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Network Working Group F. Adrangi, Ed.
Request for Comments: 4093 Intel
Category: Informational H. Levkowetz, Ed.
Ericsson
August 2005
Problem Statement: Mobile IPv4 Traversal of
Virtual Private Network (VPN) Gateways
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Deploying Mobile-IP v4 in networks that are connected to the Internet
through a Virtual Private Network (VPN) gateway presents some
problems that do not currently have well-described solutions. This
document aims to describe and illustrate these problems, and to
propose some guidelines for possible solutions.
Table of Contents
1. Introduction ....................................................2
1.1. Overview of the Problem ....................................3
1.2. Specification of Requirements ..............................3
1.3. Terminology ................................................3
2. MIP and VPN Deployment Scenarios ................................4
2.1. MIPv4 HA(s) Inside the Intranet behind a VPN Gateway .......5
2.2. VPN Gateway and MIPv4 HA(s) on the VPN Domain Border .......6
2.3. Combined VPN Gateway and MIPv4 HA ..........................7
2.4. MIPv4 HA(s) Outside the VPN Domain .........................8
2.5. Combined VPN Gateway and MIPv4 HA(s) on the Local Link .....9
3. Deployment Scenarios Selection ..................................9
4. Problem Statement ..............................................10
4.1. Registering in Co-Located Mode ............................11
4.2. Registering via an FA .....................................12
4.3. Summary: MIP Incompatibilities with IPsec-Based
VPN Gateways ..............................................13
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
5. Solution Guidelines ............................................14
5.1. Preservation of Existing VPN Infrastructure ...............14
5.2. Software Upgrades to Existing VPN Client and Gateways .....14
5.3. IPsec Protocol ............................................14
5.4. Multi-Vendor Interoperability .............................14
5.5. MIPv4 Protocol ............................................15
5.6. Handoff Overhead ..........................................15
5.7. Scalability, Availability, Reliability, and Performance ...15
5.8. Functional Entities .......................................15
5.9. Implications of Intervening NAT Gateways ..................15
5.10. Security Requirements ....................................16
6. Security Considerations ........................................16
7. Acknowledgements ...............................................16
8. References .....................................................17
8.1. Normative References ......................................17
8.2. Informative References ....................................17
1. Introduction
Mobile IP [RFC3344] agents are being deployed in enterprise networks
to enable mobility across wired and wireless LANs while roaming
inside the enterprise Intranet. With the growing deployment of IEEE
802.11 access points ("hot spots") in public places such as hotels,
airports, and convention centers, and with wireless WAN data networks
such as General Packet Radio Service (GPRS), the need is increasing
for enabling mobile users to maintain their transport connections and
constant reachability while connecting back to their target "home"
networks protected by Virtual Private Network (VPN) technology. This
implies that Mobile IP and VPN technologies have to coexist and
function together in order to provide mobility and security to the
enterprise mobile users.
The goal of this document is to:
o Identify and describe practical deployment scenarios for Mobile IP
and VPN in enterprise and operator environments.
o Identify example usage scenarios for remote users roaming outside
the "home" network protected by a VPN gateway.
o Articulate the problems resulting from Mobile IP and VPN
coexistence.
o Specify a set of framework guidelines to evaluate proposed
solutions for supporting multi-vendor seamless IPv4 mobility
across IPsec-based VPN gateways.
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
1.1. Overview of the Problem
Access to the Intranet is typically guarded by both a firewall and a
VPN device. The Intranet can only be accessed by respecting the
security policies in the firewall and the VPN device.
When MIP is deployed in a corporate Intranet (also referred to as a
VPN domain), roaming between the Intranet (i.e., trusted domain) and
the Internet (i.e., untrusted domain) becomes problematic. It would
be desirable to have seamless session mobility between the two
domains, because MIP was designed for session mobility regardless of
the network point of attachment. Unfortunately, the current MIP
standards fall short of this promise for an important customer
segment: corporate users (using VPN for remote access) who desire to
add mobility support because of a need to have continuous access to
Intranet resources while roaming outside the Intranet from one subnet
to another, or between the VPN domain and the Internet.
