Internet Engineering Task Force (IETF) R. Droms

Internet Engineering Task Force (IETF) R. Droms
Request for Comments: 6276 P. Thubert
Category: Standards Track Cisco
ISSN: 2070-1721 F. Dupont
Internet Systems Consortium
W. Haddad
Ericsson
C. Bernardos
UC3M
July 2011

DHCPv6 Prefix Delegation for Network Mobility (NEMO)

Abstract

One aspect of network mobility support is the assignment of a prefix
or prefixes to a mobile router for use on the links in the mobile
network. This document specifies how DHCPv6 prefix delegation can be
used for this configuration task. The mobile router plays the role
of requesting router, while the home agent assumes the role of
delegating router. When the mobile router is outside its home
network, the mobile router also assumes the role of DHCPv6 relay
agent, co-located with the requesting router function.

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/rfc6276.

Copyright Notice

Copyright (c) 2011 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

Droms, et al. Standards Track [Page 1]

RFC 6276 DHCPv6 Prefix Delegation for NEMO July 2011

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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DHCPv6 Prefix Delegation of Mobile Network Prefixes . . . . . 4
3.1. Exchanging DHCPv6 Messages When the Mobile Router Is
Not at Home . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Relay Agent Configuration . . . . . . . . . . . . . . 7
3.1.2. Transmission of DHCPv6 Messages . . . . . . . . . . . 8
3.1.3. Receipt of DHCPv6 Messages . . . . . . . . . . . . . . 8
3.2. Exchanging DHCPv6 Messages When the Mobile Router Is
at Home . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Selecting a Home Agent That Provides DHCPv6PD . . . . . . 9
3.4. Minimizing DHCPv6PD Messages . . . . . . . . . . . . . . . 10
3.5. Other DHCPv6 Functions . . . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . . 13

1. Introduction

One aspect of network mobility support is the assignment of a prefix
or prefixes to a mobile router for use on the links in Network
Mobility (NEMO). DHCPv6 prefix delegation (DHCPv6PD) [RFC3633] can
be used for this configuration task.

The model of operation of DHCPv6PD for prefix delegation is as
follows [RFC3633]. A delegating router is provided IPv6 prefixes to
be delegated to requesting routers. A requesting router requests
prefix(es) from the delegating router. The delegating router chooses
prefix(es) for delegation, and responds with prefix(es) to the
requesting router. The requesting router is then responsible for the
delegated prefix(es). Note that DHCPv6 options for prefix delegation
defined in [RFC3633] have been defined for general use across
routers, and not only for mobile routers running the NEMO Basic
Support protocol [RFC3963].

To use DHCPv6PD as a prefix assignment mechanism in mobile networks,
when the mobile router is located at home, the home agent assumes the
role of the delegating router and the mobile router assumes the role

Droms, et al. Standards Track [Page 2]

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of the requesting router. However, when the mobile router is away
from home, in addition to the roles when the mobile router is located
at home, the mobile router also assumes the role of a DHCPv6 relay
agent co-located with the requesting router function.

The DHCPv6PD server running at the home agent is provisioned with
prefixes to be assigned using any of the prefix assignment mechanisms
described in the DHCPv6PD specification [RFC3633].

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

The following terms used in this document are defined in the IPv6
Addressing Architecture document [RFC4291]:

Link-Local Unicast address

Link-Local Scope Multicast address

The following terms used in this document are defined in the Mobile
IPv6 specification [RFC6275]:

Home Agent (HA)

Home Link

Home Address (HoA)

Care-of Address (CoA)

Binding Update (BU)

Binding Acknowledgement (BA)

The following terms used in this document are defined in the Mobile
Network terminology document [RFC4885]:

Mobile Router (MR)

Mobile Network (NEMO)

Mobile Network Prefix (MNP)

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The following terms used in this document are defined in the DHCPv6
[RFC3315] and DHCPv6 prefix delegation [RFC3633] specifications:

Delegating Router (DR; acts as a DHCPv6 server)

Requesting Router (RR; acts as a DHCPv6 client)

DHCPv6 Relay Agent (DRA)

The following acronym is used in this document:

DHCPv6PD: DHCPv6 Prefix Delegation

3. DHCPv6 Prefix Delegation of Mobile Network Prefixes

The NEMO Basic Support protocol [RFC3963] extends the Mobile IPv6
protocol [RFC6275] to enable network mobility. With the NEMO Basic
Support protocol, a mobile router uses Mobile IPv6 to establish and
maintain a session with its home agent and uses bidirectional
tunneling between the mobile router and the home agent to provide a
path through which nodes attached to links in the mobile network can
maintain connectivity with nodes not in the NEMO.

