Network Working Group                                           C. Daboo
Internet-Draft                                                     Apple
Intended status: Standards Track                             M. Douglass
Expires: May 26, November 24, 2016                                           RPI
                                                       November
                                                            May 23, 2015 2016

                         Calendar Availability
                   draft-ietf-calext-availability-01
                   draft-ietf-calext-availability-02

Abstract

   This document specifies a new iCalendar calendar component that
   allows the publication of available and unavailable time periods
   associated with a calendar user.  This component can be used in
   standard iCalendar free-busy lookups, including iTIP free-busy
   requests, to generate repeating blocks of available or busy time with
   exceptions as needed.

   This document also defines extensions to CalDAV calendar-access and
   calendar-auto-schedule which specify how this new calendar component
   can be used when doing free-busy time evaluation in CalDAV.

Editorial Note (To be removed by RFC Editor before publication)

   Discussion of this specification is taking place on the mailing list
   http://lists.osafoundation.org/mailman/listinfo/ietf-caldav .

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 http://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 May 26, November 24, 2016.

Copyright Notice

   Copyright (c) 2015 2016 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
   (http://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 extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  iCalendar Extensions  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  VAVAILABILITY Component . . . . . . . . . . . . . . . . .   4
     3.2.  Busy Time Type  . . . . . . . . . . . . . . . . . . . . .  10
   4.  Combining VAVAILABILITY components  . . . . . . . . . . . . .  10
   5.  Calculating Free-Busy Time  . . . . . . . . . . . . . . . . .  11
     5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Use with iTIP . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  CalDAV Extensions . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  CalDAV Requirements Overview  . . . . . . . . . . . . . .  14
     7.2.  New features in CalDAV  . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18  20
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  18  20
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18  20
     10.1.  Component Registrations  . . . . . . . . . . . . . . . .  18  20
     10.2.  Property Registrations . . . . . . . . . . . . . . . . .  19  21
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19  21
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  19  21
   Appendix A.  Example Calendar #1  . . . . . . . . . . . . . . . .  20  22
   Appendix B.  Change History (To be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  22  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23  24

1.  Introduction

   Calendar users often have regular periods of time when they are
   either available to be scheduled or always unavailable.  For example,
   an office worker will often wish only to appear free to their work
   colleagues during normal 'office hours' (e.g., Monday through Friday,
   9 am through 5 pm).  Or, a university professor might only be
   available to students during a set period of time (e.g., Thursday
   afternoons, 2 pm through 5 pm during term time only).  Ideally users
   ought be able to specify such periods directly via their calendar
   user agent, and have them automatically considered as part of the
   normal free-busy lookup for that user.  In addition, it ought be
   possible to present different periods of available time depending on
   which user is making the request.

   iCalendar [RFC5545] defines a "VFREEBUSY" component that can be used
   to represent fixed busy time periods, but it does not provide a way
   to specify a repeating period of available or unavailable time.
   Since repeating patterns are often the case, "VFREEBUSY" components
   are not sufficient to solve this problem.

   This specification defines a new type of iCalendar component that can
   be used to publish user availability.

   CalDAV [RFC4791] provides a way for calendar users to access and
   manage calendar data and exchange this data via scheduling
   operations.  As part of this, the CalDAV calendar-access [RFC4791]
   feature provides a CALDAV:free-busy-query REPORT that returns free-
   busy information for a calendar collection or hierarchy of calendar
   collections.  Also, the CalDAV calendar-auto-schedule [RFC6638]
   feature allows free-busy information for a calendar user to be
   determined.  Both of these operations involve examining user
   calendars for events that 'block time', with the blocked out periods
   being returned in a "VFREEBUSY" component.

   This specification extends the CalDAV calendar-access and CalDAV
   calendar-auto-schedule features to allow the new iCalendar
   availability components to be stored and manipulated, and to allow
   free-busy lookups to use the information from any such components, if
   present.

2.  Conventions Used in This Document

   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
   [RFC2119].

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

3.  iCalendar Extensions

   This specification adds a new "VAVAILABILITY" calendar component to
   iCalendar.  The "VAVAILABILITY" component is itself a container for
   new "AVAILABLE" sub-components.

   The purpose of the "VAVAILABILITY" calendar component is to provide a
   grouping of available time information over a specific range of time.
   Within that, there are specific time ranges that are marked as
   available via a set of "AVAILABLE" calendar sub-components.  Together
   these can be used to specify available time that can repeat over set
   periods of time, and which can vary over time.

   An illustration of how "VAVAILABILITY" and "AVAILABLE" components
   work is shown below.

