Network Working Group                                         G. Waters
 INTERNET-DRAFT                                          Nortel Networks
                                                               June 1999
                    The Subnet Selection Option for DHCP
 Status of this Memo
   This document is an Internet-Draft and is in full conformance with all
   provisions of Section 10 of RFC2026.
   Internet-Drafts are working documents of the Internet Engineering Task
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 Copyright Notice
   Copyright (C) The Internet Society (1999).  All Rights Reserved.
   This memo defines a new DHCP option for selecting the subnet on which
   to allocate an address. This option would override a DHCP server's
   normal methods of selecting the subnet on which to allocate an address
   for a client.
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   Table of Contents
   1. Introduction......................................................2
   2. Subnet Selection Option...........................................3
   3. Intellectual Property.............................................4
   4. Acknowledgements..................................................4
   5. Security Considerations...........................................4
   6. References........................................................5
   7. Editor's Addresses................................................5
   8. Full Copyright Statement..........................................5
 1. Introduction
   This memo was produced by the DHCP Working Group and defines a new
   DHCP option that specifies the subnet that a DHCP server should use
   when selecting an address.  This option takes precedence over other
   methods that the DHCP server may use to determine the subnet on which
   to select an address. Two existing methods of determining the subnet
   on which to select an address are:
   o To use the subnet address of the giaddr field in the DHCP packet,
     or if the giaddr field is zero;
   o To use the subnet address of the local interface on which the
     packet was received by the DHCP server.
   Methods other than the two described above may exist.
   The subnet selection option is useful for, but not limited to, the
   class of devices that have a packet-handling plane (e.g.: switching,
   routing functionality) and a control plane (e.g.: device management
   and control functionality). The control plane is network connected and
   there is a DHCP server connected to that network. The packet-handling
   plane may or may not be network connected, however, in either case
   there is no network connected DHCP server available to this plane. The
   control plane is not network connected to the packet-handling plane,
   although the two planes may communicate using some method (e.g.: an
   internal data bus).
   There is a requirement to allocate addresses for devices connected to
   the networks to which the packet-handling plane is connected.
   Since there is no network connectivity between the DHCP server and the
   packet-handling plane, the control plane must allocate addresses using
   the DHCP on behalf of the packet-handling plane. Since the control
   plane is requesting the address, the DHCP server would normally
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   allocate the address on the subnet on which the control plane is
   connected, which would not be the desired result.
   If the option specified by this memo is included in the
   DHCPDISCOVER/DHCPREQUEST message then the server should allocate an
   address on the subnet or network segment that is specified by this
   option. The option would specify an address on one of the packet-
   handling plane's subnets.
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].
 2. Subnet Selection Option
   The subnet selection option is a DHCP option. The option contains a
   single IP address that is the address of a subnet. The value for the
   subnet address is determined by taking any IP address on the subnet
   and ANDing that address with the subnet mask (i.e.: the network and
   subnet bits are left alone and the remaining (address) bits are set to
   zero). When this option is present, the DHCP server MUST use either:
   o The subnet specified in the option, or;
   o A subnet on the same network segment as the subnet specified in the
   on which to allocate an address.
   The format of the option is:
        Code   Len        IP Address
       | TBD |  4  | A1  | A2  | A3  | A4  |
   In order to ensure backwards compatibility of clients that do support
   this option when communicating with DHCP servers that do not support
   this option, the DHCP client SHOULD check that an allocated address is
   on the requested subnet or network segment. The client SHOULD NOT
   respond to a DHCPOFFER or DHCPACK of an address that is not on the
   requested subnet or network segment.
   Servers supporting this option MUST return the option to any client
   that sends it, regardless of whether or not the client requests it in
   a parameter request list.   Clients using this option must discard
   DHCPOFFER or DHCPACK packets that do not contain this option.
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   This option does not require changes to operations or features of the
   DHCP server other than to select the subnet on which to allocate an
   address. For example, the handling of DHCPDISCOVER for an unknown
   subnet may continue to operate unchanged.
   When this option is present and the server supports this option, the
   server MUST NOT offer an address that is not on the requested subnet
   or network segment.
   The IP address to which a DHCP server sends a reply to MUST be the
   same as it would chose when this option is not present.
 3. Intellectual Property
   The IETF takes no position regarding the validity or scope of any
   intellectual property 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; neither does it represent that it has made any
   effort to identify any such rights.  Information on the IETF's
   procedures with respect to rights in standards-track and standards-
   related documentation can be found in BCP-11.
   Copies of claims of rights made available for publication 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 Secretariat.
   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
 4. Acknowledgements
   This document is the result of work undertaken the by DHCP working
   group. Thanks to Ted Lemon, Tim Aston and Ralph Droms for their
   helpful comments in this work.
 5. Security Considerations
   DHCP currently provides no authentication or security mechanisms.
   Potential exposures to attack are discussed is section 7 of the
   protocol specification [RFC2131].
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 6. References
   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", RFC 2119, BCP 14, March 1997.
   [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.
   [RFC2132] Alexander, S.  and Droms, R., "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, March 1997.
 7. Editor's Addresses
   Glenn Waters
   Nortel Networks
   310-875 Carling Avenue,
   Ottawa, Ontario K1S 5P1
   Phone:  +1 613-798-4925
 8. Full Copyright Statement
   Copyright (C) The Internet Society (1999).  All Rights Reserved.
   This document and translations of it may be copied and furnished to
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   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
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   translate it into languages other than English.
   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.
   This document and the information contained herein is provided on an
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