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Internet Engineering Task Force (IETF) C. Daboo
Request for Comments: 7953 Apple
Updates: 4791, 5545, 6638 M. Douglass
Category: Standards Track Spherical Cow Group
ISSN: 2070-1721 August 2016
Calendar Availability
Abstract
This document specifies a new iCalendar (RFC 5545) 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 the iCalendar
Transport-independent Interoperability Protocol (iTIP; RFC 5546)
free-busy requests, to generate repeating blocks of available or busy
time with exceptions as needed.
This document also defines extensions to the Calendaring Extensions
to WebDAV (CalDAV) calendar access protocol (RFC 4791) and the
associated scheduling protocol (RFC 6638) to specify how this new
calendar component can be used when evaluating free-busy time.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7953.
Daboo & Douglass Standards Track [Page 1]
RFC 7953 Calendar Availability August 2016
Copyright Notice
Copyright (c) 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . 12
5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Use with iTIP . . . . . . . . . . . . . . . . . . . . . . . . 15
7. CalDAV Extensions . . . . . . . . . . . . . . . . . . . . . . 15
7.1. CalDAV Requirements Overview . . . . . . . . . . . . . . 15
7.2. New Features in CalDAV . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10.1. Component Registrations . . . . . . . . . . . . . . . . 20
10.2. Property Registrations . . . . . . . . . . . . . . . . . 20
11. Normative References . . . . . . . . . . . . . . . . . . . . 20
Appendix A. Example Calendar #1 . . . . . . . . . . . . . . . . 22
Appendix B. Example Calendar #2 . . . . . . . . . . . . . . . . 23
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
Daboo & Douglass Standards Track [Page 2]
RFC 7953 Calendar Availability August 2016
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].
Daboo & Douglass Standards Track [Page 3]
RFC 7953 Calendar Availability August 2016
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" subcomponents.
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 subcomponents. 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
subcomponents that describe the availability associated with a
calendar user.
Daboo & Douglass Standards Track [Page 4]
RFC 7953 Calendar Availability August 2016
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
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RFC 7953 Calendar Availability August 2016
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" subcomponents.
"AVAILABLE" subcomponents are used to indicate periods of free
time within the time range of the enclosing "VAVAILABILITY"
component. "AVAILABLE" subcomponents 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).
Daboo & Douglass Standards Track [Page 6]
RFC 7953 Calendar Availability August 2016
If specified, the "DTSTART" and "DTEND" properties in
"VAVAILABILITY" components and "AVAILABLE" subcomponents MUST be
"DATE-TIME" values specified as either the date with UTC time or
the 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, unless [RFC7809] is in effect.
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 (but see Section 9 for when
it is appropriate to add additional data like this).
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:0428C7D2-688E-4D2E-AC52-CD112E2469DF
DTSTAMP:20111005T133225Z
BEGIN:AVAILABLE
UID:34EDA59B-6BB1-4E94-A66C-64999089C0AF
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
Daboo & Douglass Standards Track [Page 7]
RFC 7953 Calendar Availability August 2016
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:84D0F948-7FC6-4C1D-BBF3-BA9827B424B5
DTSTAMP:20111005T133225Z
DTSTART;TZID=America/Montreal:20111002T000000
DTEND;TZID=America/Montreal:20111202T000000
BEGIN:AVAILABLE
UID:7B33093A-7F98-4EED-B381-A5652530F04D
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:DF39DC9E-D8C3-492F-9101-0434E8FC1896
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 Denver, then back to Montreal. Note that each
overall period is covered by separate "VAVAILABILITY" components.
The last of these has no DTEND so it continues on "forever". This
example shows one way "blocks" of available time can be
represented. See Section 4 for another approach using priorities.
