[Docs] [txt|pdf] [draft-ietf-tzdist...]

PROPOSED STANDARD

Internet Engineering Task Force (IETF)                       M. Douglass
Request for Comments: 7808                           Spherical Cow Group
Category: Standards Track                                       C. Daboo
ISSN: 2070-1721                                                    Apple
                                                              March 2016


                  Time Zone Data Distribution Service

Abstract

   This document defines a time zone data distribution service that
   allows reliable, secure, and fast delivery of time zone data and
   leap-second rules to client systems such as calendaring and
   scheduling applications or operating systems.

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,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7808.

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.







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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   5
   3.  General Considerations  . . . . . . . . . . . . . . . . . . .   7
     3.1.  Time Zone . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Time Zone Data  . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Time Zone Metadata  . . . . . . . . . . . . . . . . . . .   7
     3.4.  Time Zone Data Server . . . . . . . . . . . . . . . . . .   7
     3.5.  Observance  . . . . . . . . . . . . . . . . . . . . . . .   7
     3.6.  Time Zone Identifiers . . . . . . . . . . . . . . . . . .   7
     3.7.  Time Zone Aliases . . . . . . . . . . . . . . . . . . . .   8
     3.8.  Time Zone Localized Names . . . . . . . . . . . . . . . .   8
     3.9.  Truncating Time Zones . . . . . . . . . . . . . . . . . .   9
     3.10. Time Zone Versions  . . . . . . . . . . . . . . . . . . .  10
   4.  Time Zone Data Distribution Service Protocol  . . . . . . . .  10
     4.1.  Server Protocol . . . . . . . . . . . . . . . . . . . . .  10
       4.1.1.  Time Zone Queries . . . . . . . . . . . . . . . . . .  11
       4.1.2.  Time Zone Formats . . . . . . . . . . . . . . . . . .  11
       4.1.3.  Time Zone Localization  . . . . . . . . . . . . . . .  12
       4.1.4.  Conditional Time Zone Requests  . . . . . . . . . . .  12
       4.1.5.  Expanded Time Zone Data . . . . . . . . . . . . . . .  14
       4.1.6.  Server Requirements . . . . . . . . . . . . . . . . .  14
       4.1.7.  Error Responses . . . . . . . . . . . . . . . . . . .  14
       4.1.8.  Extensions  . . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Client Guidelines . . . . . . . . . . . . . . . . . . . .  14
       4.2.1.  Discovery . . . . . . . . . . . . . . . . . . . . . .  14
         4.2.1.1.  SRV Service Labels for the Time Zone Data
                   Distribution Service  . . . . . . . . . . . . . .  15
         4.2.1.2.  TXT Records for a Time Zone Data Distribution
                   Service . . . . . . . . . . . . . . . . . . . . .  15
         4.2.1.3.  Well-Known URI for a Time Zone Data Distribution
                   Service . . . . . . . . . . . . . . . . . . . . .  16
           4.2.1.3.1.  Example: Well-Known URI Redirects to Actual
                       Context Path  . . . . . . . . . . . . . . . .  17
       4.2.2.  Synchronization of Time Zones . . . . . . . . . . . .  17
         4.2.2.1.  Initial Synchronization of All Time Zones . . . .  17
         4.2.2.2.  Subsequent Synchronization of All Time Zones  . .  17
         4.2.2.3.  Synchronization with Preexisting Time Zone Data .  18
   5.  Actions . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     5.1.  "capabilities" Action . . . . . . . . . . . . . . . . . .  18
       5.1.1.  Example: get capabilities . . . . . . . . . . . . . .  19
     5.2.  "list" Action . . . . . . . . . . . . . . . . . . . . . .  21
       5.2.1.  Example: List Time Zone Identifiers . . . . . . . . .  22
     5.3.  "get" Action  . . . . . . . . . . . . . . . . . . . . . .  23
       5.3.1.  Example: Get Time Zone Data . . . . . . . . . . . . .  24
       5.3.2.  Example: Conditional Get Time Zone Data . . . . . . .  25



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       5.3.3.  Example: Get Time Zone Data Using a Time Zone Alias .  25
       5.3.4.  Example: Get Truncated Time Zone Data . . . . . . . .  26
       5.3.5.  Example: Request for a Nonexistent Time Zone  . . . .  27
     5.4.  "expand" Action . . . . . . . . . . . . . . . . . . . . .  27
       5.4.1.  Example: Expanded JSON Data Format  . . . . . . . . .  29
     5.5.  "find" Action . . . . . . . . . . . . . . . . . . . . . .  30
       5.5.1.  Example: find action  . . . . . . . . . . . . . . . .  31
     5.6.  "leapseconds" Action  . . . . . . . . . . . . . . . . . .  32
       5.6.1.  Example: Get Leap-Second Information  . . . . . . . .  33
   6.  JSON Definitions  . . . . . . . . . . . . . . . . . . . . . .  34
     6.1.  capabilities Action Response  . . . . . . . . . . . . . .  34
     6.2.  list/find Action Response . . . . . . . . . . . . . . . .  37
     6.3.  expand Action Response  . . . . . . . . . . . . . . . . .  38
     6.4.  leapseconds Action Response . . . . . . . . . . . . . . .  39
   7.  New iCalendar Properties  . . . . . . . . . . . . . . . . . .  40
     7.1.  Time Zone Upper Bound . . . . . . . . . . . . . . . . . .  40
     7.2.  Time Zone Identifier Alias Property . . . . . . . . . . .  41
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  42
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  43
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  44
     10.1.  Service Actions Registration . . . . . . . . . . . . . .  45
       10.1.1.  Service Actions Registration Procedure . . . . . . .  45
       10.1.2.  Registration Template for Actions  . . . . . . . . .  46
       10.1.3.  Actions Registry . . . . . . . . . . . . . . . . . .  47
     10.2.  timezone Well-Known URI Registration . . . . . . . . . .  47
     10.3.  Service Name Registrations . . . . . . . . . . . . . . .  47
       10.3.1.  timezone Service Name Registration . . . . . . . . .  47
       10.3.2.  timezones Service Name Registration  . . . . . . . .  48
     10.4.  TZDIST Identifiers Registry  . . . . . . . . . . . . . .  48
       10.4.1.  Registration of invalid-action Error URN . . . . . .  49
       10.4.2.  Registration of invalid-changedsince Error URN . . .  49
       10.4.3.  Registration of tzid-not-found Error URN . . . . . .  50
       10.4.4.  Registration of invalid-format Error URN . . . . . .  50
       10.4.5.  Registration of invalid-start Error URN  . . . . . .  50
       10.4.6.  Registration of invalid-end Error URN  . . . . . . .  51
       10.4.7.  Registration of invalid-pattern Error URN  . . . . .  51
     10.5.  iCalendar Property Registrations . . . . . . . . . . . .  52
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  52
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  52
     11.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  55
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  56









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1.  Introduction

   Time zone data typically combines a coordinated universal time (UTC)
   offset with daylight saving time (DST) rules.  Time zones are
   typically tied to specific geographic and geopolitical regions.
   Whilst the UTC offset for particular regions changes infrequently,
   DST rules can change frequently and sometimes with very little notice
   (maybe hours before a change comes into effect).

   Calendaring and scheduling systems, such as those that use iCalendar
   [RFC5545], as well as operating systems, critically rely on time zone
   data to determine the correct local time.  As such, they need to be
   kept up to date with changes to time zone data.  To date, there has
   been no fast and easy way to do that.  Time zone data is often
   supplied in the form of a set of data files that have to be
   "compiled" into a suitable database format for use by the client
   application or operating system.  In the case of operating systems,
   often those changes only get propagated to client machines when there
   is an operating system update, which can be infrequent, resulting in
   inaccurate time zone data being present for significant amounts of
   time.  In some cases, old versions of operating systems stop being
   supported, but are still in use and thus require users to manually
   "patch" their system to keep up to date with time zone changes.

   Along with time zone data, it is also important to track the use of
   leap seconds to allow a mapping between International Atomic Time
   (TAI) and UTC.  Leap seconds can be added (or possibly removed) at
   various times of year in an irregular pattern typically determined by
   precise astronomical observations.  The insertion of leap seconds
   into UTC is currently the responsibility of the International Earth
   Rotation Service.

   This specification defines a time zone data distribution service
   protocol that allows for fast, reliable, and accurate delivery of
   time zone data and leap-second information to client systems.  This
   protocol is based on HTTP [RFC7230] using a simple JSON-based API
   [RFC7159].

   This specification does not define the source of the time zone data
   or leap-second information.  It is assumed that a reliable and
   accurate source is available.  One such source is the IANA-hosted
   time zone database [RFC6557].

1.1.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].



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   Unless otherwise indicated, UTC date-time values as specified in
   [RFC3339] use a "Z" suffix, and not fixed numeric offsets.

   This specification contains examples of HTTP requests and responses.
   In some cases, additional line breaks have been introduced into the
   request or response data to match maximum line-length limits of this
   document.

2.  Architectural Overview

   The overall process for the delivery of time zone data can be
   visualized via the diagram below.

               ====================  ====================
   (a)         |   Contributors   |  |   Contributors   |
               ====================  ====================
                         |                    |
               ====================  ====================
   (b)         |   Publisher A    |  |   Publisher B    |
               ====================  ====================
                           \           /
                        ====================
   (c)                  |  Root Provider   |
                        ====================
                       /            |       \
                      /             |        \
           ======================   |  ======================
   (d)     | Secondary Provider |   |  | Secondary Provider |
           ======================   |  ======================
             |           |          |              |
             |           |          |              |
        ==========  ==========  ==========      ==========
   (e)  | Client |  | Client |  | Client |      | Client |
        ==========  ==========  ==========      ==========

        Figure 1: Time Zone Data Distribution Service Architecture

   The overall service is made up of several layers:

   (a) Contributors:  Individuals, governments, or organizations that
       provide information about time zones to the publishing process.
       There can be many contributors.  Note this specification does not
       address how contributions are made.








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   (b) Publishers:  Publishers aggregate information from contributors,
       determine the reliability of the information and, based on that,
       generate time zone data.  There can be many publishers, each
       getting information from many different contributors.  In some
       cases, a publisher may choose to "republish" data from another
       publisher.

   (c) Root Providers:  Servers that obtain and then provide the time
       zone data from publishers and make that available to other
       servers or clients.  There can be many root providers.  Root
       providers can choose to supply time zone data from one or more
       publishers.

