Network Working Group R. Johnson Internet-Draft J. Kumarasamy Expires:
October 14, 2007May 19, 2008 K. Kinnear M. Stapp Cisco April 12,November 16, 2007 Virtual Subnet Selection Option draft-ietf-dhc-vpn-option-06.txtdraft-ietf-dhc-vpn-option-07.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 14, 2007.May 19, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This memo defines a new DHCP optionexisting usage for passingthe Virtual Subnet Selection (VSS) information between the DHCP client and the DHCP server.option. It is intended for use primarily by DHCP proxy clients in situations where VSS information needs to be passed to the DHCP server for proper address allocation to take place. The option number currently in use is TBD.221. This memo documents the current usage of the option in agreement with ,, which declares that any pre-existing usages of option numbers in the range 128 - 223 should be documented and the working group will try to officially assign those numbers to those options. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. VSS Information Definition . . . . . . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 7. InformativeReferences . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12 1. Introduction There is a growing use of Virtual Private Network (VPN) configurations. The growth comes from many areas; individual client systems needing to appear to be on the home corporate network even when traveling, ISPs providing extranet connectivity for customer companies, etc. In some of these cases there is a need for the DHCP server to know the VPN (hereafter called a "Virtual Subnet Selector" or "VSS") from which an address, and other resources, should be allocated. If the allocation is being done through a DHCP relay, then a relay suboptionsub-option could be included. In some cases, however an IP address is being sought by a DHCP proxy on behalf of a client (would(which may be assigned the address via a different protocol). In this case, there is a need to include VSS information relating to the client as a DHCP option. A good example might be a dial-in aggregation device where PPP  addresses are acquired via DHCP and then given to the removeremote customer system via IPCP.IPCP . In a network where such a device is used to aggregate PPP dial-in from multiple companies, each company may be assigned a unique VSS. This memo defines a new DHCP  option, the VSS Information option, which allows the DHCP client to specify the VSS Information needed in order to allocate an address. If the receiving DHCP server understands the VSS Information option, this information may be used in conjunction with other information in determining the subnet on which to select an address as well as other information such as DNS server, default router, etc. 2. Conventions 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 .. This document also uses the following terms: DHCP Client DHCP Client or "Client" is an Internet host using DHCP to obtain configuration parameters such as a network address. DHCP Server A DHCP Server or "Server" is an Internet host that returns configuration parameters to DHCP Clients. DHCP relay agent A DHCP relay agent is a third-party agent that transfers BOOTP and DHCP messages between clients and servers residing on different subnets, per  and . downstream Downstream is the direction from the access concentrator towards the subscriber. upstream Upstream is the direction from the subscriber towards the access concentrator. VSS information Information about a VPN necessary to allocate an address to a DHCP client on that VPN and necessary to forward a DHCP reply packet to a DHCP client on that VPN. VPN Virtual private network. A network which appears to the client to be a private network. VPN Identifier The VPN-ID is defined by  to be a sequence of 7 octets. 3. VSS Information Definition The VSS Information option is a DHCP option .. The option contains generalized VSS information in one of two formats: NVT ASCII VPN identifier, or RFC2685 VPN-ID .. The format of the option is: Code Len Type VSS Information octets +-----+-----+------+-----+-----+-----+--- | TBD221 | n | t | v1 | v2 | v3 | ... +-----+-----+------+-----+-----+-----+--- Type: 0 NVT ASCII VPN identifier 1 RFC2685 VPN-ID 2-255 Not Allowed Figure 1 The option minimum length (n) is 2. There are two types of identifiers which can be placed in the VSS Information Option. The first type of identifier which can be placed in the VSS Information Option is an NVT ASCII string. It MUST NOT be terminated with a zero byte. The second type of identifier which can be placed in the VSS Information Option is an RFC2685 VPN-ID ,, which is typically 14 hex digits7 octets (3 of VPN OUI followed by 4 of VPN index) in length (though it can be any length as far as the VSS Information Option is concerned). If the type field is set to zero (0), it indicates that all following bytes of the option contain a NVT ASCII string. This string MUST NOT be terminated with a zero byte. If the type field is set to one (1), it indicates that all following bytes should be interpreted in agreement with RFC2685 as a VPN Identifier, typically 14 hex digits.7 octets. All other values of the type field are invalid as of this memo and VSS options containing any other value than zero (0) or one (1) SHOULD be ignored. Any VSS information contained in a DHCP Relay Suboption SHOULD override the information contained in this VSS Information option.  Servers configured to supportSince this option MUST return an identical copy ofis placed in the optionpacket in order to any client that sends it, regardless of whether or not the client requestschange the option inVPN on which an IP address is allocated for a parameter request list. Clients usingparticular DHCP client, one presumes that an allocation on that VPN is necessary for correct operation. If this presumption is correct, then a client which places this option MUST discard DHCPOFFER or DHCPACK packetsin a packet and doesn't receive it in the returning packet should drop the packet since the IP address that dowas allocated will not containbe in the correct VPN. If an IP address that is not on the requested VPN is not required, then the client is free to accept the IP address that is not on the VPN that the was requested. Servers configured to support this option.option MUST return an identical copy of the option to any client that sends it, regardless of whether or not the client requests the option in