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IETF Mobile IP Working Group                                  T. Aura
INTERNET DRAFT                                              Microsoft
Expires August 2002                                          J. Arkko
                                                             Ericsson
                                                        February 2002
                   MIPv6 BU Attacks and Defenses
                 draft-aura-mipv6-bu-attacks-01.txt

Status of This Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026. Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF),
   its areas, and its working groups. Note that other groups may also
   distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet-Drafts
   as reference material or to cite them other than as "work in
   progress."

   The list of current Internet-Drafts can be accessed at
             http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
                 http://www.ietf.org/shadow.html.

Abstract

   This document overviews attacks against mobile and fixed IP nodes
   by exploiting features of the Mobile IPv6 Route Optimization. Many
   of the attacks can be prevented by authenticating the Mobile IPv6
   Binding Updates (BU) but some cannot, some depend on the details of
   the BU protocol, and some denial-of-service attacks specifically
   take advantage of authentication mechanisms. The purpose of this
   document is to help those involved in the design and
   standardization of BU authentication protocols to understand the
   attacks, to assess suggested protection mechanisms, and to choose
   between different levels of protection.












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Table of Contents

Status of This Memo...............................................1
Abstract..........................................................1
Table of Contents.................................................2
1. Introduction...................................................3
2. Attacks that Corrupt Routing Tables............................3
 2.1 Spoofing Binding Updates (o).................................4
 2.2 Attacks against Secrecy and Integrity (o)....................5
 2.3 Basic Denial of Service Attacks (o)..........................5
 2.4 Replaying and Blocking Binding Updates (o)...................6
 2.5 Bombing CoA with Unwanted Data...............................6
 2.6 Bombing HoA with unwanted data (*)...........................8
3. Authentication of Binding Updates..............................8
 3.1 Public Key Authentication (o)................................9
 3.2 Cryptographically Generated Addresses........................9
 3.3 Return Routability for HoA and CoA (*)......................10
 3.4 Assuming a safe route.......................................12
 3.5 Two Independent Routes......................................13
 3.6 Leap of Faith...............................................14
 3.7 The Role of Ingress Filtering (o)...........................14
4. DoS Attacks against BU Authentication.........................15
 4.1 Inducing Unnecessary Binding Updates........................15
 4.2 Consuming Authentication Resources..........................16
 4.3 Forcing Non-Optimized Routing (o)...........................17
 4.4 Reflection and Amplification (*)............................17
5. Preventing Resource Exhaustion................................18
 5.1 Delaying Commitment.........................................18
 5.2 Controlling Damage..........................................20
 5.3 Favoring Regular Customers..................................20
6. The Right Level of Protection.................................21
 6.1 Prioritizing the Goals (*)..................................21
 6.2 Multiple Levels of Authentication (*).......................22
7. Security Considerations.......................................24
8. Conclusions...................................................25
Acknowledgments..................................................25
References.......................................................25
Authors' Addresses............................................27













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1.  Introduction

   This document describes attacks against Mobile IPv6 [JP01] Route
   Optimization and related protection mechanisms. The goal of the
   attacker can be to corrupt the correspondent host's routing table
   (the Binding Cache) and to cause packets to be routed to a wrong
   address. This can compromise secrecy and integrity of communication
   and cause denial-of-service (DoS) both at the communicating parties
   and at the address that receives the unwanted packets. The attacker
   may also exploit features of a Binding Update (BU) protocol to
   exhaust the resources of either the mobile or the correspondent.
   The aim of this document is to describe the major attacks and to
   overview various protocol mechanisms and their limitations.

   In particular, we want to make known several attacks which should
   be considered when designing a protocol for authenticating BUs but
   which have only lately received attention at the Working Group
   (e.g. they were not mentioned in [MPH+01]). First, data flows can
   be redirected to flood a third party who is not taking part in the
   BU protocol (Sec. 2.5 -2.6 ). Second, some proposed BU
   authentication protocols can be broken by attackers located on the
   route from the correspondent to the Home Agent (Sec. 3.5 ). Third,
   an attacker can consume the resources of any mobile or
   correspondent by inducing authentic but unnecessary Binding Updates
   (Sec. 4.1 ).

