Internet Engineering Task Force S. Venaas
Internet Draft University of Southampton
Expiration Date: July 2005
T. Chown
University of Southampton
B. Volz
Cisco Systems, Inc.
January 2005
Information Refresh Time Option for DHCPv6
draft-ietf-dhc-lifetime-03.txt
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Copyright (C) The Internet Society (2005). All Rights Reserved.
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Abstract
This document describes a DHCPv6 option for specifying an upper bound
for how long a client should wait before refreshing information
retrieved from DHCPv6. It is used with stateless DHCPv6 as there are
no addresses or other entities with lifetimes that can tell the
client when to contact the DHCPv6 server to refresh its
configuration.
Table of Contents
1. Introduction ............................................... 2
2. Terminology ................................................ 3
3. Information refresh time option definition ................. 3
3.1. Constants .............................................. 4
3.2. Client behaviour ....................................... 4
3.3. Server behaviour ....................................... 5
3.4. Option format .......................................... 5
4. IANA Considerations ........................................ 6
5. Acknowledgements ........................................... 6
6. Security Considerations .................................... 6
7. References ................................................. 6
7.1. Normative References ................................... 6
7.2. Informative References ................................. 7
Authors' Addresses ............................................. 7
Intellectual Property and Copyright Statements ................. 7
1. Introduction
DHCPv6 [RFC 3315] specifies stateful autoconfiguration for IPv6
hosts. However, many hosts will use stateless autoconfiguration as
specified in [RFC 2462] for address assignment, and use DHCPv6 only
for other configuration data, see [RFC 3736]. This other
configuration data will typically have no associated lifetime, hence
there may be no information telling a host when to refresh its DHCPv6
configuration data. Therefore, an option that can be used from
server to client to inform the client when it should refresh the
other configuration data is needed.
This option is useful in many situations:
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- Unstable environments where unexpected changes are likely to
occur.
- For planned changes, including renumbering. An administrator
can gradually decrease the time as the event nears.
- Limit the amount of time before new services or servers are
available to the client, such as the addition of a new NTP
server or a change of address of a DNS server. See
[RENUMREQS].
2. Terminology
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 BCP 14, RFC 2119 [RFC
2119].
3. Information refresh time option definition
The information refresh time option specifies an upper bound for how
long a client should wait before refreshing information retrieved
from DHCPv6. It is only used in Reply messages in response to
Information-Request messages. In other messages there will usually
be other options that indicate when the client should contact the
server, e.g. addresses with lifetimes.
Note that it is only an upper bound. If the client has any reason to
make a DHCPv6 request before the refresh time expires, it should
attempt to refresh all the data.
A client may contact the server before the refresh time expires.
Reasons it may do this include the need for additional configuration
parameters (such as by an application), a new IPv6 prefix announced
by a router, or that it has an indication it may have moved to a new
link.
The refresh time option specifies a common refresh time for all the
data. It doesn't make sense to have different refresh time values
for different data, since when the client has reason to refresh some
of its data, it should also refresh the remaining data. Because of
this, the option must only appear in the options area of the Reply
message.
The expiry of the refresh time in itself does not in any way mean
that the client should remove the data. The client should keep its
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current data while attempting to refresh it. The client is however
free to fall back to other mechanisms than DHCPv6 if it cannot
refresh the data within a reasonable amount of time.
When a client receives a Reply to an Information-Request that
contains configuration information, it should install that new
configuration information after removing any previously received
configuration information. It should also remove information that is
missing from the new information set, e.g. an option might be left
out or contain only a subset of what it did previously.
3.1. Constants
We define two constants for use by the protocol. How they are used
is specified in the sections below.
IRT_DEFAULT 86400
In some cases the client uses a default refresh time
IRT_DEFAULT. The recommended value for IRT_DEFAULT is 86400
(24 hours). The client implementation SHOULD allow for this
value to be configurable.
IRT_MINIMUM 600
This defines a minimum value for the refresh time.
3.2. Client behaviour
A client MUST request this option in the Option Request Option (ORO)
when sending Information-Request messages to the DHCPv6 server. A
client MUST NOT request this option in the ORO in any other messages.
If the Reply to an Information-Request message does not contain this
option, the client MUST behave as if the option with value
IRT_DEFAULT was provided.
A client MUST use the refresh time IRT_MINIMUM if it receives the
option with a value less than IRT_MINIMUM.
As per section 5.6 of [RFC 3315], the value 0xffffffff is taken to
mean "infinity" and implies that the client should not refresh its
configuration data without some other trigger (such as detecting
movement to a new link).
If a client contacts the server to obtain new data or refresh some
existing data before the refresh time expires, then it SHOULD also
refresh all data covered by this option.
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When the client detects that the refresh time has expired, it SHOULD
try to update its configuration data by sending an Information-
Request as specified in section 18.1.5 of [RFC 3315], except that the
client MUST delay sending the first Information-Request by a random
amount of time between 0 and INF_MAX_DELAY.
A client MAY have a maximum value for the refresh time, where that
value is used whenever the client receives this option with a value
higher than the maximum. This also means that the maximum value is
used when the received value is "infinity". A maximum value might
make the client less vulnerable to attacks based on forged DHCP
messages. Without a maximum value, a client may be made to use wrong
information for a possibly infinite period of time. There may
however be reasons for having a very long refresh time, so it may be
useful for this maximum value to be configurable.
3.3. Server behaviour
A server sending a Reply to an Information-Request message SHOULD
include this option if it is requested in the ORO of the Information-
Request.
The option value MUST NOT be smaller than IRT_MINIMUM. The server
SHOULD give a warning if it is configured with a smaller value.
The option MUST only appear in the options area of Reply messages.
3.4. Option format
The format of the information refresh time 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-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| information-refresh-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code
OPTION_INFORMATION_REFRESH_TIME (to be decided)
option-len
4
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information-refresh-time
Time duration relative to the current time, expressed in units
of seconds
4. IANA Considerations
IANA is requested to assign an option code for the information
refresh time option from the DHCPv6 option-code space [RFC 3315].
5. Acknowledgements
The authors thank Mat Ford, Tatuya Jinmei, Ted Lemon, Thomas Narten,
Joe Quanaim and A.K. Vijayabhaskar for valuable discussions and
comments.
6. Security Considerations
Section 23 of [RFC 3315] outlines the DHCPv6 security considerations.
This option does not change these in any significant way. An
attacker could send faked Reply messages with a low information
refresh time value, which would trigger use of IRT_MINIMUM to
minimize this threat. Another attack might be to send a very large
value, to make the client use forged information for a long period of
time. A possible maximum limit at the client is suggested, which
would reduce this problem.
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2462] S. Thomson, T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC 3315] R. Droms, Ed., J. Bound, B. Volz, T. Lemon, C. Perkins,
M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[RFC 3736] R. Droms, "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
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7.2. Informative References
[RENUMREQS] T. Chown, S. Venaas, A.K. Vijayabhaskar, "Renumbering
Requirements for Stateless DHCPv6", work-in-progress,
draft-ietf-dhc-stateless-dhcpv6-renumbering-01,
March 2004.
Authors' Addresses
Stig Venaas
University of Southampton
School of Electronics and Computer Science
Southampton, Hampshire SO17 1BJ
United Kingdom
EMail: sv@ecs.soton.ac.uk
Tim Chown
University of Southampton
School of Electronics and Computer Science
Southampton, Hampshire SO17 1BJ
United Kingdom
EMail: tjc@ecs.soton.ac.uk
Bernard Volz
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
EMail: volz@cisco.com
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