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RFC 4093 - Problem Statement: Mobile IPv4 Traversal of Virtual P


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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

   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.

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.

   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

   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

   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.

   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

   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.

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

   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.

     ....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

   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).

   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

   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.

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].

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.

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.

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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