From the beginning, one explicitly stated restriction was that it was
assumed that installed firewalls and VPN gateways had to be kept
unchanged, rather than replaced or upgraded, because they have much
wider deployments than MIP at the time of writing. Therefore, any
solutions would need to minimize the impact on existing VPN and
firewall deployments, related standards, and "de facto" standards.
1.2. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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].
1.3. Terminology
MIPv4 Mobile IP for IPv4 [RFC3344]
MIPv6 Mobile IP for IPv6
VPN Virtual Private Network
GW Gateway
VPN Domain An Intranet protected by a VPN gateway.
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
DMZ (Demilitarized Zone) A small network inserted as a
"neutral zone" between a company's private network and
the outside public network to prevent outside users
from getting direct access to the company's private
network.
Home Network A network, possibly virtual, having a network prefix
matching that of a mobile node's home address.
Home Agent A router on a mobile node's home network which tunnels
datagrams for delivery to the mobile node when it is
away from home, and maintains current location
information for the mobile node.
MN Refers to a mobile node that runs both MIP and IPsec-
based VPN client software.
MIPv4 inside IPsec-ESP tunnel
MIPv4 packets are encapsulated in an IPsec-ESP tunnel
established between the Mobile Node and the VPN
gateway.
IPsec-ESP inside MIPv4 tunnel
IPsec-ESP packets are encapsulated in a MIPv4 tunnel
established between the Mobile Node and the home agent.
2. MIP and VPN Deployment Scenarios
This section describes a set of deployment scenarios wherein MIP
agents and VPN gateways have to coexist to provide mobility and
security. The intention is to identify practical deployment
scenarios for MIP and VPNs where MIP technology might be extended to
solve problems resulting from the desire for co-existence.
The network topology in the following diagrams consists of an
Intranet connected to the public network (i.e., the Internet). Here,
the word "Intranet" refers to a private network (where private
addresses [RFC1918] are typically used) protected by a VPN gateway
and perhaps by a layer-3 transparent or non-transparent firewall.
When private addresses are used, the non-transparent firewall also
functions as a Network Address Translator (NAT) or Network Address
Port Translator (NAPT) bridging between the two address realms (i.e.,
the Intranet and the Internet).
Firewalls may be placed on either side of the VPN gateway; these are
referred to as inner and outer firewalls. The inner and outer
firewalls typically inspect outbound traffic (i.e., from the Intranet
to the Internet) and inbound traffic (i.e., from the Internet to the
Adrangi & Levkowetz Informational [Page 4]
RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
Intranet), respectively. When a firewall is present, it MUST be
configured to allow Mobile IP traffic (both control and tunneled data
packets) to go through. As our focus here is the relationship
between MIP and VPN, we have purposely omitted firewalls from the
following scenarios in order to keep things simple.
It is assumed that encryption is not enforced inside the VPN domain
because: 1) the VPN domain (Intranet) is viewed as a trusted network,
and users allowed inside the Intranet are also trusted, and 2) it is
a common VPN deployment practice where the VPN is used to guard the
Intranet resources from unauthorized users attached to an untrusted
network, and to provide a secure communication channel for authorized
users to access resources inside the Intranet from outside.
The following sub-sections introduce five representative combinations
of MIPv4 HA and VPN gateway placement.
In order to give a reasonably complete survey of MIPv4 and VPN co-
existence scenarios, those in Sections 2.3 and 2.5 are included even
though, as covered in more detail below, there are no co-existence
problems to be solved in these two cases.
2.1. MIPv4 HA(s) Inside the Intranet behind a VPN Gateway
MIPv4 HAs are deployed inside the Intranet protected by a VPN
gateway, and are not directly reachable by the MNs outside the
Intranet.
..Foreign Network.. .....VPN Domain..(Intranet).....
. . . .
. +----+ +----+ . +----+ +-------+ +-------+ .
. |MNs | | FA | . | VPN| | Router| | HA | .
. |away| | | .<=========>| | | 1..n | | 1..n | .
. +----+ +----+ . | GW | +-------+ +-------+ .
. . +----+ .
................... . +-------+ +-------+ .
. | CN | | MNs | .
. | 1..n | | home | .
. +-------+ +-------+ .
. .
................................
