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rfc:rfc6607

Internet Engineering Task Force (IETF) K. Kinnear Request for Comments: 6607 R. Johnson Updates: 3046 M. Stapp Category: Standards Track Cisco Systems ISSN: 2070-1721 April 2012

       Virtual Subnet Selection Options for DHCPv4 and DHCPv6

Abstract

 This memo defines a DHCPv4 Virtual Subnet Selection (VSS) option, a
 DHCPv6 VSS option, and the DHCPv4 VSS and VSS-Control sub-options
 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-options, this
 memo documents and extends existing usage as per RFC 3942.  This memo
 updates RFC 3046 regarding details relating to the copying of sub-
 options (see Section 8).

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6607.

Kinnear, et al. Standards Track [Page 1] RFC 6607 Virtual Subnet Selection Options April 2012

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 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.

Kinnear, et al. Standards Track [Page 2] RFC 6607 Virtual Subnet Selection Options April 2012

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................4
 3. Virtual Subnet Selection Options and Sub-Options: Definitions ...6
    3.1. DHCPv4 Virtual Subnet Selection Option .....................6
    3.2. DHCPv4 Virtual Subnet Selection Sub-Option .................6
    3.3. DHCPv4 Virtual Subnet Selection Control Sub-Option .........7
    3.4. DHCPv6 Virtual Subnet Selection Option .....................7
    3.5. Virtual Subnet Selection Type and Information ..............8
 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 ...........................................15
    5.1. VPN Assignment by the DHCP Server .........................16
    5.2. DHCP Leasequery ...........................................17
 6. Client Behavior ................................................17
 7. Server Behavior ................................................19
    7.1. Returning the DHCPv4 or DHCPv6 Option .....................20
    7.2. Returning the DHCPv4 Sub-Option ...........................20
    7.3. Making Sense of Conflicting VSS Information ...............21
 8. Update to RFC 3046 .............................................22
 9. Security Considerations ........................................22
 10. IANA Considerations ...........................................23
 11. Acknowledgments ...............................................24
 12. References ....................................................25
    12.1. Normative References .....................................25
    12.2. Informative References ...................................25

1. Introduction

 There is a growing use of Virtual Private Network (VPN)
 configurations.  This 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 (also called a "Virtual Subnet Selector" or
 "VSS" in this document) from which an address, and other resources,
 should be allocated.
 This memo defines a DHCPv4 Virtual Subnet Selection (VSS) option, a
 DHCPv6 VSS option, and two VSS sub-options 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

Kinnear, et al. Standards Track [Page 3] RFC 6607 Virtual Subnet Selection Options April 2012

 understands the VSS option or sub-options, 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-options 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 messages 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.
 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 that 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.

Kinnear, et al. Standards Track [Page 4] RFC 6607 Virtual Subnet Selection Options April 2012

 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
    A DHCPv4 option is an option used to implement a capability
    defined by the DHCPv4 RFCs ([RFC2131] [RFC2132]).  This option has
    one-octet code and size fields.
 o  DHCPv4 sub-option
    As used in this document, a DHCPv4 sub-option refers to a
    sub-option of the Relay Agent Information option [RFC3046].  This
    sub-option has one-octet code and size fields.
 o  DHCPv6 option
    A DHCPv6 option is an option used to implement a capability
    defined by the DHCPv6 RFC [RFC3315].  This option has two-octet
    code and size fields.
 o  Global VPN
    This term 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 repertoire and using the
    Network Virtual Terminal encoding (see Appendix B of [RFC5198]).
 o  VSS information
    VSS information provides 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.

Kinnear, et al. Standards Track [Page 5] RFC 6607 Virtual Subnet Selection Options April 2012

 o  VPN
    This term refers to a virtual private network.  A VPN 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 Options and Sub-Options: Definitions

 The VSS 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 for both options and for one of the two sub-options.

3.1. DHCPv4 Virtual Subnet Selection Option

 The format of the option is shown below.
  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 octet.
         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 shown below.
  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 octet.
         Type and VSS Information -- see Section 3.5.