Daboo & Douglass Standards Track [Page 8]
RFC 7953 Calendar Availability August 2016
BEGIN:VAVAILABILITY
ORGANIZER:mailto:bernard@example.com
UID:BE082249-7BDD-4FE0-BDBA-DE6598C32FC9
DTSTAMP:20111005T133225Z
DTSTART;TZID=America/Montreal:20111002T000000
DTEND;TZID=America/Montreal:20111023T030000
BEGIN:AVAILABLE
UID:54602321-CEDB-4620-9099-757583263981
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:A1FF55E3-555C-433A-8548-BF4864B5621E
DTSTAMP:20111005T133225Z
DTSTART;TZID=America/Denver:20111023T000000
DTEND;TZID=America/Denver:20111030T000000
BEGIN:AVAILABLE
UID:57DD4AAF-3835-46B5-8A39-B3B253157F01
SUMMARY:Monday to Friday from 9:00 to 17:00
DTSTART;TZID=America/Denver:20111023T090000
DTEND;TZID=America/Denver:20111023T170000
RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
LOCATION:Denver
END:AVAILABLE
END:VAVAILABILITY
BEGIN:VAVAILABILITY
ORGANIZER:mailto:bernard@example.com
UID:1852F9E1-E0AA-4572-B4C4-ED1680A4DA40
DTSTAMP:20111005T133225Z
DTSTART;TZID=America/Montreal:20111030T030000
BEGIN:AVAILABLE
UID:D27C421F-16C2-4ECB-8352-C45CA352C72A
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
Daboo & Douglass Standards Track [Page 9]
RFC 7953 Calendar Availability August 2016
3.2. Busy Time Type
Property Name: BUSYTYPE
Purpose: This property specifies the default busy time type.
Value Type: TEXT
Property Parameters: IANA and nonstandard 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
Daboo & Douglass Standards Track [Page 10]
RFC 7953 Calendar Availability August 2016
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. That is,
if one component has a BUSYTYPE set to BUSY and the other 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 (the 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.
Typically, a calendar user's "default" availability (e.g., business
hours of Monday through Friday, 9:00 am to 5:00 pm) would use the
lowest level of priority: zero. Any overrides to the "default" would
use higher levels as needed. To avoid having to keep readjusting the
"PRIORITY" property value when an override has to be "inserted"
between two existing components, priority values SHOULD be "spaced
out" over the full range of values. The table below illustrates this
via an example. The first row shows the priority range from low to
high, the second row shows the corresponding "PRIORITY" property
value, and the third row shows which "VAVAILABILITY" component has
that priority. The "default" availability is created with priority
zero (shown as {a} in the table), then the first override created
with priority 5 (shown as {b} in the table), a subsequent
availability can be inserted between the two by using priority 7
(shown as {c} in the table), and another, taking precedence over all
existing ones, with priority 3 (shown as {d} in the table). As seen
in the table, additional "slots" are open for more "VAVAILABILITY"
components to be added with other priorities if needed.
Daboo & Douglass Standards Track [Page 11]
RFC 7953 Calendar Availability August 2016
+-----+----+----+-----+----+-----+----+-----+----+------+
| Low | | | | | | | | | High |
+-----+----+----+-----+----+-----+----+-----+----+------+
| 0 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
+-----+----+----+-----+----+-----+----+-----+----+------+
| {a} | | | {c} | | {b} | | {d} | | |
+-----+----+----+-----+----+-----+----+-----+----+------+
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 [RFC5546] free-busy 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 the 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 (lowest to
highest):
A. Determine if the "VAVAILABILITY" intersects the time range of
the free-busy request. If not, ignore it.
B. Determine if the "VAVAILABILITY" is completely overridden by
a higher priority component. If so, ignore it.
C. 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.
D. Append the "VAVAILABILITY" component to a list of components
for further processing in step 3, if it has not been ignored.
Daboo & Douglass Standards Track [Page 12]
RFC 7953 Calendar Availability August 2016
3. For each "VAVAILABILITY" component in the list resulting from
step 2, in order from the first item to the last item:
A. For each "AVAILABLE" component in the "VAVAILABILITY"
component:
i. 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.
ii. 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:
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.
I Represents data to be ignored in that slot (as per step 2.B
above).
5.1.1. Simple Example
Appendix A shows the user's calendar. 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.
Daboo & Douglass Standards Track [Page 13]
RFC 7953 Calendar Availability August 2016
A free-busy request for Monday, 6th November 2011, midnight to
midnight in the America/Montreal time zone would be calculated as
follows using the steps described above.
+------+----+----+----+----+----+----+----+----+----+----+----+----+
| 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
Appendix B shows another way to represent the availability of the
traveling worker shown above. Here we represent their base
availability of Monday through Friday, 8:00 am to 6:00 pm each day
with a "VAVAILABILITY" with default "PRIORITY" (there is no "DTEND"
property so that this availability is unbounded). For the week the
calendar user is working in Denver (October 23rd through October
30th), we represent their availability with a "VAVAILABILITY"
component with priority 1, which overrides the base availability.
