Timezone Options for DHCP
RFC 4833

Network Working Group                                            E. Lear
Request for Comments: 4833                            Cisco Systems GmbH
Updates: 2132                                                  P. Eggert
Category: Standards Track                                           UCLA
                                                              April 2007

                       Timezone Options for DHCP

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Two common ways to communicate timezone information are POSIX 1003.1
   timezone strings and timezone database names.  This memo specifies
   DHCP options for each of those methods.  The DHCPv4 time offset
   option is deprecated.

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1.  Introduction

   This memo specifies a means to provide hosts with more accurate
   timezone information than was previously available.  To do this we
   make use of two commonly used methods to configure timezones:

   o  POSIX TZ strings

   o  Reference to the name of the time zone entry in the TZ Database

   POSIX [1] provides a standard for how to express timezone information
   in a character string.  Use of such a string can provide accuracy for
   at least one transition into and out of daylight saving time (DST),
   and possibly for more transitions if the transitions are regular
   enough (e.g., "second Sunday in March at 02:00 local time").
   However, for accuracy over longer periods that involve daylight-
   saving rule changes or other irregular changes, a more detailed
   mechanism is necessary.

   The TZ Database [7] that is used in many operating systems provides
   backwards consistency and accuracy for almost all real-world
   locations since 1970.  The TZ database also attempts to provide a
   stable set of human readable timezone identifiers.  In addition, many
   systems already make use of the TZ database, and so the names used
   are a de facto standard.  Because the TZ database contains more
   information, one can heuristically derive the POSIX information from
   a TZ identifier (see [10] for an example), but the converse is not
   true.

   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 RFC 2119 [2].

1.1.  Related Work

   Dynamic Host Configuration Protocol (DHCP) [3] provides a means for
   hosts to receive configuration information relating to their current
   location within an IP version 4 network. [5] similarly does so for IP
   version 6 networks.  RFC 2132 [4] specifies an option to provide
   client timezone information in the form of an offset in seconds from
   UTC.  The information provided in that option is insufficient for the
   client to determine whether it is in daylight saving time, and when
   to change into and out of daylight saving time.  In order for the
   client to properly represent local wall clock time in a consistent
   and accurate fashion the DHCP server would have to time lease
   expirations of affected clients to the beginning or end of DST, thus
   effecting a self stress test (to say the least) at the appointed
   hour.

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   In addition, an offset is not sufficient to determine the actual
   timezone in which a client resides, and thus there is no means to
   derive a human readable abbreviation such as "EST" or "EDT".

   VTIMEZONE elements are defined in the iCalendar specification [9].
   Fully specified they provide a level of accuracy similar to the TZ
   database.  However, because there is currently no global registry of
   VTIMEZONE TZIDs (although one has been proposed; see [8]), complete
   accuracy requires that a full entry must be specified.  To achieve
   the same information would range from 300 octets upwards with no
   particular bound.  Furthermore, at the time of this writing the
   authors are aware of no operating system that natively takes
   advantage of VTIMEZONE entries.  It might be possible to include an
   option for a TZURL.  However, in a cold start environment, it will be
   bad enough that devices are stressing the DHCP server, and perhaps
   unwise to similarly afflict other components.

2.  New Timezone Options for DHCPv4

   The following two options are defined for DHCPv4:

            PCode  Len   TZ-POSIX String
           +-----+-----+------------------------------+
           | 100 |  N  | IEEE 1003.1 String           |
           +-----+-----+------------------------------+

            TCode  Len   TZ-Database String
           +-----+-----+------------------------------+
           | 101 |  N  | Reference to the TZ Database |
           +-----+-----+------------------------------+

   Per RFC 2939 [6], IANA allocated PCode (100) and TCode (101).

   Len is the one-octet value of the length of the succeeding string for
   each option.

   The string values that follow Len are described below.  Note that
   they are NOT terminated by an ASCII NULL.

3.  New Timezone Options for DHCPv6

   The semantics and content of the DHCPv6 encoding of these options are
   exactly the same as the encoding described for DHCPv4, other than
   necessary differences between the way options are encoded in DHCPv4
   and DHCPv6.

