====================  ====================
(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 process for the delivery of time zone data can be visualized via the diagram shown below.

The overall service is made up of several layers:

(a) Contributors:

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

(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 "re-publish" data from another publisher.

(c) Root Providers:

Servers which 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 which 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 setup their own secondary provider to allow clients within their network to make requests of that server rather than making 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.

3. General Considerations

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

3.1. Time Zone

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

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 Meta-Data

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 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, 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) time zone identifier naming scheme 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 which 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.

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 and time values, and 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 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" sub-component 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 truncation range start point. This sub-component, 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 [TZUNTIL] in the "VTIMEZONE" component to indicate the end point of the truncation range.

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 which time zone data publishers might use to publish updates to time zone data:

1. 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. e.g., a publisher producing data several times a year might use version identifiers "2015a", "2015b", etc.
2. 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. e.g., 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 which 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 and meta-data, 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 distribution service has a "context path" request-URI defined as "{/service-prefix}". This "root" prefix is discovered by the client as per Section 4.2.1. Request-URIs that target time zone data directly use the prefix "{/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, which 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 template-URI 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 meta-data in the form of a JSON [RFC7159] object. Clients can directly request the time zone meta-data, or issues queries for subsets of meta-data that match specific criteria.

Security and privacy considerations for this protocol are discussed in detail in Section 8 and Section 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 meta-data. 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 also 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 [action_get_capabilities].

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 meta-data 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 meta-data associated with one or more time zones (where the meta-data 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 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:

1. 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 meta-data for only those time zones that have changed since the last request.
2. The client will cache the new opaque synchronization token returned in the response for the next incremental update, along with the returned time zone meta-data information.
3. The client will check each time zone meta-data to see if the "etag" value is different from that of any cached time zone data it has.
4. The client will use "get" action request to update any cached time zone data for those time zone's whose ETag header field value has changed.

Note that time zone meta-data 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 meta-data 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 meta-data element will change with each new published revision, however, only a small subset of time zone data will actually change.

If a client only 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.

1. 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.
2. The client will store the value of that header field along with the request-URI used for the request.
3. 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 which 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.

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

The following are examples of response codes one would expect to be used by the server. Note, however, that unless explicitly prohibited any 2/3/4/5xx series response code may be used in a response.

• 200 (OK) - The command succeeded.
• 304 (Not Modified) - The requested data is unchanged.
• 400 (Bad Request) - The Sender has provided an invalid request parameter.
• 404 (Not Found) - The time zone was not found.

When an HTTP error response is returned to the client, the server SHOULD return a JSON "problem detail" object in the response body, as per [I-D.ietf-appsawg-http-problem]. Every JSON "problem detail" 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 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 sub-sections 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. Time Zone Data Distribution Service SRV Service Labels

[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. Time Zone Data Distribution Service TXT records

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.

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 next.

To facilitate "context path's" that might differ from user to user, 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. Time Zone Data Distribution Service Well-Known URI

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. 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 is always present on the server, even when a TXT RR [txt_rr] 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.

To facilitate "context path's" that might differ from user to user, 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).

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 first issuing a "list" action to retrieve all the time zone meta-data. The client would preserve the returned opaque token for subsequent use (see "synctoken" in Section 5.2.1). The client will store the meta-data 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 meta-data.

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 would issue a "list" action with the "changedsince" URI query parameter set to the value of the opaque token returned by the last synchronization. The client would again preserve the returned opaque token for subsequent use. The client will update its stored time zone meta-data using the new values returned in the response, which contains just the time zone meta-data for those time zones changed since the last synchronization. In addition, it will compare the "etag" value in each time zone meta-data to the ETag header field value for the corresponding time zone data resource it has previously cached, and if different, it will fetch 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 will be 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 Pre-Existing 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 meta-data, 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

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"]
  },

  "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",
          "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.

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...
  ]
}

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.

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 the time zone with a specific time zone identifier to 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

5.3.2. Example: Conditional Get time zone data

In this example the client requests the time zone with a specific time zone identifier to 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 the time zone with an aliased time zone identifier to 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
...
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 the time zone with a specific time zone identifier truncated at one of the ranges specified by the server, to 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" [TZUNTIL] iCalendar property in the "VTIMEZONE" component, indicating the upper bound on the validity 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
BEGIN:STANDARD
DTSTART:20101231T190000
TZNAME:EST
TZOFFSETFROM:-0500
TZOFFSETTO:-0500
END:STANDARD
...
END:VTIMEZONE
END:VCALENDAR

5.3.5. Example: Request for a non-existent time zone

In this example the client requests the time zone with a specific time zone identifier to 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.