                         Time-range
   <=========================================================>

      +-------------------------------------------------+
      |              VAVAILABILITY                      |
      +-------------------------------------------------+
         +------------+       +------------+
         | AVAILABLE  |       | AVAILABLE  |
         +------------+       +------------+

      <->              <----->              <-----------> Busy Time

   The overall time-range is shown at the top.  A "VAVAILABILITY"
   component spans part of the range.  The time-range covered by the
   "VAVAILABILITY" component is considered to be busy, except for the
   ranges covered by the "AVAILABLE" components within the
   "VAVAILABILITY" component.

3.1.  VAVAILABILITY Component

   Component Name:  VAVAILABILITY

   Purpose:  Provide a grouping of component properties and sub-
      components that describe the availability associated with a
      calendar user.

   Format Definition:  A "VAVAILABILITY" calendar component is defined
      by the following notation:

   availabilityc  = "BEGIN" ":" "VAVAILABILITY" CRLF
                   availabilityprop *availablec
                   "END" ":" "VAVAILABILITY" CRLF

   availabilityprop  = *(
                    ;
                    ; the following are REQUIRED,
                    ; but MUST NOT occur more than once
                    ;
                    dtstamp / uid
                    ;
                    ; the following are OPTIONAL,
                    ; but MUST NOT occur more than once
                    ;
                    busytype / class / created / description /
                    dtstart / last-mod / location / organizer /
                    priority /seq / summary / url /
                    ;
                    ; Either 'dtend' or 'duration' MAY appear
                    ; in an 'availableprop', but 'dtend' and
                    ; 'duration' MUST NOT occur in the same
                    ; 'availabilityprop'.
                    ; 'duration' MUST NOT be present if
                    ; 'dtstart' is not present
                    ;
                    dtend / duration /
                    ;
                    ; the following are OPTIONAL,
                    ; and MAY occur more than once
                    ;
                    categories / comment / contact /
                    x-prop / iana-prop
                    ;
                    )

   availablec  = "BEGIN" ":" "AVAILABLE" CRLF
                availableprop
                "END" ":" "AVAILABLE" CRLF

   availableprop  = *(
                 ;
                 ; the following are REQUIRED,
                 ; but MUST NOT occur more than once
                 ;
                 dtstamp / dtstart / uid /
                 ;
                 ; Either 'dtend' or 'duration' MAY appear in
                 ; an 'availableprop', but 'dtend' and
                 ; 'duration' MUST NOT occur in the same
                 ; 'availableprop'.
                 ;
                 dtend / duration /
                 ;
                 ; the following are OPTIONAL,
                 ; but MUST NOT occur more than once
                 ;
                 created / description / last-mod /
                 location / recurid / rrule / summary /
                 ;
                 ; the following are OPTIONAL,
                 ; and MAY occur more than once
                 ;
                 categories / comment / contact / exdate /
                 rdate / x-prop / iana-prop

                 )

   Description:  A "VAVAILABILITY" component indicates a period of time
      within which availability information is provided.  A
      "VAVAILABILITY" component can specify a start time and an end time
      or duration.  If "DTSTART" is not present, then the start time is
      unbounded.  If "DTEND" or "DURATION" are not present, then the end
      time is unbounded.  Within the specified time period, availability
      defaults to a free-busy type of "BUSY-UNAVAILABLE" (see
      Section 3.2), except for any time periods corresponding to
      "AVAILABLE" sub-components.

      "AVAILABLE" sub-components are used to indicate periods of free
      time within the time range of the enclosing "VAVAILABILITY"
      component.  "AVAILABLE" sub-components MAY include recurrence
      properties to specify recurring periods of time, which can be
      overridden using normal iCalendar recurrence behavior (i.e., use
      of the "RECURRENCE-ID" property).

      If specified, the "DTSTART" and "DTEND" properties in
      "VAVAILABILITY" components and "AVAILABLE" sub-components MUST be
      "DATE-TIME" values specified as either date with UTC time or date
      with local time and a time zone reference.

      The iCalendar object containing the "VAVAILABILITY" component MUST
      contain appropriate "VTIMEZONE" components corresponding to each
      unique "TZID" parameter value used in any DATE-TIME properties in
      all components.  Note that extensions, such as [RFC7809], might
      relax this constraint.

      When used to publish available time, the "ORGANIZER" property
      specifies the calendar user associated with the published
      available time.

      If the "PRIORITY" property is specified in "VAVAILABILITY"
      components, it is used to determine how that component is combined
      with other "VAVAILABILITY" components.  See Section 4.

      Other calendar properties MAY be specified in "VAVAILABILITY" or
      "AVAILABLE" components and are considered attributes of the marked
      block of time.  Their usage is application specific.  For example,
      the "LOCATION" property might be used to indicate that a person is
      available in one location for part of the week and a different
      location for another part of the week.

