Internet Engineering Task Force (IETF) C. Daboo
Request for Comments: 7809 Apple
Updates: 4791 March 2016
Category: Standards Track
Calendaring Extensions to WebDAV (CalDAV): Time Zones by Reference
This document defines an update to the Calendaring Extensions to
WebDAV (CalDAV) calendar access protocol (RFC 4791) to allow clients
and servers to exchange iCalendar data without the need to send full
time zone data.
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 5741.
Information about the current status of this document, any errata,
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A set of standard time zone definitions are available at the IANA-
hosted time zone database [RFC6557]. That database provides the
"raw" data for time zone definitions, and those can be converted into
iCalendar "VTIMEZONE" components for use in iCalendar applications,
as well as converted into other formats for use by other applications
(e.g., "zoneinfo" files often found on Unix-based operating systems).
A new time zone data distribution service protocol [RFC7808] is
available that allows iCalendar applications to retrieve these
standard time zone definitions in a timely and accurate fashion,
instead of relying on possibly infrequent system updates of time zone
data that frequently result in mismatched calendar data and thus
missed meetings between calendar users. Another benefit of the time
zone data distribution service is that it provides a single
"reference" for standard time zone data that CalDAV clients and
servers can make use of to "agree" on standard time zone definitions,
and thus eliminate the need to exchange the data for those.
This specification defines a new mode of operation for CalDAV clients
and servers that allows them to exchange iCalendar data without the
need to send "VTIMEZONE" components for known, standard time zone
definitions. This can significantly reduce the amount of data that
needs to be sent between client and server, giving rise to
performance and efficiency improvements for each of them.
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
Other notations used in this memo are as in [RFC4791].
3. Time Zones by Reference
Note that this specification only defines changes to iCalendar data
sent or received via the CalDAV protocol (both [RFC4791] and
[RFC6638], and extensions). These changes do not apply to other
means of exchanging calendar data, such as scheduling mechanisms
based on the iCalendar Transport-Independent Interoperability
Protocol (iTIP) [RFC5546], e.g., the iCalendar Message-Based
Interoperability Protocol (iMIP) [RFC6047], or other methods.
3.1. New Server Behavior
3.1.1. Server Advertised Capability
A server that supports this specification MUST include "calendar-no-
timezone" as a field in the DAV response header field from an
"OPTIONS" request on a calendar home collection (see Section 6.2.1 of
[RFC4791]) or calendar collection (see Section 4.2 of [RFC4791]).
Clients MUST check for the presence of that field in the DAV response
header field before changing their behavior as per Section 4.
3.1.2. Associated Time Zone Data Distribution Service
A CalDAV server supporting this specification MUST have one or more
associated time zone distribution services [RFC7808] that provide
data for the set of time zones known to the server and expected to be
used by clients. A CalDAV server advertises the set of time zone
distribution services it makes use of via a CALDAV:timezone-service-
set WebDAV property (see Section 5.1) defined on calendar home
collections. Clients can use the time zone data distribution
services listed in this property to fetch current time zone
definitions for the time zone identifiers in iCalendar data retrieved
from the server. This allows clients to keep their "built-in" time
zone definitions up to date. It also allows clients to use an "on-
demand" model for populating their local time zone definition cache,
only fetching a time zone definition when it is first seen in
calendar data, potentially allowing for savings on storage space by
eliminating the need to store time zone data that is not currently
When making use of the time zone data distribution services
advertised by a CalDAV server, clients MUST follow all the
requirements of the time zone data distribution service protocol
[RFC7808], taking care to refresh time zone data in a timely fashion.
3.1.3. Time Zones in CalDAV Responses
Servers MUST support the HTTP "CalDAV-Timezones" request header field
(see Section 7.1). If the "CalDAV-Timezones" request header field
has the value "T" on any HTTP request that returns iCalendar data,
then the server MUST include all the appropriate "VTIMEZONE"
components in the iCalendar data (all the ones that are referenced by
"TZID" property parameters). If the "CalDAV-Timezones" request
header field has the value "F" on any HTTP request that returns
iCalendar data, then the server MUST NOT return any "VTIMEZONE"
components if the time zone identifier matches one provided by any of
the advertised time zone distribution servers (see Section 3.1.2).
However, the server MUST return the appropriate "VTIMEZONE" component
for each time zone with an identifier not available on the advertised
time zone distribution servers. This behavior applies to all HTTP
requests on CalDAV resources that return iCalendar data either
directly (such as a "GET" request on a calendar object resource), or
embedded in a "structured" response such as a DAV:multistatus
returned by a "REPORT" or "PROPFIND" request.
Observation and experiments have shown that, in the vast majority of
cases, CalDAV clients have typically ignored time zone definitions in
data received from servers, and instead make use of their own "built-
in" definitions for the corresponding time zone identifier. This
means that it is reasonable for CalDAV servers to unilaterally decide
not to send "VTIMEZONE" components for standard time zones that
clients are expected to have "built-in" (i.e., IANA time zones).