Kinnear, et al. Standards Track [Page 6] RFC 6607 Virtual Subnet Selection Options April 2012

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 shown below.
     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 (152).
            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 VSS 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 (68).
 option-len        The number of octets in the option, minimum 1.
 Type and VSS Information -- see Section 3.5.

Kinnear, et al. Standards Track [Page 7] RFC 6607 Virtual Subnet Selection Options April 2012

3.5. Virtual Subnet Selection Type and Information

 All of the (sub-)options defined above that carry VSS information use
 identical payloads consisting of a Type value and additional VSS
 information, as follows:
     Type     VSS Information Format
     ------------------------------------------------------------
      0       Network Virtual Terminal (NVT) ASCII VPN identifier
      1       RFC 2685 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 -- RFC 2685 VPN-ID
    Indicates that the VSS information consists of an RFC 2685 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.

Kinnear, et al. Standards Track [Page 8] RFC 6607 Virtual Subnet Selection Options April 2012

 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
 information, each VPN defines a new address space inside the server,
 one distinct from the global or default IP address space.  A server
 that 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.  This brings up the
 question of who decides what VPN a DHCP client should be using.
 There are three entities that can insert either 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; these 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

Kinnear, et al. Standards Track [Page 9] RFC 6607 Virtual Subnet Selection Options April 2012

 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 will 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 VSS-Control sub-option into the Relay Agent Information
 option.  This is to allow the 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 [RFC4388]
 [RFC5007], 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 that inserts the VSS information into the request packet and
 that would remove it prior to forwarding the response packet.

Kinnear, et al. Standards Track [Page 10] RFC 6607 Virtual Subnet Selection Options April 2012

 The diagram below shows an example of a DHCPv4 client, DHCPv4 relay
 agent, and DHCPv4 server.  The DHCPv6 situation is similar but uses
 the DHCPv6 VSS option.
                              DHCPv4
         DHCPv4               Relay                    DHCPv4
         Client               Agent                    Server
           |                     |                       |
           | >--DHCPDISCOVER-->  |                       |
           |    on VPN "abc"     |                       |
           |                     | >--DHCPDISCOVER---->  |
           |                     |   Relay Agent Info:   |
           |                     |     VSS type 0:"abc"  |
           |                     |     VSS-Control       |
           |                     |                       |
           |                     | <----DHCPOFFER-----<  |
           |                     |   Relay Agent Info:   |
           |                     |     VSS type 0:"abc"  |
           |                     |                       |
           | <---DHCPOFFER----<  |                       |
           |    on VPN "abc"     |                       |
           |                     |                       |
           | >--DHCPREQUEST--->  |                       |
           |    on VPN "abc"     |                       |
           |                     | >--DHCPREQUEST----->  |
           |                     |   Relay Agent Info:   |
           |                     |     VSS type 0:"abc"  |
           |                     |     VSS-Control       |
           |                     |                       |
           |                     | <----DHCPACK-------<  |
           |                     |   Relay Agent Info:   |
           |                     |     VSS type 0:"abc"  |
           |                     |                       |
           | <---DHCPACK------<  |                       |
           |    on VPN "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,

Kinnear, et al. Standards Track [Page 11] RFC 6607 Virtual Subnet Selection Options April 2012

 and the DHCP server responds to the VPN specified by the relay agent.
 There is no conflict between different entities trying to specify
 different VSS information -- each entity knows its role through
 policy or configuration external to this document.
 If any misconfiguration exists, it SHOULD result in a DHCP client
 being unable to acquire an IP address.  For instance, a relay agent
 that 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-options (for DHCPv4) or the VSS option
 (for DHCPv6).  Deploying DHCPv4 relay agents that support and emit
 VSS sub-options in concert with DHCPv4 servers that 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 unusual 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.