   Example:  The following is an example of a "VAVAILABILITY" calendar
      component used to represent the availability of a user, always
      available Monday through Friday, 9:00 AM to 5:00 PM in the
      America/Montreal time zone:

   BEGIN:VAVAILABILITY
   ORGANIZER:mailto:bernard@example.com
   UID:vavail-1@example.com
   DTSTAMP:20111005T133225Z
   BEGIN:AVAILABLE
   UID:avail-1-A@example.com
   SUMMARY:Monday to Friday from 9:00 to 17:00
   DTSTART;TZID=America/Montreal:20111002T090000
   DTEND;TZID=America/Montreal:20111002T170000
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
   END:AVAILABLE
   END:VAVAILABILITY

      The following is an example of a "VAVAILABILITY" calendar
      component used to represent the availability of a user available
      Monday through Thursday, 9:00 AM to 5:00 PM at the main office,
      and Friday 9:00 AM to 12:00 PM in the branch office in the
      America/Montreal time zone between October 2nd and December 2nd
      2011:

   BEGIN:VAVAILABILITY
   ORGANIZER:mailto:bernard@example.com
   UID:vavail-1@example.com
   DTSTAMP:20111005T133225Z
   DTSTART;TZID=America/Montreal:20111002T000000
   DTEND;TZID=America/Montreal:20111202T000000
   BEGIN:AVAILABLE
   UID:avail-1-A@example.com
   SUMMARY:Monday to Thursday from 9:00 to 17:00
   DTSTART;TZID=America/Montreal:20111002T090000
   DTEND;TZID=America/Montreal:20111002T170000
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH
   LOCATION:Main Office
   END:AVAILABLE
   BEGIN:AVAILABLE
   UID:avail-1-B@example.com
   SUMMARY:Friday from 9:00 to 12:00
   DTSTART;TZID=America/Montreal:20111006T090000
   DTEND;TZID=America/Montreal:20111006T120000
   RRULE:FREQ=WEEKLY
   LOCATION:Branch Office
   END:AVAILABLE
   END:VAVAILABILITY

      The following is an example of three "VAVAILABILITY" calendar
      components used to represent the availability of a traveling
      worker: Monday through Friday, 9:00 AM to 5:00 PM each day.
      However, for three weeks the calendar user is working in Montreal,
      then one week in Los Angeles, then back to Montreal.  Note that
      each overall period is covered by separate "VAVAILABILITY"
      components.  The last of these has no DTEND so continues on "for
      ever".  This example shows one way "blocks" of available time can
      be represented.  See Section 4 for another approach using
      priorities.

   BEGIN:VAVAILABILITY
   ORGANIZER:mailto:bernard@example.com
   UID:vavail-1@example.com
   DTSTAMP:20111005T133225Z
   DTSTART;TZID=America/Montreal:20111002T000000
   DTEND;TZID=America/Montreal:20111023T030000
   BEGIN:AVAILABLE
   UID:avail-1-A@example.com
   SUMMARY:Monday to Friday from 9:00 to 17:00
   DTSTART;TZID=America/Montreal:20111002T090000
   DTEND;TZID=America/Montreal:20111002T170000
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
   LOCATION:Montreal
   END:AVAILABLE
   END:VAVAILABILITY
   BEGIN:VAVAILABILITY
   ORGANIZER:mailto:bernard@example.com
   UID:vavail-2@example.com
   DTSTAMP:20111005T133225Z
   DTSTART;TZID=America/Los_Angeles:20111023T000000
   DTEND;TZID=America/Los_Angeles:20111030T000000
   BEGIN:AVAILABLE
   UID:avail-2-A@example.com
   SUMMARY:Monday to Friday from 9:00 to 17:00
   DTSTART;TZID=America/Los_Angeles:20111023T090000
   DTEND;TZID=America/Los_Angeles:20111023T170000
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
   LOCATION:Los Angeles
   END:AVAILABLE
   END:VAVAILABILITY
   BEGIN:VAVAILABILITY
   ORGANIZER:mailto:bernard@example.com
   UID:vavail-3@example.com
   DTSTAMP:20111005T133225Z
   DTSTART;TZID=America/Montreal:20111030T030000
   BEGIN:AVAILABLE
   UID:avail-3-A@example.com
   SUMMARY:Monday to Friday from 9:00 to 17:00
   DTSTART;TZID=America/Montreal:20111030T090000
   DTEND;TZID=America/Montreal:20111030T170000
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
   LOCATION:Montreal
   END:AVAILABLE
   END:VAVAILABILITY

3.2.  Busy Time Type

   Property Name:  BUSYTYPE

   Purpose:  This property specifies the default busy time type.