   (d) Secondary Providers:  Servers that handle the bulk of the
       requests and reduce the load on root servers.  These will
       typically be simple, caches of the root server, located closer to
       clients.  For example a large Internet Service Provider (ISP) may
       choose to set up their own secondary provider to allow clients
       within their network to make requests of that server rather than
       make requests of servers outside their network.  Secondary
       servers will cache and periodically refresh data from the root
       servers.

   (e) Clients:  Applications, operating systems, etc., that make use of
       time zone data and retrieve that from either root or secondary
       providers.

   Some of those layers may be coalesced by implementors.  For example,
   a vendor may choose to implement the entire service as a single
   monolithic virtual server with the address embedded in distributed
   systems.  Others may choose to provide a service consisting of
   multiple layers of providers, many secondary servers, and a small
   number of root servers.

   This specification is concerned only with the protocol used to
   exchange data between providers and from provider to client.  This
   specification does not define how contributors pass their information
   to publishers, nor how those publishers vet that information to
   obtain trustworthy data, nor the format of the data produced by the
   publishers.











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3.  General Considerations

   This section defines several terms and explains some key concepts
   used in this specification.

3.1.  Time Zone

   A time zone is a description of the past and predicted future
   timekeeping practices of a collection of clocks that are intended to
   agree.

   Note that the term "time zone" does not have the common meaning of a
   region of the world at a specific UTC offset, possibly modified by
   daylight saving time.  For example, the "Central European Time" zone
   can correspond to several time zones "Europe/Berlin", "Europe/Paris",
   etc., because subregions have kept time differently in the past.

3.2.  Time Zone Data

   Time zone data is data that defines a single time zone, including an
   identifier, UTC offset values, DST rules, and other information such
   as time zone abbreviations.

3.3.  Time Zone Metadata

   Time zone metadata is data that describes additional properties of a
   time zone that is not itself included in the time zone data.  This
   can include such things as the publisher name, version identifier,
   aliases, and localized names (see below).

3.4.  Time Zone Data Server

   A time zone data server is a server implementing the Time Zone Data
   Distribution Service Protocol defined by this specification.

3.5.  Observance

   A time zone with varying rules for the UTC offset will have adjacent
   periods of time that use different UTC offsets.  Each period of time
   with a constant UTC offset is called an observance.

3.6.  Time Zone Identifiers

   Time zone identifiers are unique names associated with each time
   zone, as defined by publishers.  The iCalendar [RFC5545]
   specification has a "TZID" property and parameter whose value is set
   to the corresponding time zone identifier and used to identify time
   zone data and relate time zones to start and end dates in events,



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   etc.  This specification does not define what format of time zone
   identifiers should be used.  It is possible that time zone
   identifiers from different publishers overlap, and there might be a
   need for a provider to distinguish those with some form of
   "namespace" prefix identifying the publisher.  However, development
   of a standard (global) naming scheme for time zone identifiers is out
   of scope for this specification.

3.7.  Time Zone Aliases

   Time zone aliases map a name onto a time zone identifier.  For
   example, "US/Eastern" is usually mapped on to "America/New_York".
   Time zone aliases are typically used interchangeably with time zone
   identifiers when presenting information to users.

   A time zone data distribution service needs to maintain time zone
   alias mapping information and expose that data to clients as well as
   allow clients to query for time zone data using aliases.  When
   returning time zone data to a client, the server returns the data
   with an identifier matching the query, but it can include one or more
   additional identifiers in the data to provide a hint to the client
   that alternative identifiers are available.  For example, a query for
   "US/Eastern" could include additional identifiers for "America/
   New_York" or "America/Montreal".

   The set of aliases may vary depending on whether time zone data is
   truncated (see Section 3.9).  For example, a client located in the US
   state of Michigan may see "US/Eastern" as an alias for "America/
   Detroit", whereas a client in the US state of New Jersey may see it
   as an alias for "America/New_York", and all three names may be
   aliases if time zones are truncated to post-2013 data.

3.8.  Time Zone Localized Names

   Localized names are names for time zones that can be presented to a
   user in their own language.  Each time zone may have one or more
   localized names associated with it.  Names would typically be unique
   in their own locale as they might be presented to the user in a list.
   Localized names are distinct from abbreviations commonly used for UTC
   offsets within a time zone.  For example, the time zone "America/
   New_York" may have the localized name "Nueva York" in a Spanish
   locale, as distinct from the abbreviations "EST" and "EDT", which may
   or may not have their own localizations.

   A time zone data distribution service might need to maintain
   localized name information, for one or more chosen languages, as well
   as allow clients to query for time zone data using localized names.




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3.9.  Truncating Time Zones

   Time zone data can contain information about past and future UTC
   offsets that may not be relevant for a particular server's intended
   clients.  For example, calendaring and scheduling clients are likely
   most concerned with time zone data that covers a period for one or
   two years in the past on into the future, as users typically create
   new events only for the present and future.  Similarly, time zone
   data might contain a large amount of "future" information about
   transitions occurring many decades into the future.  Again, clients
   might be concerned only with a smaller range into the future, and
   data past that point might be unnecessary.

   To avoid having to send unnecessary data, servers can choose to
   truncate time zone data to a range determined by start- and end-point
   date-time values, and to provide only offsets and rules between those
   points.  If such truncation is done, the server MUST include the
   ranges it is using in the "capabilities" action response (see
   Section 6.1), so that clients can take appropriate action if they
   need time zone data for times outside of those ranges.

   The truncation points at the start and end of a range are always a
   UTC date-time value, with the start point being "inclusive" to the
   overall range, and the end point being "exclusive" to the overall
   range (i.e., the end value is just past the end of the last valid
   value in the range).  A server will advertise a truncation range for
   the truncated data it can supply or will provide an indicator that it
   can truncate at any start or end point to produce arbitrary ranges.
   In addition, the server can advertise that it supplies untruncated
   data -- that is, data that covers the full range of times available
   from the source publisher.  In the absence of any indication of
   truncated data available on the server, the server will supply only
   untruncated data.

   When truncating the start of a "VTIMEZONE" component, the server MUST
   include exactly one "STANDARD" or "DAYLIGHT" subcomponent with a
   "DTSTART" property value that matches the start point of the
   truncation range, and appropriate "TZOFFSETFROM" and "TZOFFSETTO"
   properties to indicate the correct offset in effect right before and
   after the start point of the truncation range.  This subcomponent,
   which is the first observance defined by the time zone data,
   represents the earliest valid date-time covered by the time zone data
   in the truncated "VTIMEZONE" component.

   When truncating the end of a "VTIMEZONE" component, the server MUST
   include a "TZUNTIL" iCalendar property (Section 7.1) in the
   "VTIMEZONE" component to indicate the end point of the truncation
   range.



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3.10.  Time Zone Versions

   Time zone data changes over time, and it is important for consumers
   of that data to stay up to date with the latest versions.  As a
   result, it is useful to identify individual time zones with a
   specific version number or version identifier as supplied by the time
   zone data publisher.  There are two common models that time zone data
   publishers might use to publish updates to time zone data:

   a.  with the "monolithic" model, the data for all time zones is
       published in one go, with a single version number or identifier
       applied to the entire data set.  For example, a publisher
       producing data several times a year might use version identifiers
       "2015a", "2015b", etc.

   b.  with the "incremental" model, each time zone has its own version
       identifier, so that each time zone can be independently updated
       without impacting any others.  For example, if the initial data
       has version "A.1" for time zone "A", and "B.1" for time zone "B",
       and then time zone "B" changes; when the data is next published,
       time zone "A" will still have version "A.1", but time zone "B"
       will now have "B.2".

   A time zone data distribution service needs to ensure that the
   version identifiers used by the time zone data publisher are
   available to any client, along with the actual publisher name on a
   per-time-zone basis.  This allows clients to compare publisher/
   version details on any server, with existing locally cached client
   data, and only fetch those time zones that have actually changed (see
   Section 4.2.2 for more details on how clients synchronize data from
   the server).

4.  Time Zone Data Distribution Service Protocol

4.1.  Server Protocol

   The time zone data distribution service protocol uses HTTP [RFC7230]
   for query and delivery of time zone data, metadata, and leap-second
   information.  The interactions with the HTTP server can be broken
   down into a set of "actions" that define the overall function being
   requested (see Section 5).  Each action targets a specific HTTP
   resource using the GET method, with various request-URI parameters
   altering the behavior as needed.

   The HTTP resources used for requests will be identified via URI
   templates [RFC6570].  The overall time zone data distribution service
   has a "context path" request-URI template defined as "{/service-
   prefix}".  This "root" prefix is discovered by the client as per



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   Section 4.2.1.  Request-URIs that target time zone data directly use
   the prefix template "{/service-prefix,data-prefix}".  The second
   component of the prefix template can be used to introduce additional
   path segments in the request-URI to allow for alternative ways to
   "partition" the time zone data.  For example, time zone data might be
   partitioned by publisher release dates or version identifiers.  This
   specification does not define any partitions; that is left for future
   extensions.  When the "data-prefix" variable is empty, the server is
   expected to return the current version of time zone data it has for
   all publishers it supports.

   All URI template variable values, and URI request parameters that
   contain text values, MUST be encoded using the UTF-8 [RFC3629]
   character set.  All responses MUST return data using the UTF-8
   [RFC3629] character set.  It is important to note that any "/"
   characters, which are frequently found in time zone identifiers, are
   percent-encoded when used in the value of a path segment expansion
   variable in a URI template (as per Section 3.2.6 of [RFC6570]).
   Thus, the time zone identifier "America/New_York" would appear as
   "America%2FNew_York" when used as the value for the "{/tzid}" URI
   template variable defined later in this specification.

   The server provides time zone metadata in the form of a JSON
   [RFC7159] object.  Clients can directly request the time zone
   metadata or issue queries for subsets of metadata that match specific
   criteria.

   Security and privacy considerations for this protocol are discussed
   in detail in Sections 8 and 9, respectively.

4.1.1.  Time Zone Queries

   Time zone identifiers, aliases, or localized names can be used to
   query for time zone data or metadata.  This will be more explicitly
   defined below for each action.  In general, however, if a "tzid" URI
   template variable is used, then the value may be an identifier or an
   alias.  When the "pattern" URI query parameter is used, it may be an
   identifier, an alias, or a localized name.

