the server to honour the status set by the client.

   SERVER Task Mode  The task mode value "SERVER" indicates to the
      server that it can change the "VTODO" component's status to an
      appropriate value, based on implementation defined "business
      rules", as ATTENDEE responses are processed or as deadlines
      related to the task pass.

      The server can add this property to a "VTODO" component to
      indicate to the client that it will be managing the status.

13.  Property Extensions and Clarifications

13.1.  Updated DURATION Property definition

   [RFC5545] section 3.6.2 introduced a constraint on the duration
   property requiring that a DURATION MUST be accompanied by a DTSTART.
   This constraint is dropped reverting to the situation as specified
   previously.

   Thus the text:

                     ; Either 'due' or 'duration' MAY appear in
                     ; a 'todoprop', but 'due' and 'duration'
                     ; MUST NOT occur in the same 'todoprop'.
                     ; If 'duration' appear in a 'todoprop',
                     ; then 'dtstart' MUST also appear in
                     ; the same 'todoprop'.

   is replaced by

                     ; Either 'due' or 'duration' MAY appear in
                     ; a 'todoprop', but 'due' and 'duration'
                     ; MUST NOT occur in the same 'todoprop'.

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   This allows a VTODO to only have a DURATION property.

   Furthermore, the following text:

        A "VTODO" calendar component without the "DTSTART" and "DUE" (or
        "DURATION") properties specifies a to-do that will be associated
        with each successive calendar date, until it is completed.

   is replaced by

        A "VTODO" calendar component without the "DTSTART" and "DUE"
        properties specifies a to-do that will be associated
        with each successive calendar date, until it is completed.

13.2.  Redefined STATUS Property

   The Status property is defined in Section 3.8.1.11 of [RFC5545].
   This specification extends that property to include new values
   associated with VTODO iCalendar components (See Appendix A for
   examples of the task state lifecycle).

   Format Definition  The "STATUS" property parameter list is augmented
      as follows:

   statvalue-todo = / "PENDING"    ;Indicates a to-do has been
                                   ;created and accepted, but has
                                   ; not yet started.
                   / "FAILED"      ;Indicates to-do has failed.
   ;Extended status values for "VTODO".

   Description:

   PENDING - A to-do has been created and accepted but has not yet
   started and is ready to start subject to other dependencies (e.g.
   preceding task or DTSTART).  This is the default state.

   FAILED - to-do has failed and may need some follow-up from the
   organizer to re-schedule or cancel

   Example: The following is an example of this property for a "VTODO"
   calendar component:

   STATUS:FAILED

14.  New Components

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14.1.  Status Component

   Component Name  VSTATUS

   Purpose  This component allows information to be associated with a
      status, for example comments and date stamps.

   Conformance  This component can be specified multiple times in any
      calendar component.

   Description  This component provides a way for multiple date-stamped
      statuses to be associated with a component such as a participant,
      task or event.

   This component may be added to the [RFC9073] PARTICIPANT component to
   allow participants in a task to specify their own status.

   For backwards compatibility, when a VSTATUS component is added the
   [RFC5545] STATUS property MUST be set on the parent component.

   Format Definition  This component is defined by the following
      notation:

   statusc = "BEGIN" ":" "VSTATUS" CRLF
             statusprop
             "END" ":" "VSTATUS" CRLF

   statusprop     = *(
                  ;
                  ; The following is REQUIRED,
                  ; but MUST NOT occur more than once.
                  ;
                  status /
                  ;
                  ; The following are OPTIONAL,
                  ; but MUST NOT occur more than once.
                  ;
                  description / dtstamp / reason / substate / summary
                  ;
                  ; The following are OPTIONAL,
                  ; and MAY occur more than once.
                  ;
                  comment / styleddescription / iana-prop
                  ;
                  )

   Examples

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   BEGIN:VSTATUS
   STATUS:COMPLETED
   REASON: https://example.com/reason/delivered-on-time
   DTSTAMP:20220212T120000Z
   END:VSTATUS

15.  CalDAV Support for Task Mode

   The CalDAV [RFC4791] calendar access protocol allows clients and
   servers to exchange iCalendar data.  With the introduction of the
   "TASK-MODE" property in this specification, different automated task
   management behaviours may be delegated to the server by the Task
   Organizer depending upon the value of "TASK-MODE".