Description:

This action expands the specified time zone into a list of onset start date/time (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. e.g., 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 point 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 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
    },
  ]
}

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 pattern match against the names known to the server (with an asterisk * as the wildcard character). Pattern match strings (which have to be %-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 sub-string 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. e.g., 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.

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"
        }
      ]
    },
    {
      "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. e.g., (from the example below) 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.

Possible Error Codes

No specific code.

5.6.1. Example: Get leapsecond 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": "2014-06-28",
  "publisher": "Example.com",
  "version": "2014d",
  "leapseconds": [
    {
      "utc-offset": 10,
      "onset": "1972-01-01",
    },
    {
      "utc-offset": 11,
      "onset": "1972-07-01",
    },
    ...
    {
      "utc-offset": 35,
      "onset": "2012-07-01",
    }
  ]
}

6. JSON Definitions

[RFC7159] defines the structure of JSON objects using a set of primitive elements. Those elements will be used to describe the structure of JSON objects used by this specification using a set of "rules". The rules used are:

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 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, then the name of the member, followed by 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

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

; 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)

; [RFC3339] UTC date-time value 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

; [RFC3339] UTC date-time value 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 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,

; 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

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
; [RFC3339] UTC date-time value
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

; 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

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
)

; Observance name
MEMBER oname "name" : STRING

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

; [RFC3339] UTC date-time value 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

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
; [RFC3339] full-date value
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
)

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

; [RFC3339] full-date value 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

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

tzuntilparam = *(";" other-param)

TZUNTIL:20260101T000000Z

Property Name:

TZUNTIL

Purpose:

This property specifies an upper bound for the validity 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 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" sub-components. However, [RFC5545] does not define a way to indicate an upper bound on the validity of the time zone data, which cannot be simply derived from the observance with the latest onset time. 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:

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:

7.2. Time Zone Identifier Alias Property

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

tzidaliasofparam = *(";" other-param)

;tzidprefix defined in [RFC5545].

TZID-ALIAS-OF:America/New_York

Property Name:

TZID-ALIAS-OF

Purpose:

This property specifies a time zone identifier that the main time zone identifier is an alias of.

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:

Example:

The following is an example of this property:

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 additional, 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 and 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.

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 from time to time. One option to mitigate this would be for the client to make use of two time zone distribution servers from two independent providers, that provide time zone data from 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 setup 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 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 if 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" sub-namespace to the IETF parameters URN sub-namespace as per [RFC3553] for use with protocol related error codes.

10.1. Service Actions Registration

IANA is asked to create a new top-level category called "Time Zone Distribution Service (TZDIST) Parameters", and to put all the registries created herein into that category.

IANA is asked to create 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 will create a mailing list, tzdist-service@ietf.org, which can be used for public discussion of time zone data distribution service actions proposals prior to registration. The IESG will appoint 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 re-submitted if the concerns listed in the cause are addressed. Decisions made by the designated expert can be appealed to the IESG Applications Area Director, then to the IESG. They follow the normal appeals procedure for IESG decisions.

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.
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.

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.

10.1.3. Actions Registry

The following table is to be used to initialize the actions registry.

Action Name	Status	Reference
capabilities	Current	RFCXXXX, Section 5.1
list	Current	RFCXXXX, Section 5.2
get	Current	RFCXXXX, Section 5.3
expand	Current	RFCXXXX, Section 5.4
find	Current	RFCXXXX, Section 5.5
leapseconds	Current	RFCXXXX, Section 5.6

10.2. timezone Well-Known URI Registration

IANA is asked to make the following registration in the "Well-Known URIs" [RFC5785] registry:

URI suffix:

timezone

Change controller:

IETF.

Specification document(s):

This RFC.

Related information:

10.3. Service Name Registrations

IANA is asked to add 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:

[This Draft]

Assignment Note:

This is an extension of the http service. Defined TXT keys: path=<context path>

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:

[This Draft]

Assignment Note:

This is an extension of the https service. Defined TXT keys: path=<context path>

10.4. tzdist URN sub-namespace

IANA is requested to register a new URN sub-namespace within the IETF URN Sub-namespace for Registered Protocol Parameter Identifiers defined in [RFC3553].

Registry name:

tzdist

Specification):

This RFC

Repository:

Time Distribution Service protocol elements registry.