Figure 1
Direct application of MIPv4 standards [RFC3344] is successfully used
to provide mobility for users inside the Intranet. However, mobile
users outside the Intranet can only access the Intranet resources
(e.g., MIP agents) through the VPN gateway, which will allow only
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
authenticated IPsec traffic inside. This implies that the MIPv4
traffic has to run inside IPsec, which leads to two distinct
problems:
1. When the foreign network has an FA deployed (e.g., as in CDMA
2000), MIPv4 registration becomes impossible. This is because
the MIPv4 traffic between MN and VPN gateway is encrypted, and
the FA (which is likely in a different administrative domain)
cannot inspect the MIPv4 headers needed for relaying the MIPv4
packets. Please see Section 4.2 for more details.
2. In co-located mode, successful registration is possible but the
VPN tunnel has to be re-negotiated every time the MN changes its
point of network attachment.
These problems are articulated in Section 4.
This deployment scenario may not be common yet, but it is practical
and is becoming important as there is an increasing need for
providing corporate remote users with continuous access to the
Intranet resources.
2.2. VPN Gateway and MIPv4 HA(s) on the VPN Domain Border
A MIPv4 HA is deployed on the VPN domain border (e.g., in the DMZ)
together with the VPN gateway, and it is directly reachable by MNs
inside or outside the Intranet.
..Foreign Network.. .....VPN Domain..(Intranet).....
. . . .
. +----+ +----+ . +----+ +-------+ .
. |MNs | | FA | . | VPN| | Router| .
. |away| | | .<=========>| | | 1..n | .
. +----+ +----+ . /\ | GW | +-------+ .
. . || +----+ .
. . || +----+ +-------+ +-------+ .
. . ++====>| HA | | CN | | MNs | .
................... | | | 1..n | | home | .
+----+ +-------+ +-------+ .
. .
................................
Figure 2
Please note that in deployments where the security policy prohibits
direct communication between the MN (roaming outside the Intranet)
and outside machines, the HA can be configured to forward only
encrypted traffic from/to the MN.
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
The MIPv4 HA has a public interface connected to the Internet, and a
private interface attached to the Intranet. Mobile users will most
likely have a virtual home network associated with the MIPv4 HA's
private interface, so that the mobile users are always away from home
and thus registered with the MIPv4 HA. Furthermore, in deployments
where the VPN gateway and the HA are placed in a corporate DMZ, this
implies that MIPv4 traffic will always be routed through the DMZ
(regardless of whether MNs are located outside or inside the
Intranet), which may not be acceptable to IT departments in large
corporations.
This deployment can be used with two different configurations: "MIPv4
inside IPsec-ESP tunnel" and "IPsec-ESP inside MIPv4 tunnel". The
"MIPv4 inside IPsec-ESP tunnel" has the same problems as the scenario
in Section 2.1. (Namely, MIPv4 registration becomes impossible when
the registration is to be done via an FA, and furthermore, in co-
located mode, the VPN tunnel has to be re-negotiated every time the
MN changes its point of attachment.) The "IPsec-ESP inside MIPv4
tunnel" does not have the problems described in Section 2.1; however,
it will require some modifications to the routing logic of the MIPv4
HA or the VPN gateway.
2.3. Combined VPN Gateway and MIPv4 HA
This is similar to the deployment scenario described in Section 2.2,
with the exception that the VPN gateway and MIPv4 HA are running on
the same physical machine.
..Foreign Network.. .....VPN Domain..(Intranet).....
. . . .
. +----+ +----+ . +----+ +-------+ .
. |MNs | | FA | . | VPN| | Router| .
. |away| | | .<==========| GW | | 1..n | .
. +----+ +----+ . | + | +-------+ .
. . | HA | .
................... +----+ +-------+ +-------+ .
. | CN | | MNs | .
. | 1..n | | home | .
. +-------+ +-------+ .
. .
................................
Figure 3
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
Running MIPv4 HA and VPN on the same machine resolves routing-related
issues that exist in Section 2.2 when a "IPsec-ESP inside MIPv4
tunnel" configuration is used. However, it does not promote multi-
vendor interoperability in environments where MIPv4 HA and VPN
technologies must be acquired from different vendors.