There is also a two hour meeting starting at 12:00 pm (in the
America/Denver time zone).
A free-busy request for Monday, 24th October 2011, midnight to
midnight in the America/Montreal time zone, would be calculated as
follows using the steps described above. Note that there is a two
hour offset in the in the available time, compared to the previous
example, due to the two hour difference between the time zone of the
free-busy request and the time zone of the user's availability and
meeting. "2.P0" shows the base availability, and "2.P1" shows the
higher priority availability. "3.P1" only shows the higher priority
availability contributing to the overall free-busy since the default
availability is ignored (as per step 2.B described above).
+------+----+----+----+----+----+----+----+----+----+----+----+----+
| 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.P0 | I | I | I | I | I | I | I | I | I | I | I | I |
| 2.P1 | U | U | U | U | U | U | U | U | U | U | U | U |
| 3.P1 | U | U | U | U | U | F | F | F | F | F | U | U |
| 4. | U | U | U | U | U | F | F | B | F | F | U | U |
+------+----+----+----+----+----+----+----+----+----+----+----+----+
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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 "VAVAILABILITY" components in stored calendar data. A
server:
o MUST advertise support for "VAVAILABILITY" components in
CALDAV:supported-calendar-component-set properties on calendars
that 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 Web Distributed Authoring and
Versioning (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.
Processing of "VAVAILABILITY" components MUST conform to all the
requirements CalDAV imposes on calendar object resources (see
Section 4.1 of [RFC4791]).
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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 indicates to clients that the server supports all
the 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.
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+------------------------------------------------------------+
| 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.
7.2.4. 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 only one "VAVAILABILITY" component,
and optionally, "VTIMEZONE" components - other iCalendar
components MUST NOT be present. "VTIMEZONE" components SHOULD NOT
be present if [RFC7809] is in effect. For more complex
availability scenarios, clients can store multiple "VAVAILABILITY"
components in the calendar user's calendar collections.
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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.
Definition:
<!ELEMENT calendar-availability (#PCDATA) >
; Data value MUST be an 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:VAVAILABILITY
UID:9BADC1F6-0FC4-44BF-AC3D-993BEC8C962A
DTSTAMP:20111005T133225Z
DTSTART;TZID=America/Montreal:20111002T000000
BEGIN:AVAILABLE
UID:6C9F69C3-BDA8-424E-B2CB-7012E796DDF7
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>
7.2.5. iTIP Free-Busy Requests
The CalDAV calendar-auto-schedule feature (see Section 5 of
[RFC6638]) includes a mechanism for free-busy information to be
requested via the CalDAV protocol. Any "VAVAILABILITY" components in
any calendar collections targeted during such a request MUST be
included as part of the calculation of the overall free-busy
information. In addition, the "VAVAILABILITY" component specified in
the CALDAV:calendar-availability property on the owner's Inbox MUST
also be included in the free-busy calculation. Processing of all
such "VAVAILABILITY" components is done as per Section 5.
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8. Security Considerations
Calculation of availability information, particularly with multiple
overlapping time ranges, can be complex, and CalDAV servers MUST
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.
Security considerations in [RFC5545], [RFC5546], [RFC4791],
[RFC6638], and [RFC7809] MUST also be adhered to.
9. Privacy Considerations
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.
When "VAVAILABILITY" components are sent to or shared with other
calendar users, care has to be taken not to expose more information
than is needed by each recipient. For example, a business owner will
likely not want their customers to know where they might be or what
they might be doing, but family members might be willing to expose
such information to each other. Thus, calendaring systems allowing
"VAVAILABILITY" components to be sent or shared to other calendar
users MUST provide a way for nonessential properties to be removed
(e.g., "SUMMARY", "LOCATION", and "DESCRIPTION").
iCalendar "VFREEBUSY" 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.
Privacy considerations in [RFC5545], [RFC5546], [RFC4791], [RFC6638],
and [RFC7809] MUST also be adhered to.
