CalDAV: Time Zones by Reference
draft-ietf-tzdist-caldav-timezone-ref-00

Network Working Group                                           C. Daboo
Internet-Draft                                                     Apple
Intended status: Standards Track                         August 26, 2014
Expires: February 27, 2015

                    CalDAV: Time Zones by Reference
                draft-ietf-tzdist-caldav-timezone-ref-00

Abstract

   This document defines an extension to the CalDAV calendar access
   protocol to allow clients and servers to exchange iCalendar data
   without the need to send full time zone data.

Status of This Memo

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

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   This Internet-Draft will expire on February 27, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Time Zones by Reference . . . . . . . . . . . . . . . . . . .   3
     3.1.  New Server Behavior . . . . . . . . . . . . . . . . . . .   3
   4.  New Client Behavior . . . . . . . . . . . . . . . . . . . . .   6
   5.  New WebDAV Properties . . . . . . . . . . . . . . . . . . . .   7
     5.1.  CALDAV:timezone-service-set . . . . . . . . . . . . . . .   7
     5.2.  CALDAV:calendar-timezone-id . . . . . . . . . . . . . . .   7
   6.  XML Element Definitions . . . . . . . . . . . . . . . . . . .   8
     6.1.  CALDAV:calendar-query XML Element . . . . . . . . . . . .   8
     6.2.  CALDAV:timezone-id XML Element  . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  Change History (To be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The CalDAV [RFC4791] Calendar Access protocol allows clients to
   access calendar data stored on a server in the iCalendar [RFC5545]
   data format.  In iCalendar, calendar data that uses local time in any
   of its date and/or time values is specified as a date-time value in
   combination with a time zone identifier ("TZID" property parameter).
   The time zone identifier refers to a time zone definition (a
   "VTIMEZONE" component) that has all of the rules required to
   determine local time UTC offsets for the corresponding time zone.  In
   many cases, these "VTIMEZONE" components can be larger, octet-wise,
   than the events or tasks which make use of them.  However, iCalendar
   currently requires all iCalendar objects ("VCALENDAR" components)
   that refer to a time zone via its identifier to also include the
   corresponding "VTIMEZONE" component.  This leads to inefficiencies in
   the CalDAV protocol because large amounts of "VTIMEZONE" data are
   continuously being exchanged, and for the most part these time zone
   definitions are unchanging.  This is of particular problem to mobile
   or limited devices, with limited network bandwidth, cpu processing,
   and power constraints.

   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).

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   A new Time Zone Data Distribution Service [I-D.ietf-tzdist-service]
   protocol 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
   resulting 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 which allow 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.

   Discussion of this Internet-Draft is taking place on the tzdist
   working group mailing list <tzdist@ietf.org>.

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].

   Other notations used in this memo follow the notations of [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).  If CalDAV clients and servers also offer
   alternative means of sending or receiving iCalendar data, they MUST
   follow the requirements of those alternative protocols in regards to
   whether time zone definitions are required or not.

3.1.  New Server Behavior

3.1.1.  Associated Time Zone Data Distribution Service

   A CalDAV server MUST have one or more associated time zone services
   [I-D.ietf-tzdist-service] that provides data for the set of standard
   time zones known to the server and expected to be used by clients.  A
   CalDAV server advertises the set of time zone services it makes use
   of via a CALDAV:timezone-service-set WebDAV property (see

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   Section 5.1) defined on calendar home collections.  Clients can use
   the time zone data distribution services listed in this property to
   fetch up to date time zone definitions for the standard 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.  This can be
   beneficial to limited devices that might not be able to store the
   full set of standard time zone definitions, and might only "ship"
   with a limited set based on the region where the device is sold.

   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,
   taking care to refresh time zone data in a timely fashion.

3.1.2.  Do not Send Time Zones

   Observation and experiments have shown that, in the vast majority of
   cases, CalDAV clients typically ignore 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).

   Servers that follow this specification MUST NOT send "VTIMEZONE"
   components in iCalendar data to clients, when the time zone
   identifier and corresponding time zone definitions are available on
   any of the advertised time zone data distribution services.  If a
   non-standard time zone is used in the iCalendar data, servers MUST
   include the corresponding "VTIMEZONE" component, as they would in the
   absence of support for this extension.

3.1.3.  Enable Clients not to Send Time Zones

   In addition to servers not sending standard time zone definitions to
   clients in iCalendar data, this specification also allows clients not
   to include time zone definitions when sending iCalendar data to the
   server.  In order for that to work, clients need to know that the
   server is capable of handling that situation, as currently servers
   might not be able to.

   A server that allows clients to not send standard time zone
   definitions in iCalendar data MUST include "calendar-no-timezone" as
   a field in the DAV response header from an OPTIONS request on a
   calendar home collection or calendar collection.  Clients MUST check

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   for the presence of that field in the DAV response header before
   changing their behavior as per Section 4.

