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IETF RFC 6607
Last modified on Monday, April 30th, 2012
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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/RFC 6607.
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
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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
[RFC 3633] 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 [RFC 2119].
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 [RFC 951], [RFC 1542], and [RFC 3315].
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 ([RFC 2131] [RFC 2132]). 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 [RFC 3046]. 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 [RFC 3315]. 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 [RFC 5198]).
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 [RFC 2685] 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 [RFC 3046].
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 [RFC 3046].
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
[RFC 2685], 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 [RFC 4388]
[RFC 5007], 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 [RFC 3046], 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 [RFC 3046] 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
[RFC 3046] 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 [RFC 4388]
[RFC 5007] 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 [RFC 4388] or DHCPv6 [RFC 5007] 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 [RFC 3633] 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 RFC 2685
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 [RFC 2132] 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
[RFC 3315] or the OPTION_ERO [RFC 4994] 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 [RFC 3046] 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
[RFC 3118]. Potential exposures to attack are discussed in Section 7
of the DHCP protocol specification [RFC 2131].
Implementations should consider using the DHCPv4 Authentication
option [RFC 3118] 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 [RFC 4030]
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
[RFC 2131].
For use of the VSS option by DHCPv6, the Security Considerations
section of [RFC 3315] details the general threats to DHCPv6, and thus
to messages using the VSS option. The "Authentication of DHCP
Messages" section of [RFC 3315] 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
[RFC 3942].
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 [RFC 3046], in accordance with the spirit of
[RFC 3942]. While [RFC 3942] doesn't explicitly mention the sub-option
space for the DHCP Relay Agent Information option [RFC 3046],
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 [RFC 5226]. 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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC 2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP
Vendor Extensions", RFC 2132, March 1997.
[RFC 2685] Fox, B. and B. Gleeson, "Virtual Private Networks
Identifier", RFC 2685, September 1999.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
[RFC 3315] 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.
[RFC 3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC 4994] Zeng, S., Volz, B., Kinnear, K. and J. Brzozowski,
"DHCPv6 Relay Agent Echo Request Option", RFC 4994,
September 2007.
12.2. Informative References
[RFC 951] Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985.
[RFC 1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
[RFC 3118] Droms, R., Ed., and W. Arbaugh, Ed., "Authentication for
DHCP Messages", RFC 3118, June 2001.
[RFC 3942] 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
[RFC 4030] Stapp, M. and T. Lemon, "The Authentication Suboption for
the Dynamic Host Configuration Protocol (DHCP) Relay
Agent Option", RFC 4030, March 2005.
[RFC 4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", RFC 4388, February 2006.
[RFC 5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
"DHCPv6 Leasequery", RFC 5007, September 2007.
[RFC 5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC 5226] 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
RFC TOTAL SIZE: 60312 bytes
PUBLICATION DATE: Monday, April 30th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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