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).
AbstractTwo 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.
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  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  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 . 3] provides a means for hosts to receive configuration information relating to their current location within an IP version 4 network.  similarly does so for IP version 6 networks. RFC 2132  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.
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 . 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 ), 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. RFC 2939 , 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.
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. 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.
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.
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.
 "Standard for Information Technology - Portable Operating System Interface (POSIX) - Base Definitions", IEEE Std 1003.1-2004, December 2004.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997.  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.  Droms, R., "Procedures and IANA Guidelines for Definition of New DHCP Options and Message Types", BCP 43, RFC 2939, September 2000.  Eggert, P. and A. Olson, "Sources for Time Zone and Daylight Saving Time Data", <http://www.twinsun.com/tz/tz-link.htm>.  Vaillancourt, S., "Calconnect.org TC Timezone Technical Committee: Timezone Registry and Service Recommendations", April 2006.  Dawson, F. and Stenerson, D., "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", RFC 2445, November 1998.  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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