4.1.2.  Time Zone Formats

   The default media type [RFC2046] format for returning time zone data
   is the iCalendar [RFC5545] data format.  In addition, the iCalendar-
   in-XML [RFC6321] and iCalendar-in-JSON [RFC7265] representations are
   available.  Clients use the HTTP Accept header field (see
   Section 5.3.2 of [RFC7231]) to indicate their preference for the
   returned data format.  Servers indicate the available formats that
   they support via the "capabilities" action response (Section 5.1).



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4.1.3.  Time Zone Localization

   As per Section 3.8, time zone data can support localized names.
   Clients use the HTTP Accept-Language header field (see Section 5.3.5
   of [RFC7231]) to indicate their preference for the language used for
   localized names in the response data.

4.1.4.  Conditional Time Zone Requests

   When time zone data or metadata changes, it needs to be distributed
   in a timely manner because changes to local time offsets might occur
   within a few days of the publication of the time zone data changes.
   Typically, the number of time zones that change is small, whilst the
   overall number of time zones can be large.  Thus, when a client is
   using more than a few time zones, it is more efficient for the client
   to be able to download only those time zones that have changed (an
   incremental update).

   Clients initially request a full list of time zones from the server
   using a "list" action request (see Section 5.2).  The response to
   that request includes two items the client caches for use with
   subsequent "conditional" (incremental update) requests:

   1.  An opaque synchronization token in the "synctoken" JSON member.
       This token changes whenever there is a change to any metadata
       associated with one or more time zones (where the metadata is the
       information reported in the "list" action response for each time
       zone).

   2.  The HTTP ETag header field value for each time zone returned in
       the response.  The ETag header field value is returned in the
       "etag" JSON member, and it corresponds to the ETag header field
       value that would be returned when executing a "get" action
       request (see Section 5.3) against the corresponding time zone
       data resource.

   For subsequent updates to cached data, clients can use the following
   procedure:

   a.  Send a "list" action request with a "changedsince" URI query
       parameter with its value set to the last opaque synchronization
       token returned by the server.  The server will return time zone
       metadata for only those time zones that have changed since the
       last request.

   b.  The client will cache the new opaque synchronization token
       returned in the response for the next incremental update, along
       with the returned time zone metadata information.



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   c.  The client will check each time zone metadata to see if the
       "etag" value is different from that of any cached time zone data
       it has.

   d.  The client will use a "get" action request to update any cached
       time zone data for those time zones whose ETag header field value
       has changed.

   Note that time zone metadata will always change when the
   corresponding time zone data changes.  However, the converse is not
   true: it is possible for some piece of the time zone metadata to
   change without the corresponding time zone data changing. e.g., for
   the case of a "monolithic" publisher (see Section 3.10), the version
   identifier in every time zone metadata element will change with each
   new published revision; however, only a small subset of time zone
   data will actually change.

   If a client needs data for only one or a small set of time zones
   (e.g., a clock in a fixed location), then it can use a conditional
   HTTP request to determine if the time zone data has changed and
   retrieve the new data.  The full details of HTTP conditional requests
   are described in [RFC7232]; what follows is a brief summary of what a
   client typically does.

   a.  When the client retrieves the time zone data from the server
       using a "get" action (see Section 5.3), the server will include
       an HTTP ETag header field in the response.

   b.  The client will store the value of that header field along with
       the request-URI used for the request.

   c.  When the client wants to check for an update, it issues another
       "get" action HTTP request on the original request-URI, but this
       time it includes an If-None-Match HTTP request header field, with
       a value set to the ETag header field value from the previous
       response.  If the data for the time zone has not changed, the
       server will return a 304 (Not Modified) HTTP response.  If the
       data has changed, the server will return a normal HTTP success
       response that will include the changed data, as well as a new
       value for the ETag header field.

   Clients SHOULD poll for changes, using an appropriate conditional
   request, at least once a day.  A server acting as a secondary
   provider, caching time zone data from another server, SHOULD poll for
   changes once per hour.  See Section 8 on expected client and server
   behavior regarding high request rates.





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4.1.5.  Expanded Time Zone Data

   Determining time zone offsets at a particular point in time is often
   a complicated process, as the rules for daylight saving time can be
   complex.  To help with this, the time zone data distribution service
   provides an action that allows clients to request the server to
   expand a time zone into a set of "observances" over a fixed period of
   time (see Section 5.4).  Each of these observances describes a UTC
   onset time and UTC offsets for the prior time and the observance
   time.  Together, these provide a quick way for "thin" clients to
   determine an appropriate UTC offset for an arbitrary date without
   having to do full time zone expansion themselves.

4.1.6.  Server Requirements

   To enable a simple client implementation, servers SHOULD ensure that
   they provide or cache data for all commonly used time zones, from
   various publishers.  That allows client implementations to configure
   a single server to get all time zone data.  In turn, any server can
   refresh any of the data from any other server -- though the root
   servers may provide the most up-to-date copy of the data.

4.1.7.  Error Responses

   When an HTTP error response is returned to the client, the server
   SHOULD return a JSON "problem details" object in the response body,
   as per [RFC7807].  Every JSON "problem details" object MUST include a
   "type" member with a URI value matching the applicable error code
   (defined for each action in Section 5).

4.1.8.  Extensions

   This protocol is designed to be extensible through a standards-based
   registration mechanism (see Section 10).  It is anticipated that
   other useful time zone actions will be added in the future (e.g.,
   mapping a geographical location to time zone identifiers, getting
   change history for time zones), and so, servers MUST return a
   description of their capabilities.  This will allow clients to
   determine if new features have been installed and, if not, fall back
   on earlier features or disable some client capabilities.

4.2.  Client Guidelines

4.2.1.  Discovery

   Client implementations need to either know where the time zone data
   distribution service is located or discover it through some
   mechanism.  To use a time zone data distribution service, a client



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   needs a Fully Qualified Domain Name (FQDN), port, and HTTP request-
   URI path.  The request-URI path found via discovery is the "context
   path" for the service itself.  The "context path" is used as the
   value of the "service-prefix" URI template variable when executing
   actions (see Section 5).

   The following subsections describe two methods of service discovery
   using DNS SRV records [RFC2782] and an HTTP "well-known" [RFC5785]
   resource.  However, alternative mechanisms could also be used (e.g.,
   a DHCP server option [RFC2131]).

4.2.1.1.  SRV Service Labels for the Time Zone Data Distribution Service

   [RFC2782] defines a DNS-based service discovery protocol that has
   been widely adopted as a means of locating particular services within
   a local area network and beyond, using SRV RR records.  This can be
   used to discover a service's FQDN and port.

   This specification adds two service types for use with SRV records:

   timezone:  Identifies a time zone data distribution server that uses
      HTTP without Transport Layer Security ([RFC2818]).

   timezones:  Identifies a time zone data distribution server that uses
      HTTP with Transport Layer Security ([RFC2818]).

   Clients MUST honor "TTL", "Priority", and "Weight" values in the SRV
   records, as described by [RFC2782].

   Example: service record for server without Transport Layer Security.

   _timezone._tcp SRV 0 1 80 tz.example.com.

   Example: service record for server with transport layer security.

   _timezones._tcp SRV 0 1 443 tz.example.com.

4.2.1.2.  TXT Records for a Time Zone Data Distribution Service

   When SRV RRs are used to advertise a time zone data distribution
   service, it is also convenient to be able to specify a "context path"
   in the DNS to be retrieved at the same time.  To enable that, this
   specification uses a TXT RR that follows the syntax defined in
   Section 6 of [RFC6763] and defines a "path" key for use in that
   record.  The value of the key MUST be the actual "context path" to
   the corresponding service on the server.





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   A site might provide TXT records in addition to SRV records for each
   service.  When present, clients MUST use the "path" value as the
   "context path" for the service in HTTP requests.  When not present,
   clients use the ".well-known" URI approach described in
   Section 4.2.1.3.

   As per Section 8, the server MAY require authentication when a client
   tries to access the path URI specified by the TXT RR (i.e., the
   server would return a 401 status response to the unauthenticated
   request from the client, then return a redirect response after a
   successful authentication by the client).

   Example: text record for service with Transport Layer Security.

   _timezones._tcp TXT path=/timezones

4.2.1.3.  Well-Known URI for a Time Zone Data Distribution Service

   A "well-known" URI [RFC5785] is registered by this specification for
   the Time Zone Data Distribution service, "timezone" (see Section 10).
   This URI points to a resource that the client can use as the initial
   "context path" for the service they are trying to connect to.  The
   server MUST redirect HTTP requests for that resource to the actual
   "context path" using one of the available mechanisms provided by HTTP
   (e.g., using an appropriate 3xx status response).  Clients MUST
   handle HTTP redirects on the ".well-known" URI, taking into account
   security restrictions on redirects described in Section 8.  Servers
   MUST NOT locate the actual time zone data distribution service
   endpoint at the ".well-known" URI as per Section 1.1 of [RFC5785].
   The "well-known" URI MUST be present on the server, even when a TXT
   RR (Section 4.2.1.2) is used in the DNS to specify a "context path".

   Servers SHOULD set an appropriate Cache-Control header field value
   (as per Section 5.2 of [RFC7234]) in the redirect response to ensure
   caching occurs as needed, or as required by the type of response
   generated.  For example, if it is anticipated that the location of
   the redirect might change over time, then an appropriate "max-age"
   value would be used.

   As per Section 8, the server MAY require authentication when a client
   tries to access the ".well-known" URI (i.e., the server would return
   a 401 status response to the unauthenticated request from the client,
   then return the redirect response after a successful authentication
   by the client).







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4.2.1.3.1.  Example: Well-Known URI Redirects to Actual Context Path

   A time zone data distribution server has a "context path" that is
   "/servlet/timezone".  The client will use "/.well-known/timezone" as
   the path for the service after it has first found the FQDN and port
   number via an SRV lookup or via manual entry of information by the
   user.  When the client makes its initial HTTP request against
   "/.well-known/timezone", the server would issue an HTTP 301 redirect
   response with a Location response header field using the path
   "/servlet/timezone".  The client would then "follow" this redirect to
   the new resource and continue making HTTP requests there.  The client
   would also cache the redirect information, subject to any Cache-
   Control directive, for use in subsequent requests.

4.2.2.  Synchronization of Time Zones

   This section discusses possible client synchronization strategies
   using the various protocol elements provided by the server for that
   purpose.