2.4. MIPv4 HA(s) Outside the VPN Domain
In this scenario, MIPv4 HAs are deployed outside the Intranet (e.g.,
in an operator network), as depicted in Figure 4, below.
..Foreign Network.. .....VPN Domain..(Intranet).....
. . . .
. +----+ +----+ . +----+ +-------+ .
. |MNs | | FA | . | VPN| | Router| .
. |away| | | .<==========| GW | | 1..n | .
. +----+ +----+ . /\ | | +-------+ .
. . || | | .
................... || | | +-------+ +-------+ .
|| | | | CN | | MNs | .
.....MIPv4 Home.... || | | | 1..n | | home | .
. .<===++ | | +-------+ +-------+ .
. +------+ . +----+ .
. | HAs | . . .
. | 1..n | . ................................
. +------+ .
...................
Figure 4
The IPsec tunnel endpoints will be the MN and the VPN gateway. The
'home network' will most likely be a virtual home network, located at
the HA, through which authorized remote users (i.e., those that have
successfully established a connection to the corporate VPN) can reach
the Corporate Intranet and maintain their transport session
connectivity while roaming outside the Intranet from one subnet to
another. Please note that this deployment scenario does not support
mobility inside the Intranet.
In this case, it is most practical to run IPsec-ESP inside a MIPv4
tunnel (i.e., the MIPv4 tunnel endpoints are the MN and the HA; the
IPsec-ESP packet from the MN and to the VPN gateway is encapsulated
in the MIPv4 tunnel). This is because the MNs can register with the
HA without establishing an IPsec tunnel to the VPN gateway.
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
2.5. Combined VPN Gateway and MIPv4 HA(s) on the Local Link
This is similar to the deployment scenario described in Section 2.3,
with the difference that the VPN gateway/HA is sitting on the local
link. In this case, the VPN gateway and HA would most naturally be
co-located in the same box, although this is in no way a requirement.
The VPN/HA is assumed to be reachable from the external network;
i.e., it is assumed to have a public IP address, and the firewall is
assumed to be configured to allow direct access to the VPN/HA from
the external network.
..Foreign Network.. .....VPN Domain..(Intranet).....
. . . .
. +----+ +----+ . +------+ +-------+ +-------+ .
. |MNs | | FA | . | Fire | | Router| | VPN/HA| .
. |away| | | .<=======>| wall | | 1..n | | 1..n | .
. +----+ +----+ . | | +-------+ +-------+ .
. . | NAT | .
................... +------+ +-------+ +-------+ .
. | CN | | MNs | .
. | 1..n | | home | .
. +-------+ +-------+ .
. .
................................
Figure 5
This deployment works today without any technical problems with
IPsec-ESP running inside a MIPv4 tunnel. If you were to run MIPv
inside the IPsec-ESP tunnel, it would have the same problems as in
Section 2.1, so it is deployed with the IPsec-ESP running inside the
MIPv4 tunnel. This deployment is not practical for large deployments
(on the order of thousands of users) because of the large and
distributed security perimeter.
3. Deployment Scenarios Selection
The deployment scenarios described in Section 2 were evaluated to
identify those most in need of solving. The evaluation was done
based on two main criteria: 1) Is the deployment scenario common and
practical? and 2) Does the deployment scenario reveal any problems
resulting from MIPv4 and VPN coexistence?
The authors believe that the scenario in Section 2.1 is the most
important and practical one because of a rising need for providing
corporate remote users with continuous access to their Intranet
resources. After analyzing each scenario, one realizes that problems
Adrangi & Levkowetz Informational [Page 9]
RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
occurring in scenarios in Sections 2.2 and 2.4 are either the same as
those in the scenario in Section 2.1 or a subset of them. Therefore,
solving the scenario in Section 2.1 will also solve the scenarios in
Sections 2.2 and 2.4. The scenarios in Sections 2.3 and 2.5 do not
introduce functional problems resulting from MIPv4 and VPN co-
existence, and thus there is no need to seek a solution. A solution
for the deployment scenario in Section 2.1 is therefore seen as
essential, and this in turn can also be applied to solve problems in
other scenarios. In subsequent sections, we will articulate the
roaming scenarios, the problems, and the solution guidelines relevant
to the scenario in Section 2.1.
