





Network Working Group                                            C. Vogt
Request for Comments: 4832                   Universitaet Karlsruhe (TH)
Category: Informational                                         J. Kempf
                                                         DoCoMo USA Labs
                                                              April 2007


              Security Threats to Network-Based Localized
                      Mobility Management (NETLMM)

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 IETF Trust (2007).

Abstract

   This document discusses security threats to network-based localized
   mobility management.  Threats may occur on two interfaces: the
   interface between a localized mobility anchor and a mobile access
   gateway, as well as the interface between a mobile access gateway and
   a mobile node.  Threats to the former interface impact the localized
   mobility management protocol itself.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Threats to Interface between LMA and MAG . . . . . . . . . . .  3
     2.1.  LMA Compromise or Impersonation  . . . . . . . . . . . . .  3
     2.2.  MAG Compromise or Impersonation  . . . . . . . . . . . . .  4
     2.3.  Man-in-the-Middle Attack . . . . . . . . . . . . . . . . .  6
   3.  Threats to Interface between MAG and Mobile Node . . . . . . .  6
     3.1.  Mobile Node Compromise or Impersonation  . . . . . . . . .  7
     3.2.  Man-in-the-Middle Attack . . . . . . . . . . . . . . . . .  9
   4.  Threats from the Internet  . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10





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1.  Introduction

   The network-based localized mobility management (NETLMM) architecture
   [1] supports movement of IPv6 mobile nodes locally within a domain
   without requiring mobility support in the mobile nodes' network
   stacks.  A mobile node can keep its IP address constant as it moves
   from link to link, avoiding the signaling overhead and latency
   associated with changing the IP address.  Software specifically for
   localized mobility management is not required on the mobile node,
   whereas IP-layer movement detection software may be necessary, and
   driver software for link-layer mobility is prerequisite.

   The IP addresses of mobile nodes have a prefix that routes to a
   localized mobility anchor (LMA) [3].  The LMA maintains an individual
   route for each registered mobile node.  Any particular mobile node's
   route terminates at a mobile access gateway (MAG) [3], to which the
   mobile node attaches at its current access link.  MAGs are
   responsible for updating the mobile node's route on the LMA as the
   mobile node moves.  A MAG detects the arrival of a mobile node on its
   local access link based on handoff signaling that the mobile node
   pursues.  The MAG may additionally monitor connectivity of the mobile
   node in order to recognize when the mobile node has left the local
   access link.  The localized mobility management architecture
   therefore has two interfaces:

   1.  The interface between a MAG and an LMA where route update
       signaling occurs.

   2.  The interface between a mobile node and its current MAG where
       handoff signaling and other link maintenance signaling occur.

   The localized mobility management architecture demands no specific
   protocol for a MAG to detect the arrival or departure of mobile nodes
   to and from its local access link and accordingly initiate route
   update signaling with an LMA.  An appropriate mechanism may be
   entirely implemented at the link layer, such as is common for
   cellular networks.  In that case, the IP layer never detects any
   movement, even when a mobile node moves from one link to another
   handled by a different MAG.  If the link layer does not provide the
   necessary functionality, the mobile node must perform IP-layer
   movement detection and auto-configuration signaling, thereby
   providing the trigger for the MAG to update its route on the LMA.  A
   mobile node identity, established by the localized mobility
   management domain when the mobile node initially connects and
   authenticates, enables the MAG to ascribe the decisive link- or IP-
   layer signaling to the correct mobile node.  Some wireless access
   technologies may require the mobile node identity to be reestablished
   on every link-layer handoff.



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   Vulnerabilities in either interface of the localized mobility
   management architecture may entail new security threats that go
   beyond those that already exist in IPv6.  Potential attack objectives
   may be to consume network services at the cost of a legitimate mobile
   node, interpose in a mobile node's communications and possibly
   impersonate the mobile node from a position off-link, operate under
   the disguise of a false or non-existing identity, or cause denial of
   service to a mobile node or to the localized mobility management
   domain as a whole.  This document identifies and discusses security
   threats on both interfaces of the localized mobility management
   architecture.  It is limited to threats that are peculiar to
   localized mobility management; threats to IPv6 in general are
   documented in [4].