Kinnear, et al. Standards Track [Page 12] RFC 6607 Virtual Subnet Selection Options April 2012

 The diagram below shows this approach using DHCPv4.  The DHCPv6
 situation is similar but uses the DHCPv6 VSS option instead.
                              DHCPv4
         DHCPv4               Relay                    DHCPv4
         Client               Agent                    Server
           |                     |                       |
           | >--DHCPDISCOVER-->  |                       |
           |    on unknown VPN   |                       |
           |                     | >--DHCPDISCOVER---->  |
           |                     |                       |
           |                     | <----DHCPOFFER-----<  |
           |                     |   Relay Agent Info:   |
           |                     |     VSS type 0:"abc"  |
           |                     |                       |
           | <---DHCPOFFER----<  |                       |
           |    on VPN "abc"     |                       |
           |                     |                       |
           | >--DHCPREQUEST--->  |                       |
           |    on VPN "abc"     |                       |
           |                     | >--DHCPREQUEST----->  |
           |                     |   Relay Agent Info:   |
           |                     |     VSS type 0:"abc"  |
           |                     |     VSS-Control       |
           |                     |                       |
           |                     | <----DHCPACK-------<  |
           |                     |   Relay Agent Info:   |
           |                     |     VSS type 0:"abc"  |
           |                     |                       |
           | <---DHCPACK------<  |                       |
           |    on VPN "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.

Kinnear, et al. Standards Track [Page 13] RFC 6607 Virtual Subnet Selection Options April 2012

 It is important that both the relay agent and the DHCP server support
 the VSS option and sub-options (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.

4.3. Required Support

 DHCP relay agents and servers MUST support the approach discussed in
 Section 4.1.  DHCP relay agents and servers 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 that 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 not likely 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 requires that each
 entity know 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 whether it should
 simply ignore any VSS information that 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.

Kinnear, et al. Standards Track [Page 14] RFC 6607 Virtual Subnet Selection Options April 2012

5. Relay Agent Behavior

 Implementers MAY provide a policy or configuration capability to
 enable or disable VSS support.
 A relay agent that receives a DHCP request from a DHCP client on a
 VPN SHOULD include VSS information in the DHCP packet prior to
 forwarding the packet to the DHCP server unless inhibited from doing
 so by configuration information or policy to the contrary.
 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 VSS 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 to 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

Kinnear, et al. Standards Track [Page 15] RFC 6607 Virtual Subnet Selection Options April 2012

 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.
 Any time a relay agent places a VSS option or sub-option in a DHCP
 request, it SHOULD send it only to a DHCP server that 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
 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 option or sub-option 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 VSS 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 VSS 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

Kinnear, et al. Standards Track [Page 16] RFC 6607 Virtual Subnet Selection Options April 2012

 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
 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

 A relay agent sometimes 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 networks 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
 ("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 communication 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 that 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.

Kinnear, et al. Standards Track [Page 17] RFC 6607 Virtual Subnet Selection Options April 2012

 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 be a traditional DHCPv4 or DHCPv6 client
 requesting an IP address binding for itself.
 DHCPv4 or DHCPv6 clients 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
 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 that 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 the values sent by the client.  In
 addition, a client may receive a response from a DHCP server with a
 VSS option when none was sent 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.

Kinnear, et al. Standards Track [Page 18] RFC 6607 Virtual Subnet Selection Options April 2012

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
 default VPN.  This, of course, is also the apparent behavior of any
 DHCP server that 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 VSS 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 VSS 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.

Kinnear, et al. Standards Track [Page 19] RFC 6607 Virtual Subnet Selection Options April 2012

 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 that 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
 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 that 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 that is simply copying
 Relay Agent Information option sub-options from an incoming to an
 outgoing Relay Agent Information option from a DHCPv4 server that

Kinnear, et al. Standards Track [Page 20] RFC 6607 Virtual Subnet Selection Options April 2012

 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 that does not support the VSS sub-option will copy
 both sub-options into the outgoing Relay Agent Information option,
 thus signaling to the DHCPv4 relay agent that it did not understand
 the VSS sub-option.
 A DHCPv4 server that 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
 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 outermost 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.

Kinnear, et al. Standards Track [Page 21] RFC 6607 Virtual Subnet Selection Options April 2012

 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 these
 relay agents should not specify any conflicting VSS information.
 In situations where multiple VSS options 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 VSS-Control sub-option of a DHCPv4
 Relay Agent Information option).