   In order for a CalDAV client to know what task modes are available, a
   CalDAV server advertises a CALDAV:supported-task-mode-set WebDAV
   property on calendar home or calendar collections if it supports the
   use of the "TASK-MODE" property as described in this specification.
   The server can advertise a specific set of supported task modes by
   including one or more CALDAV:supported-task-mode XML elements within
   the CALDAV:supported-task-mode-set XML element.  If no
   CALDAV:supported-task-mode XML elements are included in the WebDAV
   property, then clients can try any task mode, but need to be prepared
   for a failure when attempting to store the calendar data.

   Clients MUST NOT attempt to store iCalendar data containing "TASK-
   MODE" elements if the CALDAV:supported-task-mode-set WebDAV property
   is not advertised by the server.

   The server SHOULD return an HTTP 403 response with a DAV:error
   element containing a CALDAV:supported-task-mode XML element, if a
   client attempts to store iCalendar data with an "TASK-MODE" element
   value not supported by the server.

   It is possible for a "TASK-MODE" value to be present in calendar data
   on the server being accessed by a client that does not support the
   "TASK-MODE" property.  It is expected that existing clients, unaware
   of "TASK-MODE", will fail gracefully by ignoring the calendar
   property.

15.1.  CALDAV:supported-task-mode-set Property

   Name  supported-task-mode-set

   Namespace  urn:ietf:params:xml:ns:caldav

   Purpose  Enumerates the set of supported iCalendar "TASK-MODE"
      element values supported by the server.

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   Protected  This property MUST be protected and SHOULD NOT be returned
      by a PROPFIND allprop request (as defined in Section 14.2 of
      [RFC4918]).

   Description  See above.

   Definition

   <!ELEMENT supported-task-mode-set(supported-task-mode*)>
   <!ELEMENT supported-task-mode (#PCDATA)>
   <!-- PCDATA value: string - case insensitive but
   uppercase preferred -->

   Example

   <C:supported-task-mode-set xmlns:C="urn:ietf:params:xml:ns:caldav">
     <C:supported-task-mode>AUTOMATIC-COMPLETION</C:supported-task-mode>
     <C:supported-task-mode>AUTOMATIC-FAILURE</C:supported-task-mode>
     <C:supported-task-mode>SERVER</C:supported-task-mode>
     <C:supported-task-mode>CLIENT</C:supported-task-mode>
   </C:supported-task-mode-set>

16.  Security Considerations

   This specification introduces no new security considerations beyond
   those identified in [RFC5545], [RFC5546] and [RFC4791].

17.  IANA Considerations

17.1.  New and updated iCalendar Elements Registration

   This specification updates [RFC5545] by adding and updating a number
   of elements according to the procedures and templates specified in
   Section 8.2 of [RFC5545].

17.1.1.  Initialization of the Status registry

   This specification updates [RFC5545] by adding a Status value
   registry to the iCalendar Elements registry located here:
   https://www.iana.org/assignments/icalendar> and initializing it as
   per [RFC5545].

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        +==============+=========+===============================+
        | Name         | Status  | Reference                     |
        +==============+=========+===============================+
        | CANCELLED    | Current | Section 3.8.1.11 of [RFC5545] |
        +--------------+---------+-------------------------------+
        | COMPLETED    | Current | Section 3.8.1.11 of [RFC5545] |
        +--------------+---------+-------------------------------+
        | CONFIRMED    | Current | Section 3.8.1.11 of [RFC5545] |
        +--------------+---------+-------------------------------+
        | DRAFT        | Current | Section 3.8.1.11 of [RFC5545] |
        +--------------+---------+-------------------------------+
        | FINAL        | Current | Section 3.8.1.11 of [RFC5545] |
        +--------------+---------+-------------------------------+
        | IN-PROCESS   | Current | Section 3.8.1.11 of [RFC5545] |
        +--------------+---------+-------------------------------+
        | NEEDS-ACTION | Current | Section 3.8.1.11 of [RFC5545] |
        +--------------+---------+-------------------------------+
        | TENTATIVE    | Current | Section 3.8.1.11 of [RFC5545] |
        +--------------+---------+-------------------------------+

                  Table 1: Initial Status Value Registry

17.1.2.  Update of the Status registry

   This specification further updates the Status registry with
   additional values defined in this document.

