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rfc:rfc5265

Network Working Group S. Vaarala Request for Comments: 5265 Codebay Category: Standards Track E. Klovning

                                                              Birdstep
                                                             June 2008
       Mobile IPv4 Traversal across IPsec-Based VPN Gateways

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document outlines a solution for the Mobile IPv4 (MIPv4) and
 IPsec coexistence problem for enterprise users.  The solution
 consists of an applicability statement for using Mobile IPv4 and
 IPsec for session mobility in corporate remote access scenarios, and
 a required mechanism for detecting the trusted internal network
 securely.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.3.  Related Work . . . . . . . . . . . . . . . . . . . . . . .  5
   1.4.  Terms and Abbreviations  . . . . . . . . . . . . . . . . .  5
   1.5.  Requirement Levels . . . . . . . . . . . . . . . . . . . .  6
   1.6.  Assumptions and Rationale  . . . . . . . . . . . . . . . .  7
   1.7.  Why IPsec Lacks Mobility . . . . . . . . . . . . . . . . .  8
 2.  The Network Environment  . . . . . . . . . . . . . . . . . . .  9
   2.1.  Access Mode: 'c' . . . . . . . . . . . . . . . . . . . . . 12
   2.2.  Access Mode: 'f' . . . . . . . . . . . . . . . . . . . . . 13
   2.3.  Access Mode: 'cvc' . . . . . . . . . . . . . . . . . . . . 13
   2.4.  Access Mode: 'fvc' . . . . . . . . . . . . . . . . . . . . 14
   2.5.  NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . 14
 3.  Internal Network Detection . . . . . . . . . . . . . . . . . . 15
   3.1.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . 16
   3.2.  Implementation Requirements  . . . . . . . . . . . . . . . 16
     3.2.1.  Separate Tracking of Network Interfaces  . . . . . . . 16
     3.2.2.  Connection Status Change . . . . . . . . . . . . . . . 16
     3.2.3.  Registration-Based Internal Network Detection  . . . . 17

Vaarala & Klovning Standards Track [Page 1] RFC 5265 MIPv4-VPN June 2008

     3.2.4.  Registration-Based Internal Network Monitoring . . . . 17
   3.3.  Proposed Algorithm . . . . . . . . . . . . . . . . . . . . 19
   3.4.  Trusted Networks Configured (TNC) Extension  . . . . . . . 20
   3.5.  Implementation Issues  . . . . . . . . . . . . . . . . . . 20
   3.6.  Rationale for Design Choices . . . . . . . . . . . . . . . 21
     3.6.1.  Firewall Configuration Requirements  . . . . . . . . . 21
     3.6.2.  Registration-Based Internal Network Monitoring . . . . 22
     3.6.3.  No Encryption When Inside  . . . . . . . . . . . . . . 22
   3.7.  Improvements . . . . . . . . . . . . . . . . . . . . . . . 22
 4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 23
   4.1.  Mobile Node Requirements . . . . . . . . . . . . . . . . . 23
   4.2.  VPN Device Requirements  . . . . . . . . . . . . . . . . . 23
   4.3.  Home Agent Requirements  . . . . . . . . . . . . . . . . . 24
 5.  Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   5.1.  Comparison against Guidelines  . . . . . . . . . . . . . . 24
   5.2.  Packet Overhead  . . . . . . . . . . . . . . . . . . . . . 26
   5.3.  Latency Considerations . . . . . . . . . . . . . . . . . . 27
   5.4.  Firewall State Considerations  . . . . . . . . . . . . . . 27
   5.5.  Intrusion Detection Systems (IDSs) . . . . . . . . . . . . 28
   5.6.  Implementation of the Mobile Node  . . . . . . . . . . . . 28
   5.7.  Non-IPsec VPN Protocols  . . . . . . . . . . . . . . . . . 28
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   6.1.  Internal Network Detection . . . . . . . . . . . . . . . . 29
   6.2.  Mobile IPv4 versus IPsec . . . . . . . . . . . . . . . . . 30
 7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
 8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 32
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 33
 Appendix A.  Packet Flow Examples  . . . . . . . . . . . . . . . . 34
   A.1.  Connection Setup for Access Mode 'cvc' . . . . . . . . . . 34

Vaarala & Klovning Standards Track [Page 2] RFC 5265 MIPv4-VPN June 2008

1. Introduction

 The Mobile IP working group set out to explore the problem and
 solution spaces of IPsec and Mobile IP coexistence.  The problem
 statement and solution requirements for Mobile IPv4 case were first
 documented in [RFC4093].  This document outlines a solution for IPv4.
 The document contains two parts:
 o  a basic solution that is an applicability statement of Mobile IPv4
    and IPsec to provide session mobility between enterprise intranets
    and external networks, intended for enterprise mobile users; and
 o  a technical specification and a set of requirements for secure
    detection of the internal and the external networks, including a
    new extension that must be implemented by a mobile node and a home
    agent situated inside the enterprise network.
 There are many useful ways to combine Mobile IPv4 and IPsec.  The
 solution specified in this document is most applicable when the
 assumptions documented in the problem statement [RFC4093] are valid;
 among others that the solution:
 o  must minimize changes to existing firewall/VPN/DMZ (DeMilitarized
    Zone) deployments;
 o  must ensure that traffic is not routed through the DMZ when the
    mobile node is inside (to avoid scalability and management
    issues);
 o  must support foreign networks with only foreign agent access;
 o  should not require changes to existing IPsec or key exchange
    protocols;
 o  must comply with the Mobile IPv4 protocol (but may require new
    extensions or multiple instances of Mobile IPv4); and
 o  must propose a mechanism to avoid or minimize IPsec re-negotiation
    when the mobile node moves.

1.1. Overview

 Typical corporate networks consist of three different domains: the
 Internet (untrusted external network), the intranet (trusted internal
 network), and the DMZ, which connects the two networks.  Access to
 the internal network is guarded both by a firewall and a VPN device;

Vaarala & Klovning Standards Track [Page 3] RFC 5265 MIPv4-VPN June 2008

 access is only allowed if both firewall and VPN security policies are
 respected.
 Enterprise mobile users benefit from unrestricted seamless session
 mobility between subnets, regardless of whether the subnets are part
 of the internal or the external network.  Unfortunately, the current
 Mobile IPv4 and IPsec standards alone do not provide such a service
 [tessier].
 The solution is to use standard Mobile IPv4 (except for a new
 extension used by the home agent in the internal network to aid in
 network detection) when the mobile node is in the internal network,
 and to use the VPN tunnel endpoint address for the Mobile IPv4
 registration when outside.  IPsec-based VPN tunnels require re-
 negotiation after movement.  To overcome this limitation, another
 layer of Mobile IPv4 is used underneath IPsec, in effect making IPsec
 unaware of movement.  Thus, the mobile node can freely move in the
 external network without disrupting the VPN connection.
 Briefly, when outside, the mobile node:
 o  detects that it is outside (Section 3);
 o  registers its co-located or foreign agent care-of address with the
    external home agent;
 o  establishes a VPN tunnel using, e.g., Internet Key Exchange
    Protocol (IKE) (or IKEv2) if security associations are not already
    available;
 o  registers the VPN tunnel address as its co-located care-of address
    with the internal home agent; this registration request is sent
    inside the IPsec tunnel.
 The solution requires control over the protocol layers in the mobile
 node.  It must be capable of (1) detecting whether it is inside or
 outside in a secure fashion, and (2) controlling the protocol layers
 accordingly.  For instance, if the mobile node is inside, the IPsec
 layer needs to become dormant.
 Except for the new Mobile IPv4 extension to improve security of
 internal network detection, current Mobile IPv4 and IPsec standards,
 when used in a suitable combination, are sufficient to implement the
 solution.  No changes are required to existing VPN devices or foreign
 agents.
 The solution described is compatible with different kinds of IPsec-
 based VPNs, and no particular kind of VPN is required.  Because the

Vaarala & Klovning Standards Track [Page 4] RFC 5265 MIPv4-VPN June 2008

 appropriate Security Policy Database (SPD) entries and other IKE and
 IPsec specifics differ between deployed IPsec-based VPN products,
 these details are not discussed in the document.

1.2. Scope

 This document describes a solution for IPv4 only.  The downside of
 the described approach is that an external home agent is required and
 that the packet overhead (see Section 5) and overall complexity
 increase.  Optimizations would require significant changes to Mobile
 IPv4 and/or IPsec, and are out of scope of this document.
 VPN, in this document, refers to an IPsec-based remote access VPN.
 Other types of VPNs are out of scope.

