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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 6607

DHC Working Group                                            Kim Kinnear
Internet Draft                                           Richard Johnson
Updates: 3046                                                 Mark Stapp
Intended Status: Standards Track                           Cisco Systems
Expires: July 26, 2012                                    Jay Kumarasamy
                                                        January 26, 2012


         Virtual Subnet Selection Options for DHCPv4 and DHCPv6
                   <draft-ietf-dhc-vpn-option-15.txt>

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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Copyright Notice


   Copyright (c) 2012 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.




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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Abstract

   This memo defines a Virtual Subnet Selection (VSS) option for each of
   DHCPv4 and DHCPv6, and a VSS sub-option carried in the DHCPv4 relay-
   agent-information option.  These are intended for use by DHCP
   clients, relay agents, and proxy clients in situations where VSS
   information needs to be passed to the DHCP server for proper address
   or prefix allocation to take place.

   For the DHCPv4 option and relay-agent-information sub-option, this
   memo documents existing usage as per RFC 3942 [RFC3942].  This memo
   updates RFC 3046 [RFC3046] regarding details relating to copying of
   sub-options (see Section 8).

Table of Contents


    1.  Introduction................................................. 3
    2.  Terminology.................................................. 4
    3.  Virtual Subnet Selection Option and Sub-Options Definitions.. 5
    3.1.  DHCPv4 Virtual Subnet Selection Option..................... 5
    3.2.  DHCPv4 Virtual Subnet Selection Sub-Option................. 6
    3.3.  DHCPv4 Virtual Subnet Selection Control Sub-Option......... 6
    3.4.  DHCPv6 Virtual Subnet Selection Option..................... 7
    3.5.  Virtual Subnet Selection Type and Information.............. 7
    4.  Overview of Virtual Subnet Selection Usage................... 8
    4.1.  VPN assignment by the DHCP relay agent..................... 9
    4.2.  VPN assignment by the DHCP server.......................... 12
    4.3.  Required Support........................................... 14
    4.4.  Alternative VPN assignment approaches...................... 14
    5.  Relay Agent Behavior......................................... 14
    5.1.  VPN assignment by the DHCP server.......................... 16
    5.2.  DHCP Leasequery............................................ 17
    6.  Client Behavior.............................................. 17
    7.  Server Behavior.............................................. 18




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    7.1.  Returning the DHCPv4 or DHCPv6 Option...................... 19
    7.2.  Returning the DHCPv4 Sub-Option............................ 20
    7.3.  Making sense of conflicting VSS information................ 21
    8.  Updates to RFC 3046.......................................... 21
    9.  Security..................................................... 22
    10.  IANA Considerations......................................... 23
    11.  Acknowledgments............................................. 23
    12.  References.................................................. 24
    12.1.  Normative References...................................... 24
    12.2.  Informative References.................................... 24


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.

   This memo defines a Virtual Subnet Selection (VSS) option for each of
   DHCPv4 and DHCPv6, and a VSS sub-option carried in the DHCPv4 relay-
   agent-information option.  These are intended for use by DHCP
   clients, relay agents, and proxy clients in situations where VSS
   information needs to be passed to the DHCP server for proper address
   or prefix allocation to take place.  If the receiving DHCP server
   understands the VSS option or sub-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.

   If the allocation is being done through a DHCPv4 relay, then the
   relay-agent-information sub-option defined here should be included.
   In some cases, however, an IP address is being sought by a DHCPv4
   proxy on behalf of a client (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 DHCPv4 option.

   If the allocation is being done through a DHCPv6 relay, then the
   DHCPv6 VSS option defined in this document should be included in the
   Relay-forward and Relay-reply message going between the DHCPv6 relay
   and server.  In some cases, addresses or prefixes are being sought by
   a DHCPv6 proxy on behalf of a client.  In this case, there is a need
   for the client itself to supply the VSS information using the DHCPv6
   VSS option in the messages that it sends to the DHCPv6 server.



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   In the remaining text of this document, when a DHCPv6 address is
   indicated the same information applies to DHCPv6 Prefix Delegation
   [RFC3633] as well.

   In the remaining text of this document, when the term VSS sub-option
   is used, it refers to the VSS sub-option carried in the DHCPv4
   relay-agent-information option.


2.  Terminology

   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 RFC 2119 [RFC2119].

   This document uses the following terms:

      o "DHCP client"

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

      o "DHCP proxy"

        A DHCP proxy is a DHCP client which acquires IP addresses not
        for its own use, but rather on behalf of another entity.  There
        are a variety of ways that a DHCP proxy can supply the addresses
        it acquires to other entities that need them.

      o "DHCP relay agent"

        A DHCP relay agent is an agent that transfers BOOTP and DHCP
        messages between clients and servers residing on different
        subnets, per [RFC951], [RFC1542], and [RFC3315].

      o "DHCP server"

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

      o "DHCPv4 option"

        An option used to implement a capability defined by the DHCPv4
        RFCs [RFC2131][RFC2132].  These options have one-octet code and
        size fields.

      o "DHCPv4 sub-option"




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        As used in this document, a DHCPv4 sub-option refers to a sub-
        option of the relay-agent-information option [RFC3046].  These
        sub-options have one-octet code and size fields.

      o "DHCPv6 option"

        An option used to implement a capability defined by the DHCPv6
        RFC [RFC3315].  These options have two-octet code and size
        fields.

      o "Global VPN"

        Indicates that the address being described belongs to the set of
        addresses not part of any VPN.  In other words, the normal
        address space operated on by DHCP.  This includes private
        addresses, for example the 10.x.x.x addresses as well as the
        other private subnets that are not routed on the open internet.

      o "NVT ASCII Identifier"

        A Network Virtual Terminal (NVT) identifier is an identifier
        containing only characters from the ASCII repetoire and using
        the Network Virtual Terminal encoding (see Appendix B in
        [RFC5198]).