The requirements for Network Mobility [RFC4885] include the ability
of the mobile router to receive delegated prefixes that can then be
assigned to links in the mobile network. DHCPv6PD can be used to
meet this requirement for prefix delegation.

To use DHCPv6PD for mobile networks, when the mobile router is
located at home, the home agent assumes the role of the delegating
router and the mobile router assumes the role of the requesting
router. However, when the mobile router is away from home, in
addition to the roles when the mobile router is located at home, the
mobile router also assumes the role of a DHCPv6 relay agent
co-located with the requesting router function.

When the mobile router is not at home, the home agent and the mobile
router exchange DHCPv6PD protocol messages as specified in [RFC6275].
This means that the messages sent by the mobile router MUST include
the Home Address destination option and messages sent by the home
agent MUST make use of a Routing Header type 2. See Figure 1 for the
deployment topologies when the MR is at home and when it is visiting
a foreign network.

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RFC 6276 DHCPv6 Prefix Delegation for NEMO July 2011

------ ------
| MR |----------------| HA |
|(RR)| (home network) |(DR)|
------ ------

------- /-----------\ ------
| MR |----| Internet |-----| HA |
|(RR) | \-----------/ |(DR)|
|(DRA)| ------
-------
(visited network)

Figure 1: Deployment topologies of the use of DHCPv6PD for delegation
of Mobile Network Prefixes

The DHCPv6PD server is provisioned with prefixes to be assigned using
any of the prefix assignment mechanisms described in the DHCPv6PD
specifications. Other updates to the home agent data structures
required as a side effect of prefix delegation are specified by the
particular network mobility protocol. For example, in the case of
NEMO Basic Network Mobility Support [RFC3963], the HA would add an
entry in its binding cache registering the delegated prefix to the
mobile router to which the prefix was delegated.

3.1. Exchanging DHCPv6 Messages When the Mobile Router Is Not at Home

The case when the mobile router is away from home is described in
this section. Section 3.2 describes the protocol operation for the
case when the mobile router is attached to its home link.

The mobile router MUST register at the home agent (i.e., by sending a
Binding Update to the home agent) before initiating a DHCPv6 message
exchange for prefix delegation. The mobile router MUST use implicit
BU signaling, since the mobile router may not have yet requested any
prefixes.

If the mobile router does not have any active delegated prefixes
(with unexpired leases), the mobile router MUST initiate a DHCPv6
message exchange with a DHCPv6 Solicit message as described in
Section 17 of [RFC3315] and Section 11.1 of [RFC3633]. The
delegating router at the home agent responds with an Advertise
message. Then, the mobile router MUST request a set of prefixes by
sending a Request message. The delegating router includes the
delegated prefixes in a Reply message. Note that in this case, the
mobile router has previously sent a Binding Update to the home agent
without knowing yet the set of prefixes that it can use as mobile
network prefixes. The home agent, upon reception of the implicit
Binding Update from the mobile router, MUST select (in case this was

Droms, et al. Standards Track [Page 5]

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not pre-configured already) the prefixes that would then be delegated
to the mobile router via DHCPv6PD. The home agent, once the DHCPv6
signaling has been completed, MUST add an entry in its binding cache
including the delegated prefixes.

In case the mobile router has one or more active delegated prefixes
-- for example, as if the mobile router reboots or the mobile network
prefix(es) currently used by the mobile router is about to expire --
the mobile router MUST initiate a DHCPv6 message exchange with a
DHCPv6 Rebind message as described in Section 18.1.2 of [RFC3315] and
Section 12.1 of [RFC3633].