              +=========+=========+=========================+
              | Value   | Status  | Reference               |
              +=========+=========+=========================+
              | PENDING | Current | This Spec, Section 13.2 |
              +---------+---------+-------------------------+
              | FAILED  | Current | This Spec, Section 13.2 |
              +---------+---------+-------------------------+

                   Table 2: Updated Status Value Registry

17.1.3.  Sub-State value registry

   The following table has been used to initialize the Sub-State
   registry.

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             +===========+=========+=========================+
             | Substate  | Status  | Reference               |
             +===========+=========+=========================+
             | OK        | Current | This Spec, Section 12.3 |
             +-----------+---------+-------------------------+
             | ERROR     | Current | This Spec, Section 12.3 |
             +-----------+---------+-------------------------+
             | SUSPENDED | Current | This Spec, Section 12.3 |
             +-----------+---------+-------------------------+

                        Table 3: Sub-State registry

17.1.4.  Task Mode value registry

   The following table has been used to initialize the Task Mode
   registry.

       +======================+=========+=========================+
       | Task Mode            | Status  | Reference               |
       +======================+=========+=========================+
       | AUTOMATIC-COMPLETION | Current | This Spec, Section 12.4 |
       +----------------------+---------+-------------------------+
       | AUTOMATIC-FAILURE    | Current | This Spec, Section 12.4 |
       +----------------------+---------+-------------------------+
       | CLIENT               | Current | This Spec, Section 12.4 |
       +----------------------+---------+-------------------------+
       | SERVER               | Current | This Spec, Section 12.4 |
       +----------------------+---------+-------------------------+

                    Table 4: Task Mode Value Registry

17.1.5.  Participation Statuses registry

   The following table has been used to update the Participation
   Statuses registry defined in Section 8.3.7 of [RFC5545] and located
   here: https://www.iana.org/assignments/icalendar>

              +========+=========+=========================+
              | Value  | Status  | Reference               |
              +========+=========+=========================+
              | FAILED | Current | This Spec, Section 11.1 |
              +--------+---------+-------------------------+

                 Table 5: Participation Statuses Registry

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17.1.6.  Components Registry

   The following table has been used to update the Components registry
   defined in Section 8.3.1 of [RFC5545] and located here:
   https://www.iana.org/assignments/icalendar>.

             +===========+=========+=========================+
             | Component | Status  | Reference               |
             +===========+=========+=========================+
             | VSTATUS   | Current | This Spec, Section 14.1 |
             +-----------+---------+-------------------------+

                    Table 6: Updated Components Registry

17.1.7.  Properties registry

   The following table has been used to update the Properties registry
   defined in Section 8.3.2 of [RFC5545] and located here:
   https://www.iana.org/assignments/icalendar>.

        +====================+=========+=========================+
        | Property           | Status  | Reference               |
        +====================+=========+=========================+
        | ESTIMATED_DURATION | Current | This Spec, Section 12.1 |
        +--------------------+---------+-------------------------+
        | REASON             | Current | This Spec, Section 12.2 |
        +--------------------+---------+-------------------------+
        | SUBSTATE           | Current | This Spec, Section 12.3 |
        +--------------------+---------+-------------------------+
        | STATUS             | Current | This Spec, Section 13.2 |
        +--------------------+---------+-------------------------+
        | TASK-MODE          | Current | This Spec, Section 12.4 |
        +--------------------+---------+-------------------------+

                   Table 7: Updated Properties Registry

18.  Acknowledgements

   The authors would like to thank the members of the Calendaring and
   Scheduling Consortium technical committees and the following
   individuals for contributing their ideas, support and comments:

   John Chaffee, Marten Gajda, Ken Murchison

   The authors would also like to thank CalConnect, the Calendaring and
   Scheduling Consortium, for advice with this specification.

19.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", IETF, DOI 10.17487/RFC2119, BCP 14,
              RFC 2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", IETF,
              DOI 10.17487/RFC4791, RFC 4791, March 2007,
              <https://www.rfc-editor.org/info/rfc4791>.