4.2.2.1.  Initial Synchronization of All Time Zones

   When a secondary service or a client wishing to cache all time zone
   data first starts, or wishes to do a full refresh, it synchronizes
   with another server by issuing a "list" action to retrieve all the
   time zone metadata.  The client preserves the returned opaque token
   for subsequent use (see "synctoken" in Section 5.2.1).  The client
   stores the metadata for each time zone returned in the response.
   Time zone data for each corresponding time zone can then be fetched
   and stored locally.  In addition, a mapping of aliases to time zones
   can be built from the metadata.  A typical "list" action response
   size is about 50-100 KB of "pretty printed" JSON data, for a service
   using the IANA time zone database [RFC6557], as of the time of
   publication of this specification.

4.2.2.2.  Subsequent Synchronization of All Time Zones

   A secondary service or a client caching all time zones needs to
   periodically synchronize with a server.  To do so, it issues a "list"
   action with the "changedsince" URI query parameter set to the value
   of the opaque token returned by the last synchronization.  The client
   again preserves the returned opaque token for subsequent use.  The
   client updates its stored time zone metadata using the new values
   returned in the response, which contains just the time zone metadata
   for those time zones changed since the last synchronization.  In
   addition, it compares the "etag" value in each time zone metadata to
   the ETag header field value for the corresponding time zone data
   resource it has previously cached; if they are different, it fetches



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   the new time zone data.  Note that if the client presents the server
   with a "changedsince" value that the server does not support, all
   time zone data is returned, as it would for the case where the
   request did not include a "changedsince" value.

   Publishers should take into account the fact that the "outright"
   deletion of time zone names will cause problems to simple clients,
   and so aliasing a deleted time zone identifier to a suitable
   alternate one is preferable.

4.2.2.3.  Synchronization with Preexisting Time Zone Data

   A client might be pre-provisioned with time zone data from a source
   other than the time zone data distribution service it is configured
   to use.  In such cases, the client might want to minimize the amount
   of time zone data it synchronizes by doing an initial "list" action
   to retrieve all the time zone metadata, but then only fetch time zone
   data for those time zones that do not match the publisher and version
   details for the pre-provisioned data.

5.  Actions

   Servers MUST support the following actions.  The information below
   shows details about each action: the request-URI the client targets
   (in the form of a URI template [RFC6570]), a description, the set of
   allowed query parameters, the nature of the response, and a set of
   possible error codes for the response (see Section 4.1.7).

   For any error not covered by the specific error codes defined below,
   the "urn:ietf:params:tzdist:error:invalid-action" error code is
   returned to the client in the JSON "problem details" object.

   The examples in the following subsections presume that the timezone
   context path has been discovered to be "/servlet/timezone" (as in the
   example in Section 4.2.1.3.1).

5.1.  "capabilities" Action

   Name:  capabilities

   Request-URI Template:
      {/service-prefix}/capabilities

   Description:  This action returns the capabilities of the server,
      allowing clients to determine if a specific feature has been
      deployed and/or enabled.

   Parameters:  None



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   Response:  A JSON object containing a "version" member, an "info"
      member, and an "actions" member; see Section 6.1.

   Possible Error Codes:  No specific code.

5.1.1.  Example: get capabilities

   >> Request <<

   GET /servlet/timezone/capabilities HTTP/1.1
   Host: tz.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Wed, 4 Jun 2008 09:32:12 GMT
   Content-Type: application/json; charset="utf-8"
   Content-Length: xxxx

   {
     "version": 1,

     "info": {
       "primary-source": "Olson:2011m",
       "formats": [
         "text/calendar",
         "application/calendar+xml",
         "application/calendar+json"
       ],
       "truncated" : {
         "any": false,
         "ranges": [
           {
             "start": "1970-01-01T00:00:00Z",
             "end": "*"
           },
           {
             "start":"2010-01-01T00:00:00Z",
             "end":"2020-01-01T00:00:00Z"
           }
         ],
         "untruncated": true
       },
       "provider-details": "http://tz.example.com/about.html",
       "contacts": ["mailto:tzs@example.org"]
     },





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     "actions": [
       {
         "name": "capabilities",
         "uri-template": "/servlet/timezone/capabilities",
         "parameters": []
       },

       {
         "name": "list",
         "uri-template": "/servlet/timezone/zones{?changedsince}",
         "parameters": [
           {
             "name": "changedsince",
             "required": false,
             "multi": false
           }
         ]
       },

       {
         "name": "get",
         "uri-template": "/servlet/timezone/zones{/tzid}{?start,end}",
         "parameters": [
           {
             "name": "start",
             "required": false,
             "multi": false
           },
           {
             "name": "end",
             "required": false,
             "multi": false
           }
         ]
       },

       {
         "name": "expand",
         "uri-template":
           "/servlet/timezone/zones{/tzid}/observances{?start,end}",
         "parameters": [
           {
             "name": "start",
             "required": true,
             "multi": false
           },
           {
             "name": "end",



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             "required": true,
             "multi": false
           }
         ]
       },

       {
         "name": "find",
         "uri-template": "/servlet/timezone/zones{?pattern}",
         "parameters": [
           {
             "name": "pattern",
             "required": true,
             "multi": false
           }
         ]
       },

       {
         "name": "leapseconds",
         "uri-template": "/servlet/timezone/leapseconds",
         "parameters": []
       }
     ]
   }

5.2.  "list" Action

   Name:  list

   Request-URI Template:
      {/service-prefix,data-prefix}/zones{?changedsince}

   Description:  This action lists all time zone identifiers in summary
      format, with publisher, version, aliases, and optional localized
      data.  In addition, it returns an opaque synchronization token for
      the entire response.  If the "changedsince" URI query parameter is
      present, its value MUST correspond to a previously returned
      synchronization token value.  When "changedsince" is used, the
      server MUST return only those time zones that have changed since
      the specified synchronization token.  If the "changedsince" value
      is not supported by the server, the server MUST return all time
      zones, treating the request as if it had no "changedsince".

   Parameters:

      changedsince
         OPTIONAL, and MUST NOT occur more than once.



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   Response:  A JSON object containing a "synctoken" member and a
      "timezones" member; see Section 6.2.

   Possible Error Codes:

      urn:ietf:params:tzdist:error:invalid-changedsince

         The "changedsince" URI query parameter appears more than once.

5.2.1.  Example: List Time Zone Identifiers

   In this example the client requests the full set of time zone
   identifiers.

   >> Request <<

   GET /servlet/timezone/zones HTTP/1.1
   Host: tz.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Wed, 4 Jun 2008 09:32:12 GMT
   Content-Type: application/json; charset="utf-8"
   Content-Length: xxxx

   {
     "synctoken": "2009-10-11T09:32:11Z",
     "timezones": [
       {
         "tzid": "America/New_York",
         "etag": "123456789-000-111",
         "last-modified": "2009-09-17T01:39:34Z",
         "publisher": "Example.com",
         "version": "2015a",
         "aliases":["US/Eastern"],
         "local-names": [
           {
             "name": "America/New_York",
             "lang": "en_US"
           }
         ]
       },
       ...other time zones...
     ]
   }





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5.3.  "get" Action

   Name:  get

   Request-URI Template:
      {/service-prefix,data-prefix}/zones{/tzid}{?start,end}

      The "tzid" variable value is REQUIRED in order to distinguish this
      action from the "list" action.

   Description:  This action returns a time zone.  The response MUST
      contain an ETag response header field indicating the current value
      of the strong entity tag of the time zone resource.

      In the absence of any Accept HTTP request header field, the server
      MUST return time zone data with the "text/calendar" media type.

      If the "tzid" variable value is actually a time zone alias, the
      server will return the matching time zone data with the alias as
      the identifier in the time zone data.  The server MAY include one
      or more "TZID-ALIAS-OF" properties (see Section 7.2) in the time
      zone data to indicate additional identifiers that have the
      matching time zone identifier as an alias.

   Parameters:

      start=<date-time>
         OPTIONAL, and MUST NOT occur more than once.  Specifies the
         inclusive UTC date-time value at which the returned time zone
         data is truncated at its start.

      end=<date-time>
         OPTIONAL, and MUST NOT occur more than once.  Specifies the
         exclusive UTC date-time value at which the returned time zone
         data is truncated at its end.

   Response:  A document containing all the requested time zone data in
      the format specified.

   Possible Error Codes:

      urn:ietf:params:tzdist:error:tzid-not-found
         No time zone associated with the specified "tzid" path segment
         value was found.







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      urn:ietf:params:tzdist:error:invalid-format
         The Accept request header field supplied by the client did not
         contain a media type for time zone data supported by the
         server.

      urn:ietf:params:tzdist:error:invalid-start
         The "start" URI query parameter has an incorrect value, or
         appears more than once, or does not match one of the fixed
         truncation range start values advertised in the "capabilities"
         action response.

      urn:ietf:params:tzdist:error:invalid-end
         The "end" URI query parameter has an incorrect value, or
         appears more than once, or has a value less than or equal to
         the "start" URI query parameter, or does not match one of the
         fixed truncation range end values advertised in the
         "capabilities" action response.

5.3.1.  Example: Get Time Zone Data

   In this example, the client requests that the time zone with a
   specific time zone identifier be returned.

   >> Request <<

   GET /servlet/timezone/zones/America%2FNew_York HTTP/1.1
   Host: tz.example.com
   Accept:text/calendar

   >> Response <<

   HTTP/1.1 200 OK
   Date: Wed, 4 Jun 2008 09:32:12 GMT
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx
   ETag: "123456789-000-111"

   BEGIN:VCALENDAR
   ...
   BEGIN:VTIMEZONE
   TZID:America/New_York
   ...
   END:VTIMEZONE
   END:VCALENDAR







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5.3.2.  Example: Conditional Get Time Zone Data

   In this example the client requests that the time zone with a
   specific time zone identifier be returned, but uses an If-None-Match
   header field in the request, set to the value of a previously
   returned ETag header field, or the value of the "etag" member in a
   JSON "timezone" object returned from a "list" action response.  In
   this example, the data on the server has not changed, so a 304
   response is returned.

   >> Request <<

   GET /servlet/timezone/zones/America%2FNew_York HTTP/1.1
   Host: tz.example.com
   Accept:text/calendar
   If-None-Match: "123456789-000-111"

   >> Response <<

   HTTP/1.1 304 Not Modified
   Date: Wed, 4 Jun 2008 09:32:12 GMT

5.3.3.  Example: Get Time Zone Data Using a Time Zone Alias

   In this example, the client requests that the time zone with an
   aliased time zone identifier be returned, and the server returns the
   time zone data with that identifier and two aliases.