      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 [RFC2685] to be a sequence of 7 octets.


3.  Virtual Subnet Selection Option and Sub-Options Definitions

   The Virtual Subnet Selection options and sub-options contain a
   generalized way to specify the VSS information about a VPN.  There
   are two options and two sub-options defined in this section.  The
   actual VSS information is identical both options and one of the two
   sub-options.



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3.1.  DHCPv4 Virtual Subnet Selection Option

   The format of the option is:


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Code      |    Length     |     Type      | VSS Info ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Code     The option code (221).

              Length   The option length, minimum 1 octets.

              Type and VSS Information -- see Section 3.5



3.2.  DHCPv4 Virtual Subnet Selection Sub-Option

   This is a sub-option of the relay-agent-information option [RFC3046].
   The format of the sub-option is:


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Code      |    Length     |     Type      | VSS Info. ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Code     The sub-option code (151).

              Length   The sub-option length, minimum 1 octets.

              Type and VSS Information -- see Section 3.5.



3.3.  DHCPv4 Virtual Subnet Selection Control Sub-Option

   This is a sub-option of the relay-agent-information option [RFC3046].
   The format of the sub-option is:








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       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Code      |    Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Code     The sub-option code (TBD).

              Length   The sub-option length, 0.


   This sub-option only appears in the DHCPv4 relay-agent-information
   option.  In a DHCP request, it indicates that a DHCPv4 VSS sub-option
   is also present in the relay-agent-information option.  In a DHCP
   reply, if it appears in the relay-agent-information option, it
   indicates that the DHCP server did not understand any DHCPv4 VSS
   sub-option that also appears in the relay-agent-information option.


3.4.  DHCPv6 Virtual Subnet Selection Option

   The format of the DHCPv6 Virtual Subnet Selection option is shown
   below.  This option may be included by a client or relay-agent (or
   both).


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           OPTION_VSS          |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Type    |   VSS Information ...                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_VSS (TBD).

      option-len        The number of octets in the option, minimum 1.

      Type and VSS Information -- see Section 3.5




3.5.  Virtual Subnet Selection Type and Information

   All of the (sub)options defined above carry identical payloads,
   consisting of a type and additional VSS information as follows:



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          Type     VSS Information format:

           0       NVT ASCII VPN identifier
           1       RFC2685 VPN-ID
           2-254   Unassigned
           255     Global, default VPN.



      o Type 0 -- Network Virtual Terminal (NVT) ASCII VPN identifier

        Indicates that the VSS information consists of an NVT ASCII
        string.  It MUST NOT be terminated with a zero byte.

      o Type 1 -- RFC2685 VPN-ID

        Indicates that the VSS information consists of an RFC2685 VPN-ID
        [RFC2685], which is defined to be 7 octets in length.

      o Type 255 -- Global, default VPN

        Indicates that there is no explicit, non-default VSS information
        but rather that this option references the normal, global,
        default address space.  In this case, there MUST NOT be any VSS
        Information included in the VSS option or sub-option and the
        length of the option or sub-option MUST be 1.

      All other values of the Type field are unassigned.

4.  Overview of Virtual Subnet Selection Usage

   At the highest level, the VSS option or sub-option determines the VPN
   on which a DHCP client is supposed to receive an IP address.  How the
   option or sub-option is entered and processed is discussed below, but
   the point of all of the discussion is to determine the VPN on which
   the DHCP client resides.  This will affect a relay agent, in that it
   will have to ensure that DHCP packets sent to and received from the
   DHCP client flow over the correct VPN.  This will affect the DHCP
   server in that it determines the IP address space used for the IP
   address allocation.

   A DHCP server has as part of its configuration some IP address space
   from which it allocates IP addresses to DHCP clients.  These
   allocations are typically for a limited time, and thus the DHCP
   client gets a lease on the IP address.  In the absence of any VPN
   information, the IP address space is in the global or default VPN
   used throughout the Internet.  When a DHCP server deals with VPN



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   information, each VPN defines a new address space inside the server,
   one distinct from the global or default IP address space.  A server
   which supports the VSS option or sub-option thereby supports
   allocation of IP addresses from multiple different VPNs.  Supporting
   IP address allocation from multiple different VPNs means that the
   DHCP server must be prepared to configure multiple different address
   spaces (one per distinct VPN) and allocate IP addresses from these
   different address spaces.

   These address spaces are typically independent, so that the same IP
   address (consisting of the same string of bytes) could be allocated
   to one client in the global, default VPN, and to a different client
   residing in a different VPN.  There is no conflict in this
   allocation, since the clients have essentially different addresses,
   even though these addresses consist of the same string of bytes,
   because the IPv4 or IPv6 address is qualified by the VPN.

