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Internet Engineering Task Force (IETF)                        K. KinnearRequest for Comments: 6607                                    R. JohnsonUpdates:3046                                                   M. StappCategory: Standards Track                                  Cisco SystemsISSN: 2070-1721                                               April 2012Virtual Subnet Selection Options for DHCPv4 and DHCPv6Abstract   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 perRFC 3942.  This memo   updatesRFC 3046 regarding details relating to the copying of sub-   options (seeSection 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6607.Kinnear, et al.              Standards Track                    [Page 1]

RFC 6607            Virtual Subnet Selection Options          April 2012Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 2012Table of Contents1. Introduction ....................................................32. Terminology .....................................................43. Virtual Subnet Selection Options and Sub-Options: Definitions ...63.1. DHCPv4 Virtual Subnet Selection Option .....................63.2. DHCPv4 Virtual Subnet Selection Sub-Option .................63.3. DHCPv4 Virtual Subnet Selection Control Sub-Option .........73.4. DHCPv6 Virtual Subnet Selection Option .....................73.5. Virtual Subnet Selection Type and Information ..............84. Overview of Virtual Subnet Selection Usage ......................84.1. VPN Assignment by the DHCP Relay Agent .....................94.2. VPN Assignment by the DHCP Server .........................124.3. Required Support ..........................................144.4. Alternative VPN Assignment Approaches .....................145. Relay Agent Behavior ...........................................155.1. VPN Assignment by the DHCP Server .........................165.2. DHCP Leasequery ...........................................176. Client Behavior ................................................177. Server Behavior ................................................197.1. Returning the DHCPv4 or DHCPv6 Option .....................207.2. Returning the DHCPv4 Sub-Option ...........................207.3. Making Sense of Conflicting VSS Information ...............218. Update toRFC 3046 .............................................229. Security Considerations ........................................2210. IANA Considerations ...........................................2311. Acknowledgments ...............................................2412. References ....................................................2512.1. Normative References .....................................2512.2. Informative References ...................................251.  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 serverKinnear, 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 inRFC 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 (seeAppendix 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 -- seeSection 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 -- seeSection 3.5.Kinnear, et al.              Standards Track                    [Page 6]

RFC 6607            Virtual Subnet Selection Options          April 20123.3.  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 -- seeSection 3.5.Kinnear, et al.              Standards Track                    [Page 7]

RFC 6607            Virtual Subnet Selection Options          April 20123.5.  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        1RFC 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 anRFC 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 itKinnear, 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.  SeeSection 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 inSection 4.1.  DHCP relay agents and servers SHOULD support the   approach discussed inSection 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 20125.  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 fromKinnear, 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 (perhapsKinnear, 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 inSection 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 inSection 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 20127.  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 theRFC2685   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 thatKinnear, 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 toRFC 3046   This document updates the specification of the   Relay Agent Information option inSection 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 inSection 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 inSection 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 inSection 3.1, in accordance with   [RFC3942].   IANA has assigned sub-option number 151 to the DHCPv4 Virtual Subnet   Selection sub-option defined inSection 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 inSection 3.3.   IANA has assigned the value of 68 for the DHCPv6 Virtual Subnet   Selection option defined inSection 3.4 from the DHCP Option Codes   registry.   The Type byte defined inSection 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 201212.  References12.1.  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.comKinnear, et al.              Standards Track                   [Page 26]

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