A DHPCv6 relay agent function [RFC3315] MUST be used at the mobile
router. This relay agent function is co-located in the mobile router
with the DHCPv6 client function (see Figure 2). The DHCPv6 signaling
between the mobile router and the home agent is exchanged between the
DHCPv6 relay agent in the mobile router and the DHCPv6 server on the
home agent. DHCPv6 messages from the mobile router to the home agent
are unicast packets sent from the unicast home address of the mobile
router to the global unicast address of the home agent, and therefore
the Home Address destination option MUST be used. DHCPv6 replies
from the home agent to the mobile router MUST be sent using the
Routing Header type 2, as specified in [RFC6275]. The DHCPv6 client
in the mobile router MUST hand any outbound DHCPv6 messages to the
co-located relay agent. Responses from the DHCPv6 server are
delivered to the relay agent function in the mobile router, which
MUST extract the encapsulated message and deliver it to the DHCPv6
client in the mobile router.

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----------------------------- --------
| MR | | HA |
| (RR) (DRA) | | (DR) |
---------------------------- --------
| | Binding Update |
| |------------------------>|
| | (HoA, CoA) |
| | |
| | Binding Ack |
| |<------------------------|
| | |
| DHCPv6 Solicit | DHCPv6 Solicit |
|..................>|--=====================->|
| | |
| DHCPv6 Advertise | DHCPv6 Advertise |
|<..................|<-=====================--|
| | |
| DHCPv6 Request | DHCPv6 Request |
|..................>|--=====================->|
| | |
| DHCPv6 Reply | DHCPv6 Reply |
|<..................|<-=====================--|
| | (Mobile Network Prefix) |
| | |

Figure 2: Signaling sequence when the mobile router is not at home

Note that a mobile router using DHCPv6PD to obtain the set of
prefixes to be used as mobile network prefixes cannot derive its home
address from one of its mobile network prefix(es) (as the mobile
router does not know them before registering to the home agent).
Therefore, the mobile router MUST assign its home address from the
prefix on its Home Link.

3.1.1. Relay Agent Configuration

The use of the relay agent function in the mobile router allows the
mobile router to unicast DHCPv6 messages to the DHCPv6 server. The
relay agent MUST be configured with the address of the DHCPv6 server.
For the purposes of this specification, the relay agent assumes that
the home agent for the mobile router hosts the DHCPv6 server.
Therefore, the mobile router MUST configure the DHCPv6 relay agent to
forward DHCPv6 messages to the home agent.

The DHCPv6 specification supports in certain scenarios the use of
unicast between the client and the server. However, its use presents
some difficulties, as the client has to first receive a Server
Unicast option (Section 22.12 of [RFC3315]) from the server, which

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means that a Solicit/Advertise message exchange is required in
advance. That signaling exchange would require the presence of a
relay agent on the mobile router, and therefore little gain would be
achieved in this case from the use of the Server Unicast option.

3.1.2. Transmission of DHCPv6 Messages

When the DHCPv6 client in the mobile router sends a message, it MUST
hand the message to the DHCPv6 relay agent in the mobile router. The
way in which the message is passed to the DHCP relay agent is beyond
the scope of this document. The relay agent encapsulates the message
from the client according to [RFC3315] in a Relay-forward message and
sends the resulting DHCPv6 message to the home agent. The relay
agent sets the fields in the Relay-forward message as follows:

msg-type RELAY-FORW

hop-count 1

link-address The home address of the mobile router

peer-address The home address of the mobile router

options MUST include a "Relay Message option" [RFC3315]; MAY
include other options added by the relay agent.

3.1.3. Receipt of DHCPv6 Messages

Messages from the DHCPv6 server will be returned to the DHCPv6 relay
agent, with the message for the DHCPv6 client encapsulated in the
Relay Message option [RFC3315] in a Relay-reply message. The relay
agent function MUST extract the message for the client from the Relay
Message option and hand the message to the DHCPv6 client in the
mobile router. The way in which the message is passed to the client
is beyond the scope of this document.

3.2. Exchanging DHCPv6 Messages When the Mobile Router Is at Home

When the mobile router is on its home link, the home agent MUST use
the home link to exchange DHCPv6PD messages with the mobile router
(Figure 3). In this case, the DHCPv6 co-located relay function MUST
be disabled. It is the responsibility of the implementation to
determine when the mobile router is on its home link. The Home Link
Detection mechanism is described in Section 11.5.2 of [RFC6275].