   [RFC4918]  Dusseault, L., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", IETF,
              DOI 10.17487/RFC4918, RFC 4918, June 2007,
              <https://www.rfc-editor.org/info/rfc4918>.

   [RFC5545]  Desruisseaux, B., "Internet Calendaring and Scheduling
              Core Object Specification (iCalendar)", IETF,
              DOI 10.17487/RFC5545, RFC 5545, September 2009,
              <https://www.rfc-editor.org/info/rfc5545>.

   [RFC5546]  Daboo, C., "iCalendar Transport-Independent
              Interoperability Protocol (iTIP)", IETF,
              DOI 10.17487/RFC5546, RFC 5546, December 2009,
              <https://www.rfc-editor.org/info/rfc5546>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", IETF, DOI 10.17487/RFC8174, BCP 14,
              RFC 8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9073]  Douglass, M., "Event Publishing Extensions to iCalendar",
              IETF, DOI 10.17487/RFC9073, RFC 9073, August 2021,
              <https://www.rfc-editor.org/info/rfc9073>.

   [RFC9074]  Daboo, C. and K. Murchison, ""VALARM" Extensions for
              iCalendar", IETF, DOI 10.17487/RFC9074, RFC 9074, August
              2021, <https://www.rfc-editor.org/info/rfc9074>.

   [RFC9253]  Douglass, M., "Support for iCalendar Relationships", IETF,
              DOI 10.17487/RFC9253, RFC 9253, August 2022,
              <https://www.rfc-editor.org/info/rfc9253>.

20.  Informative References

   [BPMN]     "Business Process Model and Notation", OMG BPMN 2.0.2,
              January 2014,
              <https://www.omg.org/spec/BPMN/2.0.2/About-BPMN>.

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   [TARCH]    Apthorp, A., Daboo, C., and M. Douglass, "CalConnect, Task
              Architecture V1.0", (CalConnect Task Architecture V1.0,
              <https://www.calconnect.org/architectures/
              Task%20Architecture%201.0.pdf>.

   [WfRP]     Russell, N., ter Hofstede, A.H.M., Edmond, T., and W.M.P.
              van der Aalst,, "Workflow Resource Patterns", WfRP, 2004,
              <http://www.workflowpatterns.com/patterns/resource/>.

Appendix A.  Examples of Task State Lifecycle

A.1.  Simple Case Status Change

      +===+==============+==============+===========================+
      |   | STATUS       | PARTSTAT     | Action                    |
      +===+==============+==============+===========================+
      | 1 | -            | -            | Organizer draft           |
      +---+--------------+--------------+---------------------------+
      | 2 | NEEDS-ACTION | NEEDS-ACTION | Organizer sends iTIP      |
      |   |              |              | request                   |
      +---+--------------+--------------+---------------------------+
      | 3 | NEEDS-ACTION | ACCEPTED     | Attendee reply            |
      +---+--------------+--------------+---------------------------+
      | 4 | PENDING      | ACCEPTED     | Task accepted but waiting |
      |   |              |              | on some "trigger" to      |
      |   |              |              | start (e.g. another task  |
      |   |              |              | has to finish first)      |
      +---+--------------+--------------+---------------------------+
      | 5 | IN-PROCESS   | IN-PROCESS   | Attendee reply now        |
      |   |              |              | working on the task       |
      +---+--------------+--------------+---------------------------+
      | 6 | IN-PROCESS   | COMPLETED    | Attendee reply completed  |
      +---+--------------+--------------+---------------------------+
      | 7 | COMPLETED    | COMPLETED    | Organizer changes overall |
      |   |              |              | state                     |
      +---+--------------+--------------+---------------------------+

            Table 8: Example of status changes in assigning and
                    performing a task with one attendee.

A.2.  Example for multiple Attendees

   Example of status changes in assigning and performing a task with two
   attendees (A1 and A2).