Index value::

This specification defines the single "error" sub-parameter, which itself has sub-parameters representing 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).

10.5. iCalendar Property Registrations

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

Property	Status	Reference
TZUNTIL	Current	RFCXXXX, Section 7.1
TZID-ALIAS-OF	Current	RFCXXXX, Section 7.2

11. 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, 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.

12. References

12.1. Normative References

[I-D.ietf-appsawg-http-problem]	Nottingham, M. and E. Wilde, "Problem Details for HTTP APIs", Internet-Draft draft-ietf-appsawg-http-problem-00, September 2014.
[RFC2046]	Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996.
[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782]	Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
[RFC2818]	Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3339]	Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002.
[RFC3553]	Mealling, M., Masinter, L., Hardie, T. and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", BCP 73, RFC 3553, June 2003.
[RFC3629]	Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
[RFC4985]	Santesson, S., "Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name", RFC 4985, August 2007.
[RFC5226]	Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC5246]	Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5545]	Desruisseaux, B., "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", RFC 5545, September 2009.
[RFC5785]	Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known Uniform Resource Identifiers (URIs)", RFC 5785, April 2010.
[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, March 2011.
[RFC6265]	Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011.
[RFC6321]	Daboo, C., Douglass, M. and S. Lees, "xCal: The XML Format for iCalendar", RFC 6321, August 2011.
[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, August 2011.
[RFC6557]	Lear, E. and P. Eggert, "Procedures for Maintaining the Time Zone Database", BCP 175, RFC 6557, February 2012.
[RFC6570]	Gregorio, J., Fielding, R., Hadley, M., Nottingham, M. and D. Orchard, "URI Template", RFC 6570, March 2012.
[RFC6698]	Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, August 2012.
[RFC6763]	Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, February 2013.
[RFC7159]	Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, March 2014.
[RFC7230]	Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014.
[RFC7231]	Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.
[RFC7232]	Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", RFC 7232, June 2014.
[RFC7234]	Fielding, R., Nottingham, M. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June 2014.
[RFC7235]	Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.
[RFC7265]	Kewisch, P., Daboo, C. and M. Douglass, "jCal: The JSON Format for iCalendar", RFC 7265, May 2014.
[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, May 2015.

12.2. Informative References

[RFC2131]

Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.

Appendix A. Change History (to be removed prior to publication as an RFC)

Changes for -07

1. AD review: various nits: Sections 3.3, 3.5, 3.9, 4.2.1.3.1, 4.2.2.1, 5.x.
2. AD review: Section 4.1 now refers to security/privacy sections.
3. AD review: Section 4.1.4 re-written for clarity.
4. AD review: Section 4.2.1.3 now uses "max-age" not "no-cache".
5. AD review: Section 4.2.1.2 now includes text about per-user redirect.
6. AD review: Section 4.2.1.3 clarifies well-know is present even when TXT RR exists.
7. AD review: added /servlet/timezone as URI prefix in examples
8. AD review: Section 5.5 make use of "*" in the middle of a pattern an error.
9. AD review: Section 5.5 underscore and case mapping now a MUST.
10. AD review: Section 5.5 additional text about %-encoding the pattern.
11. AD review: Section 8 re-worded client TLS requirements and added reference to DANE.
12. AD review: Section 10 various clarifications for IANA instructions.
13. AD review: Changed to "Specification Required" policy for new registry and added guidelines for designated expert.

Changes for -07

1. Added comment to check publisher data to see if fake time zone has been used.
2. Tweaked non-DNSSEC SRV text to indicate a service->target match is based on sub-domain and that any mismatch is always a failure.
3. SECDIR: Now recommend that TLS certs include an SRV-ID field.
4. SECDIR: Additional privacy text tweaks.

Changes for -06

1. WGLC: Added addition text about problems with operating system only updates.
2. WGLC: Use "Root" and "Secondary" as prefix for "Providers" in Figure 1.
3. WGLC: various editorial tweaks and example fixes.
4. WGLC: "invalid-changedsince" error description fixed.
5. WGLC: use "full-date" to describe RFC3339 date only values.
6. WGLC: changed security considerations to prevent clients from falling back to non-TLS.
7. SECDIR: added Privacy Considerations with a bunch of recommendations for clients.
8. SECDIR: require TLS for server-to-server requests.
9. SECDIR: require clients to stick with TLS once they start using it (don't downgrade, redirect etc).
10. SECDIR: added reference to TLS BCP document.