8. Update to RFC 3046

 This document updates the specification of the
 Relay Agent Information option in Section 2.2 of RFC 3046, in the
 first sentence of the second paragraph, as follows:
 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 Considerations

 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 [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.

Kinnear, et al. Standards Track [Page 22] RFC 6607 Virtual Subnet Selection Options April 2012

 Message authentication in DHCPv4 relay agents as defined in [RFC4030]
 should be considered for DHCPv4 relay agents employing the
 sub-options defined in this document.  Potential exposures to attack
 are discussed in Section 7 of the DHCP protocol specification
 [RFC2131].
 For use of the VSS option by DHCPv6, 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 VSS options and the VSS
 sub-option should be aware of this possibility and use whatever
 techniques 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.
 One possible defense would be for the DHCP relay agent 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 has assigned DHCPv4 option number 221 to the DHCPv4 Virtual
 Subnet Selection option defined in Section 3.1, in accordance with
 [RFC3942].
 IANA has assigned sub-option number 151 to the DHCPv4 Virtual Subnet
 Selection 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],

Kinnear, et al. Standards Track [Page 23] RFC 6607 Virtual Subnet Selection Options April 2012

 sub-option 151 is already in use by existing implementations of this
 sub-option, and this document is essentially upward-compatible with
 these current implementations.
 IANA has assigned the value of 152 to the DHCPv4 Virtual Subnet
 Selection Control sub-option defined in Section 3.3.
 IANA has assigned the value of 68 for the DHCPv6 Virtual Subnet
 Selection option defined in Section 3.4 from the DHCP Option Codes
 registry.
 The Type byte defined in Section 3.5 defines a number space for which
 IANA has created and will maintain a new sub-registry entitled "VSS
 Type Options".  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 the VSS sub-option MUST have identical
 definitions.
 New values for the Type byte may only be defined by IETF Review, as
 described in [RFC5226].  Basically, this means that they are defined
 by RFCs approved by the IESG.

11. Acknowledgments

 Jay Kumarasamy contributed to earlier versions of this document.
 Bernie Volz recommended consolidation of the DHCPv4 option and
 sub-option documents after extensive review of those former
 documents, and provided valuable assistance in structuring and
 reviewing this document.  Alper Yegin expressed interest in the
 DHCPv6 VSS option, resulting in this combined document covering all
 three areas.  Alfred Hoenes provided assistance with editorial review
 and also raised substantive protocol issues.  David Hankins and
 Bernie Volz each raised important protocol issues that resulted in a
 clarified document.  Josh Littlefield provided editorial assistance.
 Several IESG reviewers took the time to substantially review this
 document, resulting in much-improved clarity.

Kinnear, et al. Standards Track [Page 24] RFC 6607 Virtual Subnet Selection Options April 2012

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. and B. Gleeson, "Virtual Private Networks
             Identifier", RFC 2685, September 1999.
 [RFC3046]   Patrick, M., "DHCP Relay Agent Information Option",
             RFC 3046, January 2001.
 [RFC3315]   Droms, R., Ed., 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, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
             September 1985.
 [RFC1542]   Wimer, W., "Clarifications and Extensions for the
             Bootstrap Protocol", RFC 1542, October 1993.
 [RFC3118]   Droms, R., Ed., and W. Arbaugh, Ed., "Authentication for
             DHCP Messages", RFC 3118, June 2001.
 [RFC3942]   Volz, B., "Reclassifying Dynamic Host Configuration
             Protocol version 4 (DHCPv4) Options", RFC 3942,
             November 2004.

Kinnear, et al. Standards Track [Page 25] RFC 6607 Virtual Subnet Selection Options April 2012

 [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, MA  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
 Mark Stapp
 Cisco Systems
 1414 Massachusetts Ave.
 Boxborough, MA  01719
 Phone: (978) 936-0000
 EMail: mjs@cisco.com

Kinnear, et al. Standards Track [Page 26]

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