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   +====+==============+==============+==============+================+
   |    | STATUS       | PARTSTAT     | PARTSTAT     | Action         |
   |    |              | (A1)         | (A2)         |                |
   +====+==============+==============+==============+================+
   | 1  | -            | -            | -            | Organizer      |
   |    |              |              |              | draft.         |
   +----+--------------+--------------+--------------+----------------+
   | 2  | NEEDS-ACTION | NEEDS-ACTION | NEEDS-ACTION | Organizer      |
   |    |              |              |              | sends iTIP     |
   |    |              |              |              | request.       |
   +----+--------------+--------------+--------------+----------------+
   | 4  | NEEDS-ACTION | ACCEPTED     | NEEDS-ACTION | Attendee 1     |
   |    |              |              |              | reply.         |
   +----+--------------+--------------+--------------+----------------+
   | 5  | NEEDS-ACTION | ACCEPTED     | ACCEPTED     | Attendee 2     |
   |    |              |              |              | reply.         |
   +----+--------------+--------------+--------------+----------------+
   | 6  | PENDING      | ACCEPTED     | ACCEPTED     | Task accepted  |
   |    |              |              |              | but waiting on |
   |    |              |              |              | some"trigger"  |
   |    |              |              |              | to start (e.g. |
   |    |              |              |              | another task   |
   |    |              |              |              | has to finish  |
   |    |              |              |              | first)         |
   +----+--------------+--------------+--------------+----------------+
   | 7  | IN-PROCESS   | ACCEPTED     | IN-PROCESS   | Attendee 2     |
   |    |              |              |              | reply now      |
   |    |              |              |              | working on the |
   |    |              |              |              | task.          |
   +----+--------------+--------------+--------------+----------------+
   | 8  | IN-PROCESS   | IN-PROCESS   | IN-PROCESS   | Attendee 1     |
   |    |              |              |              | reply now      |
   |    |              |              |              | working on the |
   |    |              |              |              | task.          |
   +----+--------------+--------------+--------------+----------------+
   | 9  | IN-PROCESS   | COMPLETED    | IN-PROCESS   | Attendee 1     |
   |    |              |              |              | reply          |
   |    |              |              |              | Completed      |
   |    |              |              |              | (overall       |
   |    |              |              |              | status still   |
   |    |              |              |              | IN-PROCESS).   |
   +----+--------------+--------------+--------------+----------------+
   | 10 | IN-PROCESS   | COMPLETED    | COMPLETED    | Attendee 2     |
   |    |              |              |              | reply          |
   |    |              |              |              | Completed      |
   +----+--------------+--------------+--------------+----------------+
   | 11 | COMPLETED    | COMPLETED    | COMPLETED    | Organizer      |
   |    |              |              |              | changes        |

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   |    |              |              |              | overall state  |
   |    |              |              |              | once both      |
   |    |              |              |              | attendees are  |
   |    |              |              |              | finished.      |
   +----+--------------+--------------+--------------+----------------+

                 Table 9: Example for multiple Attendees

      |  NOTE: The logic for determining the status change to the VTODO
      |  is determined by the task organizer based on the ATTENDEE
      |  status and other business logic.

A.3.  Example of Failure

   Example of status changes for a task that fails.

    +===+==============+==============+==============================+
    |   | STATUS       | PARTSTAT     | Action                       |
    +===+==============+==============+==============================+
    | 1 | -            | -            | Organizer draft              |
    +---+--------------+--------------+------------------------------+
    | 2 | NEEDS-ACTION | NEEDS-ACTION | Organizer sends iTIP request |
    +---+--------------+--------------+------------------------------+
    | 3 | NEEDS-ACTION | ACCEPTED     | Attendee reply               |
    +---+--------------+--------------+------------------------------+
    | 4 | IN-PROCESS   | IN-PROCESS   | Attendee reply now working   |
    |   |              |              | on the task                  |
    +---+--------------+--------------+------------------------------+
    | 5 | IN-PROCESS   | FAILED       | Attendee reply task failed   |
    +---+--------------+--------------+------------------------------+
    | 6 | FAILED       | FAILED       | Organizer changes overall    |
    |   |              |              | state                        |
    +---+--------------+--------------+------------------------------+

                       Table 10: Example of Failure

Authors' Addresses

   Adrian Apthorp
   DHL Express
   Fritz-Erler-Str. 5
   Bonn
   Germany
   Email: adrian.apthorp@dhl.com

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   Michael Douglass
   Bedework Commercial Services
   226 3rd Street
   Troy, NY
   United States of America
   Email: mdouglass@bedework.com

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