   It is essential to understand that some of the threats are more
   serious than others, some can be mitigated but not removed, and
   some may be acceptable or too expensive to prevent. There are also
   various security mechanisms that provide different levels of
   guarantees for different security properties. We are particularly
   interested in mechanisms that allow authentication between
   arbitrary Internet nodes without pre-established trust
   relationships, public-key infrastructure (PKI), or trusted third
   parties (TTP). This means that some compromises must be made. The
   question for the protocol designer is, which design choices give an
   acceptable level of protection at an acceptable cost. Our goal is
   to help answering this question.

   We have marked with (o) the sections that contain only well-known
   material, which most readers are likely to be familiar with but
   which is included for completeness. Sections with major changes or
   additions since the previous version have been marked with a star
   (*).

2.  Attacks that Corrupt Routing Tables

   Route optimization and Binding Updates create a new opportunity for
   attackers. By sending false BUs, they can create false entries in


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   the correspondent host's Binding Cache and, thus, reroute IP
   packets to wrong destinations. If the data in the packets is not
   protected cryptographically, this can lead to compromise of secrecy
   and integrity. The attacker may also cause denial-of-service by
   keeping data from arriving at the right destination and by bombing
   a target host or network with unwanted data.

   We consider only active attackers. The rationale behind this is
   that in order to corrupt a routing table, the attacker must sooner
   or later send one or more messages. Thus, it makes little sense to
   consider attackers that only observe messages but do not send any.
   In fact, the active attacks are easier to for the average attacker
   than passive ones would be. In most active attacks, the attacker
   can initiate the BU protocol execution at any time while more
   passive attacks would require the attacker to wait for suitable
   messages to be sent by the targets hosts.

2.1  Spoofing Binding Updates (o)

   If Binding Updates are not authenticated, an attacker can send
   spoofed BUs. All Internet hosts are vulnerable to this attack
   because they all must support the correspondent functionality.
   There is also no way of telling which addresses belong to mobile
   hosts that really could send BUs. Consider an IP host A sending IP
   packets to another IP host B. The attacker can redirect the packets
   to an arbitrary address C by sending to A a Binding Update where
   the home address (HoA) is B and the care-of address (CoA) is C.
   After receiving this BU, A will send all packets intended for B to
   the address C.

   The attacker may select the CoA to be either its own current
   address (or another address in its local network) or any other IP
   address. If the attacker selects a local CoA where it can receive
   packets, it will be able to send further packets to a
   correspondent, which the correspondent believes to be coming from
   the mobile. Ingress filtering at the attacker's local network does
   not prevent the spoofing of Binding Updates but forces the attacker
   either to choose a CoA from inside its own network or to use the
   Alternate CoA sub-option. This may make it easier for the attack
   targets to selectively filter the spurious BUs at a firewall.

   The correspondent stores the HoA-CoA pair in its Binding Cache. The
   attacker needs to send a new BU every few minutes to refresh the
   cache entry.

   The attacker needs to know or guess the IP addresses of both the
   sender and receiver. This means that it is difficult to redirect
   all packets to or from a host because the attacker would need to
   know the IP addresses of all the hosts with which it is


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   communicating. Hosts with well-known addresses, such as servers and
   those using stateless auto-configuration, are most vulnerable.
   Hosts that are a part of the network infrastructure, such as DNS
   servers, are particularly interesting targets for attackers.

   Hosts with frequently changing random addresses and no DNS names
   are relatively safe. However, hosts that visit publicly accessible
   networks such as airport wireless LANs risk revealing their
   addresses. IPv6 addressing privacy features [ND01] mitigate these
   risks to an extent but it should be noted that addresses cannot be
   completely recycled while there are still open sessions that use
   those addresses.

2.2  Attacks against Secrecy and Integrity (o)

   By spoofing Binding Updates, an attacker can redirect packets
   between two IP hosts to itself. By sending a spoofed BU to A, it
   can capture the data intended to B.  It can pretend to be B and
   highjack B's connections with A, or establish new spoofed
   connections. The attacker can also send spoofed BUs to both A and B
   and insert itself to the middle of all connections between them
   (man-in-the-middle attack). Consequently, the attacker is able to
   see and modify the packets sent between A and B. The attacks are
   possible if the target host or its correspondent supports Route
   Optimization and the attacker knows their IP addresses.