   Value Type:  TEXT

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

   Conformance:  This property can be specified within "VAVAILABILITY"
      calendar components.

   Format Definition:  This property is defined by the following
      notation:

   busytype      = "BUSYTYPE" busytypeparam ":" busytypevalue CRLF

   busytypeparam = *(";" other-param)

   busytypevalue = "BUSY" / "BUSY-UNAVAILABLE" /
                   "BUSY-TENTATIVE" / iana-token / x-name
                  ; Default is "BUSY-UNAVAILABLE".

   Description:  This property is used to specify the default busy time
      type.  The values correspond to those used by the "FBTYPE"
      parameter used on a "FREEBUSY" property, with the exception that
      the "FREE" value is not used in this property.  If not specified
      on a component that allows this property, the default is "BUSY-
      UNAVAILABLE".

   Example:  The following is an example of this property:

   BUSYTYPE:BUSY

4.  Combining VAVAILABILITY components

   The "VAVAILABILITY" component allows a calendar user to describe
   their availability over extended periods of time through the use of
   recurrence patterns.  This availability might be relatively constant
   from year to year.

   However, there is usually some degree of irregularity, as people take
   vacations or perhaps spend a few weeks at a different office.  For
   that period of time there is a need to redefine their availability.

   Rather than modify their existing availability, the "PRIORITY"
   property allows new "VAVAILABILITY" components to override others of
   lower ordinal priority.  Note that iCalendar [RFC5545] defines the
   "PRIORITY" property such that a value of 0 is undefined, 1 is the
   highest priority and 9 is the lowest.

   When combining "VAVAILABILITY" components, an absence of a "PRIORITY"
   property or a value of 0 implies the lowest level of priority.  When
   two or more VAVAILABILITY components overlap, and they have the same
   PRIORITY value, the overlapping busy time type is determined by the
   following order: BUSY > BUSY-UNAVAILABLE > BUSY-TENTATIVE. i.e., if
   one component has a BUSYTYPE set to BUSY, and the another has
   BUSYTYPE set to BUSY-UNAVAILABLE, then the effective busy time type
   over the time range that they overlap would be BUSY.  It is up to the
   creator of such components to ensure that combining them produces a
   consistent and expected result.

   To calculate the available time, order the intersecting
   "VAVAILABILITY" components by priority (i.e., lowest to highest
   "PRIORITY" values are 0, 9, 8, 7, 6, 5, 4, 3, 2, 1).

   Step through the resulting list of "VAVAILABILITY" components.  For
   each, the time range covered by the "VAVAILABILITY" component is set
   to busy and then portions of it defined by the "AVAILABLE" components
   in the "VAVAILABILITY" component are set to free.

   Note that, if any "VAVAILABILITY" component completely covers the
   date range of interest, then any lower priority "VAVAILABILITY"
   components can be ignored.

5.  Calculating Free-Busy Time

   This section describes how free-busy time information for a calendar
   user is calculated in the presence of "VAVAILABILITY" calendar
   components.

   An iCalendar "VFREEBUSY" component is used to convey "rolled-up"
   free-busy time information for a calendar user.  This can be
   generated as the result of an iTIP free-busy [RFC5546] request or
   through some other mechanism (e.g., a CalDAV calendar-access
   CALDAV:free-busy-query REPORT).

   When one or more "VAVAILABILITY" components are present and intersect
   the time-range for the free-busy request, first available time is
   calculated, as outlined in Section 4.  Once that is done, regular
   "VEVENT" and "VFREEBUSY" components can be "overlaid" in the usual
   way to block out time.

   An example procedure for this is as follows:

   1.  Initially mark the entire period of the free-busy request as
       free.

   2.  For each "VAVAILABILITY" component ordered by PRIORITY:

       1.  Determine if the "VAVAILABILITY" intersects the time-range of
           the free-busy request.  If not ignore it.

       2.  Determine if the "VAVAILABILITY" is completely overridden by
           a higher priority component.  If so ignore it.

       3.  For the time period covered by the "VAVAILABILITY" component,
           mark time in the free-busy request result set as busy, using
           the busy time type derived from the "BUSYTYPE" property in
           the "VAVAILABILITY" component.

   3.  For each remaining "VAVAILABILITY" component in order:

       1.  For each "AVAILABLE" component in the "VAVAILABILITY"
           component:

           1.  Expand all recurring instances, taking into account
               overridden instances, ignoring instances or parts of
               instances that fall outside of the free-busy request
               time-range or the time period specified by the
               "VAVAILABILITY" component.