   Thus a VSS option or sub-option is a way of signaling the use of a
   VPN other than the global or default VPN.  The next question is: who
   decides what VPN a DHCP client should be using?

   There are three entities which can either insert a VSS option or
   sub-option into a DHCPv4 packet or DHCPv6 message; a DHCP client, a
   relay agent, or a DHCPv4 or DHCPv6 server.  While all of these
   entities could include a different VSS option or sub-option in every
   request or response, this situation is neither typical nor useful.
   There are two known paradigms for use of the VSS option or sub-
   option, which are discussed below.

4.1.  VPN assignment by the DHCP relay agent

   The typical use of the VSS option or sub-option is for the relay
   agent to know the VPN on which the DHCP client is operating.  The
   DHCP client itself does not, in this approach, know the VPN on which
   it resides.  The relay agent is responsible for mediating the access
   between the VPN on which the DHCP client resides and the DHCP server.
   In this situation, the relay agent will insert two DHCPv4 relay-
   agent-information sub-options (one VSS sub-option, and one VSS-
   Control sub-option) into the relay-agent-information option or a
   DHCPv6 VSS option into the Relay-forward message of every request it
   forwards from the DHCP client.  The server will use the DHCPv6 VSS
   option or DHCPv4 VSS sub-option to determine the VPN on which the
   client resides, and use that VPN information to select the address
   space within its configuration from which to allocate an IP address
   to the DHCP client.

   When, using this approach, a DHCPv4 relay agent inserts a VSS sub-
   option into the relay-agent-information option it MUST also insert a



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   VSS-Control sub-option into the relay-agent-information-option.  This
   is to allow determination of whether or not the DHCPv4 server
   actually processes the VSS information provided by the DHCPv4 relay
   agent.  If the DHCPv4 server supports the VSS capabilities described
   in this document, it will remove the VSS-Control sub-option from the
   relay-agent-information option that it returns to the DHCPv4 relay
   agent.  See Section 5 for more information.

   In this approach, the relay agent might also send a VSS option or
   sub-option in either a DHCPv4 or DHCPv6 Leasequery request, but in
   this case, it would use the VSS option in the Leasequery request to
   select the correct address space for the Leasequery.  In this
   approach, the relay agent would be acting as a DHCP client from a
   Leasequery standpoint, but it would not be as if a DHCP client were
   sending in a VSS option in a standard DHCP address allocation
   request, say a DHCPDISCOVER.

   In this approach, only one relay agent would mediate the VPN access
   for the DHCP client to the DHCP server, and it would be the relay
   agent which inserts the VSS information into the request packet and
   would remove it prior to forwarding the response packet on.

   In the diagram below is an example of a DHCPv4 client, DHCPv4 relay
   agent, and DHCPv4 server.  The DHCPv6 situation is similar, but uses
   the DHCPv6 VSS option.


























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                                   DHCPv4
              DHCPv4               Relay                    DHCPv4
              Client               Agent                    Server

                |                     |                       |
                | >--DHCPDISCOVER-->  |                       |
                |    on VRF "abc"     |                       |
                |                     | >--DHCPDISCOVER---->  |
                |                     |   relay-agent-info:   |
                |                     |     VSS type VRF:"abc"|
                |                     |     VSS-Control       |
                |                     |                       |
                |                     | <----DHCPOFFER-----<  |
                |                     |   relay-agent-info:   |
                |                     |     VSS type VRF:"abc"|
                |                     |                       |
                | <---DHCPOFFER----<  |                       |
                |    on VRF "abc"     |                       |
                |                     |                       |
                | >--DHCPREQUEST--->  |                       |
                |    on VRF "abc"     |                       |
                |                     | >--DHCPREQUEST----->  |
                |                     |   relay-agent-info:   |
                |                     |     VSS type VRF:"abc"|
                |                     |     VSS-Control       |
                |                     |                       |
                |                     | <----DHCPACK-------<  |
                |                     |   relay-agent-info:   |
                |                     |     VSS type VRF:"abc"|
                |                     |                       |
                | <---DHCPACK------<  |                       |
                |    on VRF "abc"     |                       |
                |                     |                       |
               ...                   ...                     ...

           Figure 4.1-1:  DHCPv4 - Relay Agent knows VPN


   The DHCP server would know that it should respond to VPN information
   specified in a VSS option or sub-option, and it would be configured
   with appropriate VPN address spaces to service the projected client
   requirements.  Thus, in this common approach, the DHCP client knows
   nothing of any VPN access, the relay agent has been configured in
   some way that allows it to determine the VPN of the DHCP client and
   transmit that using a VSS option or sub-option to the DHCP server,
   and the DHCP server responds to the VPN specified by the relay agent.
   There is no conflict between different entities trying to specify



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   different VSS information -- each entity knows its role through
   policy or configuration external to this document.

   If any mis-configuration exists, it SHOULD result in a DHCP client
   being unable to acquire an IP address.  For instance, a relay agent
   which supports VPN access SHOULD couple transmission of VSS options
   or sub-options to the configuration of VPN support, and not allow one
   without the other.