1.1.  Terminology

   The terminology in this document follows the definitions in [2], with
   those revisions and additions from [1].  In addition, the following
   definition is used:

   Mobile Node Identity

      An identity established for the mobile node when initially
      connecting to the localized mobility management domain.  It allows
      the localized mobility management domain to definitively and
      unambiguously identify the mobile node upon handoff for route
      update signaling purposes.  The mobile node identity is
      conceptually independent of the mobile node's IP or link-layer
      addresses, but it must be securely bound to the mobile node's
      handoff signaling.

2.  Threats to Interface between LMA and MAG

   The localized mobility management protocol executed on the interface
   between an LMA and a MAG serves to establish, update, and tear down
   routes for data plane traffic of mobile nodes.  Threats to this
   interface can be separated into compromise or impersonation of a
   legitimate LMA, compromise or impersonation of a legitimate MAG, and
   man-in-the-middle attacks.

2.1.  LMA Compromise or Impersonation

   A compromised LMA can ignore route updates from a legitimate MAG in
   order to deny service to a mobile node.  It may also be able to trick
   a legitimate MAG into creating a new, incorrect route, thereby
   preparing the MAG to receive redirected traffic of a mobile node; it
   may cause the traffic forwarded by a MAG to be redirected to a
   different LMA; or it may simply have the MAG drop an existing route



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   in order to deny the mobile node service.  Since data plane traffic
   for mobile nodes routes through the LMA, a compromised LMA can also
   intercept, inspect, modify, or drop such traffic, or redirect it to a
   destination in collusion with the attacker.  The attack can be
   conducted transiently to selectively disable traffic for any
   particular mobile node or MAG at particular times.

   Moreover, a compromised LMA may manipulate its routing table such
   that all packets are directed towards a single MAG.  This may result
   in a denial-of-service attack against that MAG and its attached
   access link.

   These threats also emanate from an attacker which tricks a MAG into
   believing that it is a legitimate LMA.  This attacker can cause the
   MAG to conduct route update signaling with the attacker instead of
   with the legitimate LMA, enabling it to ignore route updates from the
   MAG, or induce incorrect route changes at the MAG as described above,
   in order to redirect or deny a mobile node's traffic.  The attacker
   does not necessarily have to be on the original control plane path
   between the legitimate LMA and the MAG, provided that it can somehow
   make its presence known to the MAG.  Failure to mutually authenticate
   when establishing an association between an LMA and a MAG would allow
   an attacker to establish itself as a rogue LMA.

   The attacker may further be able to intercept, inspect, modify, drop,
   or redirect data plane traffic to and from a mobile node.  This is
   obvious if the attacker is on the original data plane path between
   the legitimate LMA and the mobile node's current MAG, which may
   happen independently of whether the attacker is on the original
   control plane path.  If the attacker is not on this path, it may be
   able to leverage the localized mobility management protocol to
   redefine the prefix that the mobile node uses in IP address
   configuration.  The attacker can then specify a prefix that routes to
   itself.  Whether or not outgoing data plane packets sourced by the
   mobile node can be interfered with by an attacker off the original
   data plane path depends on the specific data plane forwarding
   mechanism within the localized mobility management domain.  For
   example, if IP-in-IP encapsulation or an equivalent approach is used
   for outbound data plane packets, the packets can be forced to be
   routed through the attacker.  On the other hand, standard IP routing
   may cause the packets to be relayed via a legitimate LMA and hence to
   circumvent the attacker.

2.2.  MAG Compromise or Impersonation

   A compromised MAG can redirect a mobile node's traffic onto its local
   access link arbitrarily, without authorization from the mobile node.
   This threat is similar to an attack on a typical routing protocol



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   where a malicious stub router injects a bogus host route for the
   mobile node.  In general, forgery of a subnet prefix in link state or
   distance vector routing protocols requires support of multiple
   routers in order to obtain a meaningful change in forwarding
   behavior.  But a bogus host route is likely to take precedence over
   the routing information advertised by legitimate routers, which is
   usually less specific; hence, the attack should succeed even if the
   attacker is not supported by other routers.  A difference between
   redirection in a routing protocol and redirection in localized
   mobility management is that the former impacts the routing tables of
   multiple routers, whereas the latter involves only the compromised
   MAG and an LMA.

   Moreover, a compromised MAG can ignore the presence of a mobile node
   on its local access link and refrain from registering the mobile node
   at an LMA.  The mobile node then loses its traffic.  The compromised
   MAG may further be able to cause interruption to a mobile node by
   deregistering the mobile node at the serving LMA, pretending that the
   mobile node has powered down.  The mobile node then needs to
   reinitiate the network access authentication procedure, which the
   compromised MAG may prevent repeatedly until the mobile node moves to
   a different MAG.  The mobile node should be able to handle this
   situation, but the recovery process may be lengthy and hence impair
   ongoing communication sessions to a significant extent.

