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Versions: 00 01 02 03 04 05 06 RFC 5908

Network Working Group                                         R. Gayraud
Internet-Draft                                                      None
Intended status: Standards Track                            B. Lourdelet
Expires: April 5, 2010                               Cisco Systems, Inc.
                                                         October 2, 2009


          Network Time Protocol (NTP) Server Option for DHCPv6
                  draft-ietf-ntp-dhcpv6-ntp-opt-05.txt

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   The NTP Server Option for Dynamic Host Configuration Protocol for
   IPv6 (DHCPv6) provides NTP (Network Time Protocol version 4) Server
   location information to DHCPv6 hosts.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements notation . . . . . . . . . . . . . . . . . . . . . 3
   3.  Related Work and Usage Model  . . . . . . . . . . . . . . . . . 3
   4.  NTP Server Option for DHCPv6  . . . . . . . . . . . . . . . . . 3
     4.1.  NTP Server Address Suboption  . . . . . . . . . . . . . . . 5
     4.2.  NTP Multicast Address Suboption . . . . . . . . . . . . . . 5
     4.3.  NTP Server FQDN Suboption . . . . . . . . . . . . . . . . . 6
   5.  Appearance of this Option . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  RFC 4075 Deprecation  . . . . . . . . . . . . . . . . . . . . . 7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8





















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1.  Introduction

   This document defines a DHCPv6 option and associated suboptions to
   provide Network Time Protocol version 4 [draft-ntpv4] or greater
   Server location information to DHCPv6 hosts.


2.  Requirements notation

   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 [RFC2119].


3.  Related Work and Usage Model

   The NTP service is publicly offered on the Internet by a number of
   organizations.  Those Servers can be used but not abused, so any
   method which is tasked to disseminate locations of NTP Servers must
   act responsibly in a manner that does not lead to public Server
   overloading.  When using DHCPv6 to offer NTP Server location, and if
   there is a need to distribute a host with a hardcoded configuration,
   this configuration MUST NOT include Server location that is not part
   of the organization that distributes this device.  Typical usage of
   this option is to specify an NTP Server that is part of the
   organization that operates the DHCPv6 Server.

   The location of the NTP service, like any other Internet service, can
   be specified by an IP address or a Fully Qualified Domain Name
   (FQDN).  By design, DHCP offers information to multiple devices and
   is prone to amplification of mistakes, so great care must be taken to
   define its configuration.  Specification of the NTP service by FQDN
   offers a level of indirection that works as a possible mitigation
   tool in case of misconfiguration.  DNS can be used to redirect
   misconfigured clients to an IPv6 address that is not configured on
   any host instead of having to change the address of the NTP Server
   itself.

   While the NTP specification defines a comprehensive set of
   configuration parameters, modification of those parameters is best
   left to the decision of the client itself.  The DHCPv6 option for NTP
   is therefore restricted to Server location.


4.  NTP Server Option for DHCPv6

   This option serves as a container for Server location information
   related to one NTP Server or Simple Network Time Protocol (SNTP)



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   [RFC4330] Server.  This option can appear multiple times in a DHCPv6
   message.  Each instance of this option is to be considered by the NTP
   client or SNTP client as a Server to include in its configuration.

   The option itself does not contain any value.  Instead, it contains
   one or several suboptions that carry NTP Server or SNTP Server Server
   location.  This option MUST include one, and only one, time source
   suboption.  The currently defined time source suboptions are:
   NTP_OPTION_SRV_ADDR, NTP_OPTION_SRV_MC_ADDR, NTP_OPTION_SRV_FQDN.  It
   carries the NTP Server or SNTP Server location, as a unicast or
   multicast IPv6 address or as an NTP Server or SNTP Server FQDN.  More
   time source suboptions may be defined in the future.  While the FQDN
   option offers the most deployment flexibility, resiliency as well as
   security, the IP address options are defined to cover cases where a
   DNS dependancy is not desirable.

   If the NTP Server or SNTP Server location is an IPv6 multicast
   address, the client SHOULD use this address as an NTP multicast group
   address and listen to messages sent to this group in order to
   synchronize its clock.

   The format of the NTP Server 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_NTP_SERVER        |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         suboption-1                           |
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         suboption-2                           |
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         suboption-n                           |
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       option-code: OPTION_NTP_SERVER (TBD_IANA),

       option-len: Total length of the included suboptions.

   This document does not define any priority relationship between the
   client's embedded configuration (if any) and the NTP or SNTP Servers
   discovered via this option.  In particular, the client is allowed to
   simultaneously use its own configured NTP Servers or SNTP Servers and



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   the Servers discovered via DHCP.

4.1.  NTP Server Address Suboption

   This suboption is intended to appear inside the OPTION_NTP_SERVER
   option.  It specifies the IPv6 unicast address of an NTP Server or
   SNTP Server available to the client.