   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
   various choices:

   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 or other clients who subsequently request iCalendar data.

   2.  Servers can refuse to accept any unknown/non-standard time zones,
       in which case they MUST reject the HTTP request containing such
       data using a WebDAV precondition code of CALDAV:valid-timezone.

   3.  Servers can, with appropriate knowledge, map the unknown/non-
       standard time zone to a standard time zone definition that
       accurately matches the one supplied by the client.  In doing so,
       servers will need to re-write the iCalendar data to make use of
       the new standard time zone identifier chosen by the mapping
       procedure.

3.1.4.  Support 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 setting 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. i.e., 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

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   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.

3.1.5.  Support 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, that 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
   clients providing a time zone identifier for use in a calendar query
   REPORT using this new element.

   If a client attempts use 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 pre-condition 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

   Clients MUST expect servers not to include standard time zone
   definitions in any iCalendar data they receive from the server.
   Clients MUST retrieve standard time zone definitions from the set of
   time zone services advertised from the server, or a known, trusted,
   third party service.

   When a server advertises the "calendar-no-timezone" field in a DAV
   response header (as per Section 3.1.3):

   1.  Clients SHOULD remove standard time zone definitions from
       iCalendar data they send to the server, provided the
       corresponding time zone identifier is one available via any of
       the server's advertised time zone services.

   2.  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 services.  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 3.1.3).

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   3.  Clients SHOULD make use of the CALDAV:calendar-timezone-id WebDAV
       property (see Section 3.1.4) and CalDAV:timezone-id XML element
       (see Section 3.1.5) for specifying default and specific time
       zones to use in calendar queries executed by the server.

5.  New WebDAV Properties

5.1.  CALDAV:timezone-service-set

   Name:  timezone-service-set

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

   Purpose:  Specifies one or more time zone data distribution servers
      being used by the CalDAV server to provide standard time zone
      data.

   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
      [RFC4918]).

   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 3.1.1 for
      more details.

   Definition:

   <!ELEMENT timezone-service-set (DAV:href+)>
   DAV:href value: URI of a time zone data distribution service
   as defined by this specification.

   Example:

   <C:timezone-service-set
       xmlns:D="DAV:"
       xmlns:C="urn:ietf:params:xml:ns:caldav">
     <D:href>https://timezones.example.com</D:href>
   </C:timezone-service-set>

5.2.  CALDAV:calendar-timezone-id

   Name:  calendar-timezone-id

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

   Purpose:  Specifies a time zone identifier for a calendar collection.

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   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 3.1.4 for more details.

   Definition:

   <!ELEMENT calendar-timezone-id (#PCDATA)>
   PCDATA value: an time zone identifier.

   Example:

   <C:calendar-timezone-id
       xmlns:C="urn:ietf:params:xml:ns:caldav">US-Eastern<
   /C:calendar-timezone-id>

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:

   Definition:

   <!ELEMENT calendar-query ((DAV:allprop |
                              DAV:propname |
                              DAV:prop)?, filter,
                              (timezone | timezone-id)?)>

6.2.  CALDAV:timezone-id XML Element

   Name:  timezone-id

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

   Purpose:  Specifies the time zone identifier for a time zone
      component to use when determining the results of a report.

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   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 3.1.5 for more
      details.

   Definition:

   <!ELEMENT timezone-id (#PCDATA)>
   PCDATA value: an time zone identifier.

7.  Security Considerations

   This specification does not introduce any new security concerns
   beyond those addressed in CalDAV [RFC4791] and iCalendar [RFC5545].

8.  IANA Considerations

   No IANA actions are needed.

9.  Acknowledgments

   Thanks to Mike Douglass, Andrew McMillan, and Ken Murchison.  This
   specification came about via discussions at the Calendaring and
   Scheduling Consortium.

10.  Normative References

   [I-D.ietf-tzdist-service]
              Douglass, M. and C. Daboo, "Time Zone Data Distribution
              Service", draft-ietf-tzdist-service-00 (work in progress),
              August 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              March 2007.

   [RFC4918]  Dusseault, L., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

   [RFC5545]  Desruisseaux, B., "Internet Calendaring and Scheduling
              Core Object Specification (iCalendar)", RFC 5545,
              September 2009.

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   [RFC6557]  Lear, E. and P. Eggert, "Procedures for Maintaining the
              Time Zone Database", BCP 175, RFC 6557, February 2012.

   [RFC6638]  Daboo, C. and B. Desruisseaux, "Scheduling Extensions to
              CalDAV", RFC 6638, June 2012.

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

   Changes in -00:

   1.  Initial WG draft derived from draft-daboo-caldav-timezone-ref-01,
       with some terminology changes to match WG name.

   2.  "timezone" -> "time zone" (https://tools.ietf.org/wg/tzdist/trac/
       ticket/6).

Author's Address

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

   Email: cyrus@daboo.name
   URI:   http://www.apple.com/

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