Network Working Group E. Lear
Internet-Draft Cisco Systems GmbH
Expires: August 31, 2006 P. Eggert
UCLA
February 27, 2006
A Timezone Option for DHCP
draft-lear-dhc-timezone-option-02.txt
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Copyright (C) The Internet Society (2006).
Abstract
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
hour.
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
necessary.
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
string.
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
format:
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
Suboption
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:
"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 the timezone option MUST support
this suboption.
5. The TZ Database Suboption
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
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
notification.
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 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,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", <http://www.twinsun.com/tz/tz-link.htm>.
[7] "The Microsoft TimeZone Element",
<http://msdn.microsoft.com/library/>.
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
above)
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
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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