   It is important to ensure that the relay agent and DHCP server both
   support the VSS option and sub-option (for DHCPv4) or the VSS option
   (for DHCPv6).  Deploying DHCPv4 relay agents which support and emit
   VSS sub-options in concert with DHCPv4 servers which do not support
   the VSS option or sub-option as defined in this document SHOULD NOT
   be done, as such an ensemble will not operate correctly.  Should this
   situation occur, however, the relay agent can detect the problem
   (since the VSS-Control sub-option will appear in the packets it
   receives from the DHCPv4 server, indicating the server did not
   effectively process the VSS sub-option), and it can issue appropriate
   diagnostic messages.

4.2.  VPN assignment by the DHCP server

   In this approach, the DHCP server would be configured in some way to
   know the VPN on which a particular DHCP client should be given
   access.  The DHCP server would in this case include the VSS sub-
   option in the relay-agent-information option for DHCPv4 or the VSS
   option in the Relay-reply message for DHCPv6.  The relay agent
   responsible for mediating VPN access would use this information to
   select the correct VPN for the DHCP client.  In the unusal event that
   there were more than one relay agent involved in this transaction,
   some external configuration or policy would be needed to inform the
   DHCPv6 server into which Relay-reply message the VSS option should
   go.

   Once the relay agent has placed the DHCP client into the proper VPN,
   it SHOULD begin including VSS information in requests that it
   forwards to the DHCP server.  Since this information does not
   conflict with the DHCP server's idea of the proper VPN for the
   client, everything works correctly.

   The diagram below shows this approach using DHCPv4.  The DHCPv6
   situation is similar, but uses the DHCPv6 VSS option instead.








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                                   DHCPv4
              DHCPv4               Relay                    DHCPv4
              Client               Agent                    Server

                |                     |                       |
                | >--DHCPDISCOVER-->  |                       |
                |    on unknown VPN   |                       |
                |                     | >--DHCPDISCOVER---->  |
                |                     |                       |
                |                     | <----DHCPOFFER-----<  |
                |                     |   relay-agent-info:   |
                |                     |     VSS type VRF:"abc"|
                |                     |                       |
                | <---DHCPOFFER----<  |                       |
                |    on VRF "abc"     |                       |
                |                     |                       |
                | >--DHCPREQUEST--->  |                       |
                |    on VRF "abc"     |                       |
                |                     | >--DHCPREQUEST----->  |
                |                     |   relay-agent-info:   |
                |                     |     VSS type VRF:"abc"|
                |                     |     VSS-Control       |
                |                     |                       |
                |                     | <----DHCPACK-------<  |
                |                     |   relay-agent-info:   |
                |                     |     VSS type VRF:"abc"|
                |                     |                       |
                | <---DHCPACK------<  |                       |
                |    on VRF "abc"     |                       |
                |                     |                       |
                |                     |                       |
               ...                   ...                     ...

           Figure 4.2-1:  DHCPv4 - DHCPv4 Server knows VPN


   In this approach, the DHCP client is again unaware of any VPN
   activity.  In this case, however, the DHCP server knows the VPN for
   the client, and the relay agent responds to the VSS information
   specified by the DHCP server.  Similar to the previous approach, each
   entity knows its role through a means external to this document and
   no two entities try to specify VSS information in conflict.

   It is important that both the relay agent as well as the DHCP server
   both support the VSS option and sub-option (for DHCPv4) and the VSS
   option (for DHCPv6).  Deploying and configuring VPN support in one
   element and not in the other is not a practical approach.



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4.3.  Required Support

   DHCP relay agents and servers MUST support the approach discussed in
   Section 4.1.  DHCP relay agents and server SHOULD support the
   approach discussed in Section 4.2.  DHCP relay agents and servers
   SHOULD NOT be configured to operate with both approaches
   simultaneously.

4.4.  Alternative VPN assignment approaches

   There are many other approaches which can be created with multiple
   relay agents each inserting VSS information into different Relay-
   forward messages, relay agent VSS information conflicting with client
   VSS information, or DHCP server VSS information conflicting with
   relay agent and client VSS information.  Since these approaches do
   not describe situations that are useful today, specifying precisely
   how to resolve all of these conflicts is unlikely to be valuable in
   the event that these approaches actually become practical in the
   future.

   The current use of the VSS option and sub-option require that each
   entity knows the part that it plays in dealing with VPN data.  Each
   entity -- client, relay agent or agents, and server -- SHOULD know
   through some policy or configuration beyond the scope of this
   document whether it is responsible for specifying VPN information
   using the VSS option or sub-option or responsible for responding to
   VSS information specified by another entity, or simply ignoring any
   VSS information which it might see.

   Some simple conflict resolution approaches are discussed below, in
   the hopes that they will cover simple cases that may arise from
   situations beyond those envisioned today.  However, for more complex
   situations, or simple situations where appropriate conflict
   resolution strategies differ from those discussed in this document, a
   document detailing the usage situations and appropriate conflict
   resolution strategies SHOULD be created and submitted for discussion
   and approval.

5.  Relay Agent Behavior

   Implementers MAY provide a policy or configuration capability to
   enable or disable VSS support.

   A relay agent which receives a DHCP request from a DHCP client on a
   VPN SHOULD include Virtual Subnet Selection information in the DHCP
   packet prior to forwarding the packet on to the DHCP server unless
   inhibited from doing so by configuration information or policy to the
   contrary.