   Strong encryption and integrity protection, such as authenticated
   IPSec, can prevent all the attacks against data secrecy and
   integrity. When the data is cryptographically protected, spoofed
   BUs can result in denial of service but not in disclosure or
   corruption of sensitive data beyond revealing the existence of the
   traffic flows. Two fixed hosts could also protect communication
   between themselves by refusing to accept BUs from each other.
   Ingress filtering, on the other hand, does not help because the
   attacker is using its own address as the CoA and is not spoofing
   source IP addresses.

   The attacks described above are a serious threat to the Internet
   for two reasons. First, a lot of Internet traffic is unprotected.
   Second, an attacker anywhere on the network can mount the attacks
   against any Internet hosts, also non-mobile ones. If Binding
   Updates did not exist, the attacker would need to be on the route
   between the attack targets in order to listen to the traffic
   between them.

2.3  Basic Denial of Service Attacks (o)

   By sending spoofed BUs, the attacker can redirect all packets sent
   between two IP hosts to a random or nonexistent address. This way,


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   it may be able to stop or disrupt communication between the hosts.
   The requirements are that the target host or its correspondent must
   support Route Optimization and the attacker must know their IP
   addresses.

   The attack is serious because any Internet host can be targeted,
   also fixed hosts. Hosts belonging to the infrastructure necessary
   for other hosts to communicate are also vulnerable. Again, two
   hosts can protect the communication between themselves by refusing
   BUs from each other or by establishing an authenticated IPSec
   tunnel for the BUs.

2.4  Replaying and Blocking Binding Updates (o)

   Any protocol for authenticating BUs will have to consider replay
   attacks. That is, an attacker may be able to replay recent
   authenticated BUs to the correspondent and, that way, direct
   packets to the mobile host's previous location. Like spoofed BUs,
   this can be used both for capturing packets and for DoS. The
   attacker can capture the packets and impersonate the mobile node if
   it reserves the mobile's previous address after the mobile has
   moved away and then replays the previous BU to redirect packets
   back to the previous location.

   The replays are a concern if a timestamps are used for checking the
   freshness of BUs and the mobile is moving so frequently that it
   sends the next BU before the timestamp in the previous BU has
   expired. Sequence numbers in authenticated BUs usually prevent the
   attack. The authentication protocol needs to be is carefully
   designed to avoid more complex replay attacks.

   In a related attack, the attacker blocks binding updates from the
   mobile at its new location, e.g. by jamming the radio link or by
   mounting a flooding attack, and takes over its connections at the
   old location. The attacker will be able to capture the packets sent
   to the mobile and to impersonate the mobile until the
   correspondent's Binding Cache entry expires.

   Both of the above attacks require the attacker to be on the same
   local network with the mobile, where it can relatively easily
   observe packets and block them even if the mobile does not move to
   a new location. Therefore, we believe that these attacks are not as
   serious as ones that can be mounted from remote locations.

2.5  Bombing CoA with Unwanted Data

   By sending spoofed BUs, the attacker can redirect traffic to an
   arbitrary IP address. This can be used to bomb an arbitrary
   Internet address with excessive amounts of data. The attacker can


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   also target a network by redirecting data to one or more IP
   addresses within the network.

   In the simplest attack, the attacker knows that there is a heavy
   data stream from host A to B and redirects this to the target
   address C. A will soon stop sending the data because it is not
   receiving acknowledgments from B.

   A more sophisticated attacker acts itself as B. It first subscribes
   to a data stream (e.g. a video stream from a news web site) and
   then redirects this to the target address C. The attacker may even
   be able to spoof the acknowledgements. For example, consider a
   constant-rate TCP stream. The attacker performs the TCP handshake
   itself and thus knows the initial sequence numbers. After
   redirecting the data to C, it suffices to send one spoofed
   acknowledgment per window. In a constant-rate stream, the attacker
   is able to predict the sequence numbers, the window size, and the
   right times for sending the acknowledgments. This way, the attacker
   is able to redirect a steady stream of unwanted data to the target
   address without doing much work itself. It can carry on opening
   more streams and refresh the Binding Cache entries by sending a new
   BU every few minutes.

   (TCP provides some protection against this attack: If C is the
   address of a real host, it will respond with TCP Reset, which
   should prompt A to close the connection. On the other hand, if C is
   a non-existent address, A will receive both the spoofed
   acknowledgments and ICMP Destination Unreachable messages.
   Depending on the implementation, A may ignore the error messages.
   Other transport-layer protocols might behave less gracefully.)