           2.  For each instance, mark the corresponding time in the
               free-busy request result set as free.

   4.  For each "VEVENT" or "VFREEBUSY" component apply normal free-busy
       processing within the free-busy request time-range.

5.1.  Examples

   In the examples below a table is used to represent time slots for the
   period of a free-busy request.  Each time slot is two hours long.
   The column header represents the hours from midnight local time.
   Each row below the column headers represents a step in the free-busy
   result set determination, following the procedure outlined above.

   Each cell in the rows below the column header contains a single
   character that represents the free-busy type for the corresponding
   time period at the end of the process step represented by the row.
   The characters in the row are:

     +-----------+--------------------------------------------------+
     | Character | Meaning                                          |
     +-----------+--------------------------------------------------+
     | F         | Represents "FREE" time in that slot.             |
     | B         | Represents "BUSY" time in that slot.             |
     | U         | Represents "BUSY-UNAVAILABLE" time in that slot. |
     | T         | Represents "BUSY-TENTATIVE" time in that slot.   |
     +-----------+--------------------------------------------------+

5.1.1.  Simple Example

   A free-busy request for Monday, 6th November 2011, midnight to
   midnight in the America/Montreal timezone.

   The user's calendar is as shown in Appendix A.  This includes one
   "VAVAILABILITY" component giving available time within the requested
   time-range of 8:00 AM to 6:00 PM, together with one "VEVENT"
   component representing a two hour meeting starting at 12:00 PM.

   +------+----+----+----+----+----+----+----+----+----+----+----+----+
   | Step | 0  | 2  | 4  | 6  | 8  | 10 | 12 | 14 | 16 | 18 | 20 | 22 |
   +------+----+----+----+----+----+----+----+----+----+----+----+----+
   | 1.   | F  | F  | F  | F  | F  | F  | F  | F  | F  | F  | F  | F  |
   | 2.   | U  | U  | U  | U  | U  | U  | U  | U  | U  | U  | U  | U  |
   | 3.   | U  | U  | U  | U  | F  | F  | F  | F  | F  | U  | U  | U  |
   | 4.   | U  | U  | U  | U  | F  | F  | B  | F  | F  | U  | U  | U  |
   +------+----+----+----+----+----+----+----+----+----+----+----+----+

5.1.2.  Further Example

   The following is another way to represent the availability of the
   traveling worker shown above.  Here we represent their base
   availability of Monday through Friday, 9:00 AM to 5:00 PM each day
   with a "VAVAILABILITY" with default "PRIORITY" (there is no "DTEND"
   property so that this availability is unbounded).  For the three
   weeks the calendar user is working in Los Angeles, we represent their
   availability with a "VAVAILABILITY" component with priority 1, which
   overrides the base availability.

   BEGIN:VAVAILABILITY
   ORGANIZER:mailto:bernard@example.com
   UID:vavail-1@example.com
   DTSTAMP:20111005T133225Z
   DTSTART;TZID=America/Montreal:20111002T000000
   BEGIN:AVAILABLE
   UID:avail-1-A@example.com
   SUMMARY:Monday to Friday from 9:00 to 17:00
   DTSTART;TZID=America/Montreal:20111002T090000
   DTEND;TZID=America/Montreal:20111002T170000
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
   LOCATION:Montreal
   END:AVAILABLE
   END:VAVAILABILITY
   BEGIN:VAVAILABILITY
   ORGANIZER:mailto:bernard@example.com
   UID:vavail-2@example.com
   DTSTAMP:20111005T133225Z
   DTSTART;TZID=America/Los_Angeles:20111023T000000
   DTEND;TZID=America/Los_Angeles:20111030T000000
   BEGIN:AVAILABLE
   UID:avail-2-A@example.com
   SUMMARY:Monday to Friday from 9:00 to 17:00
   DTSTART;TZID=America/Los_Angeles:20111023T090000
   DTEND;TZID=America/Los_Angeles:20111023T170000
   PRIORITY:1
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
   LOCATION:Los Angeles
   END:AVAILABLE
   END:VAVAILABILITY

6.  Use with iTIP

   This specification does not define how "VAVAILABILITY" components are
   used in scheduling messages sent using the iTIP [RFC5546] protocol.
   It is expected that future specifications will define how iTIP
   scheduling can make use of "VAVAILABILITY" components.