4. Problem Statement
This section describes roaming scenarios corresponding to the
deployment scenario in Section 2.1 where an MN needs to have
continuous access to the Intranet resources regardless of whether it
is roaming inside or outside the Intranet, and their associated
problems. The scenarios are constructed based on a multi-subnetted,
MIPv4-enabled Intranet (hereafter referred to as Intranet or VPN
domain) protected by an IPsec-based VPN gateway as depicted in
Figure 6.
....Internet....... .....VPN Domain..(Intranet).....
. . . .
. +----+ . +----+ +-------+ +-------+ .
. |MNs | . | VPN| | Router| | VPN/HA| .
. |away| .<=========>| | | 1..n | | 1..n | .
. +----+ . | GW | +-------+ +-------+ .
. . +----+ .
................... . +-------+ +-------+ .
. | CN | | MNs | .
. | 1..n | | home | .
. +-------+ +-------+ .
. .
................................
Figure 6: Intranet protected by a VPN gateway
The Intranet, as depicted in Figure 6, may include both wired (IEEE
802.3) and IEEE 802.11 wireless LAN deployments. However, it is also
possible to see IEEE 802.11 deployments outside the Intranet due to
the perceived lack of current 802.11 security, as depicted in
Figure 7.
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
....Internet....... .....VPN Domain..(Intranet).....
. . . .
. +----+ . +----+ +-------+ +-------+ .
. |MNs | . | VPN| | Router| | VPN/HA| .
. |away| .<=========>| | | 1..n | | 1..n | .
. +----+ . | GW | +-------+ +-------+ .
. . | | .
................... | | +-------+ +-------+ .
| | | CN | | MNs | .
..802.11 Wireless.. <====>| | | 1..n | | home | .
. Network . +----+ +-------+ +-------+ .
. . . .
................... ................................
Figure 7: IEEE 802.11 Wireless deployment outside the home network
4.1. Registering in Co-Located Mode
In co-located mode, the IPsec tunnel endpoints would be at the MN and
the VPN gateway, which (supposing we have the scenario described in
Section 2.1) results in the mobile-ip tunnel from MN to HA being
encapsulated inside the IPsec tunnel. See Figure 8 below. This
scenario is still possible, but has some major drawbacks.
....Internet....... .....VPN Domain..(Intranet).....
. . . .
. +----+ . +----+ +-------+ +-------+ .
. |MNs | . | VPN| | Router| | VPN/HA| .
. |away|<###################>| |-----| 1..n |->| 1..n | .
. +----+ . \ | GW | +-------+ +-------+ .
. . \ +----+ .
................... mip . +-------+ +-------+ .
inside . | CN | | MNs | .
IPsec . | 1..n | | home | .
. +-------+ +-------+ .
. .
................................
Figure 8
The MN obtains an address at its point of attachment (via DHCP
[RFC2131] or some other means), and then sets up an IPsec tunnel to
the VPN gateway, after which it can successfully register with its HA
through the IPsec tunnel. The IPsec tunnel SA (Security Association)
is identified by a triplet consisting of SPI (Security Parameter
Index), MN's IP destination address (i.e., the address obtained at
the point of attachment), and Security Protocol (AH or ESP)
Identifier as described in [RFC2401]. This means that as the MN's IP
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
destination address changes on each IP subnet handoff, the IPsec
tunnel needs to be re-established. This could have noticeable
performance implications on real-time applications and in resource-
constrained wireless networks. In effect, we don't have mobility
support for the tunnel endpoint changes associated with MN movements.
4.2. Registering via an FA
In the case where a mobile node is in a network where mobility
support is provided through the use of an FA, and no DHCP allocated
address and co-located mode is possible, we run into severe trouble.
This is illustrated in Figure 9 and explained below:
..Foreign Network.. .....VPN Domain..(Intranet).....
. . . .
. +----+ +----+ . +----+ +-------+ +-------+ .
. |MNs | | FA | . | VPN| | Router| | VPN/HA| .
. |away|<??| |<###########>| |-----| 1..n |->| 1..n | .
. +----+ \ +----+ . \ | GW | +-------+ +-------+ .