   The attack is serious because the target can be any host or
   network, not only mobile one. What makes it particularly serious
   compared to the other attacks is that the target itself cannot do
   anything to prevent the attack. For example, it does not help if
   the target network stops using Route Optimization. The damage is
   the worst if these techniques are used to amplify the effect of a
   distributed denial of service (DDoS) attack. Ingress filtering in
   the attacker's local network prevents the spoofing of source
   addresses but the attack is still possible by setting the Alternate
   CoA sub-option to the target address.

   The attacker needs to find a correspondent that is willing to send
   data streams to unauthenticated recipients. Many popular web sites
   provide such streams. If the target is a single host, the attacker
   needs to know or guess the target's IP address. On the other hand,
   if the target is an entire network, the attacker can congest the
   link toward that network by bombing random addresses within its
   routing prefix or group of prefixes. In some cases, a firewall on


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   the border of the target network may be able filter out data that
   is sent to nonexistent addresses. Whether this is possible depends
   on the way that the addresses within that network are managed. It
   seems likely that IPv6 addressing privacy features would preclude
   such filtering in the general case.

2.6  Bombing HoA with unwanted data (*)

   A variation of the bombing attack targets the HoA instead of the
   CoA. The attacker claims to be a mobile with the HoA equal to the
   target address. It starts downloading a data stream. The attacker
   then sends a BU cancellation (i.e. a request to delete the binding
   from the Binding Cache), or allows the cache entry to expire, which
   redirects the data stream to the HoA. Just like when bombing the
   CoA, the attacker can keep the stream alive by spoofing
   acknowledgments.

   When BUs are not authenticated, the attacker can choose an
   arbitrary address as the HoA and thus target any Internet node. BU
   authentication usually limits the attacker's choice of target
   address but care must be taken when designing the protocol. For
   example, one draft protocol verified the correctness of HoA (using
   a return routability test, see Sec. 3.3 ) only when a Binding Cache
   entry was created. This would allow the attacker to target any
   network where it has once owned an address, e.g. a public access
   LAN. A limit on the Binding Cache entry lifetimes or more frequent
   verification of HoA virtually prevents the attacks.

   When successful, the bombing attack against HoA is just as serious
   as the one against CoA. One mechanism is not usually enough to
   prevent both types of bombing attacks and they should be considered
   separately when designing a BU authentication protocol.

3.  Authentication of Binding Updates

   In order to prevent the corruption of correspondent routing tables,
   the Binding Updates must be authenticated. The two first
   subsections (Sec. 3.1 -3.2 ) discuss relatively strong, public-key
   authentication methods. The later subsections go into progressively
   weaker authentication methods that would be labeled as insecure in
   the traditional black-and-white network security thinking.
   Nevertheless, some of them (Sec. 3.3 -3.4 ) do provide well-defined
   levels of assurance in the real networks and can complement or even
   replace the stronger methods. The motivation is that instead of
   trying to prevent all attacks, the best strategy is often to limit
   the number of potential attackers that can attack a particular
   target, and to reduce the number of targets a potential attacker
   can threaten. In particular, limiting the number of attackers is
   essential in defending against denial-of-service attacks on the


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   Internet because it is rarely possible to entirely prevent these
   attacks.

   The current authors regard the cryptographically generated
   addresses (Sec. 3.2 ) and return routability tests (Sec. 3.3 ) as
   the most promising solutions for BU authentication. In situations
   where public-key cryptography is considered too expensive, the best
   solution may be be to assume a safe network route (Sec. 3.4 ).

3.1  Public Key Authentication (o)

   An obvious solution to Mobile IPv6 authentication would be to use a
   suite of strong generic authentication mechanisms such as IPSec,
   IKE, and a PKI. The current lack of a standard BU authentication
   protocol can thus be seen as a consequence of the commercial
   failure of PKIs and the slow pace of the IPSec standardization
   process.

   There are, however, other reasons (than unavailability) why the
   generic protocol suites may not be good for BU authentication.
   First, the generic authentication protocols have usually been
   designed with general-purpose computers and application-level
   security in mind. The computation and communication overhead of
   these protocols may be too high for low-end mobile devices and for
   a network-level signaling protocol. Second, the Mobile IPv6
   protocol is carefully designed to accommodate any Internet host as
   mobile and all hosts as correspondents. This means that a single
   PKI should cover the entire Internet, which is clearly a formidable
   goal when even local infrastructures have failed to emerge at the
   expected rate. Therefore, it is necessary to look for alternative
   solutions that do not rely on such global infrastructures.

