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Versions: 00 01 draft-ietf-tzdist-caldav-timezone-ref

Network Working Group                                           C. Daboo
Internet-Draft                                                     Apple
Intended status: Standards Track                          March 15, 2014
Expires: September 16, 2014


                     CalDAV: Timezones by Reference
                   draft-daboo-caldav-timezone-ref-01

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 timezone data.

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
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   This Internet-Draft will expire on September 16, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Timezones by Reference . . . . . . . . . . . . . . . . . . . .  4
     3.1.  New Server Behavior  . . . . . . . . . . . . . . . . . . .  4
   4.  New Client Behavior  . . . . . . . . . . . . . . . . . . . . .  7
   5.  New WebDAV Properties  . . . . . . . . . . . . . . . . . . . .  7
     5.1.  CALDAV:timezone-service-set  . . . . . . . . . . . . . . .  7
     5.2.  CALDAV:calendar-timezone-id  . . . . . . . . . . . . . . .  8
   6.  XML Element Definitions  . . . . . . . . . . . . . . . . . . .  9
     6.1.  CALDAV:calendar-query XML Element  . . . . . . . . . . . .  9
     6.2.  CALDAV:timezone-id XML Element . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

































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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 timezone identifier ("TZID" property parameter).
   The timezone identifier refers to a timezone definition (a
   "VTIMEZONE" component) that has all of the rules required to
   determine local time UTC offsets for the corresponding timezone.  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 timezone 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 timezone
   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 timezone definitions are available at the IANA
   Timezone Registry [RFC6557].  That registry provides the "raw" data
   for timezone 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
   Timezone Service [I-D.douglass-timezone-service] protocol is
   available that allows iCalendar applications to retrieve these
   standard timezone definitions in a timely and accurate fashion,
   instead of relying on possibly infrequent system updates of timezone
   data that frequently result in mismatched calendar data and resulting
   missed meetings between calendar users.  Another benefit of the
   Timezone Service is that it provides a single "reference" for
   standard timezone data that CalDAV clients and servers can make use
   of to "agree" on standard timezone 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 timezone
   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 mailing list
   <https://www.ietf.org/mailman/listinfo/caldav>.




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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.  Timezones 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 timezone definitions are required or not.

3.1.  New Server Behavior

3.1.1.  Associated Timezone Service

   A CalDAV server MUST have one or more associated timezone services
   [I-D.douglass-timezone-service] that provides data for the set of
   standard timezones known to the server and expected to be used by
   clients.  A CalDAV server advertises the set of timezone 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 timezone services listed in this property to fetch up to date
   timezone definitions for the standard timezone identifiers in
   iCalendar data retrieved from the server.  This allows clients to
   keep their "built-in" timezone definitions up to date.  It also
   allows clients to use an "on-demand" model for populating their local
   timezone definition cache, only fetching a timezone 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
   timezone definitions, and might only "ship" with a limited set based
   on the region where the device is sold.

   When making use of the timezone services advertised by a CalDAV
   server, clients MUST follow all the requirements of the timezone
   service protocol, taking care to refresh timezone data in a timely
   fashion.







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3.1.2.  Do not Send Timezones

   Observation and experiments have shown that, in the vast majority of
   cases, CalDAV clients typically ignore timezone definitions in data
   received from servers, and instead make use of their own "built-in"
   definitions for the corresponding timezone identifier.  This means
   that it is reasonable for CalDAV servers to unilaterally decide not
   to send "VTIMEZONE" components for standard timezones that clients
   are expected to have "built-in" (i.e., IANA timezones).

   Servers that follow this specification MUST NOT send "VTIMEZONE"
   components in iCalendar data to clients, when the timezone identifier
   and corresponding timezone definitions are available on any of the
   advertised timezone services.  If a non-standard timezone 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 Timezones

   In addition to servers not sending standard timezone definitions to
   clients in iCalendar data, this specification also allows clients not
   to include timezone 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 timezone
   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
   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 timezone definitions
   for timezones that are not advertised by any of the timezone services
   associated with the server.  In that case, servers have various
   choices:

   1.  Servers can preserve the original timezone 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 timezones,
       in which case they MUST reject the HTTP request containing such
       data using a WebDAV precondition code of CALDAV:valid-timezone.





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   3.  Servers can, with appropriate knowledge, map the unknown/
       non-standard timezone to a standard timezone 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 timezone identifier chosen by the mapping
       procedure.

3.1.4.  Support Timezone Identifiers in WebDAV Properties

   CalDAV defines a CALDAV:calendar-timezone WebDAV property that is
   used by clients to set a default timezone 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 timezone
   to be set via its timezone 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 timezone 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 timezone 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 timezone being set.

3.1.5.  Support Timezone 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 timezone 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 timezone to
   be set via its timezone identifier, rather than providing the full
   "VTIMEZONE" component (see Section 6.2).  Servers MUST support
   clients providing a timezone identifier for use in a calendar query
   REPORT using this new element.



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   If a client attempts use a CALDAV:timezone-id XML element with a
   value that does not correspond to a timezone 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 timezone being used.


4.  New Client Behavior

   Clients MUST expect servers not to include standard timezone
   definitions in any iCalendar data they receive from the server.
   Clients MUST retrieve standard timezone definitions from the set of
   timezone 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 timezone definitions from
       iCalendar data they send to the server, provided the
       corresponding timezone identifier is one available via any of the
       server's advertised timezone services.

   2.  Clients MUST send timezone definitions in iCalendar data for any
       timezone identifiers not available via any of the server's
       advertised timezone services.  Clients MUST be prepared for the
       server to reject such data or map the timezone to one in the set
       of standard timezones provided by the server's associated
       timezone services (as per Section 3.1.3).

   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 timezones
       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







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   Purpose:  Specifies one or more timezone servers being used by the
      CalDAV server to provide standard timezone 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 timezone servers that the CalDAV server is using to provide
      its set of timezone data.  See Section 3.1.1 for more details.

   Definition:

   <!ELEMENT timezone-service-set (DAV:href+)>
   DAV:href value: URI of a timezone 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 timezone 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
      timezone using only a timezone identifier.  It also indicates to
      the client, the timezone identifier of the current default
      timezone.  See Section 3.1.4 for more details.






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   Definition:

   <!ELEMENT calendar-timezone-id (#PCDATA)>
   PCDATA value: an timezone 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 timezone identifier for a timezone 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 timezone using a timezone identifier rather than providing the
      full iCalendar timezone data.  See Section 3.1.5 for more details.

   Definition:

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





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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.douglass-timezone-service]
              Douglass, M. and C. Daboo, "Timezone Service Protocol",
              draft-douglass-timezone-service-11 (work in progress),
              March 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.

   [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.







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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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