Network Working Group E. Lear Internet-Draft Cisco Systems GmbH Expires: August 28, 2006 February 24, 2006 A Timezone Option for DHcP draft-lear-dhc-timezone-option-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 28, 2006. Copyright Notice 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. 1. Introduction Dynamic Host Configuration Protocol (DHCP) [2] provides a means for Lear Expires August 28, 2006 [Page 1]
Internet-Draft A Timezone option for DHCP February 2006 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 savings time and when to change into and out of daylight savings 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 Olson 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 forward and backward accuracy for a total of one year. For backward accuracy beyond a year a more detailed mechanism is necessary. The so-called "Olson database" [7] that is used in many operating systems provides backwards consistency and accuracy. The Olson database also attempts to provide a stable set of human readable timezone identifiers. In addition, many systems already make use of the Olson 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.[6] Fully specified they provide a level of accuracy similar to the Olson database. However, because there is no global registry of VTIMEZONE TZIDs, 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. Lear Expires August 28, 2006 [Page 2]
Internet-Draft A Timezone option for DHCP February 2006 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 | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ option-code: OPTION_NEW_TIMEZONE(TBD) option-length: variable based on the number and value of suboptions. 4. The POSIX Suboption String Lear Expires August 28, 2006 [Page 3]
Internet-Draft A Timezone option for DHCP February 2006 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 IEEE-1003.1-2004 Section 8.3. An example might be "EST5EDT4,M4.1.1,M11.1.1". In this case, the string is interpretted as a timezone with display name "EST" which is normally five hours behind UTC, and hours behind UTC during DST, which runs from the first Sunday in April through the first Sunday in November. Clients and servers implementing the timezone option MUST support this suboption. 5. The Olson Database Suboption Suboption number Len TZ Name +---------+-----+--------------+ | 2 | N | | +---------+-----+--------------+ Suboption number is an octet with a value of 2. Len is a single octet that contains the number of octets of the following string. TZ Name is an index to the database commonly referred to as the Olson Timezone 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. Lear Expires August 28, 2006 [Page 4]
Internet-Draft A Timezone option for DHCP February 2006 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 [8]. 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 should it request and receive a response with this option from multiple subnets where the timezone information conflicts. When a POSIX string is used, 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 author does 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. Lear Expires August 28, 2006 [Page 5]
Internet-Draft A Timezone option for DHCP February 2006 9. Security Considerations It is unclear what trouble an attacker could cause by providing erronious 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. In such cases, the client operating system might do well to confirm timezone changes with a human. 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. 11. Acknowledgments The author claims little expertise in either DHCP or time, and thanks 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] "Standord for Information Technology - Portable Operating System Interface", IEEE 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. Lear Expires August 28, 2006 [Page 6]
Internet-Draft A Timezone option for DHCP February 2006 12.2. Informational References [6] Dawson, F. and Stenerson, D., "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", RFC 2445, November 1998. URIs [7] <http://elsie.nci.nih.gov> [8] <http://msdn.microsoft.com/library/default.asp?url=/library/ en-us/spptsdk/html/tscamltimezone_sv01028158.asp> Appendix A. Changes o -01; clarify uses of each suboption; reset suboption sizes; add explanation for not using VTIMEZONEs; add acknowlegments. o initial revision Lear Expires August 28, 2006 [Page 7]
Internet-Draft A Timezone option for DHCP February 2006 Author's Address Eliot Lear Cisco Systems GmbH Glatt-com Glattzentrum, ZH CH-8301 Switzerland Phone: +41 1 878 9200 Email: lear@cisco.com Lear Expires August 28, 2006 [Page 8]
Internet-Draft A Timezone option for DHCP February 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Lear Expires August 28, 2006 [Page 9]