1.3. Related Work

 Related work has been done on Mobile IPv6 in [RFC3776], which
 discusses the interaction of IPsec and Mobile IPv6 in protecting
 Mobile IPv6 signaling.  The document also discusses dynamic updating
 of the IPsec endpoint based on Mobile IP signaling packets.
 The "transient pseudo-NAT" attack, described in [pseudonat] and
 [mipnat], affects any approach that attempts to provide security of
 mobility signaling in conjunction with NAT devices.  In many cases,
 one cannot assume any cooperation from NAT devices, which thus have
 to be treated as any other networking entity.
 The IKEv2 Mobility and Multihoming Protocol (MOBIKE) [RFC4555]
 provides better mobility for IPsec.  This would allow the external
 Mobile IPv4 layer described in this specification to be removed.
 However, deploying MOBIKE requires changes to VPN devices, and is
 thus out of scope of this specification.

1.4. Terms and Abbreviations

 co-CoA:   co-located care-of address.
 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.
 external network:   the untrusted network (i.e., Internet).  Note
    that a private network (e.g., another corporate network) other
    than the mobile node's internal network is considered an external
    network.

Vaarala & Klovning Standards Track [Page 5] RFC 5265 MIPv4-VPN June 2008

 FA:   mobile IPv4 foreign agent.
 FA-CoA:   foreign agent care-of address.
 FW:   firewall.
 internal network:   the trusted network; for instance, a physically
    secure corporate network where the i-HA is located.
 i-FA:   Mobile IPv4 foreign agent residing in the internal network.
 i-HA:   Mobile IPv4 home agent residing in the internal network;
    typically has a private address [privaddr].
 i-HoA:   home address of the mobile node in the internal home agent.
 MN:   mobile node.
 NAI:   Network Access Identifier [RFC4282].
 R:   router.
 VPN:   Virtual Private Network based on IPsec.
 VPN-TIA:   VPN tunnel inner address, the address(es) negotiated
    during IKE phase 2 (quick mode), assigned manually, using IPsec-
    DHCP [RFC3456], using the "de facto" standard Internet Security
    Association and Key Management Protocol (ISAKMP) configuration
    mode, or by some other means.  Some VPN clients use their current
    care-of address as their Tunnel Inner Address (TIA) for
    architectural reasons.
 VPN tunnel:   an IPsec-based tunnel; for instance, IPsec tunnel mode
    IPsec connection, or Layer 2 Tunneling Protocol (L2TP) combined
    with IPsec transport connection.
 x-FA:   Mobile IPv4 foreign agent residing in the external network.
 x-HA:   Mobile IPv4 home agent residing in the external network.
 x-HoA:   home address of the mobile node in the external home agent.

1.5. Requirement Levels

 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 BCP 14, RFC 2119
 [RFC2119].

Vaarala & Klovning Standards Track [Page 6] RFC 5265 MIPv4-VPN June 2008

1.6. Assumptions and Rationale

 The solution is an attempt to solve the problem described in
 [RFC4093].  The major assumptions and their rationale is summarized
 below.
 Changes to existing firewall and VPN deployments should be minimized:
 o  The current deployment of firewalls and IPsec-based VPNs is much
    larger than corresponding Mobile IPv4 elements.  Thus, a solution
    should work within the existing VPN infrastructure.
 o  Current enterprise network deployments typically centralize
    management of security and network access into a compact DMZ.
 When the mobile node is inside, traffic should not go through the DMZ
 network:
 o  Routing all mobile node traffic through the DMZ is seen as a
    performance problem in existing deployments of firewalls.  The
    more sophisticated firewall technology is used (e.g., content
    scanning), the more serious the performance problem is.
 o  Current deployments of firewalls and DMZs in general have been
    optimized for the case where only a small minority of total
    enterprise traffic goes through the DMZ.  Furthermore, users of
    current VPN remote access solutions do not route their traffic
    through the DMZ when connected to an internal network.
 A home agent inside the enterprise cannot be reached directly from
 outside, even if the home agent contains IPsec functionality:
 o  Deployment of current combined IPsec/MIPv4 solutions are not
    common in large installations.
 o  Doing decryption in the home agents "deep inside" the enterprise
    effectively means having a security perimeter much larger than the
    typical, compact DMZ used by a majority of enterprises today.
 o  In order to maintain a security level equal to current firewall/
    DMZ deployments, every home agent decapsulating IPsec would need
    to do the same firewalling as the current DMZ firewalls (content
    scanning, connection tracking, etc.).

Vaarala & Klovning Standards Track [Page 7] RFC 5265 MIPv4-VPN June 2008

 Traffic cannot be encrypted when the mobile node is inside:
 o  There is a considerable performance impact on home agents (which
    currently do rather light processing) and mobile nodes (especially
    for small devices).  Note that traffic throughput inside the
    enterprise is typically an order (or more) of magnitude larger
    than the remote access traffic through a VPN.
 o  Encryption consumes processing power and has a significant impact
    on device battery life.
 o  There is also a usability issue involved; the user needs to
    authenticate the connection to the IPsec layer in the home agent
    to gain access.  For interactive authentication mechanisms (e.g.,
    SecurID), this always means user interaction.
 o  Furthermore, if there is a separate VPN device in the DMZ for
    remote access, the user needs to authenticate to both devices, and
    might need to have separate credentials for both.
 o  Current Mobile IPv4 home agents do not typically incorporate IPsec
    functionality, which is relevant for the solution when we assume
    zero or minimal changes to existing Mobile IPv4 nodes.
 o  Note, however, that the assumption (no encryption when inside)
    does not necessarily apply to all solutions in the solution space;
    if the above mentioned problems were resolved, there is no
    fundamental reason why encryption could not be applied when
    inside.

1.7. Why IPsec Lacks Mobility

 IPsec, as currently specified [RFC4301], requires that a new IKE
 negotiation be done whenever an IPsec peer moves, i.e., changes
 care-of address.  The main reason is that a security association is
 unidirectional and identified by a triplet consisting of (1) the
 destination address (which is the outer address when tunnel mode is
 used), (2) the security protocol (Encapsulating Security Payload
 (ESP) or Authentication Header (AH)), and (3) the Security Parameter
 Index (SPI) ([RFC4301], Section 4.1).  Although an implementation is
 not required to use all of these for its own Security Associations
 (SAs), an implementation cannot assume that a peer does not.
 When a mobile IPsec peer sends packets to a stationary IPsec peer,
 there is no problem; the SA is "owned" by the stationary IPsec peer,
 and therefore the destination address does not need to change.  The
 (outer) source address should be ignored by the stationary peer
 (although some implementations do check the source address as well).

Vaarala & Klovning Standards Track [Page 8] RFC 5265 MIPv4-VPN June 2008

 The problem arises when packets are sent from the stationary peer to
 the mobile peer.  The destination address of this SA (SAs are
 unidirectional) is established during IKE negotiation, and is
 effectively the care-of address of the mobile peer at time of
 negotiation.  Therefore, the packets will be sent to the original
 care-of address, not a changed care-of address.
 The IPsec NAT traversal mechanism can also be used for limited
 mobility, but UDP tunneling needs to be used even when there is no
 NAT in the route between the mobile and the stationary peers.
 Furthermore, support for changes in current NAT mapping is not
 required by the NAT traversal specification [RFC3947].
 In summary, although the IPsec standard does not as such prevent
 mobility (in the sense of updating security associations on-the-fly),
 the standard does not include a built-in mechanism (explicit or
 implicit) for doing so.  Therefore, it is assumed throughout this
 document that any change in the addresses comprising the identity of
 an SA requires IKE re-negotiation, which implies too heavy
 computation and too large latency for useful mobility.
 The IKEv2 Mobility and Multihoming Protocol (MOBIKE) [RFC4555]
 provides better mobility for IPsec.  This would allow the external
 Mobile IPv4 layer described in this specification to be removed.
 However, deploying MOBIKE requires changes to VPN devices, and is
 thus out of scope of this specification.

2. The Network Environment

 Enterprise users will access both the internal and external networks
 using different networking technologies.  In some networks, the MN
 will use FAs and in others it will anchor at the HA using co-located
 mode.  The following figure describes an example network topology
 illustrating the relationship between the internal and external
 networks, and the possible locations of the mobile node (i.e., (MN)).