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   In this situation, a DHCPv4 relay agent MUST include a DHCPv4 VSS
   sub-option in a relay-agent-information option [RFC3046], while a
   DHCPv6 relay agent MUST include a DHCPv6 VSS option in the Relay-
   forward message.

   The value placed in the Virtual Subnet Selection sub-option or option
   would typically 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.  In some cases, the information in
   the VSS sub-option or option might be an index into some internal
   table held in the relay agent, though this document places no
   requirement on a relay agent to have any such internal state.

   A DHCPv4 relay agent MUST, in addition, include a DHCPv4 VSS-Control
   sub-option (which has a length of zero) in the relay-agent-
   information option [RFC3046] whenever it includes a VSS sub-option in
   the relay-agent-information option.  The inclusion of the VSS sub-
   option and the VSS-Control sub-option in the relay-agent-information
   option will allow the DHCPv4 relay agent to determine whether the
   DHCPv4 server actually processed the information in the VSS sub-
   option when it receives the relay-agent-information option in the
   reply from the DHCPv4 server.

   The reason to include this additional VSS DHCPv4 sub-option is that
   [RFC3046] specifies (essentially) that a DHCPv4 server should copy
   all sub-options that it receives in a relay-agent-information option
   in a request into a corresponding relay-agent-information option in
   the response.  Thus, a server that didn't support the DHCPv4 VSS
   sub-option would normally just copy it to the response packet,
   leaving the relay agent to wonder if in fact the DHCPv4 server
   actually used the VSS information when processing the request.

   To alleviate this potential confusion, a DHCPv4 relay agent instead
   sends in two sub-options: one VSS sub-option, and one VSS-Control
   sub-option.  If both sub-options appear in the response from the
   DHCPv4 server, then the DHCPv4 relay agent MUST assume that the
   DHCPv4 server did not act on the VSS information in the VSS sub-
   option.  If only the VSS sub-option appears in the response from the
   DHCPv4 server and no VSS-Control sub-option appears in the response
   from the DHCPv4 server, then the relay agent SHOULD assume that the
   DHCPv4 server acted successfully on the VSS sub-option.

   Anytime a relay agent places a VSS option or sub-option in a DHCP
   request, it SHOULD send it only to a DHCP server which supports the
   VSS option or sub-option, and it MUST check the response to determine
   if the DHCP server actually honored the requested VSS information.

   In the DHCPv6 case, the appearance of the option in the Relay-reply



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   packet indicates that the DHCPv6 server understood and acted upon the
   contents of the VSS option in the Relay-forward packet.  In the
   DHCPv4 case, as discussed above, the appearance of the VSS sub-option
   without the appearance of a VSS-Control sub-option indicates that the
   DHCPv4 server successfully acted upon the VSS sub-option.

   This document does not create a requirement that a relay agent
   remember the contents of a VSS DHCPv4 sub-option or VSS DHCPv6 option
   sent to a DHCP server.  In many cases, the relay agent may simply use
   the value of the VSS returned by the DHCP server to forward the
   response to the DHCP client.  If the VSS information, the IP address
   allocated, and the VPN capabilities of the relay agent all
   interoperate correctly, then the DHCP client will receive a working
   IP address.  Alternatively, if any of these items don't interoperate
   with the others, the DHCP client will not receive a working address.

   Note that in some environments a relay agent may choose to always
   place a VSS option or sub-option into packets and messages that it
   forwards in order to forestall any attempt by a relay agent closer to
   the client or the client itself to specify VSS information.  In this
   case, a type field of 255 is used to denote the global, default VPN.
   When the type field of 255 is used, there MUST NOT be any additional
   VSS information in the VSS option or sub-option.  In the DHCPv4 case,
   an additional VSS-Control sub-option would be required, as discussed
   above.

5.1.  VPN assignment by the DHCP server

   In some cases, a DHCP server may use the Virtual Subnet Selection
   sub-option or 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 or option with the
   appropriate information to the relay agent in the relay-agent-
   information option for DHCPv4 or the Relay-reply message in DHCPv6.
   If the relay agent cannot respond correctly to the DHCP server's
   requirement to place the DHCP client into that VPN (perhaps because
   it has not been configured with a VPN that matches the VSS
   information received from the DHCP server) it MUST drop the packet
   and not send it to the DHCP client.

   In this situation, once the relay agent has placed the DHCP client
   into the VPN specified by the DHCP server, it will insert a VSS
   option or sub-option when forwarding packets from the client.  The
   DHCP server in normal operation will echo this VSS information into
   the outgoing replies.

   In the event that the relay agent doesn't include VSS information on
   subsequent requests after the DHCP server has included VSS



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   information in a reply to the relay agent, the DHCP server can
   conclude that the relay agent doesn't support VSS processing, and the
   DHCP server SHOULD stop processing this transaction and not respond
   to the request.

5.2.  DHCP Leasequery

   Sometimes a relay-agent needs to submit a DHCP Leasequery [RFC4388]
   [RFC5007] packet to the DHCP server in order to recover information
   about existing DHCP allocated IP addresses on other than the normal,
   global VPN.  In the context of a DHCP Leasequery the relay agent is a
   direct client of the DHCP server and is not relaying a packet for
   another DHCP client. Thus, the instructions in Section 6 on Client
   Behavior should be followed to include the necessary VSS information.