Thus, in the absence of a "CalDAV-Timezones" request header field,
servers advertising the "calendar-no-timezone" capability MAY opt to
not send standard "VTIMEZONE" components. Servers that do that will
need to provide an administrator configuration setting to override
the new default behavior based on client "User-Agent" request header
field values, or other suitable means of identifying the client
software in use.
3.1.4. Time Zones in CalDAV Requests
In addition to servers not sending time zone definitions to clients
in iCalendar data, this specification also allows clients to not
include time zone definitions when sending iCalendar data to the
server, as per Section 4. This behavior applies to all HTTP requests
on CalDAV resources that include iCalendar data either directly in
the request body (such as a "PUT" request on a calendar object
resource) or embedded in a "structured" request body such as a one
used by a "PROPPATCH" request.
Note that, as per Section 4, clients might send time zone definitions
for time zones that are not advertised by any of the time zone
services associated with the server. In that case, servers have
1. Servers can preserve the original time zone definitions in the
iCalendar data sent by the client, so that those can be returned
to that client or other clients who subsequently request
2. Servers can refuse to accept any unknown/nonstandard time zones
-- in which case, they MUST reject the HTTP request containing
such data using a WebDAV precondition code of
3. Servers can, with appropriate knowledge, map the unknown/
nonstandard time zone to a standard time zone definition that
accurately matches the one supplied by the client. In doing so,
servers will need to rewrite the iCalendar data to make use of
the new standard time zone identifier chosen by the mapping
procedure. Any subsequent request to fetch the calendar data
would see the new time zone identifier in the calendar data.
Note there is one important situation where this remapping is not
appropriate: an attendee's copy of an event. In that case, the
original time zone definition needs to be preserved as the
organizer's calendar user agent will expect to see that in any
iTIP [RFC5546] replies sent by the attendee.
3.1.5. Support of Time Zone Identifiers in WebDAV Properties
CalDAV defines a CALDAV:calendar-timezone WebDAV property that is
used by clients to set a default time zone for the server to use when
doing time-based queries on calendar data (see Section 5.3.2 of
[RFC4791]). The content of that WebDAV property is an iCalendar
"VTIMEZONE" component. This specification defines a new
CALDAV:calendar-timezone-id WebDAV property that allows the default
time zone to be set via its time zone identifier, rather than
providing the full "VTIMEZONE" component (see Section 5.2). This
WebDAV property MUST be present on all resources that also support
the CALDAV:calendar-timezone WebDAV property. Its value MUST match
the value of the "TZID" iCalendar property in the "VTIMEZONE"
component in the CALDAV:calendar-timezone WebDAV property on the same
resource. The server MUST accept clients that set either the
CALDAV:calendar-timezone or the CALDAV:calendar-timezone-id, and it
MUST adjust the value of the alternate property to reflect any
changes. That is, if a client sets the CALDAV:calendar-timezone-id
WebDAV property value to "America/New_York", then the server will
return the full "VTIMEZONE" data for that time zone in the
CALDAV:calendar-timezone WebDAV property.
If a client attempts to update the CALDAV:calendar-timezone-id with a
value that does not correspond to a time zone that is known to the
server, the server MUST reject the property update using a
CALDAV:valid-timezone pre-condition error. In such cases, clients
MAY repeat the request using the CALDAV:calendar-timezone instead,
and provide the full iCalendar data for the time zone being set.
3.1.6. Support of Time Zone Identifiers in CALDAV:calendar-query REPORT
CalDAV calendar query reports support a CALDAV:timezone XML element
that is used by clients to set a specific time zone for the server to
use when doing time-based queries on calendar data (see Sections 7.3
and 9.8 of [RFC4791]). The content of that XML element is an
iCalendar "VTIMEZONE" component. This specification defines a new
CALDAV:timezone-id XML element that can be used as an alternative to
the CALDAV:timezone XML element; it allows a specific time zone to be
set via its time zone identifier, rather than providing the full
"VTIMEZONE" component (see Section 6.2). Servers MUST support a
client's ability to provide a time zone identifier for use in a
calendar query "REPORT" using this new element.
If a client attempts use of a CALDAV:timezone-id XML element with a
value that does not correspond to a time zone that is known to the
server, the server MUST reject the request with a CALDAV:valid-
timezone precondition error. In such cases, clients MAY repeat the
request using the CALDAV:timezone XML element instead, and provide
the full iCalendar data for the time zone being used.
4. New Client Behavior
When a server advertises the "calendar-no-timezone" field in a DAV
response header field (as per Section 3.1.1):
1. Clients SHOULD include an HTTP "CalDAV-Timezones" request header
field with a value of "F" to ensure that the CalDAV server does
not include "VTIMEZONE" components in any iCalendar data returned
in a response (see Section 3.1.3), for those time zones whose
identifier is one provided by any of the advertised time zone
distribution servers (see Section 3.1.2). In this case, clients
will have to retrieve the missing standard time zone definitions
either from their own cache of standard time zones or from the
set of time zone distribution servers advertised by the CalDAV
server (see Section 3.1.2).
2. Clients can include an HTTP "CalDAV-Timezones" request header
field with a value of "T" to ensure that the CalDAV server does
include all "VTIMEZONE" components in any iCalendar data returned
in a response (see Section 3.1.3).
