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Versions: 00 01 02 03 04 05 06                                          
Network Working Group                                        Kim Kinnear
Internet Draft                                                Mark Stapp
Intended Status: Standards Track                         Richard Johnson
                                                          Jay Kumarasamy
                                                           Cisco Systems

                                                           February 2007
                                                     Expires August 2007

                  Virtual Subnet Selection Sub-Option
           for the Relay Agent Information Option for DHCPv4

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
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   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on August 28, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   In some environments, a relay agent resides in a network element
   which also has access to one or more virtual private networks (VPNs).
   If one DHCP server wishes to offer service to DHCP clients on those

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   different VPNs the DHCP server needs to know information about the
   VPN on which each client resides. The virtual-subnet-selection sub-
   option of the relay-agent-information option is used by the relay
   agent to tell the DHCP server important information about the VPN
   (called the Virtual Subnet Selection information, or VSS) for every
   DHCP request it passes on to the DHCP server, and is also used to
   properly forward any DHCP reply that the DHCP server sends back to
   the relay agent.

1.  Introduction

   There exist situations where there are multiple VPNs serviced by one
   or more network elements which also contain relay agents.   These
   VPNs contain DHCP clients, and there is a desire to allow one DHCP
   server to supply the full range of DHCP services to these DHCP

   The network element which contains the relay agent typically is also
   the network element which knows about the VPN association of the DHCP
   client and could include information about the VPN in the relay-
   agent-information option in the client's DHCP requests. This
   information about the VPN is called the Virtual Subnet Selection
   information, or VSS information.  This document defines a sub-option
   for the relay-agent-information option which contains this VSS
   information, and which allows the relay agent to communicate the VSS
   information to the DHCP server.

   When the DHCP server sends its response to the relay agent for
   forwarding back to the DHCP client, the relay agent will also need to
   use the virtual-subnet-selection sub-option to determine to which VPN
   to send the DHCP response.

   This sub-option can also be used by the DHCP server to inform a relay
   agent that a particular DHCP client is associated with a particular
   VPN by sending the virtual-subnet-selection sub-option in the relay-
   agent-information option back to the relay agent.

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   Consider the following architecture:

          +--------+         +---------------+
          |  DHCP  |     IP x|  Relay Agent  | IP z
          | Server |-.......-|     and       +---+-------+-------+
          +--------+         |  VPN manager  |   |       |       |
                             +---+-----------+   |       |       |
                                 |IP y        +-----+ +--+--+ +--+--+
                               +-+-----+      |Host1| |Host2| |Host3|
                               |       |      +-----+ +-----+ +-----+
                               |       |
                            +-----+ +--+--+         VPN 2
                            |Host1| |Host2|
                            +-----+ +-----+

                                VPN 1

   In this architecture, the relay agent knows the VPN for each of the
   DHCP clients, and inserts the VSS information about the VPN in the
   virtual-subnet-selection sub-option in every DHCP request it forwards
   on to the DHCP server.

   When the DHCP server copies over the relay-agent-information option
   from the request to the reply packet, it will copy over the virtual-
   subnet-selection sub-option as well.

   When the relay agent receives a DHCP reply packet from the server
   with a virtual-subnet-selection sub-option, it will forward the
   packet onto the proper VPN based on the VSS information in the
   virtual-subnet-selection sub-option.


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC 2119].

   This document uses the following terms:

      o "DHCP client"

        A DHCP client is an Internet host using DHCP to obtain
        configuration parameters such as a network address.

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      o "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 [RFC 951] and [RFC 1542].

      o "DHCP server"

        A DHCP server is an Internet host that returns configuration
        parameters to DHCP clients.

      o "downstream"

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

      o "upstream"

        Upstream is the direction from the subscriber towards the access

      o "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.

      o "VPN"

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

      o "VPN Identifier"

        The VPN-ID is defined by [RFC 2685] to be a sequence of 14 hex

3.  Virtual Subnet Selection Sub-Option Definition

   The virtual-subnet-selection sub-option MAY be used by any DHCP relay
   agent which desires to specify the VSS information about a VPN from
   which a DHCP client request was sent.

   The virtual-subnet-selection sub-option contains a generalized way to
   specify the VSS information about a VPN.

   The format of the option is:

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           SubOpt   Len   Type    VPN identifier
          | TBD  |   n  |  t   |  id1 |  id2 |  id3 | ...

          Type:    0      NVT ASCII VPN identifier
                   1      RFC2685 VPN-ID
                   2-255  Not Allowed

   There are two types of identifiers which can be placed in the
   virtual-subnet-selection sub-option.  The first type of identifier
   which can be placed in the virtual-subnet-selection sub-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 virtual-
   subnet-selection sub-option is an RFC2685 VPN-ID [RFC 2685], which is
   typically 14 hex digits in length (though it can be any length as far
   as the virtual-subnet-selection sub-option is concerned).