Vaarala & Klovning Standards Track [Page 9] RFC 5265 MIPv4-VPN June 2008

    (MN) {fvc}                            {home} (MN)   [i-HA]
     !                                             \     /
  .--+---.                                        .-+---+-.
 (        )                                      (         )
  `--+---'                      [VPN]             `--+----'
      \                           !                  !
    [R/FA]        [x-HA]       .--+--.              [R]
         \         /          (  DMZ  )              !
        .-+-------+--.         `--+--'         .-----+------.
       (              )           !           (              )
       ( external net +---[R]----[FW]----[R]--+ internal net )
       (              )                       (              )
        `--+---------'                         `---+---+----'
          /                                       /     \
[DHCP]  [R]                              [DHCP] [R]     [R]    [i-FA]
   \    /                                   \   /         \    /
   .+--+---.                               .-+-+--.     .--+--+-.
  (         )                             (        )   (         )
   `---+---'                               `--+---'     `---+---'
       !                                      !             !
      (MN) {cvc}                             (MN) {c}      (MN) {f}
    Figure 1:  Basic topology, possible MN locations, and access modes
 In every possible location described in the figure, the mobile node
 can establish a connection to the corresponding HA(s) by using a
 suitable "access mode".  An access mode is here defined to consist
 of:
 1.  a composition of the mobile node networking stack (i-MIP or
     x-MIP/VPN/i-MIP); and
 2.  registration mode(s) of i-MIP and x-MIP (if used); i.e., co-
     located care-of address or foreign agent care-of address.
 Each possible access mode is encoded as "xyz", where:
 o  "x" indicates whether the x-MIP layer is used, and if used, the
    mode ("f" indicates FA-CoA, "c" indicates co-CoA, absence
    indicates not used);
 o  "y" indicates whether the VPN layer is used ("v" indicates VPN
    used, absence indicates not used); and
 o  "z" indicates mode of i-MIP layer ("f" indicates FA-CoA, "c"
    indicates co-CoA).

Vaarala & Klovning Standards Track [Page 10] RFC 5265 MIPv4-VPN June 2008

 This results in four access modes:
       c:  i-MIP with co-CoA
       f:  i-MIP with FA-CoA
     cvc:  x-MIP with co-CoA, VPN-TIA as i-MIP co-CoA
     fvc:  x-MIP with FA-CoA, VPN-TIA as i-MIP co-CoA
 This notation is more useful when optimizations to protocol layers
 are considered.  The notation is preserved here so that work on the
 optimizations can refer to a common notation.
 The internal network is typically a multi-subnetted network using
 private addressing [privaddr].  Subnets may contain internal home
 agent(s), DHCP server(s), and/or foreign agent(s).  Current IEEE
 802.11 wireless LANs are typically deployed in the external network
 or the DMZ because of security concerns.
 The figure leaves out a few details worth noticing:
 o  There may be multiple NAT devices anywhere in the diagram.
  • When the MN is outside, the NAT devices may be placed between

the MN and the x-HA or the x-HA and the VPN.

  • There may also be NAT(s) between the VPN and the i-HA, or a NAT

integrated into the VPN. In essence, any router in the figure

       may be considered to represent zero or more routers, each
       possibly performing NAT and/or ingress filtering.
  • When the MN is inside, there may be NAT devices between the MN

and the i-HA.

 o  Site-to-site VPN tunnels are not shown.  Although mostly
    transparent, IPsec endpoints may perform ingress filtering as part
    of enforcing their policy.
 o  The figure represents a topology where each functional entity is
    illustrated as a separate device.  However, it is possible that
    several network functions are co-located in a single device.  In
    fact, all three server components (x-HA, VPN, and i-HA) may be co-
    located in a single physical device.
 The following issues are also important when considering enterprise
 mobile users:
 o  Some firewalls are configured to block ICMP messages and/or
    fragments.  Such firewalls (routers) cannot be detected reliably.

Vaarala & Klovning Standards Track [Page 11] RFC 5265 MIPv4-VPN June 2008

 o  Some networks contain transparent application proxies, especially
    for HTTP.  Like firewalls, such proxies cannot be detected
    reliably in general.  IPsec and Mobile IPv4 are incompatible with
    such networks.
 Whenever a mobile node obtains either a co-CoA or an FA-CoA, the
 following conceptual steps take place:
 o  The mobile node detects whether the subnet where the care-of
    address was obtained belongs to the internal or the external
    network using the method described in Section 3 (or a vendor-
    specific mechanism fulfilling the requirements described).
 o  The mobile node performs necessary registrations and other
    connection setup signaling for the protocol layers (in the
    following order):
  • x-MIP (if used);
  • VPN (if used); and
  • i-MIP.
 Note that these two tasks are intertwined to some extent: detection
 of the internal network results in a successful registration to the
 i-HA using the proposed network detection algorithm.  An improved
 network detection mechanism not based on Mobile IPv4 registration
 messages might not have this side effect.
 The following subsections describe the different access modes and the
 requirements for registration and connection setup phase.

2.1. Access Mode: 'c'

 This access mode is standard Mobile IPv4 [RFC3344] with a co-located
 address, except that:
 o  the mobile node MUST detect that it is in the internal network;
    and
 o  the mobile node MUST re-register periodically (with a configurable
    interval) to ensure it is still inside the internal network (see
    Section 4).

Vaarala & Klovning Standards Track [Page 12] RFC 5265 MIPv4-VPN June 2008

2.2. Access Mode: 'f'

 This access mode is standard Mobile IPv4 [RFC3344] with a foreign
 agent care-of address, except that
 o  the mobile node MUST detect that it is in the internal network;
    and
 o  the mobile node MUST re-register periodically (with a configurable
    interval) to ensure it is still inside the internal network (see
    Section 4).

2.3. Access Mode: 'cvc'

 Steps:
 o  The mobile node obtains a care-of address.
 o  The mobile node detects it is not inside and registers with the
    x-HA, where
  • T-bit MAY be set (reverse tunneling), which minimizes the

probability of firewall-related connectivity problems

 o  If the mobile node does not have an existing IPsec security
    association, it uses IKE to set up an IPsec security association
    with the VPN gateway, using the x-HoA as the IP address for IKE/
    IPsec communication.  How the VPN-TIA is assigned is outside the
    scope of this document.
 o  The mobile node sends a MIPv4 Registration Request (RRQ) to the
    i-HA, registering the VPN-TIA as a co-located care-of address,
    where
  • T-bit SHOULD be set (reverse tunneling) (see discussion below)
 Reverse tunneling in the inner Mobile IPv4 layer is often required
 because of IPsec security policy limitations.  IPsec selectors define
 allowed IP addresses for packets sent inside the IPsec tunnel.
 Typical IPsec remote VPN selectors restrict the client address to be
 VPN-TIA (remote address is often unrestricted).  If reverse tunneling
 is not used, the source address of a packet sent by the MN will be
 the MN's home address (registered with i-HA), which is different from
 the VPN-TIA, thus violating IPsec security policy.  Consequently, the
 packet will be dropped, resulting in a connection black hole.

Vaarala & Klovning Standards Track [Page 13] RFC 5265 MIPv4-VPN June 2008

 Some types of IPsec-based VPNs, in particular L2TP/IPsec VPNs (PPP-
 over-L2TP-over-IPsec), do not have this limitation and can use
 triangular routing.
 Note that although the MN can use triangular routing, i.e., skip the
 inner MIPv4 layer, it MUST NOT skip the VPN layer for security
 reasons.

2.4. Access Mode: 'fvc'

 Steps:
 o  The mobile node obtains a foreign agent advertisement from the
    local network.
 o  The mobile node detects it is outside and registers with the x-HA,
    where
  • T-bit MAY be set (reverse tunneling), which minimizes the

probability of firewall-related connectivity problems

 o  If necessary, the mobile node uses IKE to set up an IPsec
    connection with the VPN gateway, using the x-HoA as the IP address
    for IKE/IPsec communication.  How the VPN-TIA is assigned is
    outside the scope of this document.
 o  The mobile node sends a MIPv4 RRQ to the i-HA, registering the
    VPN-TIA as a co-located care-of address, where
  • T-bit SHOULD be set (reverse tunneling) (see discussion in

Section 2.3)

 Note that although the MN can use triangular routing, i.e., skip the
 inner MIPv4 layer, it MUST NOT skip the VPN layer for security
 reasons.

2.5. NAT Traversal

 NAT devices may affect each layer independently.  Mobile IPv4 NAT
 traversal [mipnat] SHOULD be supported for x-MIP and i-MIP layers,
 while IPsec NAT traversal [RFC3947][RFC3948] SHOULD be supported for
 the VPN layer.
 Note that NAT traversal for the internal MIPv4 layer may be necessary
 even when there is no separate NAT device between the VPN gateway and
 the internal network.  Some VPN implementations NAT VPN tunnel inner
 addresses before routing traffic to the intranet.  Sometimes this is
 done to make a deployment easier, but in some cases this approach

Vaarala & Klovning Standards Track [Page 14] RFC 5265 MIPv4-VPN June 2008

 makes VPN client implementation easier.  Mobile IPv4 NAT traversal is
 required to establish a MIPv4 session in this case.