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RFC 6276 DHCPv6 Prefix Delegation for NEMO July 2011

-------- --------
| MR | | HA |
| (RR) | | (DR) |
-------- --------
| |
| DHCPv6 Solicit |
|------------------------>|
| |
| DHCPv6 Advertise |
|<------------------------|
| |
| DHCPv6 Request |
|------------------------>|
| |
| DHCPv6 Reply |
|<------------------------|
| (Mobile Network Prefix) |
| |

Figure 3: Signaling sequence for the case the home agent is at home

3.3. Selecting a Home Agent That Provides DHCPv6PD

Not all nodes that are willing to act as a home agent are required to
provide DHCPv6PD. Therefore, when selecting a home agent, a mobile
router that requires DHCPv6PD service MUST identify a home agent that
will provide the service. The mobile router can determine if a home
agent provides DHCPv6PD by initiating a DHCPv6 message exchange
(i.e., sending a Solicit message) in which the mobile router requests
delegated prefix(es). If the home agent does not respond or responds
but does not delegate any prefix(es) in its response, the mobile
router assumes that the home agent does not provide DHCPv6PD service.
The mobile router continues to query all candidate home agents until
it finds one that provides DHCPv6PD. Note that in this particular
case and if the mobile router is away from home, the mobile router
has to have already performed a Mobile IPv6 registration with the
home agent it queries.

Querying a home agent to determine if it provides DHCPv6PD requires
different operational variables than those recommended by the DHCPv6
specification. [RFC3315] recommends that under normal circumstances,
a host will continue to send DHCPv6 Solicit messages until it
receives a response (see Section 17 of [RFC3315]), i.e., the Maximum
Retransmission Duration (MRD) and Maximum Retransmission Count (MRC)
are both set to zero. However, a home agent may not respond to the
Solicit messages from the mobile router because the home agent does
not support DHCPv6 prefix delegation. Therefore, when querying a
home agent to determine if the home agent provides DHCPv6PD service,

Droms, et al. Standards Track [Page 9]

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it is RECOMMENDED that MRD and MRC be set to non-zero values so that
the mobile router discontinues sending Solicit messages to the home
agent after sending 6 Solicit messages, and conclude that the home
agent will not provide DHCPv6PD service. Sending 6 queries provides
enough reliability for scenarios in which the wireless connectivity
is lost for a short period after sending the first Binding Update
message.

It is RECOMMENDED that the mobile router uses a sequential probing of
the home agents for DHCPv6PD service.

3.4. Minimizing DHCPv6PD Messages

The use DHCPv6PD in a mobile network can be combined with the Rapid
Commit option [RFC3315] to provide DHCPv6 prefix delegation with a
two-message exchange between the mobile router and the DHCPv6PD
delegating router.

3.5. Other DHCPv6 Functions

The DHCPv6 messages exchanged between the mobile router and the home
agent MAY also be used for other DHCPv6 functions in addition to
DHCPv6PD. For example, the home agent MAY assign global addresses to
the mobile router and MAY pass other configuration information such
as a list of available DNS recursive name servers [RFC3646] to the
mobile router using the same DHCPv6 messages as used for DHCPv6PD.

The home agent MAY act as a DHCPv6 relay agent for mobile nodes while
it acts as a delegating router for mobile routers.

4. Security Considerations

This document describes the use of DHCPv6 for prefix delegation in
mobile networks. In addition to the security considerations for
DHCPv6 described in the "Security Considerations" section of the
DHCPv6 base specification [RFC3315] and the "Security Considerations"
of the DHCPv6 Prefix Delegation specification [RFC3633], there are
two aspects that need to be considered.

First, the NEMO Basic Support specification requires the home agent
to prevent a mobile router from claiming mobile network prefixes
belonging to another mobile router. Upon reception of an implicit
Binding Update from a mobile router, the home agent MUST only add
prefixes into the mobile router's Binding Cache Entry if the mobile
router has a valid DHCPv6 Prefix Delegation lease for said prefixes.
If the mobile router does not have a valid DHCPv6 Prefix Delegation
lease, the home agent MUST NOT add any prefixes into the mobile
router's Binding Cache Entry. Upon the mobile router obtaining a

Droms, et al. Standards Track [Page 10]

RFC 6276 DHCPv6 Prefix Delegation for NEMO July 2011

valid DHCPv6 Prefix Delegation lease for a given set of prefixes, the
home agent MUST add these prefixes to the mobile router's Binding
Cache Entry. This avoids the home agent forwarding traffic addressed
to prefixes that have not been yet delegated to the mobile router.