7.  CalDAV Extensions

7.1.  CalDAV Requirements Overview

   This section lists what functionality is required of a CalDAV server
   which supports "VAVAILBILITY" components in stored calendar data.  A
   server:

   o  MUST advertise support for "VAVAILABILITY" components in
      CALDAV:supported-calendar-component-set properties on calendars
      which allow storing of such components;

   o  MUST support CALDAV:free-busy-query REPORTs that aggregate the
      information in any "VAVAILABILITY" components in the calendar
      collections targeted by the request;

   o  MUST support "VAVAILABILITY" components stored in a
      CALDAV:calendar-availability WebDAV property on a CALDAV
      scheduling inbox collection, if the CALDAV calendar-auto-schedule
      feature is supported;

   o  MUST support iTIP [RFC5546] free-busy requests that aggregate the
      information in any "VAVAILABILITY" components in calendar
      collections that contribute to free-busy, or in any
      "VAVAILABILITY" components stored in the CALDAV:calendar-
      availability property on the CALDAV scheduling inbox collection of
      the calendar user targeted by the iTIP free-busy request, if the
      CalDAV calendar-auto-schedule feature is available.

   "VAVAILABILITY" components are treated in a manner similar to
   "VEVENT" and "VTODO" components, and MUST satisfy the other
   requirements CalDAV imposes on calendar object resources (see
   Section 4.1 of [RFC4791]).

7.2.  New features in CalDAV

7.2.1.  Calendar Availability Support

   A server supporting the features described in this document MUST
   include "calendar-availability" as a field in the DAV response header
   from an OPTIONS request.  A value of "calendar-availability" in the
   DAV response header MUST indicate that the server supports all MUST
   level requirements specified in this document.

7.2.1.1.  Example: Using OPTIONS for the Discovery of Calendar
          Availability Support

   >> Request <<

   OPTIONS /home/bernard/calendars/ HTTP/1.1
   Host: cal.example.com
   >> Response <<

   HTTP/1.1 200 OK
   Allow: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE
   Allow: PROPFIND, PROPPATCH, LOCK, UNLOCK, REPORT, ACL
   DAV: 1, 2, 3, access-control, calendar-access,
   calendar-availability
   Date: Fri, 11 Nov 2005 09:32:12 GMT
   Content-Length: 0

   In this example, the OPTIONS method returns the value "calendar-
   availability" in the DAV response header to indicate that the
   collection "/home/bernard/calendars/" supports the new features
   defined in this specification.

7.2.2.  CALDAV Time Range Queries

   Section 9.9 of [RFC4791] describes how to specify time ranges to
   limit the set of calendar components returned by the server.  This
   specification extends [RFC4791] to describe how to apply time range
   filtering to "VAVAILABILITY" components.

   A "VAVAILABILITY" component is said to overlap a given time range if
   the condition for the corresponding component state specified in the
   table below is satisfied.  The conditions depend on the presence of
   the "DTSTART", "DTEND", and "DURATION" properties in the
   "VAVAILABILITY" component.  Note that, as specified above, the
   "DTEND" value MUST be a "DATE-TIME" value equal to or after the
   "DTSTART" value, if specified.

       +------------------------------------------------------------+
       | VAVAILABILITY has the DTSTART property?                    |
       |   +--------------------------------------------------------+
       |   | VAVAILABILITY has the DTEND property?                  |
       |   |   +----------------------------------------------------+
       |   |   | VAVAILABILITY has the DURATION property?           |
       |   |   |   +------------------------------------------------+
       |   |   |   | Condition to evaluate                          |
       +---+---+---+------------------------------------------------+
       | Y | Y | N | (start < DTEND  AND  end > DTSTART)            |
       +---+---+---+------------------------------------------------+
       | Y | N | Y | (start < DTSTART+DURATION  AND  end > DTSTART) |
       +---+---+---+------------------------------------------------+
       | Y | N | N | (end > DTSTART)                                |
       +---+---+---+------------------------------------------------+
       | N | Y | N | (start < DTEND)                                |
       +---+---+---+------------------------------------------------+
       | N | N | * | TRUE                                           |
       +---+---+---+------------------------------------------------+

7.2.3.  CALDAV:free-busy-query REPORT

   A CALDAV:free-busy-query REPORT can be executed on a calendar
   collection that contains iCalendar "VAVAILABILITY" components.  When
   that occurs, the server MUST aggregate the information in any
   "VAVAILABILITY" components when generating the free-busy response, as
   described in Section 5.

7.2.3.

7.2.4.  CALDAV:calendar-availability Property

   Name:  calendar-availability

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

   Purpose:  Defines a "VAVAILABILITY" component that will be used in
      calculating free-busy time when an iTIP free-busy request is
      targeted at the calendar user who owns the Inbox.

   Conformance:  This property MAY be protected and SHOULD NOT be
      returned by a PROPFIND DAV:allprop request.  Support for this
      property is REQUIRED.  The value of this property MUST be a valid
      iCalendar object containing "VAVAILABILITY" components and
      "VTIMEZONE" components (if required) only.