3. Internal Network Detection

 Secure detection of the internal network is critical to prevent
 plaintext traffic from being sent over an untrusted network.  In
 other words, the overall security (confidentiality and integrity of
 user data) relies on the security of the internal network detection
 mechanism in addition to IPsec.  For this reason, security
 requirements are described in this section.
 In addition to detecting entry into the internal network, the mobile
 node must also detect when it has left the internal network.  Entry
 into the internal network is easier security-wise: the mobile node
 can ensure that it is inside the internal network before sending any
 plaintext traffic.  Exit from the internal network is more difficult
 to detect, and the MN may accidentally leak plaintext packets if the
 event is not detected in time.
 Several events can cause the mobile node to leave the internal
 network, including:
 o  a routing change upstream;
 o  a reassociation of 802.11 on layer 2 that the mobile node software
    does not detect;
 o  a physical cable disconnect and reconnect that the mobile node
    software does not detect.
 Whether the mobile node can detect such changes in the current
 connection reliably depends on the implementation and the networking
 technology.  For instance, some mobile nodes may be implemented as
 pure layer three entities.  Even if the mobile node software has
 access to layer 2 information, such information is not trustworthy
 security-wise, and depends on the network interface driver.
 If the mobile node does not detect these events properly, it may leak
 plaintext traffic into an untrusted network.  A number of approaches
 can be used to detect exit from the internal network, ranging from
 frequent re-registration to the use of layer two information.
 A mobile node MUST implement a detection mechanism fulfilling the
 requirements described in Section 3.2; this ensures that basic
 security requirements are fulfilled.  The basic algorithm described
 in Section 3.3 is one way to do that, but alternative methods may be
 used instead or in conjunction.  The assumptions that the

Vaarala & Klovning Standards Track [Page 15] RFC 5265 MIPv4-VPN June 2008

 requirements and the proposed mechanism rely upon are described in
 Section 3.1.

3.1. Assumptions

 The enterprise firewall MUST be configured to block traffic
 originating from external networks going to the i-HA.  In other
 words, the mobile node MUST NOT be able to perform a successful
 Registration Request/Registration Reply (RRQ/RRP) exchange (without
 using IPsec) unless it is connected to the trusted internal network;
 the mobile node can then stop using IPsec without compromising data
 confidentiality.
 If this assumption does not hold, data confidentiality is compromised
 in a potentially silent and thus dangerous manner.  To minimize the
 impact of this scenario, the i-HA is also required to check the
 source address of any RRQ to determine whether it comes from a
 trusted (internal network) address.  The i-HA needs to indicate to
 the MN that it supports the checking of trusted source addresses by
 including a Trusted Networks Configured extension in its registration
 reply.  This new extension, which needs to be implemented by both
 i-HA and the MN, is described in Section 3.4.
 The firewall MAY be configured to block registration traffic to the
 x-HA originating from within the internal network, which makes the
 network detection algorithm simpler and more robust.  However, as the
 registration request is basically UDP traffic, an ordinary firewall
 (even a stateful one) would typically allow the registration request
 to be sent and a registration reply to be received through the
 firewall.

3.2. Implementation Requirements

 Any mechanism used to detect the internal network MUST fulfill the
 requirements described in this section.  An example of a network
 detection mechanism fulfilling these requirements is given in
 Section 3.3.

3.2.1. Separate Tracking of Network Interfaces

 The mobile node implementation MUST track each network interface
 separately.  Successful registration with the i-HA through interface
 X does not imply anything about the status of interface Y.

3.2.2. Connection Status Change

 When the mobile node detects that its connection status on a certain
 network interface changes, the mobile node MUST:

Vaarala & Klovning Standards Track [Page 16] RFC 5265 MIPv4-VPN June 2008

 o  immediately stop relaying user data packets;
 o  detect whether this interface is connected to the internal or the
    external network; and
 o  resume data traffic only after the internal network detection and
    necessary registrations and VPN tunnel establishment have been
    completed.
 The mechanisms used to detect a connection status change depends on
 the mobile node implementation, the networking technology, and the
 access mode.

3.2.3. Registration-Based Internal Network Detection

 The mobile node MUST NOT infer that an interface is connected to the
 internal network unless a successful registration has been completed
 through that particular interface to the i-HA, the i-HA registration
 reply contained a Trusted Networks Configured extension
 (Section 3.4), and the connection status of the interface has not
 changed since.

3.2.4. Registration-Based Internal Network Monitoring

 Some leak of plaintext packets to a (potentially) untrusted network
 cannot always be completely prevented; this depends heavily on the
 client implementation.  In some cases, the client cannot detect such
 a change, e.g., if upstream routing is changed.
 More frequent re-registrations when the MN is inside is a simple way
 to ensure that the MN is still inside.  The MN SHOULD start re-
 registration every (T_MONITOR - N) seconds when inside, where N is a
 grace period that ensures that re-registration is completed before
 T_MONITOR seconds are up.  To bound the maximum amount of time that a
 plaintext leak may persist, the mobile node must fulfill the
 following security requirements when inside:
 o  The mobile node MUST NOT send or receive a user data packet if
    more than T_MONITOR seconds have elapsed since the last successful
    (re-)registration with the i-HA.
 o  If more than T_MONITOR seconds have elapsed, data packets MUST be
    either dropped or queued.  If the packets are queued, the queues
    MUST NOT be processed until the re-registration has been
    successfully completed without a connection status change.

Vaarala & Klovning Standards Track [Page 17] RFC 5265 MIPv4-VPN June 2008

 o  The T_MONITOR parameter MUST be configurable, and have the default
    value of 60 seconds.  This default is a trade-off between traffic
    overhead and a reasonable bound to exposure.
 This approach is reasonable for a wide range of mobile nodes (e.g.,
 laptops), but has unnecessary overhead when the mobile node is idle
 (not sending or receiving packets).  If re-registration does not
 complete before T_MONITOR seconds are up, data packets must be queued
 or dropped as specified above.  Note that re-registration packets
 MUST be sent even if bidirectional user data traffic is being
 relayed: data packets are no substitute for an authenticated re-
 registration.
 To minimize traffic overhead when the mobile node is idle, re-
 registrations can be stopped when no traffic is being sent or
 received.  If the mobile node subsequently receives or needs to send
 a packet, the packet must be dropped or queued (as specified above)
 until a re-registration with the i-HA has been successfully
 completed.  Although this approach adds packet processing complexity,
 it may be appropriate for small, battery-powered devices, which may
 be idle much of the time.  (Note that ordinary re-registration before
 the mobility binding lifetime is exhausted should still be done to
 keep the MN reachable.)
 T_MONITOR is required to be configurable so that an administrator can
 determine the required security level for the particular deployment.
 Configuring T_MONITOR in the order of a few seconds is not practical;
 alternative mechanisms need to be considered if such confidence is
 required.
 The re-registration mechanism is a worst-case fallback mechanism.  If
 additional information (such as layer two triggers) is available to
 the mobile node, the mobile node SHOULD use the triggers to detect MN
 movement and restart the detection process to minimize exposure.
 Note that re-registration is required by Mobile IPv4 by default
 (except for the atypical case of an infinite binding lifetime);
 however, the re-registration interval may be much larger when using
 an ordinary Mobile IPv4 client.  A shorter re-registration interval
 is usually not an issue, because the internal network is typically a
 fast, wired network, and the shortened re-registration interval
 applies only when the mobile node is inside the internal network.
 When outside, the ordinary Mobile IPv4 re-registration process (based
 on binding lifetime) is used.

