Network Working Group A.K. Vijayabhaskar
Internet-Draft Hewlett-Packard
Expires: August 26, 2003 26 Feb 2003
Time Configuration Options for DHCPv6
draft-ietf-dhc-dhcpv6-opt-timeconfig-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the options for Time related configuration
information in DHCPv6: NTP Servers and Timezone specifier.
1. Introduction
This document describes the options for time related configuration
information in DHCPv6 [1].
2. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [4]
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3. Terminology
This document uses terminology specific to IPv6 and DHCPv6 as defined
in section "Terminology" of the DHCP specification.
4. Network Time Protocol (NTP) Servers option
The Network Time Protocol Servers option provides a list of one or
more IP addresses of NTP [2] servers available to the client. The
NTP servers SHOULD be listed in the order of preference.
The format of the Network Time Protocol Servers option is as shown
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_NTP_SERVERS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NTP server (IP address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NTP server (IP address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_NTP_SERVERS (tbd)
option-len: Length of the 'NTP server' fields in octets; It must be
a multiple of 16
NTP server: IP address of NTP server
5. Timezone option
The Timezone option is used by the server to convey client's timezone
information to the client.
The format of the Timezone option is:
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_TIME_ZONE | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time-zone |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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option-code: OPTION_TIME_ZONE (tbd)
option-len: Length of the 'time-zone' field in octets
time-zone: Time zone of the client in the format as explained below.
Std[Offset[Dst[Offset],[Start[/Time],End[/Time]]]]
where '[' and ']' enclose optional fields, '|' indicates choice
of exactly one of the alternatives, ',' and '/' represent literal
characters present in the string.
If "Offset" is specified, then the time-zone is represented in the
IEEE 1003.1 POSIX timezone format [3].
Std Three or more octets for the standard timezone (Std).
Any character (or case) except a leading colon, digits,
comma, minus or plus sign is allowed. If the time-zone
is not represented in IEEE 1003.1 POSIX timezone format [3],
then Std is treated as the index to the timezone database,
for example, a file name, from where additional information
about the timezone may be obtained.
Offset Indicates the value one must add to local time to
arrive at UTC, of the form: [+|-]hh[:mm[:ss]]. Offset
following Std is required, if the timezone is represented
in IEEE 1003.1 POSIX timezone format. Digits are always
interpreted as decimal number. If preceded by a '-', the
timezone is east of the Prime Meridian, otherwise it is west
('+' is optional) The permissible values for hh[:mm[:ss]] are
as follows:
hh 0 <= hh <= 23
mm 0 <= mm <= 60
ss 0 <= ss <= 60
Dst Three or more octets for the daylight savings timezone.
If Dst is missing, then daylight savings time does not
apply in this locale. If no Offset follows Dst, then
Dst is assumed to be one hour ahead of standard time.
Any character (or case) except a leading colon, digits,
comma, minus or plus sign is allowed.
Start Indicates the day of the year, in one of the formats
indicated below, when to change to daylight savings time.
The ``Time'' field (which follows immediately after a
``/'' character, if present) indicates when the change is
made, in local time.
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End Indicates the day of the year, in one of the formats
indicated below, when to change back from daylight
savings time. The ``Time'' field (which follows
immediately after a ``/'' character, if present)
indicates when the change is made, in local time.
Time Time has the same format as Offset, except that no
leading ``-'' or ``+'' is permitted. The default is
02:00:00.
The day of the year needs to be given in any of the following
formats:
Jn The julian day n, (1 <= n <= 365). Leap days are not
counted.
n Zero-based julian day, (0 <= n <= 365). Leap days are
counted so it is possible to refer to Feb 29.
Mm.n.d The ``d''th day, (0 <= d <= 6) of week ``n'' of month
``m'' of the year (1 <= n <= 5, 1 <= m <= 12, where week
5 means last ``d'' day in month ``m'' which may occur in
either the fourth or the fifth week. Week ``1'' is the
first week in which the ``d'' day occurs. Day ``0'' refers
Sunday, day ``1'' refers Monday and so on.
Examples:
i) Indian Standard Time zone is represented as:
IST-5:30
Here, ``IST'' refers the standard timezone and ``-5:30'' is the offset.
`-' sign in the offset says that the timezone is 5 hours and 30 minutes
ahead of UTC. Absence of ``Dst'' says that daylight savings doesn't apply
to this locale.
ii) For Eastern USA time zone, 1986, the timezone string is as shown below:
EST5EDT4,116/02:00:00,298/02:00:00
It says:
The standard time zone is in 5 hours behind UTC. The Daylight Savings
Timezone is 4 hours behind UTC. Day light savings starts at 116 day, i.e.,
April 27 02:00 AM standard time and ends at 298th day, i.e., October 26
02:00 AM daylight time.
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It can also represented as:
EST5EDT,116/02:00:00,298/02:00:00
Since no offset follows the ``Dst'', daylight savings time is 1 hour
ahead of standard time, thus, it is 4 hours behind UTC.
iii) Representing ii) in the non POSIX standard way is:
America/New-York
It says that the locale belongs to New-York timezone in America, which
will be used as the index in to a timezone database to get more
information of the timezone.
6. Appearance of these option
The NTP servers and Timezone options MUST appear only in the following
messages: Solicit, Advertise, Request, Confirm, Renew, Rebind,
Information-Request and Reply.
The option number for these options MAY appear in the Option Request
Option [1] in the following messages: Solicit, Request, Confirm, Renew,
Rebind, Information-Request and Reconfigure.
7. Security Considerations
The NTP servers option may be used by an intruder DHCP server to
cause DHCP clients to contact an intruder NTP server, resulting in
invalid synchronization of time in client and finally leading to
time critical applications running inaccurately in client machine.
The time accuracy can be crucial to some security algorithms. For
example, it may cause expired certificates to gain a new life, making
the application less secured.
The Timezone option may be used by an intruder DHCP server to assign
invalid time zones, leading to timing issues for the applications running
on the client machine.
To avoid attacks through these options, the DHCP client SHOULD use
authenticated DHCP (see section "Authentication of DHCP messages"
in the DHCPv6 specification [1]).
8. IANA Considerations
IANA is requested to assign an option code to these options from the
option-code space defined in section "DHCPv6 Options" of the DHCPv6
specification [1].
9. Normative References
[1] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress), November
2002.
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10. Informative References
[2] D. Mills. Simple Network Time Protocol (SNTP) Version 4 for
IPv4, IPv6 and OSI. Request for Comments (Informational) 2030,
Internet Engineering Task Force, October 1996.
[3] IEEE, "1003.1 POSIX Timezone Specification", 1988.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Author's Addresses
Vijayabhaskar A K
Hewlett-Packard ESD-I
29, Cunningham Road
Bangalore - 560052
India
Phone: +91-80-2053085
E-Mail: vijayak@india.hp.com
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Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society. Thanks to the DHC Working Group for their time and
input into the specification. In particular, thanks to (in alphabetical
order) Bernie Volz, Jim Bound, Ralph Droms, Robert Elz for their thorough
review. Special thanks to Robert Elz for his suggestions and help in
making this document more readable. Thanks to Mike Carney for his abstract
on Time Zone option.
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