   There are nevertheless situations where it is possible, and
   advisable, to apply the generic authentication solutions. In closed
   user groups and high-security environments, it may be possible to
   set up a PKI and require BUs to be strongly authenticated.

3.2  Cryptographically Generated Addresses

   A recently discovered technique provides an intermediate level of
   security below strong public-key authentication and above routing-
   based weak methods (which will be described in the following
   sections). The idea, originally introduced in a BU authentication
   protocol called CAM [OR01], is to form the last 64 bits of the IP
   address (the interface identifier) by hashing the host's public
   signature key. Binding Updates can then be signed with this key. A
   secure one-way hash function makes it difficult for the attacker to
   come up with a key that matches a given address and to forge signed
   BUs. The attraction of this technique is that it provides public-


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   key authentication independent of any trusted third parties, PKI,
   or other global infrastructure.

   The main weakness of the scheme is that only 62-64 bits of the IP
   address can be used for the hash. Thus, an attacker may be able to
   mount a brute force attack and find a matching signature key by
   trial and error. It is relatively expensive to generate strong
   signature keys but the attacker does not need to care about the
   strength of the keys. There may be relatively fast ways of
   generating weak signature keys, which the correspondent may not
   able to tell apart from strong ones. In order to make such brute-
   force attacks less attractive, one should include the routing
   prefix of the network into the hash. This forces the attacker to
   perform the search separately for each prefix. Without this, a
   global table of all hash values and their corresponding keys might
   become feasible in the future, even if that seems out of practical
   reach given current storage technology. A mechanism for generating
   new public keys and changing addresses at regular intervals should
   also discourage brute-force attacks against individual hosts.

   Another limitation of the cryptographically generated addresses
   (CGA) is that although they prevent the theft of another host's
   address, they do not stop the attacker from inventing new false
   addresses with an arbitrary routing prefix. The attacker can
   generate a public key and a matching IP address in any network and
   use it to mount bombing attacks (as described in Sec. 2.5 -2.6 ).

   While the public-key protocols (both PKI-based and CGA-based ones)
   provide a reasonable protection against unauthentic BUs, they are
   computationally intensive and therefore expose the participants to
   denial-of-service attacks (see Sec. 4.1 -4.2 ).

3.3  Return Routability for HoA and CoA (*)

   One way to limit the number of attackers and their targets is to
   test the return routability (RR) of the HoA and CoA. That is, the
   correspondent sends packets to both addresses and accepts Binding
   Updates only from mobiles that are able to receive them. Some
   malicious entities (e.g. ones on the correspondent's local network)
   may be able to capture one or both packets but most Internet users
   do not have such capabilities. A typical attacker is able to attack
   only the correspondents in its own local network. The RR test is,
   in fact, a variation of the cookie exchange, which has been used as
   part of the TCP handshake [SKK+97] and in authentication protocols,
   including Photuris [KS99] and IKE [HC98].

   Arguments have been made as to whether the RR test should be done
   for HoA, CoA, or both. In the following, we will explain what kind



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   of attacks each test prevents and conclude that both tests are
   needed.

   The RR test for HoA can be seen as a weaker alternative to CGA or
   other public-key authentication. In order to subvert the
   authentication, the attacker must be on the route between the
   correspondent and the HoA. But as we will see below, the
   routability test for HoA provides other guarantees in addition to
   mobile authentication, which make it is complementary, rather than
   alternative, to CGA authentication.

   The RR for CoA prevents the bombing of third parties at CoA (Sec.
   2.5 ). The attacker has to be able to receive the packet sent to
   the CoA, which means that the attacker must have recently visited
   the target network. This is a way of checking that the mobile is
   not lying about its location. Thus, the test has to be repeated
   every time the mobile moves to a new location. This provides a
   level of authentication but is not entirely secure because the
   attacker could be somewhere on the path from the correspondent to
   the CoA.

   The bombing attacks against HoA (Sec. 2.6 ) gets around the RR test
   for CoA (and other the defense mechanisms that somehow verify the
   correctness the CoA). The first logical reaction to this would be
   to verify the new location of the mobile no matter if it arrives to
   a new CoA or to HoA. Unfortunately, it may be impossible to execute
   a RR test for HoA when a Binding Cache entry is cancelled or
   expires. The mobile may be unreachable at that time or it may
   refuse to co-operate in the authentication protocol.