   >> Request <<

   GET /servlet/timezone/zones/US%2FEastern HTTP/1.1
   Host: tz.example.com
   Accept:text/calendar

   >> Response <<

   HTTP/1.1 200 OK
   Date: Wed, 4 Jun 2008 09:32:12 GMT
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx
   ETag: "123456789-000-111"

   BEGIN:VCALENDAR
   ...
   BEGIN:VTIMEZONE
   TZID:US/Eastern
   TZID-ALIAS-OF:America/New_York
   TZID-ALIAS-OF:America/Montreal



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   ...
   END:VTIMEZONE
   END:VCALENDAR

5.3.4.  Example: Get Truncated Time Zone Data

   Assume the server advertises a "truncated" object in its
   "capabilities" response that appears as:

   "truncated": {
     "any": false,
     "ranges": [
       {"start": "1970-01-01T00:00:00Z", "end": "*"},
       {"start":"2010-01-01T00:00:00Z", "end":"2020-01-01T00:00:00Z"}
     ],
     "untruncated": false
   }

   In this example, the client requests that the time zone with a
   specific time zone identifier truncated at one of the ranges
   specified by the server be returned.  Note the presence of a
   "STANDARD" component that matches the start point of the truncation
   range (converted to the local time for the UTC offset in effect at
   the matching UTC time).  Also, note the presence of the "TZUNTIL"
   (Section 7.1) iCalendar property in the "VTIMEZONE" component,
   indicating the upper bound on the validity period of the time zone
   data.

   >> Request <<

   GET /servlet/timezone/zones/America%2FNew_York
     ?start=2010-01-01T00:00:00Z&end=2020-01-01T00:00:00Z HTTP/1.1
   Host: tz.example.com
   Accept:text/calendar

   >> Response <<

   HTTP/1.1 200 OK
   Date: Wed, 4 Jun 2008 09:32:12 GMT
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: xxxx
   ETag: "123456789-000-111"

   BEGIN:VCALENDAR
   ...
   BEGIN:VTIMEZONE
   TZID:America/New_York
   TZUNTIL:20200101T000000Z



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   BEGIN:STANDARD
   DTSTART:20101231T190000
   TZNAME:EST
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0500
   END:STANDARD
   ...
   END:VTIMEZONE
   END:VCALENDAR

5.3.5.  Example: Request for a Nonexistent Time Zone

   In this example, the client requests that the time zone with a
   specific time zone identifier be returned.  As it turns out, no time
   zone exists with that identifier.

   >> Request <<

   GET /servlet/timezone/zones/America%2FPittsburgh HTTP/1.1
   Host: tz.example.com
   Accept:application/calendar+json

   >> Response <<

   HTTP/1.1 404 Not Found
   Date: Wed, 4 Jun 2008 09:32:12 GMT
   Content-Type: application/problem+json; charset="utf-8"
   Content-Language: en
   Content-Length: xxxx

   {
     "type": "urn:ietf:params:tzdist:error:tzid-not-found",
     "title": "Time zone identifier was not found on this server",
     "status": 404
   }

5.4.  "expand" Action

   Name:  expand

   Request-URI Template:
      {/service-prefix,data-prefix}/zones{/tzid}/observances{?start,end}

      The "tzid" variable value is REQUIRED.







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   Description:  This action expands the specified time zone into a list
      of onset start date/time values (in UTC) and UTC offsets.  The
      response MUST contain an ETag response header field indicating the
      current value of the strong entity tag of the time zone being
      expanded.

   Parameters:

      start=<date-time>:  REQUIRED, and MUST occur only once.  Specifies
         the inclusive UTC date-time value for the start of the period
         of interest.

      end=<date-time>:  REQUIRED, and MUST occur only once.  Specifies
         the exclusive UTC date-time value for the end of the period of
         interest.  Note that this is the exclusive end value, i.e., it
         represents the date just after the range of interest.  For if a
         client wants the expanded date just for the year 2014, it would
         use a start value of "2014-01-01T00:00:00Z" and an end value of
         "2015-01-01T00:00:00Z".  An error occurs if the end value is
         less than or equal to the start value.

   Response:  A JSON object containing a "tzid" member and an
      "observances" member; see Section 6.3.  If the time zone being
      expanded is not fully defined over the requested time range (e.g.,
      because of truncation), then the server MUST include "start" and/
      or "end" members in the JSON response to indicate the actual start
      and end points for the observances being returned.  The server
      MUST include an expanded observance representing the time zone
      information in effect at the start of the returned observance
      period.

   Possible Error Codes

      urn:ietf:params:tzdist:error:tzid-not-found
         No time zone associated with the specified "tzid" path segment
         value was found.

      urn:ietf:params:tzdist:error:invalid-start
         The "start" URI query parameter has an incorrect value, or
         appears more than once, or is missing, or has a value outside
         any fixed truncation ranges advertised in the "capabilities"
         action response.

      urn:ietf:params:tzdist:error:invalid-end
         The "end" URI query parameter has an incorrect value, or
         appears more than once, or has a value less than or equal to





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         the "start" URI query parameter, or has a value outside any
         fixed truncation ranges advertised in the "capabilities" action
         response.

5.4.1.  Example: Expanded JSON Data Format

   In this example, the client requests a time zone in the expanded
   form.

   >> Request <<

   GET /servlet/timezone/zones/America%2FNew_York/observances
    ?start=2008-01-01T00:00:00Z&end=2009-01-01T00:00:00Z HTTP/1.1
   Host: tz.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Mon, 11 Oct 2009 09:32:12 GMT
   Content-Type: application/json; charset="utf-8"
   Content-Length: xxxx
   ETag: "123456789-000-111"

   {
     "tzid": "America/New_York",
     "observances": [
       {
         "name": "Standard",
         "onset": "2008-01-01T00:00:00Z",
         "utc-offset-from": -18000,
         "utc-offset-to": -18000
       },
       {
         "name": "Daylight",
         "onset": "2008-03-09T07:00:00Z",
         "utc-offset-from": -18000,
         "utc-offset-to": -14400
       },
       {
         "name": "Standard",
         "onset": "2008-11-02T06:00:00Z",
         "utc-offset-from": -14400,
         "utc-offset-to": -18000
       },
     ]
   }





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5.5.  "find" Action

   Name:  find

   Request-URI Template:
      {/service-prefix,data-prefix}/zones{?pattern}

   Description:  This action allows a client to query the time zone data
      distribution service for a matching identifier, alias, or
      localized name, using a simple "glob" style patter match against
      the names known to the server (with an asterisk (*) as the
      wildcard character).  Pattern-match strings (which have to be
      percent-encoded and then decoded when used in the URI query
      parameter) have the following options:

      * not present:  An exact text match is done, e.g., "xyz"

      * first character only:  An ends-with text match is done, e.g.,
         "*xyz"

      * last character only:  A starts-with text match is done, e.g.,
         "xyz*"

      * first and last characters only:  A substring text match is done,
         e.g., "*xyz*"

      Escaping \ and *:  To match 0x2A ("*") and 0x5C ("\") characters
         in a time zone identifier, those characters have to be
         "escaped" in the pattern by prepending a single 0x5C ("\")
         character.  For example, a pattern "\*Test\\Time\*Zone\*" is
         used for an exact match against the time zone identifier
         "*Test\Time*Zone*".  An unescaped "*" character MUST NOT appear
         in the middle of the string and MUST result in an error.  An
         unescaped "\" character MUST NOT appear anywhere in the string
         and MUST result in an error.

      In addition, when matching:

      Underscores:  Underscore characters (0x5F) in time zone
         identifiers MUST be mapped to a single space character (0x20)
         prior to string comparison in both the pattern and time zone
         identifiers being matched.  This allows time zone identifiers
         such as "America/New_York" to match a query for "*New York*".

      Case mapping:  ASCII characters in the range 0x41 ("A") through
         0x5A ("Z") MUST be mapped to their lowercase equivalents in
         both the pattern and time zone identifiers being matched.




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   Parameters:

      pattern=<text>
         REQUIRED, and MUST occur only once.

   Response:  The response has the same format as the "list" action,
      with one result object per successful match; see Section 6.2.

   Possible Error Codes

      urn:ietf:params:tzdist:error:invalid-pattern
         The "pattern" URI query parameter has an incorrect value or
         appears more than once.

5.5.1.  Example: find action

   In this example, the client asks for data about the time zone
   "US/Eastern".

   >> Request <<

   GET /servlet/timezone/zones?pattern=US/Eastern HTTP/1.1
   Host: tz.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Wed, 4 Jun 2008 09:32:12 GMT
   Content-Type: application/json; charset="utf-8"
   Content-Length: xxxx

   {
     "synctoken": "2009-10-11T09:32:11Z",
     "timezones": [
       {
         "tzid": "America/New_York",
         "etag": "123456789-000-111",
         "last-modified": "2009-09-17T01:39:34Z",
         "publisher": "Example.com",
         "version": "2015a",
         "aliases":["US/Eastern"],
         "local-names": [
           {
             "name": "America/New_York",
             "lang": "en_US"
           }
         ]
       },



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       {
         "tzid": "America/Detroit",
         "etag": "123456789-999-222",
         "last-modified": "2009-09-17T01:39:34Z",
         "publisher": "Example.com",
         "version": "2015a",
         "aliases":["US/Eastern"],
         "local-names": [
           {
             "name": "America/Detroit",
             "lang": "en_US"
           }
         ]
       },
       ...
     ]
   }

5.6.  "leapseconds" Action

   Name:  leapseconds

   Request-URI Template:
      {/service-prefix,data-prefix}/leapseconds

   Description:  This action allows a client to query the time zone data
      distribution service to retrieve the current leap-second
      information available on the server.

   Parameters:  None

   Response:  A JSON object containing an "expires" member, a
      "publisher" member, a "version" member, and a "leapseconds"
      member; see Section 6.4.  The "expires" member in the JSON
      response indicates the latest date covered by leap-second
      information.  For example (as in Section 5.6.1), if the "expires"
      value is set to "2014-06-28" and the latest leap-second change
      indicated was at "2012-07-01", then the data indicates that there
      are no leap seconds added (or removed) between those two dates,
      and information for leap seconds beyond the "expires" date is not
      yet available.

      The "leapseconds" member contains a list of JSON objects each of
      which contains a "utc-offset" and "onset" member.  The "onset"
      member specifies the date (with the implied time of 00:00:00 UTC)
      at which the corresponding UTC offset from TAI takes effect.  In
      other words, a leap second is added or removed just prior to time
      00:00:00 UTC of the specified onset date.  When a leap second is



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      added, the "utc-offset" value will be incremented by one; when a
      leap second is removed, the "utc-offset" value will be decremented
      by one.

