Network Working Group                                         R. Johnson
Internet-Draft                                             J. Kumarasamy
Expires: October 14, 2007 May 19, 2008                                         K. Kinnear
                                                                M. Stapp
                                                          April 12,
                                                       November 16, 2007

                    Virtual Subnet Selection Option

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This memo defines a new DHCP option existing usage for passing the 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 [7], [8], 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.  Informative  References . . . . . . . . . . . . . . . . . . . . . . . . . . 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

   If the allocation is being done through a DHCP relay, then a relay
   sub-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

   A good example might be a dial-in aggregation device where PPP [10]
   addresses are acquired via DHCP and then given to the remove remote customer
   system via IPCP. IPCP [9].  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] [4] 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",
   document are to be interpreted as described in [1]. [3].

   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 [1] and [2].

      Downstream is the direction from the access concentrator towards
      the subscriber.

      Upstream is the direction from the subscriber towards the access

   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.

      Virtual private network.  A network which appears to the client to
      be a private network.

   VPN Identifier
      The VPN-ID is defined by [6] to be a sequence of 7 octets.

3.  VSS Information Definition

   The VSS Information option is a DHCP option [3]. [5].  The option contains
   generalized VSS information in one of two formats: NVT ASCII VPN
   identifier, or RFC2685 VPN-ID [4]. [6].

   The format of the option is:

    Code   Len   Type   VSS Information octets
   | TBD 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], [6], which is typically 14
   hex digits 7
   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 support

   Since this option MUST return an identical
   copy of is placed in the option packet in order to any client that sends it, regardless of whether
   or not the client requests change the option in VPN
   on which an IP address is allocated for a parameter request list.
   Clients using particular 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 packets in a packet and doesn't receive it in the
   returning packet should drop the packet since the IP address that do was
   allocated will not contain be 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 configure configured 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 Virtual Subnet Selection option and a VSS Information
   Relay Suboption Virtual
   Subnet Selection sub-option [12] are both received in a particular
   DHCP client packet, the information from the VSS Information Suboption Virtual 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 discard should 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 case DHCPACK such that the client will discard know that the DHCPOFFER
   allocated 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 [5]. [11].
   Potential exposures to attack are discussed in section 7 of the DHCP
   protocol specification in [2]. [4].

   The VSS Information option could be used by a client in order to
   obtain an IP address from a VSS VPN 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 of

   Implementations should consider using the DHCP Authentication option [5].
   [11] 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 [7].  Option 221 has been used for this [8].

   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 users so creation of option 221 identified during the
   6-month notification period specified in [7].  No assignment a registry of these
   numbers is not required by this document.  In the event that
   additional values for the type field need be made byte 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 [6]. [7].  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 Relay 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.  Informative  References

7.1.  Normative References

   [1]   Croft, B. and J. Gilmore, "Bootstrap Protocol (BOOTP)",
         RFC 951, September 1985.

   [2]   Wimer, W., "Clarifications and Extensions for the Bootstrap
         Protocol", RFC 1542, October 1993.

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


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


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


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

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


   [7]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.


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


7.2.  Informative References

   [9]   McGregor, G., "The PPP Internet Protocol Control Protocol
         (IPCP)", RFC 1332, May 1992.

   [10]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
         RFC 1661, July 1994.

   [11]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
         RFC 3118, June 2001.

   [12]  Kinnear, K., "Virtual Subnet Selection Sub-Option for the Relay
         Agent Information Option  for DHCPv4",
         draft-ietf-dhc-agent-vpn-id-05 (work in progress), March
         November 2007.

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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