   What makes the situation difficult is that if the RR test for HoA
   fails, the correspondent cannot simply drop the cache entry because
   that is exactly what the attacker would want. However, we believe
   that it is desirable for the correspondent to be able to delete the
   Binding Cache entry at any time without executing any further
   protocols. Therefore, we suggest that the RR test for HoA should be
   performed regularly during the lifetime of a Binding Cache entry.
   That way, when the cache entry needs to be deleted for any reason
   (e.g. BU cancellation, expiring cache entry, or failing
   authentication), a recent RR test for HoA has always been
   performed. This prevents bombing of HoA unless the attacker has
   recently visited that address (or other on-path location where it
   can receive packets from the correspondent to the HoA).

   We propose the following guidelines for combining the mechanisms:

   - Before creating a Binding Cache entry, do authenticated key
     exchange with CGA, RR test for HoA, and RR test for CoA.



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   - Before a Binding Cache entry is updated with new CoA, do RR test
     for CoA.

   - Periodically do RR for HoA. The best time to do this is before
     refreshing the Binding Cache entry with unchanged CoA but the
     test must not be postponed indefinitely if the mobile changes
     its CoA in every BU. (One way to implement this is to limit the
     lifetime of Binding Cache entries.)

   The RR test for HoA could executed be more frequently, e.g. every
   few minutes, in protocols that rely on the RR test for
   authentication and less often, e.g. every hour, with CGA
   authentication. This is because CGA removes the need for the RR
   authentication function and also prevents the bombing of individual
   addresses. The benefit from such optimization is, however, limited,
   because the cost of the RR test is lowest when the mobile is not
   moving and the BU protocol can be executed in the background well
   before the Binding Cache entry expires. It might, therefore, be
   simplest to do both RR tests every time when the cache entry is
   updated without changing CoA.

   If the CGA authentication or one of RR tests fails, the
   correspondent should ignore the Binding Update and allow the
   Binding Cache entry expire.

   Interestingly, there is an argument for doing RR for CoA in the
   transport layer rather than in the IP layer. The flooding problem
   is related to flow control: the correspondent is redirecting a data
   flow into a new route and needs to verify that this route can
   accept the data. For TCP streams, the natural solution would be to
   reset the window size to the initial window size (1 or 2 packets).
   This would, in effect, test return routability of the new route
   before sending large amounts of data into it. However, mandating
   secure RR testing in all transport protocols and changing existing
   implementations would not be possible in practice. Therefore, the
   best solution is to do the RR tests in the IP layer.

3.4  Assuming a safe route

   Some proposed BU authentication protocols make the assumption that
   the communication between two specific hosts is safe from
   attackers, even though it is not cryptographically protected. For
   example, the return routability test for HoA (Sec. 3.3 ) can
   replace public-key authentication of the mobile if one is prepared
   to assume that the route from the correspondent to the mobile's
   Home Agent is secure. (Note that the RR test for HoA has two
   separate functions: authentication of the mobile and protection of
   the HoA against flooding.)



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   The assumption may be reasonable because the average Internet host
   cannot listen to or modify packets on the specific routers.
   Moreover, even an attacker in control of some routers can only
   interfere with communication between a limited number of hosts
   because most Internet traffic will not be routed through the
   compromised routers.

   The assumption can be justified also by the fact that an attacker
   on the route between two fixed hosts (a mobile at home and a
   correspondent) can mount equally damaging attacks against the
   communication between them.

3.5  Two Independent Routes

   Some weak BU authentication schemes have been proposed (Bake
   [NP01], HA Cookies [TO01]) where the security essentially depends
   on sending two pieces of the authentication data between the
   correspondent and the mobile or its Home Agent through two
   independent routes and hoping that most attackers are unable to
   capture both of them. This may not help as much as has perhaps been
   hoped. In all these protocols, a single attacker on the route
   between the correspondent and Home Agent can spoof BUs by
   pretending to be both the mobile and its Home Agent. If the
   attacker uses its own address as the false CoA, it can spoof
   packets from both the mobile and the Home Agent to the
   correspondent, and it can receive messages sent by the
   correspondent to both HoA and CoA. This means that the protocols
   essentially make the same assumption about a single safe route as
   we proposed in the previous section.