   Description:  This property allows a user to specify their
      availability by including an "VAVAILABILITY" component in the
      value of this property.  If present, the server MUST use this
      "VAVAILABILITY" component when determining free-busy information
      as part of an iTIP free-busy request being handled by the server.
      Fot simplicity, only a single "VAVAILABILITY" component MUST be
      present in the property.  For more complex availability scenarios,
      clients can store multiple "VAVAILABILITY" components in the
      calendar user's calendar collections.

   Definition:

   <!ELEMENT calendar-availability (#PCDATA) >
   ; Data value MUST be iCalendar object containing
   ; "VAVAILABILITY" or "VTIMEZONE" components.

   Example:

   <C:calendar-availability xmlns:D="DAV:"
   xmlns:C="urn:ietf:params:xml:ns:caldav"
   >BEGIN:VCALENDAR
   CALSCALE:GREGORIAN
   PRODID:-//example.com//iCalendar 2.0//EN
   VERSION:2.0
   BEGIN:VTIMEZONE
   LAST-MODIFIED:20040110T032845Z
   TZID:America/Montreal
   BEGIN:DAYLIGHT
   DTSTART:20000404T020000
   RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4
   TZNAME:EDT
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0400
   END:DAYLIGHT
   BEGIN:STANDARD
   DTSTART:20001026T020000
   RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
   TZNAME:EST
   TZOFFSETFROM:-0400
   TZOFFSETTO:-0500
   END:STANDARD
   END:VTIMEZONE
   BEGIN:VAVAILABILITY
   UID:vavail-1@example.com
   DTSTAMP:20111005T133225Z
   DTSTART;TZID=America/Montreal:20111002T000000
   BEGIN:AVAILABLE
   UID:avail-1-A@example.com
   SUMMARY:Monday to Friday from 9:00 to 18:00
   DTSTART;TZID=America/Montreal:20111002T090000
   DTEND;TZID=America/Montreal:20111002T180000
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
   END:AVAILABLE
   END:VAVAILABILITY
   END:VCALENDAR
   </C:calendar-availability>

7.2.4.

7.2.5.  iTIP free-busy requests

   The [RFC6638] processing of an iTIP free-busy request targeted at the
   owner of the CALDAV:schedule-inbox will include free-busy information
   derived from "VAVAILABILITY" components in any calendar collection
   targeted during the request, as described in Section 5.  In addition,
   any "VAVAILABILITY" component specified in the CALDAV:calendar-
   availability property on the owner's Inbox, MUST be included in the
   free-busy calculation.

8.  Security Considerations

   Calculation of availability information, particularly with multiple
   overlapping time-ranges, can be complex, and CalDAV servers MAY limit
   the complexity of such data stored by a client.

   An attacker able to "inject" availability information into a calendar
   user's calendar data could ensure that the user never appears free
   for meetings, or appears free at inappropriate times.  Calendar
   systems MUST ensure that availability information for a calendar user
   can only be modified by authorized users.

   Beyond this, this specification does not add any additional security
   issues that are not already present in [RFC5545], [RFC5546],
   [RFC4791] and [RFC6638].

9.  Privacy Considerations

   Normal free-busy information generated from "VAVAILABILITY"
   components MUST NOT include information other than busy or free time
   periods.  In particular, user specified property values such as
   "SUMMARY", "LOCATION" and "DESCRIPTION" MUST NOT be copied into the
   free-busy result data.

   Free-busy and availability information can be used by attackers to
   infer the whereabouts or overall level of "activity" of the
   corresponding calendar user.  Any calendar system that allows a user
   to expose their free-busy and availability information MUST limit
   access to that information to only authorized users.

10.  IANA Considerations

10.1.  Component Registrations

   This documents defines the following new iCalendar components to be
   added to the registry defined in Section 8.2.2 of [RFC5545]:

            +---------------+---------+-----------------------+
            | Component     | Status  | Reference             |
            +---------------+---------+-----------------------+
            | VAVAILABILITY | Current | RFCXXXX, Section 3.1  |
            | AVAILABLE     | Current | RFCXXXX, Section 3.1  |
            +---------------+---------+-----------------------+

10.2.  Property Registrations

   This documents defines the following new iCalendar properties to be
   added to the registry defined in Section 8.2.3 of [RFC5545]:

              +----------+---------+-----------------------+
              | Property | Status  | Reference             |
              +----------+---------+-----------------------+
              | BUSYTYPE | Current | RFCXXXX, Section 3.2  |
              +----------+---------+-----------------------+

11.  Acknowledgments

   Thanks to the following for providing feedback: Toby Considine
   Bernard Desruisseaux, Ken Murchison, Evert Pot, Dave Thewlis.  This
   specification came about via discussions at the Calendaring and
   Scheduling Consortium.