3. Clients can expect servers not to include standard time zone
definitions in any iCalendar data they receive from the server,
if there is no "CalDAV-Timezones" request header field in the
HTTP request. Clients MUST retrieve standard time zone
definitions either from its own cache of standard time zones or
from the set of time zone distribution servers advertised by the
CalDAV server (see Section 3.1.2).
4. Clients SHOULD remove standard time zone definitions from
iCalendar data they send to the server, provided the
corresponding time zone identifier is one available on any of the
server's advertised time zone distribution servers (see
5. Clients MUST send time zone definitions in iCalendar data for any
time zone identifiers not available via any of the server's
advertised time zone distribution servers. Clients MUST be
prepared for the server to reject such data or map the time zone
to one in the set of standard time zones provided by the server's
associated time zone services (as per Section 3.1.4).
6. Clients SHOULD make use of the CALDAV:calendar-timezone-id WebDAV
property (see Section 3.1.5) and CalDAV:timezone-id XML element
(see Section 3.1.6) for specifying default and specific time
zones to use in calendar queries executed by the server.
5. New WebDAV Properties
Purpose: Specifies one or more time zone data distribution servers
being used by the CalDAV server to provide standard time zone
Conformance: This property SHOULD be defined on CalDAV calendar home
collection resources. If defined, it SHOULD NOT be returned by a
"PROPFIND" DAV:allprop request (as defined in Section 14.2 of
Description: The CALDAV:timezone-service-set property lists one or
more time zone data distribution servers that the CalDAV server is
using to provide its set of time zone data. See Section 3.1.2 for
<!ELEMENT timezone-service-set (DAV:href+)>
DAV:href value: URI of a time zone data distribution service
as defined by this specification.
Purpose: Specifies a time zone identifier for a calendar collection.
Conformance: This property SHOULD be defined on all resources where
the CALDAV:calendar-timezone property is also defined. If
defined, it SHOULD NOT be returned by a "PROPFIND" DAV:allprop
request (as defined in Section 14.2 of [RFC4918]).
Description: The CALDAV:calendar-timezone-id property is used as an
alternative to the CALDAV:calendar-timezone property (see
Section 5.3.2 of [RFC4791]). It allows clients to set the default
time zone using only a time zone identifier. It also indicates to
the client the time zone identifier of the current default time
zone. See Section 3.1.5 for more details.
<!ELEMENT calendar-timezone-id (#PCDATA)>
PCDATA value: a time zone identifier.
6. XML Element Definitions
6.1. CALDAV:calendar-query XML Element
The CALDAV:calendar-query XML element, defined in Section 9.5 of
[RFC4791], is modified to allow use of the CALDAV:timezone-id XML
element as follows.
<!ELEMENT calendar-query ((DAV:allprop |
(timezone | timezone-id)?)>
6.2. CALDAV:timezone-id XML Element
Purpose: Specifies the time zone identifier for a time zone
component to use when determining the results of a report.
Description: The CALDAV:timezone-id XML element is used as an
alternative to the CALDAV:timezone XML element (see Section 9.8 of
[RFC4791]) in calendar query reports, to allow a client to specify
a time zone using a time zone identifier rather than providing the
full iCalendar time zone data. See Section 3.1.6 for more
<!ELEMENT timezone-id (#PCDATA)>
PCDATA value: a time zone identifier.
7. Additional Message Header Fields
7.1. CalDAV-Timezones Request Header Field
The "CalDAV-Timezones" request header field provides a way for a
client to indicate to the server whether it wants "VTIMEZONE"
components returned in any iCalendar data that is part of the HTTP
response. The value "T" indicates that the client wants time zone
data returned; the value "F" indicates that it does not.
CalDAV-Timezones = "T" / "F"
8. Security Considerations
This specifications adds time zone data distribution service
[RFC7808] servers into the overall calendaring and scheduling client/
server architecture, as a critical component, and thus adds a new
vector of attack against such systems. As such, administrators of
CalDAV servers SHOULD ensure that any advertised time zone
distribution servers are protected by a level of security
commensurate with all the other components in the system.
Besides the above point, this specification does not introduce any
new security concerns beyond those addressed in CalDAV [RFC4791],
iCalendar [RFC5545], and the time zone data distribution service
9. Privacy Considerations
The privacy recommendations in Section 9 of the time zone data
distribution service specification [RFC7808] SHOULD be used to ensure
that details of clients' interactions with CalDAV servers are not
exposed to potential network observers. Note that since events can
be delivered to a calendar user from an outside source (e.g., using
iTIP [RFC5546]), and an attacker could create a calendar event with,
e.g., a time zone identifier that is fake or rarely used and that
could be used to monitor the calendar user's activity and interaction
with others, this specification increases the importance of using the
mitigations of privacy issues discussed in [RFC7808].
10. IANA Considerations
The message header field below has been added to the Permanent
Message Header Field Registry (see [RFC3864]).
Header field name: CalDAV-Timezones
Applicable protocol: http
Author/Change controller: IETF
Specification document(s): this document (Section 7.1)
Related information: none