   Without ingress filtering at the attacker's local network, the
   attacker is, in fact, able to select an arbitrary CoA. Ingress
   filtering at the attacker's network forces the attacker to use a
   CoA from its local network. (The Alternate CoA sub-option could be
   used to get around this.) Ingress filtering also prevents attacks
   against the HA Cookie protocol because the attacker could not spoof
   packets from Home Agent to the correspondent.

   A false sense of security is created if one assumes that the three
   sides of the triangle in MIPv6 triangle routing are independent
   routes. This is usually the case for an authentic mobile, but need
   not be true not for an attacker who claims to be the mobile. When
   the false mobile (i.e. the attacker) is on the route between the
   correspondent and the Home Agent, the triangle collapses. (When
   thinking about the protocols one easily starts arguing in terms of
   active and passive attackers. It is worth remembering here that all
   attacks against BU protocols  are essentially active, as we
   explained at the beginning of Sec. 2.)



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   On the other hand, if one finds it reasonable to assume a safe
   route, i.e. that the attacker cannot listen to traffic between the
   correspondent and the Home Agent, then an equivalent level of
   security is achieved by a much simpler protocol: the correspondent
   sends a secret key in plaintext to HoA and Home Agent forwards it
   through a secure tunnel to the mobile. (This is a variation of the
   RR test for HoA.) We believe that however ridiculous this may
   sound, it may sometimes be the best solution for BU authentication.

3.6  Leap of Faith

   Yet another idea that has been floating around is that if the
   mobile sends a session key insecurely to the correspondent at the
   beginning of a connection, the key can be used to authenticate
   subsequent BUs (so called leap-of-faith authentication). This is a
   flawed proposition. It fails even if we assume that attacker is
   unlikely to capture the keys sent by authentic mobiles. First, the
   attacker can send its false key before the authentic mobile sends
   the authentic key. Second, there must be a recovery mechanism for
   situations where the mobile or the correspondent loses its state,
   and the attacker can exploit this mechanism.

   The leap-of-faith authentication is suitable for situations where a
   human user, or some other factor outside the attacker's control, at
   random times initiates the protocol. The party making the leap must
   always be the one that initiates the protocol. In such situations,
   it may be reasonable to argue that an attacker is unlikely to be
   present at the time of the unauthenticated key exchange. In BU
   authentication, the protocol is usually initiated by the mobile but
   the leap in faith should be made by the correspondent. Also, the
   attacker can trigger the BU protocol at any time by sending a
   spoofed packet from the correspondent to the mobile's HoA.

3.7  The Role of Ingress Filtering (o)

   Ingress filtering is another way of limiting the number of
   potential attackers and their targets. As we have noted above, many
   of the attacks against Route Optimization involve spoofed source IP
   addresses and are, thus, prevented by ingress filtering. There are
   two well-known weaknesses in this thinking, though. Firsts, ingress
   filtering must be applied on the attacker's local network; on the
   target network it makes no difference. Second, the Home Address
   Option and the Alternate Care-of Address sub-option can be used for
   similar source spoofing. While it is advisable to apply ingress
   filtering in as many networks as possible, one cannot rely on this
   to stop all attackers.





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4.  DoS Attacks against BU Authentication

   Security protocols that successfully protect the secrecy and
   integrity of data can sometimes make the participants more
   vulnerable to denial-of-service attacks. In fact, the stronger the
   authentication, the easier it may be for an attacker to use the
   protocol features to exhaust the mobile's or the correspondent's
   resources.

4.1  Inducing Unnecessary Binding Updates

   When a mobile host receives an IP packet from a new correspondent
   via the HoA, it automatically sends a Binding Update to that
   correspondent. The attacker can exploit this by sending the mobile
   spoofed IP packets (e.g. ping or TCP SYN packets) that appear to
   come from different correspondent addresses. The attacker will
   automatically start the BU protocol with these correspondents. If
   the correspondent addresses are real addresses of existing IP
   hosts, then most instances of the BU protocol will complete
   successfully. The entries created in the Binding Cache are correct
   but useless. This way, the attacker can with little work induce the
   mobile to execute the BU protocol unnecessarily, which can drain
   the mobile's resources.

   A correspondent (i.e. any IP host) can also be attacked in a
   similar way. The attacker sends spoofed IP packets to a large
   number of mobiles with the target host's address as the source
   address. These mobiles will all send Binding Updates to the target
   host. Again, most of the BU protocol executions will complete
   successfully. By inducing a large number of unnecessary BUs, the
   attacker is able to consume the target host's resources.