   Possible Error Codes  No specific code.

5.6.1.  Example: Get Leap-Second Information

   In this example, the client requests the current leap-second
   information from the server.

   >> Request <<

   GET /servlet/timezone/leapseconds HTTP/1.1
   Host: tz.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Wed, 4 Jun 2008 09:32:12 GMT
   Content-Type: application/json; charset="utf-8"
   Content-Length: xxxx

   {
     "expires": "2015-12-28",
     "publisher": "Example.com",
     "version": "2015d",
     "leapseconds": [
       {
         "utc-offset": 10,
         "onset": "1972-01-01",
       },
       {
         "utc-offset": 11,
         "onset": "1972-07-01",
       },
       ...
       {
         "utc-offset": 35,
         "onset": "2012-07-01",
       },
       {
         "utc-offset": 36,
         "onset": "2015-07-01",
       }
     ]
   }




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6.  JSON Definitions

   [RFC7159] defines the structure of JSON objects using a set of
   primitive elements.  The structure of JSON objects used by this
   specification is described by the following set of rules:

   OBJECT  represents a JSON object, defined in Section 4 of [RFC7159].
      "OBJECT" is followed by a parenthesized list of "MEMBER" rule
      names.  If a member rule name is preceded by a "?" (0x3F)
      character, that member is optional; otherwise, all members are
      required.  If two or more member rule names are present, each
      separated from the other by a "|" (0x7C) character, then only one
      of those members MUST be present in the JSON object.  JSON object
      members are unordered, and thus the order used in the rules is not
      significant.

   MEMBER  represents a member of a JSON object, defined in Section 4 of
      [RFC7159].  "MEMBER" is followed by a rule name, the name of the
      member, a ":", and then the value.  A value can be one of
      "OBJECT", "ARRAY", "NUMBER", "STRING", or "BOOLEAN" rules.

   ARRAY  represents a JSON array, defined in Section 5 of [RFC7159].
      "ARRAY" is followed by a value (one of "OBJECT", "ARRAY",
      "NUMBER", "STRING", or "BOOLEAN"), indicating the type of items
      used in the array.

   NUMBER  represents a JSON number, defined in Section 6 of [RFC7159].

   STRING  represents a JSON string, defined in Section 7 of [RFC7159].

   BOOLEAN  represents either of the JSON values "true" or "false",
      defined in Section 3 of [RFC7159].

   ;  a line starting with a ";" (0x3B) character is a comment.

   Note, clients MUST ignore any unexpected JSON members in responses
   from the server.

6.1.  capabilities Action Response

   Below are the rules for the JSON document returned for a
   "capabilities" action request.

   ; root object
   OBJECT (version, info, actions)

   ; The version number of the protocol supported - MUST be 1
   MEMBER version "version" : NUMBER



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   ; object containing service information
   ; Only one of primary_source or secondary_source MUST be present
   MEMBER info "info" : OBJECT (
     primary_source | secondary_source,
     formats,
     ?truncated,
     ?provider_details,
     ?contacts
   )

   ; The source of the time zone data provided by a "primary" server
   MEMBER primary_source "primary-source" : STRING

   ; The time zone data server from which data is provided by a
   ; "secondary" server
   MEMBER secondary_source "secondary-source" : STRING

   ; Array of one or more media types for the time zone data formats
   ; that the server can return
   MEMBER formats "formats" : ARRAY STRING

   ; Present if the server is providing truncated time zone data.  The
   ; value is an object providing details of the supported truncation
   ; modes.
   MEMBER truncated "truncated" : OBJECT: (
     any,
     ?ranges,
     ?untruncated
   )

   ; Indicates whether the server can truncate time zone data at any
   ; start or end point.  When set to "true", any start or end point is
   ; a valid value for use with the "start" and "end" URI query
   ; parameters in a "get" action request.
   MEMBER any "any" : BOOLEAN

   ; Indicates which ranges of time the server has truncated data for.
   ; A value from this list may be used with the "start" and "end" URI
   ; query parameters in a "get" action request.  Not present if "any"
   ; is set to "true".
   MEMBER ranges "ranges" : ARRAY OBJECT (range-start, range-end)

   ; UTC date-time value (per [RFC3339]) for inclusive start of the
   ; range, or the single character "*" to indicate a value
   ; corresponding to the lower bound supplied by the publisher of the
   ; time zone data
   MEMBER range-start "start" : STRING




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   ; UTC date-time value (per [RFC3339]) for exclusive end of the range,
   ; or the single character "*" to indicate a value corresponding to
   ; the upper bound supplied by the publisher of the time zone data
   MEMBER range-end "end" : STRING

   ; Indicates whether the server can supply untruncated data.  When
   ; set to "true", indicates that, in addition to truncated data being
   ; available, the server can return untruncated data if a "get"
   ; action request is executed without a "start" or "end" URI query
   ; parameter.
   MEMBER untruncated "untruncated" : BOOLEAN

   ; A URI where human-readable details about the time zone service
   ; is available
   MEMBER provider_details "provider-details" : STRING

   ; Array of URIs providing contact details for the server
   ; administrator
   MEMBER contacts "contacts" : ARRAY STRING

   ; Array of actions supported by the server
   MEMBER actions "actions" : ARRAY OBJECT (
     action_name,
     action_params
   )

   ; Name of the action
   MEMBER action_name: "name" : STRING

   ; Array of request-URI query parameters supported by the action
   MEMBER action_params: "parameters" ARRAY OBJECT (
     param_name,
     ?param_required,
     ?param_multi,
     ?param_values
   )

   ; Name of the parameter
   MEMBER param_name "name" : STRING

   ; If true, the parameter has to be present in the request-URI
   ; default is false
   MEMBER param_required "required" : BOOLEAN

   ; If true, the parameter can occur more than once in the request-URI
   ; default is false
   MEMBER param_multi "multi" : BOOLEAN,




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   ; An array that defines the allowed set of values for the parameter
   ; In the absence of this member, any string value is acceptable
   MEMBER param_values "values" ARRAY STRING

6.2.  list/find Action Response

   Below are the rules for the JSON document returned for a "list" or
   "find" action request.

   ; root object
   OBJECT (synctoken, timezones)

   ; Server-generated opaque token used for synchronizing changes
   MEMBER synctoken "synctoken" : STRING

   ; Array of time zone objects
   MEMBER timezones "timezones" : ARRAY OBJECT (
     tzid,
     etag,
     last_modified,
     publisher,
     version,
     ?aliases,
     ?local_names,
   )

   ; Time zone identifier
   MEMBER tzid "tzid" : STRING

   ; Current ETag for the corresponding time zone data resource
   MEMBER etag "etag" : STRING

   ; Date/time when the time zone data was last modified
   ; UTC date-time value as specified in [RFC3339]
   MEMBER last_modified "last-modified" : STRING

   ; Time zone data publisher
   MEMBER publisher "publisher" : STRING

   ; Current version of the time zone data as defined by the
   ; publisher
   MEMBER version "version" : STRING

   ; An array that lists the set of time zone aliases available
   ; for the corresponding time zone
   MEMBER aliases "aliases" : ARRAY STRING





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   ; An array that lists the set of localized names available
   ; for the corresponding time zone
   MEMBER local_names "local-names" : ARRAY OBJECT (
     lname, lang, ?pref
   )

   ; Language tag for the language of the associated name
   MEMBER: lang "lang" : STRING

   ; Localized name
   MEMBER lname "name" : STRING

   ; Indicates whether this is the preferred name for the associated
   ; language default: false
   MEMBER pref "pref" : BOOLEAN

6.3.  expand Action Response

   Below are the rules for the JSON document returned for a "expand"
   action request.

   ; root object
   OBJECT (
     tzid,
     ?start,
     ?end,
     observances
   )

   ; Time zone identifier
   MEMBER tzid "tzid" : STRING

   ; The actual inclusive start point for the returned observances
   ; if different from the value of the "start" URI query parameter
   MEMBER start "start" : STRING

   ; The actual exclusive end point for the returned observances
   ; if different from the value of the "end" URI query parameter
   MEMBER end "end" : STRING

   ; Array of time zone objects
   MEMBER observances "observances" : ARRAY OBJECT (
     oname,
     ?olocal_names,
     onset,
     utc_offset_from,
     utc_offset_to
   )



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   ; Observance name
   MEMBER oname "name" : STRING

   ; Array of localized observance names
   MEMBER olocal_names "local-names" : ARRAY STRING

   ; UTC date-time value (per [RFC3339]) at which the observance takes
   ; effect
   MEMBER onset "onset" : STRING

   ; The UTC offset in seconds before the start of this observance
   MEMBER utc_offset_from "utc-offset-from" : NUMBER

   ; The UTC offset in seconds at and after the start of this observance
   MEMBER utc_offset_to "utc-offset-to" : NUMBER

6.4.  leapseconds Action Response

   Below are the rules for the JSON document returned for a
   "leapseconds" action request.

   ; root object
   OBJECT (
     expires,
     publisher,
     version,
     leapseconds
   )

   ; Last valid date covered by the data in this response
   ; full-date value as specified in [RFC3339]
   MEMBER expires "expires" : STRING

   ; Leap-second information publisher
   MEMBER publisher "publisher" : STRING

   ; Current version of the leap-second information as defined by the
   ; publisher
   MEMBER version "version" : STRING

   ; Array of leap-second objects
   MEMBER leapseconds "leapseconds" : ARRAY OBJECT (
     utc_offset,
     onset
   )






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   ; The UTC offset from TAI in seconds in effect at and after the
   ; specified date
   MEMBER utc_offset "utc-offset" : NUMBER

   ; full-date value (per [RFC3339]) at which the new UTC offset takes
   ; effect, at T00:00:00Z
   MEMBER onset "onset" : STRING

7.  New iCalendar Properties

7.1.  Time Zone Upper Bound

   Property Name:  TZUNTIL

   Purpose:  This property specifies an upper bound for the validity
      period of data within a "VTIMEZONE" component.

   Value Type:  DATE-TIME

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

   Conformance:  This property can be specified zero times or one time
      within "VTIMEZONE" calendar components.

   Description:  The value MUST be specified in the UTC time format.