6.  Client Behavior

   Typically, DHCPv4 and DHCPv6 clients have no interaction with VSS
   options or sub-options.  The VSS information is handled by exchanges
   between a DHCPv4 or DHCPv6 relay agent and the corresponding DHCPv4
   or DHCPv6 server.

   However, there are times when an entity is acting as a DHCPv4 or
   DHCPv6 client in that it is communicating directly with a DHCPv4 or
   DHCPv6 server.  In these instances -- where communications is
   occurring without employing the DHCPv4 relay-agent-information option
   or the DHCPv6 Relay-forward or Relay-reply messages, the entity is
   acting as a DHCPv4 or DHCPv6 client with regard to its communication
   with the DHCPv4 or DHCPv6 server, but not necessarily as a DHCP
   client who is requesting a DHCPv4 or DHCPv6 address for its own use.

   The client, in this context, may be requesting an IP address for
   another entity, thus acting as a DHCP proxy.  The client may be
   requesting information about another client-to-address binding, using
   the DHCPv4 [RFC4388] or DHCPv6 [RFC5007] Leasequery protocol.

   In the rest of this section, the term "client" refers to an entity
   communicating VSS information directly to a DHCPv4 or DHCPv6 server
   without using the DHCPv4 relay-agent-information option or the DHCPv6
   Relay-forward or Relay-reply messages, and there is no requirement
   that such a client is a traditional DHCPv4 or DHCPv6 client
   requesting an IP address binding for itself.

   A DHCPv4 or DHCPv6 client will employ the VSS option to communicate
   VSS information to their respective servers.  This information MUST
   be included in every message concerning any IP address on a different
   VPN than the global or default VPN.  A DHCPv4 client will place the
   DHCPv4 VSS option in its packets, and a DHCPv6 client will place the



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   DHCPv6 VSS option in its messages.

   A DHCPv6 client that needs to place a VSS option into a DHCPv6
   message SHOULD place a single VSS option into the DHCPv6 message at
   the same level as the Client Identifier option.  A DHCPv6 client MUST
   NOT include different VSS options in the same DHCPv6 message.

   Note that, as mentioned in Section 1, throughout this document when a
   DHCPv6 address is indicated the same information applies to DHCPv6
   Prefix Delegation [RFC3633] as well.

   Since this option is placed in the packet in order to change the VPN
   on which an IP address is allocated for a particular DHCP client, one
   presumes that an allocation on that VPN is necessary for correct
   operation.  Thus, a client which places this option in a packet and
   doesn't receive it or receives a different value in a returning
   packet SHOULD drop the packet since the IP address that was allocated
   will not be in the requested VPN.

   Clients should be aware that some DHCP servers will return a VSS
   option with different values than that which was sent in.  In
   addition, a client may receive a response from a DHCP server with a
   VSS option when none was sent in by the Client.

   Note that when sending a DHCP Leasequery request, a relay agent is
   acting as a DHCP client and so it SHOULD include the respective
   DHCPv4 or DHCPv6 VSS option in its DHCPv4 or DHCPv6 Leasequery packet
   if the DHCP Leasequery request is generated for other than the
   default, global VPN.  It SHOULD NOT include a DHCPv4 sub-option in
   this case.

7.  Server Behavior

   A DHCP server receiving the VSS option or sub-option SHOULD allocate
   an IP address (or use the VSS information to access an already
   allocated IP address) from the VPN specified by the included VSS
   information.

   In the case where the type field of the VSS option or sub-option is
   255, the VSS option denotes the global, default VPN.  In this case,
   there is no explicit VSS information beyond the type field.

   This document does not prescribe any particular address allocation
   policy.  A DHCP server may choose to attempt to allocate an address
   using the VSS information and, if this is impossible, to not allocate
   an address.  Alternatively, a DHCP server may choose to attempt
   address allocation based on the VSS information and, if that is not
   possible, it may fall back to allocating an address on the global or



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   default VPN.  This, of course, is also the apparent behavior of any
   DHCP server which doesn't implement support for the VSS option and
   sub-option.  Thus, DHCP clients and relay agents SHOULD be prepared
   for either of these alternatives.

   In some cases, a DHCP server may use the Virtual Subnet Selection
   sub-option or 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 or option with the
   appropriate information to the relay agent in the relay-agent-
   information option for DHCPv4 or the Relay-reply message in DHCPv6.

   In this situation, the relay agent will place the client in the
   proper VPN, and then it will insert a VSS option or sub-option in
   subsequent forwarded requests.  The DHCP server will see this VSS
   information and since it doesn't conflict in any way with the
   server's notion of the VPN on which the client is supposed to reside,
   it will process the requests based on the VPN specified in the VSS
   option or sub-option, and echo the same VSS information in the
   outgoing replies.

   The relay agent receiving a reply containing a VSS option should
   support the VSS option.  Otherwise the relay agent will end up
   attempting to use the address as though it were a global address.
   Should this happen, the subsequent DHCPREQUEST will not contain any
   VSS information, in which case the DHCP server SHOULD NOT respond
   with a DHCPACK.

   If a server uses a different VPN than what was specified in the VSS
   option or sub-option, it SHOULD send back the VPN information using
   the same type as the received type. It MAY send back a different type
   if it is not possible to use the same type (such as the RFC2685 VPN-
   ID if no ASCII VPN identifier exists).