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   Specifically, the DHCPv6 new timezone options are described below:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  OPTION_NEW_POSIX_TIMEZONE    |         option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      TZ POSIX String                          |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   option-code: OPTION_NEW_POSIX_TIMEZONE(41)

   option-length: the number of octets of the TZ POSIX String Index
   described below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  OPTION_NEW_TZDB_TIMEZONE    |          option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          TZ Name                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   option-code: OPTION_NEW_TZDB_TIMEZONE(42)

   option-length: the number of octets of the TZ Database String Index
   described below.

4.  The TZ POSIX String

   TZ POSIX string is a string suitable for the TZ variable as specified
   by [1] in Section 8.3, with the exception that a string may not begin
   with a colon (":").  This string is NOT terminated by an ASCII NULL.
   Here is an example:

   EST5EDT4,M3.2.0/02:00,M11.1.0/02:00

   In this case, the string is interpreted as a timezone that is
   normally five hours behind UTC, and four hours behind UTC during DST,
   which runs from the second Sunday in March at 02:00 local time
   through the first Sunday in November at 02:00 local time.  Normally
   the timezone is abbreviated "EST" but during DST it is abbreviated
   "EDT".

   Clients and servers implementing other timezone options MUST support
   this option for basic compatibility.

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5.  The TZ Name

   TZ Name is the name of a Zone entry in the database commonly referred
   to as the TZ database.  Specifically, in the database's textual form,
   the string refers to the name field of a zone line.  In order for
   this option to be useful, the client must already have a copy of the
   database.  This string is NOT terminated with an ASCII NULL.

   An example string is Europe/Zurich.

   Clients must already have a copy of the TZ Database for this option
   to be useful.  Configuration of the database is beyond the scope of
   this document.  A client that supports this option SHOULD prefer this
   option to POSIX string if it recognizes the TZ Name that was
   returned.  If it doesn't recognize the TZ Name, the client MUST
   ignore this option.

6.  Use of the Timezone String(s) Returned from the Server

   This specification presumes the DHCP server has some means of
   identifying which timezone the client is in.  One obvious approach
   would be to associate a subnet or group of subnets with a timezone,
   and respond with this option accordingly.

   When considering which option to implement on a client, one must
   choose between the TZ Name, which should be easier for users to
   configure and which provides accuracy over longer historical periods,
   and the TZ POSIX string, which does not require regular updating of a
   copy of the TZ Database.  The TZ Name is better for most uses, in
   particular those cases where the timezone name might persist in a
   database for long periods of time, but the TZ POSIX string may be
   more suitable for small-footprint applications that are expertly
   maintained.

   So that clients need not request both options, servers who implement
   either timezone option SHOULD implement the other one as well.  This
   association can be established by the server's administrator.  A
   basic server can transmit option values to the client without parsing
   or validating them.  A more advanced server might have a copy of the
   TZ database and validate TZ names against this copy, or derive TZ
   POSIX strings heuristically from TZ names to simplify administration.

   As a matter of practicality, the client will use this information at
   its discretion to configure the current timezone in which it resides.

   It will periodically be necessary for a DHCP server to update the
   timezone string, based on administrative changes made by local
   jurisdictions (say, for instance, counties in Indiana).  While the

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   authors do not expect this to be a lower bound on a lease time in the
   vast majority of cases, there may be times when anticipation of a
   change dictates prudence, as certain governments give little if any
   notification.

   The effect of a changed timezone on client applications is not
   specified by this memo, but it may be helpful to note common problems
   in this area.  Often, client applications consult the timezone
   setting only during process initialization, or inherit the setting
   from a parent process, so existing processes on a client may ignore a
   timezone change returned from the server.  Sometimes it is normal and
   expected for processes on the same client to have different timezone
   settings (e.g., remote logins), and so client implementations should
   consider these ramifications of changing timezone settings of
   existing processes.

7.  The New Timezone Option and Lease Times

   When a lease has expired and new information is not forthcoming, the
   client MAY continue to use timezone information returned by the
   server.  This follows the principle of least astonishment.

