Network Working Group A.K. Vijayabhaskar
Internet-Draft Hewlett-Packard
Expires: April 16, 2004 15 Oct 2003
Time Configuration Options for DHCPv6
draft-ietf-dhc-dhcpv6-opt-timeconfig-03.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: SNTP Server addresses - using which the clients
can synchronize their system time to that of the standard time
servers; Timezone specifier - used to set the timezone of the clients.
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 "Terminology" section of the DHCPv6 specification.
4. Simple Network Time Protocol (SNTP) Servers option
The Simple Network Time Protocol Servers option provides a list of
one or more IPv6 addresses of SNTP [2] servers available to the
client for synchronization. The SNTP servers SHOULD be listed in
the order of preference.
The format of the Simple 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_SNTP_SERVERS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| SNTP server (IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| SNTP server (IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_SNTP_SERVERS (tbd)
option-len: Length of the 'SNTP server' fields in octets; It must be
a multiple of 16
SNTP server: IPv6 address of SNTP server
5. Timezone option
The Timezone option is used by the server to convey the timezone
in which the client resides. The client is expected to set the
timezone in its system on receiving this option from the server.
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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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 NVT-ASCII string format.
The format of this string is 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 there is no
Offset followed by the Std, then the timezone is not
represented in IEEE 1003.1 format. In this case, the
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.
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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.
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
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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.
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. Usage of Timezone option
The Timezone option has the flexibility of providing timezone
information in formats other than POSIX timezone, because
some vendor specific databases can provide more information than
POSIX Timezone string. The server SHOULD be configurable to send any
of the format specified in Section 5.
The timezone option can be used along with the Vendor Class
Option [1] to make sure that the client and server agree upon the
meaning of the string. For example, the clients running in different
OS expect the string in different formats. Here, the Vendor Class
Option [1] sent by clients can be used by the server to distinguish
between the clients to return the proper timezone string.
If the client is not able to interpret the timezone option sent
by the server, then it SHOULD ignore the option. It MAY contact
alternative DHCPv6 servers to obtain the timezone information.
7. Appearance of these options
The SNTP servers and Timezone options MUST NOT appear in other than
the following messages: Solicit, Advertise, Request, 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, Renew, Rebind,
Information-Request and Reconfigure.
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8. Security Considerations
The SNTP servers option may be used by an intruder DHCPv6 server to
cause DHCPv6 clients to contact a rogue SNTP 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 applications running on the client machine less secure. It can
even cause clients to set their time incorrectly, making them
vulnerable to replay attacks in protocols that use time stamps to
detect replays.
The Timezone option may be used by an intruder DHCPv6 server to
assign invalid time zones, leading to timing issues for the
applications running on the client machine. For example, because of
wrongly configured timezone, there is a possibility that some critical
applications, which are supposed to start at a particular time don't
get started at that time. A delayed start of OS security update will
leave the client's machine vulnerable to security attacks.
To avoid attacks through these options, the DHCPv6 client SHOULD use
authenticated DHCPv6 (see "Authentication of DHCP messages" section
in the DHCPv6 specification [1]).
9. IANA Considerations
IANA is requested to assign an option code to the following options
from the option-code space defined in "DHCPv6 Options" section of the
DHCPv6 specification [1].
Option Name Value Described in
OPTION_SNTP_SERVERS tbd Section 4.
OPTION_TIME_ZONE tbd Section 5.
10. Normative References
[1] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
11. 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.
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Author's Addresses
Vijayabhaskar A K
Hewlett-Packard STSD-I
29, Cunningham Road
Bangalore - 560052
India
Phone: +91-80-2053085
E-Mail: vijayak@india.hp.com
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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, Margaret Wasserman, Ralph
Droms, Robert Elz and Thomas Narten 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 Timezone option.
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