a parameter request list. This option provides the DHCP server additional information upon which to make a determination of address to be assigned. The DHCP server, if it is configureconfigured to support this option, should use this information in addition to other options included in the DHCPDISCOVER packet in order to assign an IP address for DHCP client. In the event that a VSS Informmation OptionVirtual Subnet Selection option and a VSS Information Relay SuboptionVirtual Subnet Selection sub-option  are both received in a particular DHCP client packet, the information from the VSS Information SuboptionVirtual Subnet Selection sub-option MUST be used in preference to the information in the VSS Information Option.Virtual Subnet Selection option. This reasoning behind this approach is that the relay-agent is almost certainly more trusted than the DHCP client, and therefore information in the relay-agent-information option that conflicts with information in the packet generated by the DHCP client is more likely to be correct. Servers that do not understand this option will allocate an address using their normal algorithms and will not return this option in the DHCPOFFER or DHCPACK. In this case the client will discardshould consider discarding the DHCPOFFER or DHCPACK.DHCPACK, as mentioned above. Servers that understand this option but are administratively configured to ignore the option MUST ignore the option, use their normal algorithms to allocate an address, and MUST NOT return this option in the DHCPOFFER or DHCPACK. In this caseDHCPACK such that the client will discardknow that the DHCPOFFERallocated address is not in the VPN requested and will consider this information in deciding whether or DHCPACK.not to accept the DHCPOFFER. In other words, this option MUST NOT appear in a DHCPOFFER or DHCPACK from a server unless it was used by the server in making or updating the address allocation requested. 4. Security Considerations Message authentication in DHCP for intradomain use where the out-of- band exchange of a shared secret is feasible is defined in .. Potential exposures to attack are discussed in section 7 of the DHCP protocol specification in .. The VSS Information option could be used by a client in order to obtain an IP address from a VSSVPN other than the one where it should. DHCP relays MAY choose to remove the option before passing on DHCPDISCOVER packets.Another possible defense would be for the DHCP relay to insert a Relay option containing a VSS Information Suboption,Relay Sub-option, which would override the DHCP VSS Information option. This option would allow a client to perform a more complete address- pool exhaustion attack since the client would no longer be restricted to attacking address-pools on just its local subnet. Servers that implement the VSS Information option MUST by default disable use of the feature; it must specifically be enabled through configuration. Moreover, a server SHOULD provide the ability to selectively enable use of the feature under restricted conditions, e.g., by enabling use of the option only from explicitly configured client-ids, enabling its use only by clients on a particular subnet, or restricting the VSSs from which addresses may be requested. This option SHOULD NOT be used without also making use ofImplementations should consider using the DHCP Authentication option . in order to provide a higher level of security if it is deemed necessary in their environment. 5. IANA Considerations IANA is requested to assign DHCP option number 221 for this option, in accordance with . Option 221 has been used for this. While the type byte of the Virtual Subnet Selection option defines a number space that could be managed by IANA, expansion of this number space is not anticipated and there were no conflicting usersso creation of option 221 identified during the 6-month notification period specified in . No assignmenta registry of these numbers is not required by this document. In the event that additional values for the type field need be madebyte are defined in subsequent documents, IANA should at this time.that time create a registry for these type bytes. New values for the type byte may only be defined by IETF Consensus, as described in .. Basically, this means that they are defined by RFCs approved by the IESG. Moreover, any changes or additions to the type byte codes MUST be made concurrently in the type byte codes of the VSS Information Option. The type bytes and data formats of the VSS Information Option and VSS Information SuboptionRelay Sub-option MUST always be identical. 6. Acknowledgements This document is the result of work done within Cisco Systems. Thanks to Kim Kinnear, Mark Stapp, and Jay Kumarasamy for their work on this option definition and the other related work for which this is necessary. 7. InformativeReferences 7.1. Normative References  Croft, B. and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC 951, September 1985.  Wimer, W., "Clarifications and Extensions for the Bootstrap Protocol", RFC 1542, October 1993.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997.  Fox, B. and B. Gleeson, "Virtual Private Networks Identifier", RFC 2685, September 1999.  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001.  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.  Volz, B., "Reclassifying Dynamic Host Configuration Protocol version 4 (DHCPv4) Options", RFC 3942, November 2004. 7.2. Informative References  McGregor, G., "The PPP Internet Protocol Control Protocol (IPCP)", RFC 1332, May 1992.  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994.  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001.  Kinnear, K., "Virtual Subnet Selection Sub-Option for the Relay Agent Information Option for DHCPv4", draft-ietf-dhc-agent-vpn-id-04draft-ietf-dhc-agent-vpn-id-05 (work in progress), MarchNovember 2007. Authors' Addresses Richard A. Johnson Cisco Systems 170 W. Tasman Dr. San Jose, CA 95134 US Phone: +1 408 526 4000 Email: firstname.lastname@example.org Jay Kumarasamy Cisco Systems 170 W. Tasman Dr. San Jose, CA 95134 US Phone: +1 408 526 4000 Email: email@example.com Kim Kinnear Cisco Systems 250 Apollo Drive Chelmsford, MA 01824 US Phone: +1 978 244 8000 Email: firstname.lastname@example.org Mark Stapp Cisco Systems 250 Apollo Drive Chelmsford, MA 01824 US Phone: +1 978 244 8000 Email: email@example.com Full Copyright Statement Copyright (C) The IETF Trust (2007). 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