      Time zone data in a "VTIMEZONE" component might cover only a fixed
      period of time.  The start of such a period is clearly indicated
      by the earliest observance defined by the "STANDARD" and
      "DAYLIGHT" subcomponents.  However, an upper bound on the validity
      period of the time zone data cannot be simply derived from the
      observance with the latest onset time, and [RFC5545] does not
      define a way to get such an upper bound.  This specification
      introduces the "TZUNTIL" property for that purpose.  It specifies
      an "exclusive" UTC date-time value that indicates the last time at
      which the time zone data is to be considered valid.

      This property is also used by time zone data distribution servers
      to indicate the truncation range end point of time zone data (as
      described in Section 3.9).

   Format Definition:  This property is defined by the following
      notation in ABNF [RFC5234]:

      tzuntil      = "TZUNTIL" tzuntilparam ":" date-time CRLF

      tzuntilparam = *(";" other-param)



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   Example:  Suppose a time zone based on astronomical observations has
      well-defined onset times through the year 2025, but the first
      onset in 2026 is currently known only approximately.  In that
      case, the "TZUNTIL" property could be specified as follows:

   TZUNTIL:20260101T000000Z

7.2.  Time Zone Identifier Alias Property

   Property Name:  TZID-ALIAS-OF

   Purpose:  This property specifies a time zone identifier for which
      the main time zone identifier is an alias.

   Value Type:  TEXT

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

   Conformance:  This property can be specified zero or more times
      within "VTIMEZONE" calendar components.

   Description:  When the "VTIMEZONE" component uses a time zone
      identifier alias for the "TZID" property value, the "TZID-ALIAS-
      OF" property is used to indicate the time zone identifier of the
      other time zone (see Section 3.7).

   Format Definition:  This property is defined by the following
      notation in ABNF [RFC5234]:

      tzid-alias-of    = "TZID-ALIAS-OF" tzidaliasofparam ":"
                              [tzidprefix] text CRLF

      tzidaliasofparam = *(";" other-param)

      ;tzidprefix defined in [RFC5545].

   Example:  The following is an example of this property:

   TZID-ALIAS-OF:America/New_York











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8.  Security Considerations

   Time zone data is critical in determining local or UTC time for
   devices and in calendaring and scheduling operations.  As such, it is
   vital that a reliable source of time zone data is used.  Servers
   providing a time zone data distribution service MUST support HTTP
   over Transport Layer Security (TLS) (as defined by [RFC2818] and
   [RFC5246], with best practices described in [RFC7525]).  Servers MAY
   support a time zone data distribution service over HTTP without TLS.
   However, secondary servers MUST use TLS to fetch data from a primary
   server.

   Clients SHOULD use Transport Layer Security as defined by [RFC2818],
   unless they are specifically configured otherwise.  Clients that have
   been configured to use the TLS-based service MUST NOT fall back to
   using the non-TLS service if the TLS-based service is not available.
   In addition, clients MUST NOT follow HTTP redirect requests from a
   TLS service to a non-TLS service.  When using TLS, clients MUST
   verify the identity of the server, using a standard, secure mechanism
   such as the certificate verification process specified in [RFC6125]
   or DANE [RFC6698].

   A malicious attacker with access to the DNS server data, or able to
   get spoofed answers cached in a recursive resolver, can potentially
   cause clients to connect to any server chosen by the attacker.  In
   the absence of a secure DNS option, clients SHOULD check that the
   target FQDN returned in the SRV record is the same as the original
   service domain that was queried, or is a sub-domain of the original
   service domain.  In many cases, the client configuration is likely to
   be handled automatically without any user input; as such, any
   mismatch between the original service domain and the target FQDN is
   treated as a failure and the client MUST NOT attempt to connect to
   the target server.  In addition, when Transport Layer Security is
   being used, the Transport Layer Security certificate SHOULD include
   an SRV-ID field as per [RFC4985] matching the expected DNS SRV
   queries clients will use for service discovery.  If an SRV-ID field
   is present in a certificate, clients MUST match the SRV-ID value with
   the service type and domain that matches the DNS SRV request made by
   the client to discover the service.

   Time zone data servers SHOULD protect themselves against poorly
   implemented or malicious clients by throttling high request rates or
   frequent requests for large amounts of data.  Clients can avoid being
   throttled by using the polling capabilities outlined in
   Section 4.1.4.  Servers MAY require some form of authentication or
   authorization of clients (including secondary servers), as per
   [RFC7235], to restrict which clients are allowed to access their
   service or provide better identification of problematic clients.



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9.  Privacy Considerations

   The type and pattern of requests that a client makes can be used to
   "fingerprint" specific clients or devices and thus potentially used
   to track information about what the users of the clients might be
   doing.  In particular, a client that only downloads time zone data on
   an as-needed basis, will leak the fact that a user's device has moved
   from one time zone to another or that the user is receiving
   scheduling messages from another user in a different time zone.

   Clients need to be aware of the potential ways in which an untrusted
   server or a network observer might be able to track them and take
   precautions such as the following:

   1.  Always use TLS to connect to the server.

   2.  Avoid use of TLS session resumption.

   3.  Always fetch and synchronize the entire set of time zone data to
       avoid leaking information about which time zones are actually in
       use by the client.

   4.  Randomize the order in which individual time zones are fetched
       using the "get" action, when retrieving a set of time zones based
       on a "list" action response.

   5.  Avoid use of conditional HTTP requests [RFC7232] with the "get"
       action to prevent tracking of clients by servers generating
       client-specific ETag header field values.

   6.  Avoid use of cookies in HTTP requests [RFC6265].

   7.  Avoid use of authenticated HTTP requests.

   8.  When doing periodic polling to check for updates, apply a random
       (positive or negative) offset to the next poll time to avoid
       servers being able to identify the client by the specific
       periodicity of its polling behavior.

   9.  A server trying to "fingerprint" clients might insert a "fake"
       time zone into the time zone data, using a unique identifier for
       each client making a request.  The server can then watch for
       client requests that refer to that "fake" time zone and thus
       track the activity of each client.  It is hard for clients to
       identify a "fake" time zone given that new time zones are added
       occasionally.  One option to mitigate this would be for the
       client to make use of two time zone data distribution servers
       from two independent providers that provide time zone data from



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       the same publisher.  The client can then compare the list of time
       zones from each server (assuming they both have the same version
       of time zone data from the common publisher) and detect ones that
       appear to be added on one server and not the other.
       Alternatively, the client can check the publisher data directly
       to verify that time zones match the set the publisher has.

   Note that some of the above recommendations will result in less
   efficient use of the protocol due to fetching data that might not be
   relevant to the client.

   An organization can set up a secondary server within their own domain
   and configure their clients to use that server to protect the
   organization's users from the possibility of being tracked by an
   untrusted time zone data distribution server.  Clients can then use
   more-efficient protocol interactions, free from the concerns above,
   on the basis that their organization's server is trusted.  When doing
   this, the secondary server would follow the recommendations for
   clients (listed in the previous paragraph) so that the untrusted
   server is not able to gain information about the organization as a
   whole.  Note, however, that client requests to the secondary server
   are subject to tracking by a network observer, so clients ought to
   apply some of the randomization techniques from the list above.

   Servers that want to avoid accidentally storing information that
   could be used to identify clients can take the following precautions:

   1.  Avoid logging client request activity, or anonymize information
       in any logs (e.g., client IP address, client user-agent details,
       authentication credentials, etc.).

   2.  Add an unused HTTP response header to each response with a random
       amount of data in it (e.g., to pad the overall request size to
       the nearest power-of-2 or 128-byte boundary) to avoid exposing
       which time zones are being fetched when TLS is being used, via
       network traffic analysis.

10.  IANA Considerations

   This specification defines a new registry of "actions" for the time
   zone data distribution service protocol, defines a "well-known" URI
   using the registration procedure and template from Section 5.1 of
   [RFC5785], creates two new SRV service label aliases, and defines one
   new iCalendar property parameter as per the registration procedure in
   [RFC5545].  It also adds a new "TZDIST Identifiers Registry" to the
   IETF parameters URN sub-namespace as per [RFC3553] for use with
   protocol related error codes.




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10.1.  Service Actions Registration

   IANA has created a new top-level category called "Time Zone Data
   Distribution Service (TZDIST) Parameters" and has put all the
   registries created herein into that category.

   IANA has created a new registry called "TZDIST Service Actions", as
   defined below.

10.1.1.  Service Actions Registration Procedure

   This registry uses the "Specification Required" policy defined in
   [RFC5226], which makes use of a designated expert to review potential
   registrations.

   The IETF has created a mailing list, tzdist-service@ietf.org, which
   is used for public discussion of time zone data distribution service
   actions proposals prior to registration.  The IESG has appointed a
   designated expert who will monitor the tzdist-service@ietf.org
   mailing list and review registrations.

   A Standards Track RFC is REQUIRED for changes to actions previously
   documented in a Standards Track RFC; otherwise, any public
   specification that satisfies the requirements of [RFC5226] is
   acceptable.

   The registration procedure begins when a completed registration
   template, as defined below, is sent to tzdist-service@ietf.org and
   iana@iana.org.  The designated expert is expected to tell IANA and
   the submitter of the registration whether the registration is
   approved, approved with minor changes, or rejected with cause, within
   two weeks.  When a registration is rejected with cause, it can be
   resubmitted if the concerns listed in the cause are addressed.
   Decisions made by the designated expert can be appealed as per
   Section 7 of [RFC5226].

   The designated expert MUST take the following requirements into
   account when reviewing the registration:

   1.  A valid registration template MUST be provided by the submitter,
       with a clear description of what the action does.

   2.  A proposed new action name MUST NOT conflict with any existing
       registered action name.  A conflict includes a name that
       duplicates an existing one or that appears to be very similar to
       an existing one and could be a potential source of confusion.





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   3.  A proposed new action MUST NOT exactly duplicate the
       functionality of any existing actions.  In cases where the new
       action functionality is very close to an existing action, the
       designated expert SHOULD clarify whether the submitter is aware
       of the existing action, and has an adequate reason for creating a
       new action with slight differences from an existing one.

   4.  If a proposed action is an extension to an existing action, the
       changes MUST NOT conflict with the intent of the existing action,
       or in a way that could cause interoperability problems for
       existing deployments of the protocol.

   The IANA registry contains the name of the action ("Action Name") and
   a reference to the section of the specification where the action
   registration template is defined ("Reference").

10.1.2.  Registration Template for Actions

   An action is defined by completing the following template.