. \ . \ +----+ .
...........\....... mip . +-------+ +-------+ .
\ inside . | CN | | MNs | .
MN expects IPsec . | 1..n | | home | .
IPsec traffic . +-------+ +-------+ .
. .
................................
Figure 9
When arriving at the visited network on the left in this figure, the
MN has to reach the FA with registration requests in order to have
the FA send them on to the HA. However, the MN in all likelihood
cannot register with the FA because the registration requests will be
sent encrypted, and the FA will not be able to decrypt them. If the
MN would have a policy that allowed split tunneling so that it could
reach the FA with clear text messages, then the FA would still not be
able to get through the VPN gateway unless the HA is reachable from
outside and the Intranet security policy allows MIP registration
packets to bypass the VPN gateway.
Even if the HA is reachable and the MIP registration succeeds, the FA
(which is likely in a different administrative domain) will not be
able to relay packets between the MN and the VPN gateway. Packets
from the MN will be encapsulated by the FA with IP-in-IP [RFC2003],
which the VPN gateway will drop, and packets from the VPN gateway
will have ESP payloads (with IP-in-IP inside), which the FA will drop
(as it expects IP-in-IP-encapsulated traffic to the MN).
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The use of a 'trusted FA' has also been suggested in this scenario,
meaning an FA that is actually a combined VPN GW and FA. The
scenario will work fine in this case, as the tunnel end-points are at
the FA and the VPN gateway as shown in Figure 10 below. However, we
cannot expect that the FA in access networks (e.g., wireless hot-
spots or CDMA 2000 networks) will have security associations with any
given corporate network, so this is not particularly realistic in the
general mobility case.
..Foreign Network.. .....VPN Domain..(Intranet).....
. . . .
. +----+ +----+ . +----+ +-------+ +-------+ .
. | FA | | VPN| . | VPN| | Router| | VPN/HA| .
. | |<--| GW |<###########>| |-----| 1..n |->| 1..n | .
. +----+ +----+ . \ | GW | +-------+ +-------+ .
. | . \ +----+ .
. +----+ . mip . +-------+ +-------+ .
. |MNs | . inside . | CN | | MNs | .
. |away| . IPsec . | 1..n | | home | .
. +----+ . . +-------+ +-------+ .
................... . .
................................
Figure 10
Furthermore, this solution would leave the traffic between FA and MN
unprotected, and as this link in particular may be a wireless link,
this is clearly undesirable.
4.3. Summary: MIP Incompatibilities with IPsec-Based VPN Gateways
An MN roaming outside the Intranet has to establish an IPsec tunnel
to its home VPN gateway first, in order to be able to register with
its home agent. This is because the MN cannot reach its HA (inside
the private protected network) directly from the outside. This
implies that the MIPv4 traffic from the MN to a node inside the
Intranet is forced to run inside an IPsec tunnel, and thus that it
will not be in the clear. This in turn leads to two distinct
problems depending on whether the MN uses co-located or non-co-
located modes to register with its HA.
In co-located mode, the IPsec tunnel needs to be re-established on
each IP subnet handoff, which will have performance implications on
real-time applications and resource-constrained wireless networks.
In non-co-located mode (i.e., using an FA care-of address), the
problem becomes severe, as the MN may be unable to register with its
HA through the FA because the FA cannot understand MIPv4 registration
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
requests if they are encrypted in the IPsec tunnel (i.e., split
tunneling is not supported). Even if the MN could reach the FA with
non-encrypted registration requests (i.e., split tunneling is
supported), and the requests going from the FA to the HA can pass
through the VPN gateway, there would still be a problem with routing
of data packets between the Intranet and the internet. This is
because the VPN will not allow IP-in-IP-encapsulated packets from the
FA to go through. And furthermore, ESP-encapsulated packets from the
VPN gateway to the MN will be dropped by the FA, as it expects IP-
in-IP-encapsulated traffic to the MN.
5. Solution Guidelines
This section describes guidelines for a solution to MIPv4 traversal
across VPN gateways.
5.1. Preservation of Existing VPN Infrastructure
o The solution MUST work with currently deployed VPN gateways. This
is the whole raison d'etre of this investigation: Finding a way
to deploy Mobile-IP in cases where a VPN solution is already in
place.