Vaarala & Klovning Standards Track [Page 18] RFC 5265 MIPv4-VPN June 2008

3.3. Proposed Algorithm

 When the MN detects that it has changed its point of network
 attachment on a certain interface, it issues two simultaneous
 registration requests, one to the i-HA and another to the x-HA.
 These registration requests are periodically retransmitted if reply
 messages are not received.
 Registration replies are processed as follows:
 o  If a response from the x-HA is received, the MN stops
    retransmitting its registration request to the x-HA and
    tentatively determines it is outside.  However, the MN MUST keep
    on retransmitting its registration to the i-HA for a period of
    time.  The MN MAY postpone the IPsec connection setup for some
    period of time while it waits for a (possible) response from the
    i-HA.
 o  If a response from the i-HA is received and the response contains
    the Trusted Networks Configured extension (Section 3.4), the MN
    SHOULD determine that it is inside.  In any case, the MN MUST stop
    retransmitting its registration requests to both i-HA and x-HA.
 o  When successfully registered with the i-HA directly, MN SHOULD de-
    register with the x-HA.
 If the MN ends up detecting that it is inside, it MUST re-register
 periodically (regardless of binding lifetime); see Section 3.2.4.  If
 the re-registration fails, the MN MUST stop sending and receiving
 plaintext traffic, and MUST restart the detection algorithm.
 Plaintext re-registration messages are always addressed either to the
 x-HA or the i-HA, not to both.  This is because the MN knows, after
 initial registration, whether it is inside or outside.  (However,
 when the mobile node is outside, it re-registers independently with
 the x-HA using plaintext, and with the i-HA through the VPN tunnel.)
 Postponing the IPsec connection setup could prevent aborted IKE
 sessions.  Aborting IKE sessions may be a problem in some cases
 because IKE does not provide a reliable, standardized, and mandatory-
 to-implement mechanism for terminating a session cleanly.
 If the x-HA is not reachable from inside (i.e., the firewall
 configuration is known), a detection period of zero is preferred, as
 it minimizes connection setup overhead and causes no timing problems.
 Should the assumption have been invalid and a response from the i-HA
 received after a response from the x-HA, the MN SHOULD re-register
 with the i-HA directly.

Vaarala & Klovning Standards Track [Page 19] RFC 5265 MIPv4-VPN June 2008

3.4. Trusted Networks Configured (TNC) Extension

 This extension is a skippable extension.  An i-HA sending the
 extension must fulfill the requirements described in Section 4.3,
 while an MN processing the extension must fulfill the requirements
 described in Section 4.1.  The format of the extension is described
 below.  It adheres to the short extension format described in
 [RFC3344]:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |    Length     |    Sub-Type   |   Reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Type        149
        Length      2
        Sub-Type    0
        Reserved    Set to 0 when sending, ignored when receiving

3.5. Implementation Issues

 When the MN uses a parallel detection algorithm and is using an FA,
 the MN sends two registration requests through the same FA with the
 same Media Acces Control (MAC) address (or equivalent) and possibly
 even the same home address.  Although this is not in conflict with
 existing specifications, it is an unusual scenario; hence some FA
 implementations may not work properly in such a situation.  However,
 testing against deployed foreign agents seems to indicate that a
 majority of available foreign agents handle this situation.
 When the x-HA and i-HA addresses are the same, the scenario is even
 more difficult for the FA, and it is almost certain that existing FAs
 do not deal with the situation correctly.  Therefore, it is required
 that x-HA and i-HA addresses MUST be different.
 Regardless, if the MN detects that i-HA and x-HA have the same
 address, it MUST assume that it is in the external network and bypass
 network detection to avoid confusing the FA.  Because the HA
 addresses are used at different layers, achieving connectivity is
 possible without address confusion.
 The mobile node MAY use the following hints to determine that it is
 inside, but MUST verify reachability of the i-HA anyway:

Vaarala & Klovning Standards Track [Page 20] RFC 5265 MIPv4-VPN June 2008

 o  a domain name in a DHCPDISCOVER / DHCPOFFER message
 o  an NAI in a foreign agent advertisement
 o  a list of default gateway MAC addresses that are known to reside
    in the internal network (i.e., configured as such, or have been
    previously verified to be inside)
 For instance, if the MN has reason to believe it is inside, it MAY
 postpone sending a registration request to the x-HA for some time.
 Similarly, if the MN has reason to believe it is outside, it may
 start IPsec connection setup immediately after receiving a
 registration reply from the x-HA.  However, should the MN receive a
 registration reply from the i-HA after IPsec connection setup has
 been started, the MN SHOULD still switch to using the i-HA directly.

3.6. Rationale for Design Choices

3.6.1. Firewall Configuration Requirements

 The requirement that the i-HA cannot be reached from the external
 network is necessary.  If not, a successful registration with the
 i-HA (without IPsec) cannot be used as a secure indication that the
 mobile node is inside.  A possible solution to the obvious security
 problem would be to define and deploy a secure internal network
 detection mechanism based on, e.g., signed FA advertisement or signed
 DHCP messages.
 However, unless the mechanism is defined for both FA and DHCP
 messages and is deployed in every internal network, it has limited
 applicability.  In other words, the mobile node MUST NOT assume it is
 in the internal network unless it receives a signed FA or DHCP
 message (regardless of whether or not it can register directly with
 the i-HA).  If it receives an unsigned FA or DHCP message, it MUST
 use IPsec; otherwise, the mobile node can be easily tricked into
 using plaintext.
 Assuming that all FA and DHCP servers in the internal network are
 upgraded to support such a feature does not seem realistic; it is
 highly desirable to be able to take advantage of existing DHCP and FA
 deployments.  Similar analysis seems to apply regardless of what kind
 of additional security mechanism is defined.
 Because a firewall configuration error can have catastrophic data
 security consequences (silent exposure of user data to external
 attackers), a separate protection mechanism is provided by the i-HA.
 The i-HA must be configured, by the administrator, with a list of
 trusted networks.  The i-HA advertises that it knows which

Vaarala & Klovning Standards Track [Page 21] RFC 5265 MIPv4-VPN June 2008

 registration request source addresses are trusted, using a
 registration reply extension (Trusted Networks Configured extension,
 Section 3.4).  Without this extension, an MN may not rely on a
 successful registration to indicate that it is connected to the
 internal network.  This ensures that user data compromise does not
 occur unless both the firewall and the i-HA are configured
 incorrectly.  Further, occurrences of registration requests from
 untrusted addresses should be logged by the i-HA, exposing them to
 administrator review.

3.6.2. Registration-Based Internal Network Monitoring

 This issue also affects IPsec client security.  However, as IPsec
 specifications take no stand on how and when client IPsec policies
 are configured or changed (for instance, in response to a change in
 network connectivity), the issue is out of scope for IPsec.  Because
 this document describes an algorithm and requirements for (secure)
 internal network detection, the issue is in scope of the document.
 The current requirement for internal network monitoring was added as
 a fallback mechanism.

3.6.3. No Encryption When Inside

 If encryption was applied also when MN was inside, there would be no
 security reason to monitor the internal network periodically.
 The main rationale for why encryption cannot be applied when the MN
 is inside was given in Section 1.6.  In short, the main issues are
 (1) power consumption; (2) extra CPU load, especially because
 internal networks are typically switched networks and a lot of data
 may be routinely transferred; (3) existing HA devices do not
 typically integrate IPsec functionality; (4) (IPsec) encryption
 requires user authentication, which may be interactive in some cases
 (e.g., SecurID) and thus a usability issue; and (5) user may need to
 have separate credentials for VPN devices in the DMZ and the HA.

3.7. Improvements

 The registration process can be improved in many ways.  One simple
 way is to make the x-HA detect whether a registration request came
 from inside or outside the enterprise network.  If it came from
 inside the enterprise network, the x-HA can simply drop the
 registration request.
 This approach is feasible without protocol changes in scenarios where
 a corporation owns both the VPN and the x-HA.  The x-HA can simply
 determine based on the incoming interface identifier (or the router

Vaarala & Klovning Standards Track [Page 22] RFC 5265 MIPv4-VPN June 2008

 that relayed the packet) whether or not the registration request came
 from inside.
 In other scenarios, protocol changes may be needed.  Such changes are
 out of scope of this document.

4. Requirements

4.1. Mobile Node Requirements

 The mobile node MUST implement an internal network detection
 algorithm fulfilling the requirements set forth in Section 3.2.  A
 new configurable MN parameter, T_MONITOR, is required.  The value of
 this parameter reflects a balance between security and the amount of
 signaling overhead, and thus needs to be configurable.  In addition,
 when doing internal network detection, the MN MUST NOT disable IPsec
 protection unless the registration reply from the i-HA contains a
 Trusted Networks Configured extension (Section 3.4).
 The mobile node MUST support access modes c, f, cvc, fvc (Section 2).
 The mobile node SHOULD support Mobile IPv4 NAT traversal [mipnat] for
 both internal and external Mobile IP.
 The mobile node SHOULD support IPsec NAT traversal [RFC3947]
 [RFC3948].
 When the mobile node has direct access to the i-HA, it SHOULD use
 only the inner Mobile IPv4 layer to minimize firewall and VPN impact.
 When the mobile node is outside and using the VPN connection, IPsec
 policies MUST be configured to encrypt all traffic sent to and from
 the enterprise network.  The particular Security Policy Database
 (SPD) entries depend on the type and configuration of the particular
 VPN (e.g., plain IPsec vs. L2TP/IPsec, full tunneling or split
 tunneling).