The use of DHCPv6, as described in this document, requires message
integrity protection and source authentication. When the mobile
router is at home, normal DHCPv6 operation is used between the mobile
router and the home agent and therefore this specification does not
add any new security issue. While the mobile router is away from
home, the IPsec security mechanism mandated by Mobile IPv6 [RFC3776]
MUST be used to secure the DHCPv6 signaling. In the following, we
describe the Security Policy Database (SPD) and Security Association
Database (SAD) entries necessary to protect the DHCPv6 signaling. We
use the same format used by [RFC4877]. The SPD and SAD entries are
only example configurations. A particular mobile router
implementation and a home agent implementation could configure
different SPD and SAD entries as long as they provide the required
security of the DHCPv6 signaling messages.

For the examples described in this document, a mobile router with
home address "home_address_1", and a home agent with address
"home_agent_1" are assumed. If the home address of the mobile router
changes, the SPD and SAD entries need to be re-created or updated for
the new home address.

mobile router SPD-S:
- IF local_address = home_address_1 &
remote_address = home_agent_1 & proto = UDP &
local_port = any & remote_port = DHCP
Then use SA1 (OUT) and SA2 (IN)

mobile router SAD:
- SA1(OUT, spi_a, home_agent_1, ESP, TRANSPORT):
local_address = home_address_1 &
remote_address = home_agent_1 &
proto = UDP & remote_port = DHCP
- SA2(IN, spi_b, home_address_1, ESP, TRANSPORT):
local_address = home_agent_1 &
remote_address = home_address_1 &
proto = UDP & local_port = DHCP

home agent SPD-S:
- IF local_address = home_agent_1 &
remote_address = homa_address_1 & proto = UDP &
local_port = DHCP & remote_port = any
Then use SA2 (OUT) and SA1 (IN)

Droms, et al. Standards Track [Page 11]

RFC 6276 DHCPv6 Prefix Delegation for NEMO July 2011

home agent SAD:
- SA2(OUT, spi_b, home_address_1, ESP, TRANSPORT):
local_address = home_agent_1 &
remote_address = home_address_1 &
proto = UDP & local_port = DHCP
- SA1(IN, spi_a, home_agent_1, ESP, TRANSPORT):
local_address = home_address_1 &
remote_address = home_agent_1 &
proto = UDP & remote_port = DHCP

5. Acknowledgments

The authors would like to thank people who have given valuable
comments on the mailing list. Specific suggestions from Ryuji
Wakikawa, George Tsirtsis, Alexandru Petrescu, Vijay Devarapalli, and
Marcelo Bagnulo were incorporated into this document.

The authors would like to thank Julien Laganier, Michaela Vanderveen,
and Jean-Michel Combes for their review of previous versions of this
document.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.

[RFC3776] Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling Between Mobile Nodes and
Home Agents", RFC 3776, June 2004.

[RFC3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
Thubert, "Network Mobility (NEMO) Basic Support Protocol",
RFC 3963, January 2005.

Droms, et al. Standards Track [Page 12]

RFC 6276 DHCPv6 Prefix Delegation for NEMO July 2011

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.

[RFC4877] Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with
IKEv2 and the Revised IPsec Architecture", RFC 4877,
April 2007.

[RFC6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
in IPv6", RFC 6275, July 2011.

6.2. Informative References

[RFC4885] Ernst, T. and H-Y. Lach, "Network Mobility Support
Terminology", RFC 4885, July 2007.

Droms, et al. Standards Track [Page 13]

RFC 6276 DHCPv6 Prefix Delegation for NEMO July 2011

Authors' Addresses

Ralph Droms
Cisco
1414 Massachusetts Avenue
Boxborough, MA 01719
USA

Phone: +1 978.936.1674
EMail: [email protected]

Pascal Thubert
Cisco
Village d'Entreprises Green Side
400, Avenue Roumanille
Biot - Sophia Antipolis 06410
FRANCE

EMail: [email protected]

Francis Dupont
Internet Systems Consortium

EMail: [email protected]

Wassim Haddad
Ericsson
6210 Spine Road
Boulder, CO 80301
USA

Phone: +1 303.473.6963
EMail: [email protected]

Carlos J. Bernardos
Universidad Carlos III de Madrid
Av. Universidad, 30
Leganes, Madrid 28911
Spain

Phone: +34 91624 6236
EMail: [email protected]
URI: http://www.it.uc3m.es/cjbc/

Droms, et al. Standards Track [Page 14]