Network Working Group                                         R. Johnson
Internet-Draft                                             J. Kumarasamy
Expires: August 10, December 31, 2005                                    K. Kinnear
                                                                M. Stapp
                                                        February 9,
                                                           June 29, 2005

                    Virtual Subnet Selection Option

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This memo defines a new DHCP option for passing Virtual Subnet
   Selection (VSS) information between the DHCP client and the DHCP
   server.  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 221.  This memo documents the
   current usage of the option in agreement with RFC-3942[7] , 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.   VSS Information Definition . . . . . . . . . . . . . . . . .   4
   3.   Security Considerations  . . . . . . . . . . . . . . . . . .   6
   4.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .   7
   5.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .   8
   6.   References . . . . . . . . . . . . . . . . . . . . . . . . .   8
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .   8
        Intellectual Property and Copyright Statements . . . . . . .  10

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 Subject Selector"
   or "VSS") from which an address, and other resources, should be

   If the allocation is being done through a DHCP relay, then a relay
   suboption could be included.  In some cases, however an IP address is
   being sought by a DHCP proxy on behalf of a client (would 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

   A good example might be a dial-in aggregation device where PPP
   addresses are acquired via DHCP and then given to the remove customer
   system via 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 [2] 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.  VSS Information Definition

   The VSS Information option is a DHCP option [3].  The option contains
   generalized VSS information in one of two formats: NVT ASCII VPN
   identifier, or RFC2685 VPN-ID [4].

   The format of the option is:

    Code   Len   Type   VSS Information octets
   | 221 |  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 [4], which is typically 14
   hex digits 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 [4] as a VPN
   Identifier, typically 14 hex digits.

   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 support this 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.
   Clients using this option MUST discard DHCPOFFER or DHCPACK packets
   that do not contain this option.

   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 configure 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 Option and a VSS Information
   Relay Suboption are both received in a particular DHCP client packet,
   the information from the VSS Information Suboption MUST be used in
   preference to the information in the VSS Information Option.

   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 discard the
   DHCPOFFER or DHCPACK.  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 case the
   client will discard the DHCPOFFER or DHCPACK.  In other words, this
   option MUST NOT appear in a DHCPOFFER from a server unless it was
   used by the server in making the address allocation requested.

   This option SHOULD NOT be used without also making use of the DHCP
   Authentication option [5].

3.  Security Considerations

   Message authentication in DHCP for intradomain use where the out-of-
   band exchange of a shared secret is feasible is defined in [5].
   Potential exposures to attack are discussed in section 7 of the DHCP
   protocol specification in [2].

   The VSS Information option could be used by a client in order to
   obtain an IP address from a VSS 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, which would override the DHCP VSS Information option.

   This option would allow a client to perform a more complete
   address-pool 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.

4.  IANA Considerations

   No assignment of values for the type field need be made at this time.
   New values may only be defined by IETF Consensus, as described in
   [6].  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 Suboption MUST always be identical.

5.  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.


6.  References

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

   [2]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [3]  Droms, R. and S. Alexander, "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, March 1997.

   [4]  Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
        RFC 2685, September 1999.

   [5]  Droms, R., "Authentication for DHCP Messages", RFC 3118,
        June 2001.

   [6]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", RFC 2434, October 1998.

   [7]  Volz, B., "Reclassifying Dynamic Host Configuration Protocol
        version 4 (DHCPv4) Options", RFC 3942, November 2004.

Authors' Addresses

   Richard A. Johnson
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA  95134

   Phone: +1 408 526 4000
   Jay Kumarasamy
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA  95134

   Phone: +1 408 526 4000

   Kim Kinnear
   Cisco Systems
   250 Apollo Drive
   Chelmsford, MA  01824

   Phone: +1 978 244 8000

   Mark Stapp
   Cisco Systems
   250 Apollo Drive
   Chelmsford, MA  01824

   Phone: +1 978 244 8000

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