4.2. VPN Device Requirements

 The VPN security policy MUST allow communication using UDP to the
 internal home agent(s), with home agent port 434 and any remote port.
 The security policy SHOULD allow IP-IP to internal home agent(s) in
 addition to UDP port 434.
 The VPN device SHOULD implement the IPsec NAT traversal mechanism
 described in [RFC3947] and [RFC3948].

Vaarala & Klovning Standards Track [Page 23] RFC 5265 MIPv4-VPN June 2008

4.3. Home Agent Requirements

 The home agent SHOULD implement the Mobile IPv4 NAT traversal
 mechanism described in [mipnat].  (This also refers to the i-HA: NAT
 traversal is required to support VPNs that NAT VPN tunnel addresses
 or block IP-IP traffic.)
 To protect user data confidentiality against firewall configuration
 errors, the i-HA:
 o  MUST be configured with a list of trusted IP subnets (containing
    only addresses from the internal network), with no subnets being
    trusted by default.
 o  MUST reject (drop silently) any registration request coming from a
    source address that is not inside any of the configured trusted
    subnets.  These dropped registration requests SHOULD be logged.
 o  MUST include a Trusted Networks Configured extension (Section 3.4)
    in a registration reply sent in response to a registration request
    coming from a trusted address.

5. Analysis

 This section provides a comparison against guidelines described in
 Section 6 of the problem statement [RFC4093] and additional analysis
 of packet overhead with and without the optional mechanisms.

5.1. Comparison against Guidelines

 Preservation of existing VPN infrastructure
 o  The solution does not mandate any changes to existing VPN
    infrastructure, other than possibly changes in configuration to
    avoid stateful filtering of traffic.
 Software upgrades to existing VPN clients and gateways
 o  The solution described does not require any changes to VPN
    gateways or Mobile IPv4 foreign agents.
 IPsec protocol
 o  The solution does not require any changes to existing IPsec or key
    exchange standard protocols, and does not require implementation
    of new protocols in the VPN device.

Vaarala & Klovning Standards Track [Page 24] RFC 5265 MIPv4-VPN June 2008

 Multi-vendor interoperability
 o  The solution provides easy multi-vendor interoperability between
    server components (VPN device, foreign agents, and home agents).
    Indeed, these components need not be aware of each other.
 o  The mobile node networking stack is somewhat complex to implement,
    which may be an issue for multi-vendor interoperability.  However,
    this is a purely software architecture issue, and there are no
    known protocol limitations for multi-vendor interoperability.
 MIPv4 protocol
 o  The solution adheres to the MIPv4 protocol, but requires the new
    Trusted Networks Configured extension to improve the
    trustworthiness of internal network detection.
 o  The solution requires the use of two parallel MIPv4 layers.
 Handoff overhead
 o  The solution provides a mechanism to avoid VPN tunnel SA
    renegotiation upon movement by using the external MIPv4 layer.
 Scalability, availability, reliability, and performance
 o  The solution complexity is linear with the number of MNs
    registered and accessing resources inside the intranet.
 o  Additional overhead is imposed by the solution.
 Functional entities
 o  The solution does not impose any new types of functional entities
    or required changes to existing entities.  However, an external HA
    device is required.
 Implications of intervening NAT gateways
 o  The solution leverages existing MIPv4 NAT traversal [mipnat] and
    IPsec NAT traversal [RFC3947] [RFC3948] solutions and does not
    require any new functionality to deal with NATs.
 Security implications
 o  The solution requires a new mechanism to detect whether the mobile
    node is in the internal or the external network.  The security of
    this mechanism is critical in ensuring that the security level

Vaarala & Klovning Standards Track [Page 25] RFC 5265 MIPv4-VPN June 2008

    provided by IPsec is not compromised by a faulty detection
    mechanism.
 o  When the mobile node is outside, the external Mobile IPv4 layer
    may allow some traffic redirection attacks that plain IPsec does
    not allow.  Other than that, IPsec security is unchanged.
 o  More security considerations are described in Section 6.

5.2. Packet Overhead

 The maximum packet overhead depends on access mode as follows:
 o  f: 0 octets
 o  c: 20 octets
 o  fvc: 77 octets
 o  cvc: 97 octets
 The maximum overhead of 97 octets in the 'cvc' access mode consists
 of the following:
 o  IP-IP for i-MIPv4: 20 octets
 o  IPsec ESP: 57 octets total, consisting of 20 (new IP header),
    4+4+8 = 16 (SPI, sequence number, cipher initialization vector),
    7+2 = 9 (padding, padding length field, next header field), 12
    (ESP authentication trailer)
 o  IP-IP for x-MIPv4: 20 octets
 When IPsec is used, a variable amount of padding is present in each
 ESP packet.  The figures were computed for a cipher with 64-bit block
 size, padding overhead of 9 octets (next header field, padding length
 field, and 7 octets of padding; see Section 2.4 of [RFC4303]), and
 ESP authentication field of 12 octets (HMAC-SHA1-96 or HMAC-MD5-96).
 Note that an IPsec implementation MAY pad with more than a minimum
 amount of octets.
 NAT traversal overhead is not included, and adds 8 octets when IPsec
 NAT traversal [RFC3947] [RFC3948] is used and 12 octets when MIP NAT
 traversal [mipnat] is used.  For instance, when using access mode
 cvc, the maximum NAT traversal overhead is 12+8+12 = 32 octets.
 Thus, the worst case scenario (with the above mentioned ESP
 assumptions) is 129 octets for cvc.

Vaarala & Klovning Standards Track [Page 26] RFC 5265 MIPv4-VPN June 2008

5.3. Latency Considerations

 When the MN is inside, connection setup latency does not increase
 compared to standard MIPv4 if the MN implements the suggested
 parallel registration sequence (see Section 3.3).  Exchange of RRQ/
 RRP messages with the i-HA confirms the MN is inside, and the MN may
 start sending and receiving user traffic immediately.  For the same
 reason, handovers in the internal network have no overhead relative
 to standard MIPv4.
 When the MN is outside, the situation is slightly different.  Initial
 connection setup latency essentially consists of (1) registration
 with the x-HA, (2) optional detection delay (waiting for i-HA
 response), (3) IPsec connection setup (IKE), and (4) registration
 with the i-HA.  All but (4) are in addition to standard MIPv4.
 However, handovers in the external network have performance
 comparable to standard MIPv4.  The MN simply re-registers with the
 x-HA and starts to send IPsec traffic to the VPN gateway from the new
 address.
 The MN may minimize latency by (1) not waiting for an i-HA response
 before triggering IKE if the x-HA registration succeeds and (2)
 sending first the RRQ most likely to succeed (e.g., if the MN is most
 likely outside).  These can be done based on heuristics about the
 network, e.g., addresses, MAC address of the default gateway (which
 the mobile node may remember from previous access); based on the
 previous access network (i.e., optimize for inside-inside and
 outside-outside movement); etc.

5.4. Firewall State Considerations

 A separate firewall device or an integrated firewall in the VPN
 gateway typically performs stateful inspection of user traffic.  The
 firewall may, for instance, track TCP session status and block TCP
 segments not related to open connections.  Other stateful inspection
 mechanisms also exist.
 Firewall state poses a problem when the mobile node moves between the
 internal and external networks.  The mobile node may, for instance,
 initiate a TCP connection while inside, and later go outside while
 expecting to keep the connection alive.  From the point of view of
 the firewall, the TCP connection has not been initiated, as it has
 not witnessed the TCP connection setup packets, thus potentially
 resulting in connectivity problems.
 When the VPN-TIA is registered as a co-located care-of address with
 the i-HA, all mobile node traffic appears as IP-IP for the firewall.

Vaarala & Klovning Standards Track [Page 27] RFC 5265 MIPv4-VPN June 2008

 Typically, firewalls do not continue inspection beyond the IP-IP
 tunnel, but support for deeper inspection is available in many
 products.  In particular, an administrator can configure traffic
 policies in many firewall products even for IP-IP encapsulated
 traffic.  If this is done, similar statefulness issues may arise.
 In summary, the firewall must allow traffic coming from and going
 into the IPsec connection to be routed, even though they may not have
 successfully tracked the connection state.  How this is done is out
 of scope of this document.