   All other values of the type field are invalid as of this memo and
   VSS sub-options containing any other value than zero (0) or one (1)
   SHOULD be ignored.

   A relay agent which recieves a DHCP request from a DHCP client on a
   VPN SHOULD include a virtual-subnet-selection sub-option in the
   relay-agent-information option that it inserts in the DHCP packet
   prior to forwarding it on to the DHCP server.

   The value placed in the virtual-subnet-selection sub-option SHOULD be
   sufficient for the relay agent to properly route any DHCP reply
   packet returned from the DHCP server to the DHCP client for which it
   is destined.  Servers supporting this sub-option MUST return an
   instance of this sub-option in the relay-agent-info option to any
   relay-agent that sends it.  Servers SHOULD return the an exact copy
   of the sub-option unless they desire to change the VPN on which a
   client was configured, which would typically be a very unusual thing
   to do.

   In the event that a virtual-subnet-selection option and a virtual-
   subnet-selection sub-option are both received in a particular DHCP
   client packet, the information from the virtual-subnet-selection
   sub-option MUST be used in preference to the information in the
   virtual-subnet-selection option.

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   Relay agents which include this sub-option when forwarding DHCP
   client requests MUST discard DHCPOFFER or DHCPACK packets that do not
   contain this sub-option in their associated relay-agent-info options.
   This does not imply any memory of the particular packets forwarded
   with this sub-option included.  Rather, the expectation is that the
   relay agent will use whatever algorithm that it used on the
   DHCPDISCOVER and DHCPREQUEST packets to decide to include this sub-
   option on the DHCPOFFER and DHCPACK packets to decide if they MUST
   have this sub-option included in their relay-agent-info options.

   In some cases, a DHCP server may use the virtual-subnet-selection
   sub-option to inform a relay agent that a particular DHCP client is
   associated with a particular VPN.  It does this by sending the
   virtual-subnet-selection sub-option with the appropriate information
   to the relay agent in the relay-agent-information option.  If the
   relay agent is unable to honor the DHCP server's requirement to place
   the DHCP client into that VPN it MUST drop the packet and not send it
   to the DHCP client.

   This sub-option SHOULD NOT be used without also making use of some
   form of authentication for relay-agent-information option.

4.  Security

   Message authentication in DHCP for intradomain use where the out-of-
   band exchange of a shared secret is feasible is defined in [RFC
   3118].  Potential exposures to attack are discussed in section 7 of
   the DHCP protocol specification in [RFC 2131].

   The virtual-subnet-selection sub-option could be used by a client in
   order to obtain an IP address from a VPN other than the one on which
   it resides.  This attack can be partially prevented by the relay
   agent not forwarding any DHCP packet which already contains a relay-
   agent-information option.

   Any program which unicasts a DHCP packet to the DHCP server with a
   relay-agent-information option in it with a vpn-id for a different
   VPN would cause the DHCP server to allocate an address from that
   different VPN, but since the DHCP server cannot (in general)
   communicate directly back to the program that sent in the malicious
   DHCP packet, the entire cycle of creating a lease will not be
   completed.  Certainly many leases could be offered, which would
   result in a temporaty form of address-pool exhaustion.

   Servers that implement the virtual-subnet-selection sub-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

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   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 VPNs from which addresses may
   be requested.

5.  IANA Considerations

   IANA has assigned the value of TBD for the VPN Identifier sub-option
   from the DHCP Relay Agent Sub-options space [RFC 3046] for the VPN
   Identifier sub-option defined in Section 3.

   This document defines a number space for the type byte of the
   virtual-subnet-selection sub-option. Certain allowable values for
   this byte are defined in this specification (see Section 3).  New
   values may only be defined by IETF Consensus, as described in [RFC
   2434].  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 virtual-subnet-
   selection option. The type bytes and data formats of the virtual-
   subnet-selection option and virtual-subnet-selection sub-option MUST
   always be identical.

6.  Acknowledgments


7.  Normative References

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

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

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

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

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

   [RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC

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      3046, January 2001.

8.  Authors' Addresses

      Kim Kinnear
      Mark Stapp
      Cisco Systems
      1414 Massachusetts Ave.
      Boxborough, Massachusetts 01719

      Phone: (978) 936-0000

      EMail: kkinnear@cisco.com

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

      Phone: (408) 526-4000

      EMail: jayk@cisco.com

9.  Full Copyright Statement

Copyright (C) The IETF Trust (2007).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors retain
all their rights.

This document and the information contained herein are provided on an

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10.  Intellectual Property

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11.  Acknowledgment

Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).

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