   Denial of service against an LMA is another threat of MAG subversion.
   The compromised MAG can trick an LMA into believing that a high
   number of mobile nodes have attached to the MAG.  The LMA will then
   establish a routing table entry for each of the non-existing mobile
   nodes.  The unexpected growth of the routing table may eventually
   cause the LMA to reject legitimate route update requests.  It may
   also decrease the forwarding speed for data plane packets due to
   higher route lookup latencies, and it may, for the same reason, slow
   down the responsiveness to control plane packets.  Another adverse
   side effect of a high number of routing table entries is that the
   LMA, and hence the localized mobility management domain as a whole,
   becomes more susceptible to flooding packets from external attackers
   (see Section 4).  The high number of superfluous routes increase the
   probability that a flooding packet, sent to a random IP address
   within the localized mobility management domain, matches an existing
   routing table entry at the LMA and gets tunneled to a MAG, which in
   turn performs address resolution on the local access link.  At the
   same time, fewer flooding packets can be dropped directly at the LMA
   on the basis of a nonexistent routing table entry.

   All of these threats apply not just to a compromised MAG, but also to
   an attacker that manages to counterfeit the identity of a legitimate
   MAG in interacting with both mobile nodes and an LMA.  Such an



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   attacker can behave towards mobile nodes like an authorized MAG and
   engage an LMA in route update signaling.  In a related attack, the
   perpetrator eavesdrops on signaling packets exchanged between a
   legitimate MAG and an LMA, and replays these packets at a later time.
   These attacks may be conducted transiently, to selectively disable
   traffic for any particular mobile node at particular times.

2.3.  Man-in-the-Middle Attack

   An attacker that manages to interject itself between a legitimate LMA
   and a legitimate MAG can act as a man in the middle with respect to
   both control plane signaling and data plane traffic.  If the attacker
   is on the original control plane path, it can forge, modify, or drop
   route update packets so as to cause the establishment of incorrect
   routes or the removal of routes that are in active use.  Similarly,
   an attacker on the original data plane path can intercept, inspect,
   modify, drop, and redirect data plane packets sourced by or destined
   to a mobile node.

   A compromised switch or router located between an LMA and a MAG can
   cause similar damage.  Any switch or router on the control plane path
   can forge, modify, or drop control plane packets, and thereby
   interfere with route establishment.  Any switch or router on the data
   plane path can intercept, inspect, modify, and drop data plane
   packets, or rewrite IP headers so as to divert the packets from their
   original path.

   An attacker between an LMA and a MAG may further impersonate the MAG
   towards the LMA, and vice versa in route update signaling.  The
   attacker can interfere with a route establishment even if it is not
   on the original control plane path between the LMA and the MAG.  An
   attacker off the original data plane path may undertake the same to
   cause inbound data plane packets destined to the mobile node to be
   routed first from the LMA to the attacker, then to the mobile node's
   MAG, and finally to the mobile node itself.  As explained in
   Section 2.1, here, too, it depends on the specific data plane
   forwarding mechanism within the localized mobility management domain
   whether or not the attacker can influence the route of outgoing data
   plane packets sourced by the mobile node.

3.  Threats to Interface between MAG and Mobile Node

   A MAG monitors the arrival and departure of mobile nodes to and from
   its local access link based on link- or IP-layer mechanisms.
   Whatever signaling on the access link is thereby decisive must be
   securely bound to the mobile node identity.  A MAG uses this binding
   to ascribe the signaling to the mobile node and accordingly initiate
   route update signaling with an LMA.  The binding must be robust to



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   spoofing because it would otherwise facilitate impersonation of the
   mobile node by a third party, denial of service, or man-in-the-middle
   attacks.

3.1.  Mobile Node Compromise or Impersonation

   An attacker that is able to forge the mobile node identity of a
   mobile node can trick a MAG into redirecting data plane packets for
   the mobile node to the attacker.  The attacker can launch such an
   impersonation attack against a mobile node that resides on the same
   link as the attacker, or against a mobile node on a different link.
   If the attack is on-link, the redirection of packets from the mobile
   node to the attacker is internal to the MAG, and it involves no route
   update signaling between the MAG and an LMA.  On-link attacks are
   possible in a regular IPv6 network [4] that does not use Secure
   Neighbor Discovery [5].