5.5. Intrusion Detection Systems (IDSs)

 Many firewalls incorporate intrusion detection systems monitoring
 network traffic for unusual patterns and clear signs of attack.
 Since traffic from a mobile node implementing this specification is
 UDP to i-HA port 434, and possibly IP-IP traffic to the i-HA address,
 existing IDSs may treat the traffic differently than ordinary VPN
 remote access traffic.  Like firewalls, IDSs are not standardized, so
 it is impossible to guarantee interoperability with any particular
 IDS system.

5.6. Implementation of the Mobile Node

 Implementation of the mobile node requires the use of three tunneling
 layers, which may be used in various configurations depending on
 whether that particular interface is inside or outside.  Note that it
 is possible that one interface is inside and another interface is
 outside, which requires a different layering for each interface at
 the same time.
 For multi-vendor implementation, the IPsec and MIPv4 layers need to
 interoperate in the same mobile node.  This implies that a flexible
 framework for protocol layering (or protocol-specific APIs) is
 required.

5.7. Non-IPsec VPN Protocols

 The solution also works for VPN tunneling protocols that are not
 IPsec-based, provided that the mobile node is provided IPv4
 connectivity with an address suitable for registration.  However,
 such VPN protocols are not explicitly considered.

Vaarala & Klovning Standards Track [Page 28] RFC 5265 MIPv4-VPN June 2008

6. Security Considerations

6.1. Internal Network Detection

 If the mobile node by mistake believes it is in the internal network
 and sends plaintext packets, it compromises IPsec security.  For this
 reason, the overall security (confidentiality and integrity) of user
 data is a minimum of (1) IPsec security and (2) security of the
 internal network detection mechanism.
 Security of the internal network detection relies on a successful
 registration with the i-HA.  For standard Mobile IPv4 [RFC3344], this
 means HMAC-MD5 and Mobile IPv4 replay protection.  The solution also
 assumes that the i-HA is not directly reachable from the external
 network, requiring careful enterprise firewall configuration.  To
 minimize the impact of a firewall configuration problem, the i-HA is
 separately required to be configured with trusted source addresses
 (i.e., addresses belonging to the internal network), and to include
 an indication of this in a new Trusted Networks Configured extension.
 The MN is required not to trust a registration as an indication of
 being connected to the internal network, unless this extension is
 present in the registration reply.  Thus, to actually compromise user
 data confidentiality, both the enterprise firewall and the i-HA have
 to be configured incorrectly, which reduces the likelihood of the
 scenario.
 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.
 When the connection status of an interface changes, an interface
 previously connected to the trusted internal network may suddenly be
 connected to an untrusted network.  Although the same problem is also
 relevant to IPsec-based VPN implementations, the problem is
 especially relevant in the scope of this specification.
 In most cases, mobile node implementations are expected to have layer
 2 information available, making connection change detection both fast
 and robust.  To cover cases where such information is not available
 (or fails for some reason), the mobile node is required to
 periodically re-register with the internal home agent to verify that
 it is still connected to the trusted network.  It is also required
 that this re-registration interval be configurable, thus giving the
 administrator a parameter by which potential exposure may be
 controlled.

Vaarala & Klovning Standards Track [Page 29] RFC 5265 MIPv4-VPN June 2008

6.2. Mobile IPv4 versus IPsec

 MIPv4 and IPsec have different goals and approaches for providing
 security services.  MIPv4 typically uses a shared secret for
 authentication of signaling traffic, while IPsec typically uses IKE
 (an authenticated Diffie-Hellman exchange) to set up session keys.
 Thus, the overall security properties of a combined MIPv4 and IPsec
 system depend on both mechanisms.
 In the solution outlined in this document, the external MIPv4 layer
 provides mobility for IPsec traffic.  If the security of MIPv4 is
 broken in this context, traffic redirection attacks against the IPsec
 traffic are possible.  However, such routing attacks do not affect
 other IPsec properties (confidentiality, integrity, replay
 protection, etc.), because IPsec does not consider the network
 between two IPsec endpoints to be secure in any way.
 Because MIPv4 shared secrets are usually configured manually, they
 may be weak if easily memorizable secrets are chosen, thus opening up
 redirection attacks described above.  Note especially that a weak
 secret in the i-HA is fatal to security, as the mobile node can be
 fooled into dropping encryption if the i-HA secret is broken.
 Assuming the MIPv4 shared secrets have sufficient entropy, there are
 still at least the following differences and similarities between
 MIPv4 and IPsec worth considering:
 o  Both IPsec and MIPv4 are susceptible to the "transient pseudo NAT"
    attack described in [pseudonat] and [mipnat], assuming that NAT
    traversal is enabled (which is typically the case).  "Pseudo NAT"
    attacks allow an attacker to redirect traffic flows, resulting in
    resource consumption, lack of connectivity, and denial of service.
    However, such attacks cannot compromise the confidentiality of
    user data protected using IPsec.
 o  When considering a "pseudo NAT" attack against standard IPsec and
    standard MIP (with NAT traversal), redirection attacks against MIP
    may be easier because:
  • MIPv4 re-registrations typically occur more frequently than

IPsec SA setups (although this may not be the case for mobile

       hosts).
  • It suffices to catch and modify a single registration request,

whereas attacking IKE requires that multiple IKE packets are

       caught and modified.

Vaarala & Klovning Standards Track [Page 30] RFC 5265 MIPv4-VPN June 2008

 o  There may be concerns about mixing of algorithms.  For instance,
    IPsec may be using HMAC-SHA1-96, while MIP is always using HMAC-
    MD5 (RFC 3344) or prefix+suffix MD5 (RFC 2002).  Furthermore,
    while IPsec algorithms are typically configurable, MIPv4 clients
    typically use only HMAC-MD5 or prefix+suffix MD5.  Although this
    is probably not a security problem as such, it is more difficult
    to communicate to users.
 o  When IPsec is used with a Public Key Infrastructure (PKI), the key
    management properties are superior to those of basic MIPv4.  Thus,
    adding MIPv4 to the system makes key management more complex.
 o  In general, adding new security mechanisms increases overall
    complexity and makes the system more difficult to understand.

7. IANA Considerations

 This document specifies a new skippable extension (in the short
 format) in Section 3.4, whose Type and Sub-Type values have been
 assigned.
 Allocation of new Sub-Type values can be made via Expert Review and
 Specification Required [RFC5226].

8. Acknowledgements

 This document is a joint work of the contributing authors (in
 alphabetical order):
  1. Farid Adrangi (Intel Corporation)
  2. Nitsan Baider (Check Point Software Technologies, Inc.)
  3. Gopal Dommety (Cisco Systems)
  4. Eli Gelasco (Cisco Systems)
  5. Dorothy Gellert (Nokia Corporation)
  6. Espen Klovning (Birdstep)
  7. Milind Kulkarni (Cisco Systems)
  8. Henrik Levkowetz (ipUnplugged AB)
  9. Frode Nielsen (Birdstep)
  10. Sami Vaarala (Codebay)
  11. Qiang Zhang (Liqwid Networks, Inc.)
 The authors would like to thank the MIP/VPN design team, especially
 Mike Andrews, Gaetan Feige, Prakash Iyer, Brijesh Kumar, Joe Lau,
 Kent Leung, Gabriel Montenegro, Ranjit Narjala, Antti Nuopponen, Alan
 O'Neill, Alpesh Patel, Ilkka Pietikainen, Phil Roberts, Hans
 Sjostrand, and Serge Tessier for their continuous feedback and
 helping us improve this document.  Special thanks to Radia Perlman
 for giving the document a thorough read and a security review.  Tom

Vaarala & Klovning Standards Track [Page 31] RFC 5265 MIPv4-VPN June 2008

 Hiller pointed out issues with battery-powered devices.  We would
 also like to thank the previous Mobile IP working group chairs
 (Gabriel Montenegro, Basavaraj Patil, and Phil Roberts) for important
 feedback and guidance.

9. References

9.1. Normative References

 [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3344]    Perkins, C., Ed., "IP Mobility Support for IPv4",
              RFC 3344, August 2002.
 [RFC3947]    Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
              "Negotiation of NAT-Traversal in the IKE", RFC 3947,
              January 2005.
 [RFC3948]    Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and
              M. Stenberg, "UDP Encapsulation of IPsec packets",
              RFC 3948, January 2005.
 [RFC4301]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.
 [RFC4303]    Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.
 [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
 [mipnat]     Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of
              Network Address Translation (NAT) Devices", RFC 3519,
              April 2003.
 [privaddr]   Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
              G., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, February 1996.