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RFC 7953 Calendar Availability August 2016
10. IANA Considerations
10.1. Component Registrations
This document defines the following new iCalendar components, which
have been added to the registry defined in Section 8.3.1 of
[RFC5545]:
+---------------+---------+------------------------+
| Component | Status | Reference |
+---------------+---------+------------------------+
| VAVAILABILITY | Current | RFC 7953, Section 3.1 |
| AVAILABLE | Current | RFC 7953, Section 3.1 |
+---------------+---------+------------------------+
10.2. Property Registrations
This documents defines the following new iCalendar properties, which
have been added to the registry defined in Section 8.3.2 of
[RFC5545]:
+----------+---------+------------------------+
| Property | Status | Reference |
+----------+---------+------------------------+
| BUSYTYPE | Current | RFC 7953, Section 3.2 |
+----------+---------+------------------------+
11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 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, December 2009,
<http://www.rfc-editor.org/info/rfc5546>.
Daboo & Douglass Standards Track [Page 20]
RFC 7953 Calendar Availability August 2016
[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>.
Daboo & Douglass Standards Track [Page 21]
RFC 7953 Calendar Availability August 2016
Appendix A. Example Calendar #1
BEGIN:VCALENDAR
CALSCALE:GREGORIAN
PRODID:-//example.com//iCalendar 2.0//EN
VERSION:2.0
BEGIN:VEVENT
DTSTAMP:20111113T044111Z
DTSTART;TZID=America/Montreal:20111106T120000
DURATION:PT2H
SUMMARY:Meeting
UID:768CB0C2-8642-43F7-A6C4-F8BB04B829B4
END:VEVENT
BEGIN:VAVAILABILITY
UID:452DFCA7-3203-4A3D-9A9A-99753A383B41
DTSTAMP:20111005T133225Z
DTSTART;TZID=America/Montreal:20111002T000000
BEGIN:AVAILABLE
UID:466D5C68-5C4A-4078-AF5D-9C55EA9145D7
SUMMARY:Monday to Friday from 8:00 to 18:00
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
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Appendix B. Example Calendar #2
BEGIN:VCALENDAR
CALSCALE:GREGORIAN
PRODID:-//example.com//iCalendar 2.0//EN
VERSION:2.0
BEGIN:VEVENT
DTSTAMP:20111113T044111Z
DTSTART;TZID=America/Denver:20111106T120000
DURATION:PT2H
SUMMARY:Lunch meeting in Denver
UID:2346C09A-42BF-439E-916C-FC83AF869171
END:VEVENT
BEGIN:VAVAILABILITY
ORGANIZER:mailto:bernard@example.com
UID:627A87FA-E5F1-43C0-B3B1-567DA10F2A83
DTSTAMP:20111005T133225Z
DTSTART;TZID=America/Montreal:20111002T000000
BEGIN:AVAILABLE
UID:A833E850-892B-43F6-98B6-C15A6BFC5D27
SUMMARY:Monday to Friday from 9:00 to 17:00
DTSTART;TZID=America/Montreal:20111002T080000
DTEND;TZID=America/Montreal:20111002T180000
RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
LOCATION:Montreal
END:AVAILABLE
END:VAVAILABILITY
BEGIN:VAVAILABILITY
ORGANIZER:mailto:bernard@example.com
UID:F01411E3-38B8-4490-8A1F-0CCEC57A0943
DTSTAMP:20111005T133225Z
DTSTART;TZID=America/Denver:20111023T000000
DTEND;TZID=America/Denver:20111030T000000
PRIORITY:1
BEGIN:AVAILABLE
UID:A35AA091-3846-48ED-96F6-881E8A0D0A93
SUMMARY:Monday to Friday from 9:00 to 17:00
DTSTART;TZID=America/Denver:20111023T080000
DTEND;TZID=America/Denver:20111023T180000
RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR
LOCATION:Denver
END:AVAILABLE
END:VAVAILABILITY
END:VCALENDAR
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Acknowledgements
Thanks to the following for providing feedback: Toby Considine,
Bernard Desruisseaux, Alexey Melnikov, Daniel Migault, Ken Murchison,
Evert Pot, and Dave Thewlis. This specification came about via
discussions at the Calendaring and Scheduling Consortium.
Authors' Addresses
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
United States of America
Email: cyrus@daboo.name
URI: http://www.apple.com/
Michael Douglass
Spherical Cow Group
226 3rd Street
Troy, NY 12180
United States of America
Email: mdouglass@sphericalcowgroup.com
URI: http://sphericalcowgroup.com
Daboo & Douglass Standards Track [Page 24]