12.  Normative References

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

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

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

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

   [RFC6638]  Daboo, C. and B. Desruisseaux, "Scheduling Extensions to
              CalDAV", RFC 6638, DOI 10.17487/RFC6638, June 2012,
              <http://www.rfc-editor.org/info/rfc6638>.

   [RFC7809]  Daboo, C., "Calendaring Extensions to WebDAV (CalDAV):
              Time Zones by Reference", RFC 7809, DOI 10.17487/RFC7809,
              March 2016, <http://www.rfc-editor.org/info/rfc7809>.

Appendix A.  Example Calendar #1

   iCalendar object

   BEGIN:VCALENDAR
   CALSCALE:GREGORIAN
   PRODID:-//example.com//iCalendar 2.0//EN
   VERSION:2.0
   BEGIN:VTIMEZONE
   LAST-MODIFIED:20040110T032845Z
   TZID:America/Montreal
   BEGIN:DAYLIGHT
   DTSTART:20000404T020000
   RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4
   TZNAME:EDT
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0400
   END:DAYLIGHT
   BEGIN:STANDARD
   DTSTART:20001026T020000
   RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
   TZNAME:EST
   TZOFFSETFROM:-0400
   TZOFFSETTO:-0500
   END:STANDARD
   END:VTIMEZONE
   BEGIN:VEVENT
   DTSTAMP:20111113T044111Z
   DTSTART;TZID=America/Montreal:20111106T120000
   DURATION:PT1H
   SUMMARY:Meeting
   UID:60A48841ECB90F3F215FE3D2@example.com
   END:VEVENT
   BEGIN:VAVAILABILITY
   UID:vavail-1@example.com
   DTSTAMP:20111005T133225Z
   DTSTART;TZID=America/Montreal:20111002T000000
   BEGIN:AVAILABLE
   UID:avail-1-A@example.com
   SUMMARY:Monday to Friday from 9:00 8:00 to 18:00
   DTSTART;TZID=America/Montreal:20111002T090000
   DTSTART;TZID=America/Montreal:20111002T080000
   DTEND;TZID=America/Montreal:20111002T180000
   RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
   END:AVAILABLE
   END:VAVAILABILITY
   END:VCALENDAR

Appendix B.  Change History (To be removed by RFC Editor before
             publication)

   Changes in draft-ietf-calext-availability-02:

   1.  [WGLC] Change Appendix A example to start available time at
       08:00.

   2.  [WGLC] Added new section with table describing CalDAV time range
       query behavior.

   3.  Added text and reference to RFC7809.

   4.  Added location to formal syntax of components.

   Changes in draft-ietf-calext-availability-01:

   1.  Minor editorial fixes.

   2.  ABNF syntax fixes.

   3.  Clarify BUSTYPE precedence when combining components with the
       same PRIORITY values.

   4.  Added section explaining that iTIP use is not defined

   5.  Added Privacy Considerations and tweaked Security Considerations.

   6.  Added diagram to illustrate the overall concept.

   7.  Limit the calendar-availability property to a single
       "VAVAILABILITY" component.

   Changes in draft-ietf-calext-availability-00:

   1.  Re-publication as WG document.

   Changes in draft-daboo-calendar-availability-05:

   1.  Small typos.

   2.  Fix explanation of priority.

   3.  Change uid values to make legal and easier to follow.

   Changes in draft-daboo-calendar-availability-04:

   1.  Small typos.

   2.  Add prioritized example.

   Changes in draft-daboo-calendar-availability-03:

   1.  Switch authors.

   2.  CalDAV scheduling is now rfc6638.

   3.  List priority as a vavailability property and define its use.

   Changes in draft-daboo-calendar-availability-02:

   1.  Updated to 5545/5546 references.

   2.  Fixed some examples.

   3.  Added some more properties to the components

   4.  Fixed text that said dtstart was required in VAVAILABILITY

   Changes in draft-daboo-calendar-availability-01:

   1.  Allow property on Inbox for caldav-schedule.

   2.  Clarify that DURATION can only be present in VAVAILABILITY if
       DTSTART is also present, and DTEND is not.

   3.  Updated references.

   4.  Added templates.

Authors' Addresses

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

   Email: cyrus@daboo.name
   URI:   http://www.apple.com/
   Michael Douglass
   Rensselaer Polytechnic Institute
   110 8th Street
   Troy, NY  12180
   USA

   Email: douglm@rpi.edu
   URI:   http://www.rpi.edu/