   A server which receives a VSS sub-option in the DHCPv4 relay-agent-
   information option and does not receive a VSS-Control sub-option in
   the relay-agent-information option MUST process the information
   specified in the VSS sub-option in the same fashion as it would have
   if it received both sub-options.

7.1.  Returning the DHCPv4 or DHCPv6 Option

   DHCPv4 or DHCPv6 servers receiving a VSS option (for sub-option
   processing, see below) MUST return an instance of this option in the
   reply packet or message if the server successfully uses this option
   to allocate an IP address, and it MUST NOT include an instance of
   this option if the server is unable to support, is not configured to
   support, or does not implement support for VSS information in general



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   or the requested VPN in particular.

   If they echo the option (based on the criteria above), servers SHOULD
   return an exact copy of the option unless they desire to change the
   VPN on which a client was configured.

   The appearance of the DHCPv4 VSS option code in the DHCPv4 Parameter
   Request List option [RFC2132] should not change the processing or
   decision to return or not return the VSS option as specified in this
   document.  The appearance of the DHCPv6 VSS option in the OPTION_ORO
   [RFC3315] or the OPTION_ERO [RFC4994] should not change the
   processing or decision to return (or not to return) the VSS option as
   specified in this document.

7.2.  Returning the DHCPv4 Sub-Option

   The case of the DHCPv4 sub-option is a bit more complicated.  Note
   that [RFC3046] specifies that a DHCPv4 server which supports the
   relay-agent-information option SHALL copy all sub-options received in
   a relay-agent-information option into any outgoing relay-agent-
   information option.  Thus, the default behavior for any DHCPv4 server
   is to return any VSS sub-option received to the relay agent whether
   or not the DHCPv4 server understands the VSS sub-option.

   In order to distinguish a DHCPv4 server which is simply copying
   relay-agent-information option sub-options from an incoming to an
   outgoing relay-agent-informaion option from one which successfully
   acted upon the information in the VSS sub-option, DHCPv4 relay agents
   MUST include a VSS-Control sub-option in the relay-agent-information
   any time that it includes a VSS sub-option in the relay-agent-
   information option.

   A DHCPv4 server which does not support the VSS sub-option will copy
   both sub-options into the outgoing relay-agent-information option,
   thus signalling to the DHCPv4 relay agent that it did not understand
   the VSS sub-option.

   A DHCPv4 server which supports the VSS sub-option:


      o MUST copy the VSS sub-option into the outgoing relay-agent-
        information option

      o MUST NOT copy the VSS-Control sub-option into the outgoing
        relay-agent-information option

   Moreover, if a server uses different VSS information to allocate an
   IP address than it receives in a particular DHCPv4 sub-option, it



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   MUST include that alternative VSS information in the VSS sub-option
   that it returns to the DHCPv4 relay agent instead of the original VSS
   information it was given.

   If a DHCPv4 server supports this sub-option and for some reason
   (perhaps administrative control) does not honor this sub-option from
   the request then it MUST NOT echo either sub-option into the outgoing
   relay-agent-information option.

7.3.  Making sense of conflicting VSS information

   It is possible for a DHCPv4 server to receive both a VSS option and
   VSS sub-options in the same packet.  Likewise, a DHCPv6 server can
   receive multiple VSS options in nested Relay-forward messages as well
   as in the client message itself.  In either of these cases, the VSS
   information from the relay agent closest to the DHCP server SHOULD be
   used in preference to all other VSS information received.  In the
   DHCPv4 case, this means that the VSS sub-option takes precedence over
   the VSS option, and in the DHCPv6 case, this means that the VSS
   option from the outer-most Relay-forward message in which a VSS
   option appears takes precedence.

   The reasoning behind this approach is that the relay-agent closer to
   the DHCP server is almost certainly more trusted than the DHCP client
   or more distant relay agents, and therefore information in the
   relay-agent-information option or the Relay-forward message is more
   likely to be correct.

   In general, relay agents SHOULD be aware through configuration or
   policy external to this document whether or not they should be
   including VSS information in packets that they forward and so there
   should not be conflicts among relay agent specified VSS information.

   In these situations where multiple VSS option or sub-options appear
   in the incoming packet or message, when the DHCP server constructs
   the response to be sent to the DHCP client or relay agent, all
   existing VSS options or sub-options MUST be replicated in the
   appropriate places in the response and MUST contain only the VSS
   information that was used by the DHCP server to allocate the IP
   address (with, of course, the exception of a DHCPv4 relay-agent-
   information sub-option VSS-Control).

8.  Updates to RFC 3046

   This document updates the specification of the Relay Agent
   Information option in RFC 3046 as follows:

   Change the first sentence, second paragraph, section 2.2 of RFC 3046:



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      o OLD:

        DHCP servers claiming to support the Relay Agent Information
        option SHALL echo the entire contents of the Relay Agent
        Information option in all replies.

      o NEW:

        DHCP servers claiming to support the Relay Agent Information
        option SHALL echo the entire contents of the Relay Agent
        Information option in all replies, except if otherwise specified
        in the definition of specific Relay Agent Information sub-
        options.

9.  Security

   Message authentication in DHCPv4 for intradomain use where the out-
   of-band exchange of a shared secret is feasible is defined in
   [RFC3118].  Potential exposures to attack are discussed in Section 7
   of the DHCP protocol specification in [RFC2131].

