Network Working Group V. Devarapalli

Network Working Group V. Devarapalli
Request for Comments: 5266 Wichorus
BCP: 136 P. Eronen
Category: Best Current Practice Nokia
June 2008

Secure Connectivity and Mobility Using Mobile IPv4 and
IKEv2 Mobility and Multihoming (MOBIKE)

Status of This Memo

This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.

Abstract

Enterprise users require mobility and secure connectivity when they
roam and connect to the services offered in the enterprise. Secure
connectivity is required when the user connects to the enterprise
from an untrusted network. Mobility is beneficial when the user
moves, either inside or outside the enterprise network, and acquires
a new IP address. This document describes a solution using Mobile
IPv4 (MIPv4) and mobility extensions to IKEv2 (MOBIKE) to provide
secure connectivity and mobility.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Access Modes . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1. Access Mode: 'c' . . . . . . . . . . . . . . . . . . . 6
3.1.2. Access Mode: 'f' . . . . . . . . . . . . . . . . . . . 6
3.1.3. Access Mode: 'mc' . . . . . . . . . . . . . . . . . . 6
3.2. Mobility within the Enterprise . . . . . . . . . . . . . . 7
3.3. Mobility When outside the Enterprise . . . . . . . . . . . 7
3.4. Crossing Security Boundaries . . . . . . . . . . . . . . . 7
3.4.1. Operation When Moving from an Untrusted Network . . . 8
3.4.2. Operation When Moving from a Trusted Network . . . . . 9
4. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Applicability to a Mobile Operator Network . . . . . 13

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1. Introduction

A typical enterprise network consists of users connecting to the
services from a trusted network (intranet), and from an untrusted
network (Internet). The trusted and untrusted networks are typically
separated by a demilitarized zone (DMZ). Access to the intranet is
controlled by a firewall and a Virtual Private Network (VPN) gateway
in the DMZ.

Enterprise users, when roaming on untrusted networks, most often have
to authenticate themselves to the VPN gateway and set up a secure
tunnel in order to access the intranet. The use of IPsec VPNs is
very common to enable such secure connectivity to the intranet. When
the user is on the trusted network, VPNs are not used. However, the
users benefit tremendously when session mobility between subnets,
through the use of Mobile IPv4, is available.

There has been some work done on using Mobile IPv4 and IPsec VPNs to
provide roaming and secure connectivity to an enterprise [RFC5265]
[RFC4093]. The solution described in [RFC5265] was designed with
certain restrictions, including requiring no modifications to the VPN
gateways, and involves the use of two layers of MIPv4, with one home
agent inside the intranet and one in the Internet or in the DMZ
before the VPN gateway. The per-packet overhead is very high in this
solution. It is also challenging to implement and have two instances
of MIPv4 active at the same time on a mobile node. However, the
solution described here is only applicable when Internet Key Exchange
Protocol version 2 (IKEv2) IPsec VPNs are used.

This document describes an alternate solution that does not require
two layers of MIPv4. The solution described in this document uses
Mobile IPv4 when the mobile node is on the trusted network and
MOBIKE-capable IPsec VPNs when the mobile node is on the untrusted
network. The mobile node uses the tunnel inner address (TIA) given
out by the IPsec VPN gateway as the co-located care-of address (CoA)
for MIPv4 registration. This eliminates the need for using an
external MIPv4 home agent and the need for encapsulating the VPN
tunnel inside a MIPv4 tunnel.

The following assumptions are made for the solution described in this
document.

o IKEv2 [RFC4306] and IPsec [RFC4301] are used to set up the VPN
tunnels between the mobile node and the VPN gateway.

o The VPN gateway and the mobile node support MOBIKE extensions as
defined in [RFC4555].

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o When the mobile node is on the trusted network, traffic should not
go through the DMZ. Current deployments of firewalls and DMZs
consider the scenario where only a small amount of the total
enterprise traffic goes through the DMZ. Routing through the DMZ
typically involves stateful inspection of each packet by the
firewalls in the DMZ. Moreover, the DMZ architecture assumes that
the DMZ is less secure than the internal network. Therefore, the
DMZ-based architecture allows the least amount of traffic to
traverse the DMZ, that is, only traffic between the trusted
network and the external network. Requiring all normal traffic to
the mobile nodes to traverse the DMZ would negate this
architecture.

o When the mobile node is on the trusted network and uses a wireless
access technology, confidentiality protection of the data traffic
is provided by the particular access technology. In some
networks, confidentiality protection MAY be available between the
mobile node and the first hop access router, in which case it is
not required at layer 2.

