Network Working Group                                            E. Lear
Internet-Draft                                        Cisco Systems GmbH
Expires: August 31, 2006                                       P. Eggert
                                                       February 27, 2006

                       A Timezone Option for DHCP

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   DHCP defines an option for a server to deliver to a client offset
   from UTC.  This information in and of itself is not sufficient for
   devices to portray local time both accurately and consistently.  This
   memo specifies a new option for both DHCPv4 and DHCPv6 to do so.

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1.  Introduction

   Dynamic Host Configuration Protocol (DHCP) [2] provides a means for
   hosts to receive configuration information relating to their current
   location within an IP version 4 network. [4] similarly does so for IP
   version 6 networks.  RFC2132 [3] specifies an option to provide a
   client timezone information in the form of an offset in seconds from
   UTC.  The information provided in this 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 (DST).  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

   This memo specifies a means to provide hosts with more accurate
   timezone information than was previously available.  There are
   currently three well known means to configure timezones:
   o  POSIX TZ strings
   o  Reference to the TZ Database
   o  Microsoft's timezone.xml

   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, 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

   The so-called "TZ Database" [6] 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 defacto standard.

   The Microsoft TimeZone element conveys information similar to the
   POSIX string, but with an additional (presumably localized) display

1.1.  What about VTIMEZONE elements from iCalendar?

   VTIMEZONE elements are defined in the iCalendar specification.[8]
   Fully specified they provide a level of accuracy similar to the TZ
   database.  However, because there is currently no global registry of

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   VTIMEZONE TZIDs (although one has been proposed; see [9]), 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
   author is 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 Option for DHCPv4

            Code   Len   TZ Option 1       TZ Option N
           | TBD |  N  |            .     .           |

   Code is TBD and will be allocated by IANA according to RFC-2939 [5].

   Len is the two-octet sum of the size of all following TZ options.

   Suboptions are described later in this document.

3.  New Timezone Option for DHCPv6

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

   Specifically, the DHCPv6 new timezone option has the following

       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_TIMEZONE      |         option-length         |
      |                    New Timezone Suboptions                    |
      |                              ...                              |

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   option-code: OPTION_NEW_TIMEZONE(TBD)

   option-length: variable based on the number and value of suboptions.

4.  The POSIX Suboption String

             number    Len   POSIX String
           |    1    |  N  |              |

   Suboption number is an octet with a value of 1.

   Len is a single octet that contains the number of octets of the
   following string.

   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 (":").  An example might be as follows:


   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

   Clients and servers implementing the timezone option MUST support
   this suboption.

5.  The TZ Database Suboption

             number    Len   TZ Name
           |    2    |  N  |              |

   Suboption number is an octet with a value of 2.

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   Len is a single octet that contains the number of octets of the
   following string.

   TZ Name is the name of a Zone entry in the database commonly referred
   to as the TZ database.  In order for this option to be useful, the
   client must already have a copy of the database.  An example string
   would be "Europe/Zurich".

   If a client understands this option but does not recognize the TZ
   Name returned, it MUST ignore this option and MAY make use of the
   POSIX string.

6.  The Microsoft TZ Suboption

             number    TZ ID
           |    3    |  NNNN             |

   Suboption number is an octet with a value of 3.

   TZ ID is a four-octet integer in network byte order that references
   the timezone ID as defined in the TimeZone element, as specified by
   Microsoft [7].

   If a client does not have an entry corresponding to the TZ ID
   returned by the server, the client MUST ignore this sub-option, and
   MAY instead make use of the POSIX option.

7.  Use of the timezone string 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.

   The client uses this information at its discretion to configure the
   current timezone in which it resides.  This memo does not define
   client behavior, particularly whether it should request and receive a
   response with this option from multiple subnets where the timezone
   information conflicts.

   It will periodically be necessary for a DHCP server to update the

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   timezone string, based on administrative changes made by local
   jurisdictions (say, for instance, counties in Indiana).  While the
   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

8.  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.

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.  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.

   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 time
   zone 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 is requested to allocate both DHCPv4 and DHCPv6 option codes
   for this purpose and reference this document in that allocation for
   both DHCPv4 and DHCPv6.

   The IANA need not and should not retain a list of suboptions.  Any
   new suboptions require further standards action.

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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 have done this nearly thankless
   task for many years.  Thanks also go to Ralph Droms, Bernie Volz, Ted
   Lemon, Lisa Dusseault,and Simon Vaillancourt for their attempts to
   improve this work.

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

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

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

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

   [6]  Eggert, P. and A. Olson, "Sources for Time Zone and Daylight
        Saving Time Data", <>.

   [7]  "The Microsoft TimeZone Element",

12.2.  Informational References

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

   [9]  Royer, D., "Time Zone Registry",
        draft-royer-timezone-registry-03 (work in progress),
        August 2005.

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Appendix A.  Changes

   [The RFC Editor is requested to remove this section at publication.]
   o  -02; fix references to the TZ database; add additional security
      considerations; clarify POSIX example; reference Doug Royer
      registry draft; add Paul Eggert as co-author(who did all the
   o  -01; clarify uses of each suboption; reset suboption sizes; add
      explanation for not using VTIMEZONEs; add acknowlegments.
   o  initial revision

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Authors' Addresses

   Eliot Lear
   Cisco Systems GmbH
   Glattzentrum, ZH  CH-8301

   Phone: +41 1 878 9200

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

   Phone: +1 310 825 3886

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