   Off-link impersonation requires the attacker to fabricate handoff
   signaling of the mobile node and thus trick the MAG into believing
   that the mobile node has handed over onto the MAG's access link.  The
   attack is conceivable both if the attacker and the mobile node are on
   separate links that connect to different MAGs, as well as if they are
   on separate, possibly virtual per-mobile-node links that connect to
   the same MAG.  In the former case, two MAGs would think they see the
   mobile node and both would independently perform route update
   signaling with the LMA.  In the latter case, route update signaling
   is likely to be performed only once, and the redirection of packets
   from the mobile node to the attacker is internal to the MAG.  The
   mobile node can always recapture its traffic back from the attacker
   through another run of handoff signaling.  But standard mobile nodes
   are generally not prepared to counteract this kind of attack, and
   even where network stacks include suitable functionality, the attack
   may not be noticeable early enough at the link or IP layer to quickly
   institute countermeasures.  The attack is therefore disruptive at a
   minimum, and may potentially persist until the mobile node initiates
   signaling again upon a subsequent handoff.

   Impersonation attacks can be prevented at the link layer,
   particularly with cellular technologies where the handoff signaling
   between the mobile node and the network must be authenticated and is
   completely controlled by the wireless link layer.  Cellular access
   technologies provide a variety of cryptographic and non-cryptographic
   attack barriers at the link layer, which makes mounting an
   impersonation attack, both on-link and off-link, very difficult.
   However, for non-cellular technologies that do not require link-layer
   authentication and authorization during handoff, impersonation
   attacks may be possible.




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   An attacker that can forge handoff signaling may also cause denial of
   service against the localized mobility management domain.  The
   attacker can trick a MAG into believing that a large number of mobile
   nodes have attached to the local access link and thus induce it to
   initiate route update signaling with an LMA for each mobile node
   assumed on link.  The result of such an attack is both superfluous
   signaling overhead on the control plane as well as a high number of
   needless entries in the LMA's and MAG's routing tables.  The
   unexpected growth of the routing tables may eventually cause the LMA
   to reject legitimate route update requests, and it may cause the MAG
   to ignore handoffs of legitimate mobile nodes onto its local access
   link.  It may also decrease the LMA's and MAG's forwarding speed for
   inbound and outbound data plane packets due to higher route lookup
   latencies, and it may for the same reason slow down their
   responsiveness to control plane packets.  An adverse side effect of
   this attack is that the LMA, and hence the localized mobility
   management domain as a whole, becomes more susceptible to flooding
   packets from external attackers (see Section 4).  The high number of
   superfluous routes increases the probability that a flooding packet,
   sent to a random IP address within the localized mobility management
   domain, matches an existing routing table entry at the LMA and gets
   tunneled to a MAG, which in turn performs address resolution on the
   local access link.  At the same time, fewer flooding packets can be
   dropped directly at the LMA on the basis of a nonexistent routing
   table entry.

   A threat related to the ones identified above, but not limited to
   handoff signaling, is IP spoofing [6].  Attackers use IP spoofing
   mostly for reflection attacks or to hide their identities.  The
   threat can be reasonably contained by a wide deployment of network
   ingress filtering [7] in routers, especially within access networks.
   This technique prevents IP spoofing to the extent that it ensures
   topological correctness of IP source address prefixes in to-be-
   forwarded packets.  Where the technique is deployed in an access
   router, packets are forwarded only if the prefix of their IP source
   address is valid on the router's local access link.  An attacker can
   still use a false interface identifier in combination with an on-link
   prefix.  But since reflection attacks typically aim at off-link
   targets, and the enforcement of topologically correct IP address
   prefixes also limits the effectiveness of identity concealment,
   network ingress filtering has proven adequate so far.  On the other
   hand, prefixes are not limited to a specific link in a localized
   mobility management domain, so merely ensuring topological
   correctness through ingress filtering becomes insufficient.  An
   additional mechanism for IP address ownership verification is
   necessary to prevent an attacker from sending packets with an off-
   link IP source address.




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3.2.  Man-in-the-Middle Attack

   An attacker that can interpose between a mobile node and a MAG during
   link- and/or IP-layer handoff signaling may be able to mount a man-
   in-the-middle attack on the mobile node, spoofing the mobile node
   into believing that it has a legitimate connection with the localized
   mobility management domain.  The attacker can thus intercept,
   inspect, modify, or drop data plane packets sourced by or destined to
   the mobile node.