This document also presents a solution for the mobile node to detect
when it is on a trusted network, so that the IPsec tunnel can be
dropped and the mobile node can use Mobile IP in the intranet.

IPsec VPN gateways that use IKEv1 [RFC2409] are not addressed in this
document.

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 [RFC2119].

Many of the following terms are defined in [RFC5265], but are
repeated here to make this document self-contained.

FA: Mobile IPv4 foreign agent.

Co-CoA: co-located care-of address.

FA-CoA: foreign agent care-of address.

FW: firewall.

i-FA: Mobile IPv4 foreign agent residing in the trusted (intranet)
network.

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i-HA: Mobile IPv4 home agent residing in the trusted (intranet)
network.

i-MIP: The mobile node uses the home agent in the internal network.

VPN-TIA: VPN tunnel inner address. This address is given out by the
VPN gateway during IKE negotiation and is routable in the trusted
network.

mVPN: VPN with MOBIKE functionality.

The following access modes are used in explaining the protocol. The
access modes are explained in more detail in [RFC5265].

f: i-MIP with FA-CoA

c: i-MIP with Co-CoA

mc: i-MIP with MOBIKE-enabled VPN, with VPN-TIA as Co-CoA

3. Solution Overview

The mobile node is configured with a home address that remains the
same irrespective of whether the mobile node is inside or outside the
enterprise network. The mobile node is also reachable at the same
home address irrespective of its current point of attachment. When
the mobile node is connected to the intranet directly, it uses Mobile
IP for internal mobility.

When the mobile node roams and connects to an untrusted network
outside the enterprise, it sets up a VPN tunnel to the VPN gateway.
However, it still maintains a valid binding cache entry at the i-HA.
It uses the VPN-TIA, allocated by the VPN gateway, as the co-located
CoA for registration with the i-HA. If the VPN-TIA changes or if the
mobile node moves and connects to another VPN gateway, then it sends
a Registration Request to the i-HA using the new co-located CoA.

If the mobile node moves while outside the enterprise and its access
network changes, it uses the MOBIKE protocol to update the VPN
gateway of its current address. The internal home agent is not aware
of the mobile node's movement as long as the mobile node is attached
to the same VPN gateway and the TIA remains the same.

Figure 1 depicts the network topology assumed for the solution. It
also shows the possible mobile node locations and access modes.

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{home} (MN) [i-HA]
\ /
.-+---+-.
( )
[mVPN] `--+----'
! !
.--+--. [R]
( DMZ ) !
.-+-------+--. `--+--' .-----+------.
( ) ! ( )
( external net +---[R]----[FW]----[R]--+ internal net )
( ) ( )
`--+---------' `---+---+----'
/ / \
[DHCP] [R] [DHCP] [R] [R] [i-FA]
\ / \ / \ /
.+--+---. .-+-+--. .--+--+-.
( ) ( ) ( )
`---+---' `--+---' `---+---'
! ! !
(MN) {mc} (MN) {c} (MN) {f}

Figure 1: Network Topology Using MIPv4 and MOBIKE

The solution described above results in a Mobile IP tunnel inside an
IPsec tunnel. The Mobile IP tunnel is between the mobile node and
the home agent, and the IPsec tunnel is between the mobile node (MN)
and the mVPN gateway. The mobile node MUST reverse tunnel through
the home agent [RFC3024] when the Mobile IP tunnel is inside an IPsec
tunnel.

The overhead of running a Mobile IP tunnel inside an IPsec tunnel can
be avoided by having the Mobile IP foreign agent functionality on the
VPN gateway. This is out of scope for this document and is further
described in [MEGHANA].

Whenever the mobile node attaches to a new link, it may encounter a
foreign agent. The mobile node MUST not use the foreign agent
care-of address with the i-HA when attached to an untrusted access
network. The default behavior for the mobile node is to always
configure an address from the access link using DHCP. The mobile
node then checks if it is attached to a trusted access network by
sending a Registration Request to the i-HA in the co-located care-of
address mode. If the mobile node discovers that it is attached to a
trusted access network, then it MAY start using a foreign agent
care-of address with the i-HA. In order to do this, the mobile node
has to perform a new registration with the i-HA.