8.  Deprecation of Time Offset Option

   Because this option provides a superset of functionality to the
   previous IPv4 time offset option (tag 2), and in order to maintain
   consistency between IPv4 and IPv6 implementation, the older option is
   deprecated.  Current implementations that support the time offset
   IPv4 option SHOULD implement this option also.  Other implementations
   SHOULD implement this option, and SHOULD NOT implement the time
   offset IPv4 option.  As a matter of transition, clients that already
   use the time offset option MAY request the time offset option and the
   timezone option.

9.  Security Considerations

   An attacker could provide erroneous information to a client.  It is
   possible that someone might miss a meeting or otherwise show up
   early, or that heavy machinery or other critical functions might act
   at the wrong time or fail to act.  If clients have job processing
   tools, such as cron that operate on wall clock time, it is possible
   that certain jobs could be triggered either earlier or later, or even
   repeated or skipped entirely if scheduled during a DST transition.
   In such cases, the client operating system might do well to confirm
   timezone changes with a human.

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   Clients using the POSIX option should beware of any time zone setting
   specifying unusual characters (e.g., control characters) in the
   standard or daylight-saving abbreviations, as this might well trigger
   security-relevant bugs in applications.

   Clients using the POSIX option should also be suspicious of any
   timezone setting whose UTC offset exceeds 25 hours (the POSIX limit,
   if the default daylight-saving offset is used).  As of this writing,
   the maximum UTC offset is 14 hours in practice, but governments may
   extend this somewhat in the future.

10.  IANA Considerations

   The IANA has allocated DHCPv4 and DHCPv6 option codes for this
   purpose and references this document.

   The IANA has annotated the time offset IPv4 option (tag 2) as
   deprecated, with a reference to this document.

11.  Acknowledgments

   This document specifies a means to exchange timezone information.
   The hard part is actually collecting changes to the various databases
   from scattered sources around the world.  The many volunteers on the
   mailing list tz@elsie.nci.nih.gov have done this nearly thankless
   task for many years.  Thanks also go to Ralph Droms, Bernie Volz, Ted
   Lemon, Lisa Dusseault, John Hawkinson, Stig Venaas, and Simon
   Vaillancourt for their efforts to improve this work.

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12.  References

12.1.  Normative References

   [1]   "Standard for Information Technology - Portable Operating
         System Interface (POSIX) - Base Definitions",
         IEEE Std 1003.1-2004, December 2004.

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

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

   [4]   Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
         Extensions", RFC 2132, March 1997.

   [5]   Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
         Carney, "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.

   [6]   Droms, R., "Procedures and IANA Guidelines for Definition of
         New DHCP Options and Message Types", BCP 43, RFC 2939,
         September 2000.

   [7]   Eggert, P. and A. Olson, "Sources for Time Zone and Daylight
         Saving Time Data", <http://www.twinsun.com/tz/tz-link.htm>.

12.2.  Informational References

   [8]   Vaillancourt, S., "Calconnect.org TC Timezone Technical
         Committee: Timezone Registry and Service Recommendations",
         April 2006.

   [9]   Dawson, F. and Stenerson, D., "Internet Calendaring and
         Scheduling Core Object Specification (iCalendar)", RFC 2445,
         November 1998.

   [10]  Eggert, P. and E. Reingold, "cal-dst.el --- calendar functions
         for daylight savings rules", <http://cvs.savannah.gnu.org/
         viewcvs/*checkout*/emacs/lisp/calendar/cal-dst.el?root=emacs>.

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Authors' Addresses

   Eliot Lear
   Cisco Systems GmbH
   Glatt-com
   Glattzentrum, ZH  CH-8301
   Switzerland

   Phone: +41 1 878 9200
   EMail: lear@cisco.com

   Paul Eggert
   UCLA
   Computer Science Department
   4532J Boelter Hall
   Los Angeles, CA  90095
   USA

   Phone: +1 310 825 3886
   EMail: eggert@cs.ucla.edu

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RFC 4833               Timezone Options for DHCP              April 2007

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