4.  Threats from the Internet

   A localized mobility management domain uses individual host routes
   for data plane traffic of different mobile nodes, each between an LMA
   and a MAG.  Creation, maintenance, and deletion of these routes cause
   control traffic within the localized mobility management domain.
   These characteristics are transparent to mobile nodes as well as
   external correspondent nodes, but the functional differences within
   the domain may influence the impact that a denial-of-service attack
   from the outside world can have on the domain.

   A denial-of-service attack on an LMA may be launched by sending
   packets to arbitrary IP addresses that are potentially in use by
   mobile nodes within the localized mobility management domain.  Like a
   border router, the LMA is in a topological position through which a
   substantial amount of data plane traffic goes, so it must process the
   flooding packets and perform a routing table lookup for each of them.
   The LMA can discard packets for which the IP destination address is
   not registered in its routing table.  But other packets must be
   encapsulated and forwarded.  A target MAG as well as any mobile nodes
   attached to that MAG's local access link are also likely to suffer
   damage because the unrequested packets must be decapsulated and
   consume link bandwidth as well as processing capacities on the
   receivers.  This threat is in principle the same as for denial of
   service on a regular IPv6 border router, but because the routing
   table lookups may enable the LMA to drop part of the flooding packets
   early on or, on the contrary, additional tunneling workload is
   required for packets that cannot be dropped, the impact of an attack
   against localized mobility management may be different.

   In a related attack, the attacker manages to obtain a globally
   routable IP address of an LMA or a different network entity within
   the localized mobility management domain and perpetrates a denial-of-
   service attack against that IP address.  Localized mobility
   management is, in general, somewhat resistant to such an attack
   because mobile nodes need never obtain a globally routable IP address
   of any entity within the localized mobility management domain.
   Hence, a compromised mobile node cannot pass such an IP address off



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   to a remote attacker, limiting the feasibility of extracting
   information on the topology of the localized mobility management
   domain.  It is still possible for an attacker to perform IP address
   scanning if MAGs and LMAs have globally routable IP addresses, but
   the much larger IPv6 address space makes scanning considerably more
   time consuming.

5.  Security Considerations

   This document describes threats to network-based localized mobility
   management.  These may either occur on the interface between an LMA
   and a MAG, or on the interface between a MAG and a mobile node.
   Mitigation measures for the threats, as well as the security
   considerations associated with those measures, are described in the
   respective protocol specifications [3][8] for the two interfaces.

6.  Acknowledgments

   The authors would like to thank the NETLMM working group, especially
   Jari Arkko, Charles Clancy, Gregory Daley, Vijay Devarapalli,
   Lakshminath Dondeti, Gerardo Giaretta, Wassim Haddad, Andy Huang,
   Dirk von Hugo, Julien Laganier, Henrik Levkowetz, Vidya Narayanan,
   Phil Roberts, and Pekka Savola (in alphabetical order) for valuable
   comments and suggestions regarding this document.

7.  References

7.1.  Normative References

   [1]  Kempf, J., Ed., "Problem Statement for Network-Based Localized
        Mobility Management", RFC 4830, April 2007.

   [2]  Manner, J. and M. Kojo, "Mobility Related Terminology",
        RFC 3753, June 2004.

7.2.  Informative References

   [3]  Levkowetz, H., Ed., "The NetLMM Protocol", Work in Progress,
        October 2006.

   [4]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
        Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.

   [5]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
        Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [6]  CERT Coordination Center, "CERT Advisory CA-1996-21 TCP SYN
        Flooding and IP Spoofing Attacks", September 1996.



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   [7]  Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
        Denial of Service Attacks which employ IP Source Address
        Spoofing", BCP 38, RFC 2827, May 2000.

   [8]  Laganier, J., Narayanan, S., and F. Templin, "Network-based
        Localized Mobility Management Interface between Mobile Node and
        Access Router", Work in Progress, June 2006.

Authors' Addresses

   Christian Vogt
   Institute of Telematics
   Universitaet Karlsruhe (TH)
   P.O. Box 6980
   76128 Karlsruhe
   Germany

   EMail: chvogt@tm.uka.de


   James Kempf
   DoCoMo USA Labs
   3240 Hillview Avenue
   Palo Alto, CA 94304
   USA

   EMail: kempf@docomolabs-usa.com
























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