   Name:  The name of the action.

   Request-URI Template:  The URI template used in HTTP requests for the
      action.

   Description:  A general description of the action, its purpose, etc.

   Parameters:  A list of allowed request URI query parameters,
      indicating whether they are "REQUIRED" or "OPTIONAL" and whether
      they can occur only once or multiple times, together with the
      expected format of the parameter values.

   Response:  The nature of the response to the HTTP request, e.g., what
      format the response data is in.

   Possible Error Codes:  Possible error codes reported in a JSON
      "problem details" object if an HTTP request fails.














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10.1.3.  Actions Registry

   The following table provides the initial content of the actions
   registry.

                +---------------+------------------------+
                | Action Name   | Reference              |
                +---------------+------------------------+
                | capabilities  | RFC 7808, Section 5.1  |
                | list          | RFC 7808, Section 5.2  |
                | get           | RFC 7808, Section 5.3  |
                | expand        | RFC 7808, Section 5.4  |
                | find          | RFC 7808, Section 5.5  |
                | leapseconds   | RFC 7808, Section 5.6  |
                +---------------+------------------------+

10.2.  timezone Well-Known URI Registration

   IANA has added the following to the "Well-Known URIs" [RFC5785]
   registry:

   URI suffix:  timezone

   Change controller:  IESG.

   Specification document(s):  RFC 7808

   Related information:  None.

10.3.  Service Name Registrations

   IANA has added two new service names to the "Service Name and
   Transport Protocol Port Number Registry" [RFC6335], as defined below.

10.3.1.  timezone Service Name Registration

   Service Name:  timezone

   Transport Protocol(s):  TCP

   Assignee:  IESG <iesg@ietf.org>

   Contact:  IETF Chair <chair@ietf.org>

   Description:  Time Zone Data Distribution Service - non-TLS

   Reference:  RFC 7808




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   Assignment Note:  This is an extension of the http service.  Defined
      TXT keys: path=<context path> (as per Section 6 of [RFC6763]).

10.3.2.  timezones Service Name Registration

   Service Name:  timezones

   Transport Protocol(s):  TCP

   Assignee:  IESG <iesg@ietf.org>

   Contact:  IETF Chair <chair@ietf.org>

   Description:  Time Zone Data Distribution Service - over TLS

   Reference:  RFC 7808

   Assignment Note:  This is an extension of the https service.  Defined
      TXT keys: path=<context path> (as per Section 6 of [RFC6763]).

10.4.  TZDIST Identifiers Registry

   IANA has registered a new URN sub-namespace within the IETF URN Sub-
   namespace for Registered Protocol Parameter Identifiers defined in
   [RFC3553].

   Registrations in this registry follow the "IETF Review" [RFC5226]
   policy.

   Registry name:  TZDIST Identifiers

   URN prefix:  urn:ietf:params:tzdist

   Specification:  RFC 7808

   Repository:  http://www.iana.org/assignments/tzdist-identifiers

   Index value:  Values in this registry are URNs or URN prefixes that
      start with the prefix "urn:ietf:params:tzdist:".  Each is
      registered independently.  The prefix
      "urn:ietf:params:tzdist:error:" is used to represent specific
      error codes within the protocol as defined in the list of actions
      in Section 5 and used in problem reports (Section 4.1.7).

   Each registration in the "TZDIST Identifiers" registry requires the
   following information:

   URN:  The complete URN that is used or the prefix for that URN.



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   Description:  A summary description for the URN or URN prefix.

   Specification:  A reference to a specification describing the URN or
      URN prefix.

   Contact:  Email for the person or groups making the registration.

   Index Value:  As described in [RFC3553], URN prefixes that are
      registered include a description of how the URN is constructed.
      This is not applicable for specific URNs.

   The "TZDIST Identifiers" registry has the initial registrations
   included in the following sections.

10.4.1.  Registration of invalid-action Error URN

   The following URN has been registered in the "tzdist Identifiers"
   registry.

   URN:  urn:ietf:params:tzdist:error:invalid-action

   Description:  Generic error code for any invalid action.

   Specification:  RFC 7808, Section 5

   Repository:  http://www.iana.org/assignments/tzdist-identifiers

   Contact:  IESG <iesg@ietf.org>

   Index value:  N/A.

10.4.2.  Registration of invalid-changedsince Error URN

   The following URN has been registered in the "tzdist Identifiers"
   registry.

   URN:  urn:ietf:params:tzdist:error:invalid-changedsince

   Description:  Error code for incorrect use of the "changedsince" URI
      query parameter.

   Specification:  RFC 7808, Section 5.2

   Repository:  http://www.iana.org/assignments/tzdist-identifiers

   Contact:  IESG <iesg@ietf.org>

   Index value:  N/A.



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10.4.3.  Registration of tzid-not-found Error URN

   The following URN has been registered in the "tzdist Identifiers"
   registry.

   URN:  urn:ietf:params:tzdist:error:tzid-not-found

   Description:  Error code for missing time zone identifier.

   Specification:  RFC 7808, Sections 5.3 and 5.4

   Repository:  http://www.iana.org/assignments/tzdist-identifiers

   Contact:  IESG <iesg@ietf.org>

   Index value:  N/A.

10.4.4.  Registration of invalid-format Error URN

   The following URN has been registered in the "tzdist Identifiers"
   registry.

   URN:  urn:ietf:params:tzdist:error:invalid-format

   Description:  Error code for unsupported HTTP Accept request header
      field value.

   Specification:  RFC 7808, Section 5.3

   Repository:  http://www.iana.org/assignments/tzdist-identifiers

   Contact:  IESG <iesg@ietf.org>

   Index value:  N/A.

10.4.5.  Registration of invalid-start Error URN

   The following URN has been registered in the "tzdist Identifiers"
   registry.

   URN:  urn:ietf:params:tzdist:error:invalid-start

   Description:  Error code for incorrect use of the "start" URI query
      parameter.

   Specification:  RFC 7808, Sections 5.3 and 5.4

   Repository:  http://www.iana.org/assignments/tzdist-identifiers



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   Contact:  IESG <iesg@ietf.org>

   Index value:  N/A.

10.4.6.  Registration of invalid-end Error URN

   The following URN has been registered in the "tzdist Identifiers"
   registry.

   URN:  urn:ietf:params:tzdist:error:invalid-end

   Description:  Error code for incorrect use of the "end" URI query
      parameter.

   Specification:  RFC 7808, Sections 5.3 and 5.4

   Repository:  http://www.iana.org/assignments/tzdist-identifiers

   Contact:  IESG <iesg@ietf.org>

   Index value:  N/A.

10.4.7.  Registration of invalid-pattern Error URN

   The following URN has been registered in the "tzdist Identifiers"
   registry.

   URN:  urn:ietf:params:tzdist:error:invalid-pattern

   Description:  Error code for incorrect use of the "pattern" URI query
      parameter.

   Specification:  RFC 7808, Section 5.5

   Repository:  http://www.iana.org/assignments/tzdist-identifiers

   Contact:  IESG <iesg@ietf.org>

   Index value:  N/A.












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10.5.  iCalendar Property Registrations

   This document defines the following new iCalendar properties, which
   have been added to the "Properties" registry under "iCalendar Element
   Registries" [RFC5545]:

          +----------------+----------+------------------------+
          | Property       | Status   | Reference              |
          +----------------+----------+------------------------+
          | TZUNTIL        | Current  | RFC 7808, Section 7.1  |
          | TZID-ALIAS-OF  | Current  | RFC 7808, Section 7.2  |
          +----------------+----------+------------------------+

11.  References

11.1.  Normative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <http://www.rfc-editor.org/info/rfc2046>.

   [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>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <http://www.rfc-editor.org/info/rfc2782>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <http://www.rfc-editor.org/info/rfc3339>.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <http://www.rfc-editor.org/info/rfc3553>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.



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   [RFC4985]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, DOI 10.17487/RFC4985, August 2007,
              <http://www.rfc-editor.org/info/rfc4985>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [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>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <http://www.rfc-editor.org/info/rfc5785>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <http://www.rfc-editor.org/info/rfc6125>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <http://www.rfc-editor.org/info/rfc6265>.

   [RFC6321]  Daboo, C., Douglass, M., and S. Lees, "xCal: The XML
              Format for iCalendar", RFC 6321, DOI 10.17487/RFC6321,
              August 2011, <http://www.rfc-editor.org/info/rfc6321>.







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   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <http://www.rfc-editor.org/info/rfc6335>.

   [RFC6557]  Lear, E. and P. Eggert, "Procedures for Maintaining the
              Time Zone Database", BCP 175, RFC 6557,
              DOI 10.17487/RFC6557, February 2012,
              <http://www.rfc-editor.org/info/rfc6557>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <http://www.rfc-editor.org/info/rfc6570>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <http://www.rfc-editor.org/info/rfc6698>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <http://www.rfc-editor.org/info/rfc6763>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <http://www.rfc-editor.org/info/rfc7232>.







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   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <http://www.rfc-editor.org/info/rfc7234>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <http://www.rfc-editor.org/info/rfc7235>.

   [RFC7265]  Kewisch, P., Daboo, C., and M. Douglass, "jCal: The JSON
              Format for iCalendar", RFC 7265, DOI 10.17487/RFC7265, May
              2014, <http://www.rfc-editor.org/info/rfc7265>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <http://www.rfc-editor.org/info/rfc7807>.

11.2.  Informative References

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <http://www.rfc-editor.org/info/rfc2131>.

Acknowledgements

   The authors would like to thank the members of the Calendaring and
   Scheduling Consortium's Time Zone Technical Committee, and the
   participants and chairs of the IETF tzdist working group.  In
   particular, the following individuals have made important
   contributions to this work: Steve Allen, Lester Caine, Stephen
   Colebourne, Tobias Conradi, Steve Crocker, Paul Eggert, Daniel Kahn
   Gillmor, John Haug, Ciny Joy, Bryan Keller, Barry Leiba, Andrew
   McMillan, Ken Murchison, Tim Parenti, Arnaud Quillaud, Jose Edvaldo
   Saraiva, and Dave Thewlis.

   This specification originated from work at the Calendaring and
   Scheduling Consortium, which has supported the development and
   testing of implementations of the specification.






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Authors' Addresses

   Michael Douglass
   Spherical Cow Group
   226 3rd Street
   Troy, NY  12180
   United States

   Email: mdouglass@sphericalcowgroup.com
   URI:   http://sphericalcowgroup.com


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

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































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