GENWiki

Network Working Group S. Venaas Request for Comments: 4242 UNINETT Category: Standards Track T. Chown

                                             University of Southampton
                                                               B. Volz
                                                   Cisco Systems, Inc.
                                                         November 2005

                Information Refresh Time Option for
       Dynamic Host Configuration Protocol for IPv6 (DHCPv6)

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 Internet Society (2005).

Abstract

 This document describes a Dynamic Host Configuration Protocol for
 IPv6 (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 .....................................................2
 3. Information Refresh Time Option Definition ......................2
    3.1. Constants ..................................................3
    3.2. Client Behaviour ...........................................3
    3.3. Server Behaviour ...........................................4
    3.4. Option Format ..............................................5
 4. IANA Considerations .............................................5
 5. Acknowledgements ................................................5
 6. Security Considerations .........................................5
 7. References ......................................................6
    7.1. Normative References .......................................6
    7.2. Informative References .....................................6

Venaas, et al. Standards Track [Page 1] RFC 4242 Information Refresh Time Option for DHCPv6 November 2005

1. Introduction

 DHCPv6 [RFC3315] specifies stateful autoconfiguration for IPv6 hosts.
 However, many hosts will use stateless autoconfiguration as specified
 in [RFC2462] for address assignment, and use DHCPv6 only for other
 configuration data; see [RFC3736].  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:

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 [RFC4076].

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
 [RFC2119].

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

Venaas, et al. Standards Track [Page 2] RFC 4242 Information Refresh Time Option for DHCPv6 November 2005

 parameters (such as by an application), a new IPv6 prefix announced
 by a router, or that it has an indication that 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
 current data while attempting to refresh it.  However, the client is
 free to fall back to mechanisms other 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.

Venaas, et al. Standards Track [Page 3] RFC 4242 Information Refresh Time Option for DHCPv6 November 2005

 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 [RFC3315], 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.

 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 [RFC3315], 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.

Venaas, et al. Standards Track [Page 4] RFC 4242 Information Refresh Time Option for DHCPv6 November 2005

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 (32)

    option-len
       4

    information-refresh-time
       Time duration relative to the current time, expressed in units
       of seconds

4. IANA Considerations

 The IANA has assigned an option code for the information refresh time
 option from the DHCPv6 option-code space [RFC3315].

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 [RFC3315] 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.

Venaas, et al. Standards Track [Page 5] RFC 4242 Information Refresh Time Option for DHCPv6 November 2005

7. References

7.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

 [RFC2462]   Thomson, S. and T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, December 1998.

 [RFC3315]   Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
             and M. Carney, "Dynamic Host Configuration Protocol for
             IPv6 (DHCPv6)", RFC 3315, July 2003.

 [RFC3736]   Droms, R., "Stateless Dynamic Host Configuration Protocol
             (DHCP) Service for IPv6", RFC 3736, April 2004.

7.2. Informative References

 [RFC4076]   Chown, T., Venaas, S., and A. Vijayabhaskar, "Renumbering
             Requirements for Stateless Dynamic Host Configuration
             Protocol for IPv6 (DHCPv6)", RFC 4076, May 2005.

Venaas, et al. Standards Track [Page 6] RFC 4242 Information Refresh Time Option for DHCPv6 November 2005

Authors' Addresses

 Stig Venaas
 UNINETT
 Trondheim  NO 7465
 Norway

 EMail: venaas@uninett.no

 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

Venaas, et al. Standards Track [Page 7] RFC 4242 Information Refresh Time Option for DHCPv6 November 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).

 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.

 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.

Intellectual Property

 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.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Venaas, et al. Standards Track [Page 8]