Network Working Group G. Waters INTERNET-DRAFT Nortel Networks February 1999 The Subnet Selection Option for DHCP Friday, February 19, 1999, 3:01 PM 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 Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Abstract 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 which subnet on which to allocate an address for a client. Waters Expires: Feb 1999 + 6 months [Page 1] Internet Draft Subnet Selection Option February 1999 Table of Contents 1. Introduction......................................................2 2. Subnet Selection Option...........................................3 3. Intellectual Property.............................................3 4. Acknowledgements..................................................4 5. Security Considerations...........................................4 6. References........................................................4 7. Editor's Addresses................................................4 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 on which 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, 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). For the networks to which the packet-handling plane is connected, there is a requirement to allocate addresses for devices connected to those networks. 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. Because the control plane is requesting the addresses, the DHCP server would normally have the undesired result of allocating the address on the subnet on which the control plane is connected. Waters Expires: Feb 1999 + 6 months [Page 2] Internet Draft Subnet Selection Option February 1999 If the option specified by this memo is included in the DHCPDISCOVER then the server should allocate addresses on the subnet specified by this option. The option would specify an address of one of the packet- handling plane's subnets. 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]. 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 the subnet specified 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 in on the requested subnet. The client SHOULD NOT respond to an DHCPOFFER of an address which is not on the requested subnet. This option does not require any change to other 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. A server that supports this option MUST NOT offer an address that is not on the requested subnet when this option is present. Existing methods for determining where to send a reply to a DHCP client are not changed when this option is present in a request. 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 Waters Expires: Feb 1999 + 6 months [Page 3] Internet Draft Subnet Selection Option February 1999 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 Director. 4. Acknowledgements This document is the result of work undertaken the by DHCP working group. Thanks to Tim Aston and Ralph Droms for reviewing this memo. 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]. 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 Canada Waters Expires: Feb 1999 + 6 months [Page 4] Internet Draft Subnet Selection Option February 1999 Phone: +1 613-798-4925 Email: gww@nortelnetworks.com 8. Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. 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