Vaarala & Klovning Standards Track [Page 32] RFC 5265 MIPv4-VPN June 2008

9.2. Informative References

 [RFC2002]    Perkins, C., "IP Mobility Support", RFC 2002,
              October 1996.
 [RFC3456]    Patel, B., Aboba, B., Kelly, S., and V. Gupta, "Dynamic
              Host Configuration Protocol (DHCPv4) Configuration of
              IPsec Tunnel Mode", RFC 3456, January 2003.
 [RFC3776]    Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec
              to Protect Mobile IPv6 Signaling Between Mobile Nodes
              and Home Agents", RFC 3776, June 2004.
 [RFC4093]    Adrangi, F. and H. Levkowetz, "Problem Statement: Mobile
              IPv4 Traversal of Virtual Private Network (VPN)
              Gateways", RFC 4093, August 2005.
 [RFC4282]    Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.
 [RFC4555]    Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, June 2006.
 [pseudonat]  Dupont, F. and J. Bernard, "Transient pseudo-NAT attacks
              or how NATs are even more evil than you believed", Work
              in Progress, June 2004.
 [tessier]    Tessier, S., "Guidelines for Mobile IP and IPsec VPN
              Usage", Work in Progress, December 2002.

Vaarala & Klovning Standards Track [Page 33] RFC 5265 MIPv4-VPN June 2008

Appendix A. Packet Flow Examples

A.1. Connection Setup for Access Mode 'cvc'

 The following figure illustrates connection setup when the mobile
 node is outside and using a co-located care-of address.  IKE
 connection setup is not shown in full, and involves multiple round
 trips (4.5 round trips when using main mode followed by quick mode).

Vaarala & Klovning Standards Track [Page 34] RFC 5265 MIPv4-VPN June 2008

  MN-APP      MN        x-HA       VPN        i-HA        CN
   !          !          !          !          !          !
   !          ! -------> !          !          !          !
   !          !  rrq     !          !          !          !
   !          ! -----------------X  !          !          ! rrq not
   !          !  rrq     !          !          !          ! received
   !          !          !          !          !          ! by i-HA
   !          ! <------- !          !          !          !
   !          !  rrp     !          !          !          !
   !          !          !          !          !          !
   !  [wait for detection period for response from i-HA]  !
   !  [may also retransmit to i-HA, depending on config]  ! no rrp
   !          !          !          !          !          ! from i-HA
   !          ! ==(1)==> !          !          !          !
   !          !  ike {1a}! -------> !          !          !
   !          !          !  ike     !          !          !
   !          !          ! <------- !          !          !
   !          ! <==(1)== !  ike     !          !          !
   !          !  ike     !          !          !          !
   :          :          :          :          :          :
   :          :          :          :          :          :
   !          !          !          !          !          !
   !          ! ==(2)==> !          !          !          !
   !          !  rrq {2a}! ==(1)==> !          !          !
   !          !          !  rrq {2b}! -------> !          !
   !          !          !          !  rrq {2c}!          !
   !          !          !          ! <------- !          !
   !          !          ! <==(1)== !  rrp     !          !
   !          ! <==(2)== !  rrp     !          !          !
   !          !  rrp     !          !          !          !
   !          !          !          !          !          !
  [[--- connection setup ok, bidirectional connection up ---]]
   !          !          !          !          !          !
   ! -------> !          !          !          !          !
   !  pkt {3a}! ==(3)==> !          !          !          !
   !          !  pkt {3b}! ==(2)==> !          !          !
   !          !          !  pkt {3c}! ==(1)==> !          !
   !          !          !          !  pkt {3d}! -------> !
   !          !          !          !          !  pkt {3e}!
   !          !          !          !          ! <------- !
   !          !          !          ! <==(1)== !  pkt     !
   !          !          ! <==(2)== !  pkt     !          !
   !          ! <==(3)== !  pkt     !          !          !
   !  <------ !  pkt     !          !          !          !
   !   pkt    !          !          !          !          !
   :          :          :          :          :          :
   :          :          :          :          :          :

Vaarala & Klovning Standards Track [Page 35] RFC 5265 MIPv4-VPN June 2008

 The notation "==(N)==>" or "<==(N)==" indicates that the innermost
 packet has been encapsulated N times, using IP-IP, ESP, or MIP NAT
 traversal.
 Packets marked with {xx} are shown in more detail below.  Each area
 represents a protocol header (labeled).  Source and destination
 addresses or ports are shown underneath the protocol name when
 applicable.  Note that there are no NAT traversal headers in the
 example packets.
     Packet {1a}
         .------------------------------------.
         ! IP      ! IP      ! UDP   ! IKE    !
         !  co-CoA !  x-HoA  !  500  !        !
         !  x-HA   !  VPN-GW !  500  !        !
         `------------------------------------'
     Packet {2a}
         .--------------------------------------------------------.
         ! IP      ! IP      ! ESP   ! IP       ! UDP   ! MIP RRQ !
         !  co-CoA !  x-HoA  !       !  VPN-TIA !  ANY  !         !
         !  x-HA   !  VPN-GW !       !  i-HA    !  434  !         !
         `--------------------------------------------------------'
     Packet {2b}
         .----------------------------------------------.
         ! IP      ! ESP   ! IP       ! UDP   ! MIP RRQ !
         !  x-HoA  !       !  VPN-TIA !  ANY  !         !
         !  VPN-GW !       !  i-HA    !  434  !         !
         `----------------------------------------------'
     Packet {2c}
         .----------------------------.
         ! IP       ! UDP   ! MIP RRQ !
         !  VPN-TIA !  ANY  !         !
         !  i-HA    !  434  !         !
         `----------------------------'
     Packet {3a}
         .-------------------.
         ! IP     ! user     !
         !  i-HoA ! protocol !
         !  CN    !          !
         `-------------------'

Vaarala & Klovning Standards Track [Page 36] RFC 5265 MIPv4-VPN June 2008

     Packet {3b}
         .------------------------------------------------------- -
         ! IP      ! IP      ! ESP ! IP       ! IP     ! user      \
         !  co-CoA !  x-HoA  !     !  VPN-TIA !  i-HoA ! protocol../
         !  x-HA   !  VPN-GW !     !  i-HA    !  CN    !           \
         `------------------------------------------------------- -
            - - -----------------.
           \..user     ! ESP     !
           /  protocol ! trailer !
           \           !         !
            - - -----------------'
     Packet {3c}
         .--------------------------------------------------------.
         ! IP      ! ESP ! IP       ! IP     ! user     ! ESP     !
         !  x-HoA  !     !  VPN-TIA !  i-HoA ! protocol ! trailer !
         !  VPN-GW !     !  i-HA    !  CN    !          !         !
         `--------------------------------------------------------'
     Packet {3d}
         .------------------------------.
         ! IP       ! IP     ! user     !
         !  VPN-TIA !  i-HoA ! protocol !
         !  i-HA    !  CN    !          !
         `------------------------------'
     Packet {3e}
         .-------------------.
         ! IP     ! user     !
         !  i-HoA ! protocol !
         !  CN    !          !
         `-------------------'

Vaarala & Klovning Standards Track [Page 37] RFC 5265 MIPv4-VPN June 2008

 Packet {3b} with all NAT traversal headers (x-MIP, ESP, and i-MIP) is
 shown below for comparison.
     Packet {3b} (with NAT traversal headers)
         .------------------------------------------------- -
         ! IP      ! UDP  ! MIP    ! IP      ! UDP   ! ESP.. \
         !  co-CoA !  ANY ! tunnel !  x-HoA  !  4500 !       /
         !  x-HA   !  434 ! data   !  VPN-GW !  4500 !       \
         `------------------------------------------------- -
          <=== external MIPv4 ====> <=== IPsec ESP ======== = =
  1. - ———————————————— -

\..ESP ! IP ! UDP ! MIP ! IP ! user \

           /      !  VPN-TIA !  ANY ! tunnel !  i-HoA ! protocol../
           \      !  i-HA    !  434 ! data   !  CN    !           \
            - - ------------------------------------------------ -
            = ===> <==== internal MIPv4 ====> <== user packet == =
  1. - —————–.

\..user ! ESP !

           /  protocol ! trailer !
           \           !         !
            - - -----------------'
            = = ======> <= ESP =>

Authors' Addresses

 Sami Vaarala
 Codebay
 P.O. Box 63
 Espoo  02601
 FINLAND
 Phone: +358 (0)50 5733 862
 EMail: sami.vaarala@iki.fi
 Espen Klovning
 Birdstep
 Bryggegata 7
 Oslo  0250
 NORWAY
 Phone: +47 95 20 26 29
 EMail: espen@birdstep.com

Vaarala & Klovning Standards Track [Page 38] RFC 5265 MIPv4-VPN June 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Vaarala & Klovning Standards Track [Page 39]

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