   The format of the NTP Server Address Suboption 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    NTP_SUBOPTION_SRV_ADDR     |        suboption-len = 16     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                                                               |
     |                   IPv6 address of NTP Server                  |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       suboption-code: NTP_SUBOPTION_SRV_ADDR (1),

       suboption-len: 16.

4.2.  NTP Multicast Address Suboption

   This suboption is intended to appear inside the OPTION_NTP_SERVER
   option.  It specifies the IPv6 address of the IPv6 multicast group
   address used by NTP on the local network.

   The format of the NTP Multicast Address Suboption 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    NTP_SUBOPTION_MC_ADDR      |        suboption-len = 16     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                                                               |
     |                   Multicast IPv6 address                      |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       suboption-code: NTP_SUBOPTION_MC_ADDR (2),

       suboption-len: 16.




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4.3.  NTP Server FQDN Suboption

   This suboption is intended to appear inside the OPTION_NTP_SERVER
   option.  It specifies the FQDN of an NTP Server or SNTP Server
   available to the client.

 The format of the NTP Server FQDN Suboption 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    NTP_SUBOPTION_SRV_FQDN     |         suboption-len         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                      FQDN of NTP Server                       |
   :                                                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     suboption-code: NTP_SUBOPTION_SRV_FQDN (3),

     suboption-len: Length of the included FQDN field,

     FQDN: Fully Qualified Domain Name of the NTP Server or SNTP Server.
          This field MUST be encoded as described in [RFC3315],
           section 8.


5.  Appearance of this Option

   The OPTION_NTP_SERVER option can appear multiple times in a DHCPv6
   message.  The order in which these options appear is not significant.
   The client uses its usual algorithms to determine which Server(s) or
   multicast group(s) should be prefered to synchronize its clock.

   The OPTION_NTP_SERVER option MUST NOT appear in messages other than
   the following: Solicit, Advertise, Request, Renew, Rebind,
   Information-Request, and Reply.  If this option appears in messages
   other than those specified above, the receiver MUST ignore it.

   The option number for this option MAY appear in the "Option Request"
   option [RFC3315] in the following messages: Solicit, Request, Renew,
   Rebind, Information-Request, and Reconfigure.  If this option number
   appears in the "Option Request" option in messages other than those
   specified above, the receiver SHOULD ignore it.







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6.  Security Considerations

   This option could be used by an intruder to advertise the address of
   a malicious NTP Server or SNTP Server and adversely affect the clock
   of clients on the network.  The consequences of such an attack can be
   critical, because many security protocols depend on time
   synchronization to run their algorithms.  As an example, an attacker
   could break connectivity between SEND-enabled nodes [RFC3971], simply
   by affecting the clock on these nodes.

   To prevent these attacks, it is strongly advisable to secure the use
   of this option either by:

      -  using the NTPv4 Autokey public key authentication, as defined
         in [draft-autokey] or,

      -  using authenticated DHCP as described in [RFC3315] section 21.


7.  RFC 4075 Deprecation

   [RFC4075] (SNTP Configuration Option for DHCPv6) provides some degree
   of automatic time Server configuration for IPv6, as it specifies how
   to transmit SNTP [RFC4330] Servers addresses through DHCPv6.  However
   this approach is not suitable for all NTP deployments.  It is not an
   extensible mechanism and introduces some semantic confusion throught
   the use of the "SNTP" acronym.  Additionally the approach of only
   offering IPv6 addresses to specify Server location does not meet NTP
   requirements that make use of a FQDN (Fully Qualified Domain Name) as
   well.  For all the abovementioned reasons, this document makes
   [RFC4075] deprecated.


8.  IANA Considerations

   When this document is published, the IANA is requested to assign an
   option code from the "DHCPv6 Options Codes" registry for
   OPTION_NTP_SERVER.


9.  References

9.1.  Normative References

   [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.




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   [draft-ntpv4]
              Burbank, J., Kasch, W., Martin, J., and D. Mills, "Network
              Time Protocol Version 4 Protocol And Algorithms
              Specification", draft-ietf-ntp-ntpv4-proto-11 (work in
              progress), September 2008.

   [draft-autokey]
              Haberman, B. and D. Mills, "Network Time Protocol Version
              4 Autokey Specification", draft-ietf-ntp-autokey-06 (work
              in progress), July 2009.

9.2.  Informative References

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

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4075]  Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
              Configuration Option for DHCPv6", RFC 4075, May 2005.

   [RFC4330]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4
              for IPv4, IPv6 and OSI", RFC 4330, January 2006.


Authors' Addresses

   Richard Gayraud
   None

   Email: richard.gayraud@free.fr


   Benoit Lourdelet
   Cisco Systems, Inc.
   Village ent. GreenSide, Bat T3,
   400, Av de Roumanille,
   06410 BIOT - Sophia-Antipolis Cedex
   France

   Phone: +33 4 97 23 26 23
   Email: blourdel@cisco.com








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