5.2. Software Upgrades to Existing VPN Client and Gateways
o The solution SHOULD minimize changes to existing VPN
client/gateway software.
5.3. IPsec Protocol
o The solution SHOULD NOT require any changes to existing IPsec or
key-exchange standard protocols implemented by VPN gateways.
o The solution SHOULD NOT require that the VPN gateway or the VPN
client implement any new protocols in addition to the existing
standard protocols.
5.4. Multi-Vendor Interoperability
o The solution MUST provide multi-vendor interoperability, whereby
MIPv4 mobility agents, mobility clients (MN), VPN server, and VPN
client solutions may come from four different vendors. This is
typical for medium and large enterprises that purchase and deploy
best-of-breed multi-vendor solutions for IP routing, VPNs,
firewalls, etc.
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5.5. MIPv4 Protocol
o The solution MUST adhere to MIPv4 protocol [RFC3344]. That is,
the solution MUST NOT impose any changes that violate MIPv4
protocol.
o The solution MAY introduce new extensions to MIPv4 nodes per
guidelines specified in the MIPv4 protocol [RFC3344]. However, in
order to overcome barriers to deployment, it is highly desirable
to avoid any changes to MIPv4 mobility agents such as the FA and
HA.
o The solution MAY require more than one instance of MIPv4 running
in parallel (multiple encapsulation).
5.6. Handoff Overhead
o It is imperative to keep the key management overhead down to a
minimum, in order to support fast handoffs across IP subnets.
Therefore, the solution MUST propose a mechanism to avoid or
minimize IPsec tunnel SA renegotiation and IKE renegotiation as
the MN changes its current point of network attachment.
5.7. Scalability, Availability, Reliability, and Performance
o The solution complexity MUST increase at most linearly with the
number of MNs registered and accessing resources inside the
Intranet.
o The solution MAY introduce additional header or tunneling overhead
if needed.
5.8. Functional Entities
o The solution MAY introduce new MIPv4-compliant functional
entities.
5.9. Implications of Intervening NAT Gateways
o The solution MUST be able to work with the existing MIPv4 and
IPsec NAT traversal solutions [RFC3519] [RFC3715] [RFC3947].
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5.10. Security Requirements
o The solution MUST provide security that is not inferior to what is
already provided to existing "nomadic computing" remote access
users; i.e., for confidentiality, authentication, message
integrity, protection against replay attacks, and related security
services.
6. Security Considerations
This document describes an existing problem and proposes guidelines
for possible solutions; as such, its security implications are
indirect, through the guidelines it proposes for the solutions.
Section 5.10 gives the relevant security requirements.
7. Acknowledgements
The authors who contributed text to this document were, in no
particular order: Farid Adrangi, Milind Kulkarni, Gopal Dommety, Eli
Gelasco, Qiang Zhang, Sami Vaarala, Dorothy Gellert, Nitsan Baider,
and Henrik Levkowetz.
The authors would like to thank other contributors, especially
Prakash Iyer, Mike Andrews, Ranjit Narjala, Joe Lau, Kent Leung,
Alpesh Patel, Phil Roberts, Hans Sjostrand, Serge Tessier, Antti
Nuopponen, Alan O'Neill, Gaetan Feige, and Brijesh Kumar, for their
feedback and help in improving this document.
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8. References
8.1. Normative References
[RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.
8.2. Informative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC3519] Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of
Network Address Translation (NAT) Devices", RFC 3519, May
2003.
[RFC3715] Aboba, B. and W. Dixon, "IPsec-Network Address Translation
(NAT) Compatibility Requirements", RFC 3715, March 2004.
[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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Authors' Addresses
Farid Adrangi
Intel Corporation
2111 N.E. 25th Avenue
Hillsboro OR
USA
Phone: +1 503-712-1791
EMail: farid.adrangi@intel.com
Henrik Levkowetz
Ericsson Research
Torshamsgatan 23
SE-164 80 Stockholm
SWEDEN
Phone: +46 7 08 32 16 08
EMail: henrik@levkowetz.com
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RFC 4093 MIPv4 VPN Traversal Problem Statement August 2005
Full Copyright Statement
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