Changes for -05

1. Now uses its own rules for defining JSON objects.
2. Added new section on time zone versions.
3. Added publisher/version to the list action response meta-data.
4. Changed conditional request and synchronization sections to better describe how meta-data and data are updated.
5. Added the ability to retrieve leap second information from the server.
6. Added text to require servers to return all data if a "changedsince" value is not supported.
7. Switched TZUNTIL to be exclusive rather than inclusive, so that it now matches the definition of the truncation end point (also exclusive).

Changes for -04

1. Tweaked invalid-start/end for action expand to indicate outside truncation range.
2. Added text on use of Accept-Language.
3. Added text on requirement to percent-encode {/tzid}.
4. Moved /observances under /zones{/tzid}.
5. Observances response now includes start/end of actual range returned if different from what was requested.
6. Truncation end and &end= for get action are now exclusive.
7. Added capabilities action in capabilities example response.
8. Added uri-template items to capabilities action definitions.
9. Added start/end items to the observances response.
10. Error codes are now URNs (with an IANA registration for a tzdist sub-namespace) and the URNs are used as the type value in JSON problem reports.
11. Removed "changedsince" from expand action - ETag can be used instead.

Changes for -03

1. Reworked conditional list section (https://tools.ietf.org/wg/tzdist/trac/ticket/22 & https://tools.ietf.org/wg/tzdist/trac/ticket/33).
2. Moved definitions into General Considerations section.
3. Now makes use of ietf-appsawg-http-problem for error responses.
4. Switched to using a more RESTful design with resources used to identify endpoints for actions (https://tools.ietf.org/wg/tzdist/trac/ticket/29).
5. Tweaked TZUNTIL text to further address (https://tools.ietf.org/wg/tzdist/trac/ticket/15).
6. Tweaked "outright" deletion text to match latest on mailing list (https://tools.ietf.org/wg/tzdist/trac/ticket/18).
7. Added additional text suggesting other discovery mechanisms could be used (https://tools.ietf.org/wg/tzdist/trac/ticket/30).
8. Now require "end" parameter on expand to avoid issues with truncated data upper bounds (https://tools.ietf.org/wg/tzdist/trac/ticket/10).

Changes for -02

1. "time zone server" -> "time zone data server".
2. Re-worded some text containing reference to "historical" time zone data, and truncation behavior.
3. Removed "REST".
4. Use "TZID-ALIAS-OF" in place of "EQUIVALENT-TZID".
5. Added \-escape mechanism for find action.
6. Revised Section 4.2.3 to address (https://tools.ietf.org/wg/tzdist/trac/ticket/18).

Changes for -01

1. Query attribute: "name" -> "pattern" (https://tools.ietf.org/wg/tzdist/trac/ticket/4).
2. UTF-8 used for time zone ids and in all responses.
3. Added glossary term and note for "time zone" (https://tools.ietf.org/wg/tzdist/trac/ticket/12).
4. Glossary term change and alias text from (https://tools.ietf.org/wg/tzdist/trac/ticket/13).
5. "Local Provider" -> "Secondary Provider".
6. Additional security text for (https://tools.ietf.org/wg/tzdist/trac/ticket/25).
7. Added additional text to better describe localized names.
8. Added "tzid" member to expand response.
9. Added optional "provider-details" member to capabilities response, and also made "contacts" optional.
10. Definition of "invalid-action" moved to Section 6, and clarified text related to error responses in Sections 4.1.6 and 6 (https://tools.ietf.org/wg/tzdist/trac/ticket/17).
11. Added "Observance" to glossary.
12. Added "TZUNTIL" iCalendar property (part of https://tools.ietf.org/wg/tzdist/trac/ticket/15).
13. Revamped truncation to always use UTC date-time values and support end points (https://tools.ietf.org/wg/tzdist/trac/ticket/21, and https://tools.ietf.org/wg/tzdist/trac/ticket/10).
14. Expand always uses UTC date-time values for query parameters, and always returns UTC date-time onset values (https://tools.ietf.org/wg/tzdist/trac/ticket/21).

Changes for -00

1. Initial WG draft derived from draft-douglass-timezone-service-11, with some terminology changes to match WG name.
2. Updated references.
3. "timezone" -> "time zone" (https://tools.ietf.org/wg/tzdist/trac/ticket/6).
4. Glossary tweak (first part of https://tools.ietf.org/wg/tzdist/trac/ticket/13).
5. Fix iCalendar property names: UTC-OFFSET-* -> TZOFFSET*.
6. Fix invalid-truncate error code description.

Authors' Addresses