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The mobile node can use a foreign agent on a untrusted access
network, if there is an external home agent that the mobile node is
able to use. The use of an external home agent in the untrusted
access network and a home agent in the trusted access network at the
same time is described in detail in [RFC5265].

Some IPsec VPN implementations allow a host to send traffic directly
to the Internet when attached to an untrusted network. This traffic
bypasses the IPsec tunnel with the VPN gateway. This document does
not prevent such traffic from being sent out from the host, but there
will be no mobility or session continuity for the traffic. Any data
traffic that is sent through the Mobile IP tunnel with the home agent
is always sent through the VPN gateway.

3.1. Access Modes

The following access modes are used in the solution described in this
document.

3.1.1. Access Mode: 'c'

This access mode is standard Mobile IPv4 [RFC3344] with a co-located
care-of address. The mobile node must detect that it is connected to
an internal trusted network before using this mode. The co-located
care-of address is assigned by the access network to which the mobile
node is attached.

3.1.2. Access Mode: 'f'

This access mode is standard Mobile IPv4 [RFC3344] with a foreign
agent care-of address. The mobile node can use this mode only when
it detects that it is connected to an internal trusted network and
also detects a foreign agent on the access network.

3.1.3. Access Mode: 'mc'

This access mode involves using both Mobile IPv4 and a MOBIKE-enabled
IPsec VPN gateway, resulting in a Mobile IP tunnel inside an IPsec
tunnel. The mobile node uses the VPN-TIA as the co-located CoA for
registering with the home agent. This mode is used only when the
mobile node is attached to an untrusted network and is required to
set up an IPsec tunnel with a VPN gateway to gain access to the
trusted network.

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3.2. Mobility within the Enterprise

When the mobile node is inside the enterprise network and attached to
the intranet, it uses Mobile IPv4 [RFC3344] for subnet mobility. The
mobile node always configures a care-of address through DHCP on the
access link and uses it as the co-located care-of address. The
mobile node MAY use a foreign agent care-of address, if a foreign
agent is available. However, the foreign agent care-of address is
used only when the mobile node is attached to the trusted access
network. The mobile node attempts Foreign Agent discovery and CoA
address acquisition through DHCP simultaneously in order to avoid the
delay in discovering a foreign agent when there is no foreign agent
available. The mobile node maintains a valid binding cache entry at
all times at the home agent mapping the home address to the current
CoA. Whenever the mobile node moves, it sends a Registration Request
to update the binding cache entry.

The Mobile IP signaling messages between the mobile node and the home
agent are authenticated as described in [RFC3344].

The mobile node maintains a valid binding cache entry at the home
agent even when it is outside the enterprise network.

3.3. Mobility When outside the Enterprise

When the mobile node is attached to an untrusted network, it sets up
an IPsec VPN tunnel with the VPN gateway to gain access to the
enterprise network. If the mobile node moves and its IP address
changes, it initiates the MOBIKE protocol [RFC4555] to update the
address on the VPN gateway.

The mobile node maintains a binding at the home agent even when it is
outside the enterprise network. If the TIA changes due to the mobile
node re-connecting to the VPN gateway or attaching to a different VPN
gateway, the mobile node should send a Registration Request to its
home agent to update the binding cache with the new TIA.

3.4. Crossing Security Boundaries

Security boundary detection is based on the reachability of the i-HA
from the mobile node's current point of attachment. Whenever the
mobile node detects a change in network connectivity, it sends a
Registration Request to the i-HA without any VPN encapsulation. If
the mobile node receives a Registration Reply with the Trusted
Networks Configured (TNC) extension from the i-HA, then it assumes
that it is on a trusted network. The TNC extension is described in
[RFC5265]. The mobile node MUST check that the Registration Reply is
integrity protected using the mobile node-home agent mobility

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security association before concluding it is attached to a trusted
network. This security boundary detection is based on the mechanism
described in [RFC5265] to detect attachment to the internal trusted
network. The mobile node should re-transmit the Registration Request
if it does not receive the Registration Reply within a timeout
period. The number of times the mobile node should re-transmit the
Registration Request and the timeout period for receiving the
Registration Reply are configurable on the mobile node.