   Implementations should consider using the DHCPv4 Authentication
   option [RFC3118] to protect DHCPv4 client access in order to provide
   a higher level of security if it is deemed necessary in their
   environment.

   Message authentication in DHCPv4 relay agents as defined in [RFC4030]
   should be considered for DHCPv4 relay agents employing this sub-
   option.  Potential exposures to attack are discussed in Section 7 of
   the DHCP protocol specification in [RFC2131].

   For DHCPv6 use of the VSS option, the "Security Considerations"
   Section of [RFC3315] details the general threats to DHCPv6, and thus
   to messages using the VSS option.  The "Authentication of DHCP
   Messages" Section of [RFC3315] describes securing communication
   between relay agents and servers, as well as clients and servers.

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

   A DHCP server that implements these options and sub-option should be
   aware of this possibility and use whatever techniques that can be
   devised to prevent such an attack.  Information such as the giaddr in
   DHCPv4 or link address in the Relay-forward DHCPv6 message might be
   used to detect and prevent this sort of attack.



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   One possible defense would be for the DHCP relay to insert a VSS
   option or sub-option to override the DHCP client's VSS option.

   Servers that implement the VSS option and 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 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.

10.  IANA Considerations

   IANA is requested to assign DHCPv4 option number 221 for the DHCPv4
   VSS option defined in Section 3.1, in accordance with [RFC3942].

   IANA is requested to assign sub-option number 151 for the DHCPv4 VSS
   sub-option defined in Section 3.2 from the DHCP Relay Agent Sub-
   options space [RFC3046], in accordance with the spirit of [RFC3942].
   While [RFC3942] doesn't explicitly mention the sub-option space for
   the DHCP Relay Agent Information option [RFC3046], sub-option 151 is
   already in use by existing implementations of this sub-option and the
   current draft is essentially upward compatible with these current
   implementations.

   IANA is requested to assign the value of TBD for the DHCPv4 VSS-
   Control sub-option defined in Section 3.3.

   IANA is requested to assign the value of TBD for the DHCPv6 VSS
   option defined in Section 3.4 from the DHCPv6 option registry.

   The type byte defined in Section 3.5 defines a number space for which
   IANA is to create and maintain a new sub-registry entitled "VSS Type
   values".  This sub-registry needs to be related to both the DHCPv4
   and DHCPv6 VSS options and the DHCPv4 relay-agent-information option
   sub-option (all defined by this document), since the type byte in
   these two options and one sub-option MUST have identical definitions.

   New values for the type byte may only be defined by IETF Consensus,
   as described in [RFC5226].  Basically, this means that they are
   defined by RFCs approved by the IESG.

11.  Acknowledgments

   Bernie Volz recommended consolidation of the DHCPv4 option and sub-
   option drafts after extensive review of the former drafts, and
   provided valuable assistance in structuring and reviewing this
   document.  Alper Yegin expressed interest in the DHCPv6 VSS option,



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   resulting in this combined draft covering all three areas.  Alfred
   Hoenes provided assistance with editorial review as well as raising
   substantive protocol issues.  David Hankins and Bernie Volz each
   raised important protocol issues which resulted in a clarified
   document.  Josh Littlefield provided editorial assistance.  Several
   IESG reviewers took the time to substantially review this document,
   resulting in much increased clarity.


12.  References


12.1.  Normative References


   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
      Requirement Levels", RFC 2119, March 1997.

   [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
      March 1997.

   [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
      Extensions", RFC 2132, March 1997.

   [RFC2685] Fox, B., Gleeson, B., "Virtual Private Networks
      Identifier", RFC 2685, September 1999.

   [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
      3046, January 2001.

   [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and
      M. Carney, "Dynamic Host Configuration Protocol for IPv6
      (DHCPv6)", RFC 3315, July 2003.

   [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
      Host Configuration Protocol (DHCP) version 6", RFC 3633, December
      2003.

   [RFC4994] Zeng, S., Volz, B., Kinnear, K. and J. Brzozowski, "DHCPv6
      Relay Agent Echo Request Option", RFC 4994, September 2007.

12.2.  Informative References


   [RFC951] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951,
      September 1985.

   [RFC1542] Wimer, W., "Clarifications and Extensions for the Bootstrap



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      Protocol", RFC 1542, October 1993.

   [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
      Messages", RFC 3118, June 2001.

   [RFC3942] Volz, B., "Reclassifying Dynamic Host Configuration
      Protocol version 4 (DHCPv4) Options", RFC 3942, November 2004.

   [RFC4030] Stapp, M. and T. Lemon, "The Authentication Suboption for
      the Dynamic Host Configuration Protocol (DHCP) Relay Agent
      Option", RFC 4030, March 2005.

   [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
      Protocol (DHCP) Leasequery", RFC 4388, February 2006.

   [RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng, "DHCPv6
      Leasequery", RFC 5007, September 2007.

   [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
      Interchange", RFC 5198, March 2008.

   [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
      IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.


Authors' Addresses


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

      Phone: (978) 936-0000

      EMail: kkinnear@cisco.com


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

      Phone: (408) 526-4000

      EMail: raj@cisco.com





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

      Phone: (978) 936-0000

      EMail: mjs@cisco.com


      Jay Kumarasamy








































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