When the mobile node is attached to an untrusted network and is using
an IPsec VPN to the enterprise network, the ability to send a
Registration Request to the i-HA without VPN encapsulation would
require some interaction between the IPsec and MIPv4 modules on the
mobile node. This is local to the mobile node and out of scope for
this document.

If the mobile node has an existing VPN tunnel to its VPN gateway, it
MUST send a MOBIKE message at the same time as the registration
request to the i-HA whenever the IP address changes. If the mobile
node receives a response from the VPN gateway, but not from the i-HA,
it assumes it is outside the enterprise network. If it receives a
response from the i-HA, then it assumes it is inside the enterprise
network.

There could also be some out-of-band mechanisms that involve
configuring the wireless access points with some information that the
mobile node can recognize as access points that belong to the trusted
network in an enterprise network. Such mechanisms are beyond the
scope of this document.

The mobile node should not send any normal traffic while it is trying
to detect whether it is attached to the trusted or untrusted network.
This is described in more detail in [RFC5265].

3.4.1. Operation When Moving from an Untrusted Network

When the mobile node is outside the enterprise network and attached
to an untrusted network, it has an IPsec VPN tunnel with its mobility
aware VPN gateway, and a valid registration with a home agent on the
intranet with the VPN-TIA as the care-of address.

If the mobile node moves and its IP address changes, it performs the
following steps:

1a. Initiate an IKE mobility exchange to update the VPN gateway with
the current address. If the new network is also untrusted, this
will be enough for setting up the connectivity. If the new
network is trusted, and if the VPN gateway is reachable, this

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exchange will allow the mobile node to keep the VPN state alive
while on the trusted side. If the VPN gateway is not reachable
from inside, then this exchange will fail.

1b. At the same time as step 1, send a Mobile IPv4 Registration
Request to the internal home agent without VPN encapsulation.

2. If the mobile node receives a Registration Reply to the request
sent in step 1b, then the current subnet is a trusted subnet,
and the mobile node can communicate without VPN tunneling. The
mobile node MAY tear down the VPN tunnel.

3.4.2. Operation When Moving from a Trusted Network

When the mobile node is inside the enterprise and attached to the
intranet, it does not use a VPN tunnel for data traffic. It has a
valid binding cache entry at its home agent. If the VPN gateway is
reachable from the trusted network, the mobile node MAY have valid
IKEv2 security associations with its VPN gateway. The IPsec security
associations can be created when required. The mobile node may have
to re-negotiate the IKEv2 security associations to prevent them from
expiring.

If the mobile node moves and its IP address changes, it performs the
following steps:

1. Initiate an IKE mobility exchange to update the VPN gateway with
the current address, or if there is no VPN connection, then
establish a VPN tunnel with the gateway from the new local IP
address. If the new network is trusted, and if the VPN gateway
is reachable, this exchange will allow the mobile node to keep
the VPN state alive, while in the trusted side. If the new
network is trusted and if the VPN gateway is not reachable from
inside, then this exchange will fail.

2. At the same time as step 1, send a Mobile IPv4 Registration
Request to the internal home agent without VPN encapsulation.

3. If the mobile node receives a Registration Reply to the request
sent in step 2, then the current subnet is a trusted subnet, and
the mobile node can communicate without VPN tunneling, using only
Mobile IP with the new care-of address.

4. If the mobile node didn't receive the response in step 3, and if
the VPN tunnel is successfully established and registered in step
1, then the mobile node sends a Registration Request over the VPN
tunnel to the internal home agent. After receiving a
Registration Reply from the home agent, the mobile node can start

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communicating over the VPN tunnel with the Mobile IP home
address.

4. NAT Traversal

There could be a Network Address Translation (NAT) device between the
mobile node and the home agent in any of the access modes, 'c', 'f',
and 'mc', and between the mobile node and the VPN gateway in the
access mode 'mc'. Mobile IPv4 NAT traversal, as described in
[RFC3519], should be used by the mobile node and the home agent in
access modes 'c' or 'f', when there is a NAT device present. When
using access mode, 'mc', IPsec NAT traversal [RFC3947] [RFC3948]
should be used by the mobile node and the VPN gateway, if there is a
NAT device present. Typically, the TIA would be a routable address
inside the enterprise network. But in some cases, the TIA could be
from a private address space associated with the VPN gateway. In
such a case, Mobile IPv4 NAT traversal should be used in addition to
IPsec NAT traversal in the 'mc' mode.

5. Security Considerations

Enterprise connectivity typically requires very strong security, and
the solution described in this document was designed keeping this in
mind.

Security concerns related to the mobile node detecting that it is on
a trusted network and thereafter dropping the VPN tunnel are
described in [RFC5265].

When the mobile node sends a Registration Request to the i-HA from an
untrusted network that does not go through the IPsec tunnel, it will
reveal the i-HA's address, its own identity including the NAI and the
home address, and the Authenticator value in the authentication
extensions to the untrusted network. This may be a concern in some
deployments.

Please see [RFC4555] for MOBIKE-related security considerations, and
[RFC3519], [RFC3947] for security concerns related to the use of NAT
traversal mechanisms for Mobile IPv4 and IPsec.

6. Acknowledgments

The authors would like to thank Henry Haverinen, Sandro Grech, Dhaval
Shah, and John Cruz for their participation in developing this
solution.

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The authors would also like to thank Henrik Levkowetz, Jari Arkko, TJ
Kniveton, Vidya Narayanan, Yaron Sheffer, Hans Sjostrand, Jouni
Korhonen, and Sami Vaarala for reviewing the document.

7. References

7.1. Normative References

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

[RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.

[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, June 2006.

[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.

[RFC5265] Vaarala, S. and E. Klovning, "Mobile IPv4 Traversal across
IPsec-Based VPN Gateways", RFC 5265, June 2008.

7.2. Informative References

[RFC4093] Adrangi, F. and H. Levkowetz, "Problem Statement: Mobile
IPv4 Traversal of Virtual Private Network (VPN) Gateways",
RFC 4093, August 2005.

[RFC3024] Montenegro, G., "Reverse Tunneling for Mobile IP,
revised", RFC 3024, January 2001.

[MEGHANA] Sahasrabudhe, M. and V. Devarapalli, "Optimizations to
Secure Connectivity and Mobility", Work in Progress,
February 2008.

[RFC3519] Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of
Network Address Translation (NAT) Devices", RFC 3519,
April 2003.

[RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947,
January 2005.

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[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.

[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.

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Appendix A. Applicability to a Mobile Operator Network

The solution described in this document can also be applied to a
Mobile Operator's network when the Operator deploys heterogeneous
access networks and some of the access networks are considered as
trusted networks and others as untrusted networks. Figure 2
illustrates such a network topology.

+----------------------+
| +----+ |
+----------------+ | |i-HA| |
| | | +----+ |
(MN)----+ trusted +---+ |
| access network | | internal network |
+----------------+ | |
| |
+----------+-----------+
|
|
|
[mVPN]
+----------------+ |
| | |
(MN)----+ untrusted +--------------+
{mc} | access network |
+----------------+

Figure 2: Network Topology of a Mobile Operator with Trusted and
Untrusted Networks

An IPsec VPN gateway provides secure connectivity to the Operator's
internal network for mobile nodes attached to an untrusted access
network. The VPN gateway supports MOBIKE extensions so that the
IPsec tunnels survive any IP address change when the mobile node
moves while attached to the untrusted access networks.

When the mobile node is attached to the trusted access network, it
uses Mobile IP with the i-HA. It uses the IP address obtained from
the trusted access network as the co-located care-of address to
register with the i-HA. If a foreign agent is available in the
trusted access network, the mobile node may use a foreign agent
care-of address. If the mobile node moves and attaches to an
untrusted access network, it sets up an IPsec tunnel with the VPN
gateway to access the Operator's internal network. It uses the IPsec
TIA as the co-located care-of address to register with the i-HA
thereby creating a Mobile IP tunnel inside an IPsec tunnel.

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When the mobile node is attached to the trusted access network, it
can either be attached to a foreign link in the trusted network or to
the home link directly. This document does not impose any
restrictions.

Authors' Addresses

Vijay Devarapalli
Wichorus
3590 North First Street
San Jose, CA 95134
USA

EMail: [email protected]

Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland

EMail: [email protected]

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