   This attack is possible against any BU authentication protocol. The
   more resources the Binding Update protocol consumes, the more
   serious the attack. Hence, strong cryptographic authentication
   protocol is more vulnerable to the attack than a weak one or
   unauthenticated BUs.

   A host should protect itself from the attack by setting a limit on
   the amount of resources, i.e. processing time, memory, and
   communications bandwidth, which it uses for processing BUs. When
   the limit is exceeded, the host can simply stop Route Optimization.
   (See Sec. 5.2 .) Sometimes it is possible to process some BUs even
   when a host is under the attack. A mobile host may have a local
   security policy listing a limited number of addresses to which BUs
   will be sent even when the mobile host is under DoS attack. A
   correspondent (i.e. any IP host) may similarly have a local
   security policy listing a limited set of addresses from which BUs
   will be accepted even when the correspondent is under a DoS attack.


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   The host may also recognize addresses with which they have had
   meaningful communication in the past and sent BUs to or accept them
   from those addresses.

   Ingress filtering at the attacker's local network mitigates the
   attacks. When flooding a mobile, the attacker can only choose
   correspondent addresses in its own local network. When flooding a
   correspondent, the attacker can only target correspondents in its
   own network. Unfortunately, the Home Address Option (HAO) can be
   used to circumvent the ingress filtering and to spoof packets from
   any correspondent. It has been questioned whether the HAO should be
   allowed in all packets, or only on those that are associated with
   an already successfully created Binding Cache Entry. The latter
   would prevent the use of HAOs in DoS attacks against BU
   authentication.

   In the following section, we will explain some additional DoS
   attacks and defense mechanisms. However, these attacks are
   generally no more serious than the ones described in this section.
   It usually does not pay to defend against the other types of
   attacks unless one can also prevent the attacks of this section.

4.2  Consuming Authentication Resources

   Authentication protocols are often vulnerable to flooding attacks
   that exploit the protocol features to consume the target host's
   resources. Computing power is consumed by flooding the host with
   messages that cause it to perform expensive cryptographic
   operations. If a host creates state during protocol execution, it
   is also vulnerable to attacks where the attacker starts excessive
   numbers of protocol runs and never finishes them.

   In order to exhaust the computing power of modern processors, the
   attacker needs to get them to perform public-key cryptographic
   operations. To do this, they may, for example, spoof large numbers
   of signed messages where the signatures are replaced by random
   numbers. The target host must verify the signatures before
   rejecting the messages. Symmetric encryption, cryptographic hash
   functions, and non-cryptographic computation are rarely the
   performance bottleneck. However, if the cryptographic library is
   optimized only for bulk data, it may behave inefficiently when the
   functions are invoked on millions of short messages and the keys
   are changed on every invocation.

   One way protocols can avoid the unnecessary public-key operations
   is to require a weak authentication, such as a cookie exchange,
   before doing the expensive computation. Attacks against stateful
   protocols can be prevented by making the protocol parties stateless
   until the honesty of the other participant has been proved or by


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   designing the stare management with flooding attacks in mind. (See
   Sec. 5.1 .)

4.3  Forcing Non-Optimized Routing (o)

   If the BUs are not authenticated, the attacker can prevent a
   correspondent from using Route Optimization by filling its Binding
   Cache with false entries so that most entries for real mobiles are
   dropped. With authenticated BUs, the attacker can mount the same
   attack by inducing unnecessary Binding Updates that create
   authentic but unnecessary cache entries (see Sec. 4.1 ).

   Any successful DoS attack against a mobile or a correspondent can
   also prevent the processing of BUs. We have repeatedly suggested
   that the target of a DoS attack may respond by stopping Route
   Optimization for all or some communication. Obviously, an attacker
   can exploit this fallback mechanism and force the target to use the
   less efficient triangle routing. The attacker only needs to mount a
   noticeable DoS attack against the mobile or correspondent, and the
   target will default to non-optimized routing.

   The target host can mitigate the effects of the attack by reserving
   more space for the Binding Cache, by reverting to non-optimized
   routing only when it cannot otherwise cope with the DoS attack, by
   trying aggressively to return to optimized routing, and by favoring
   mobiles with which it has an established relationship. This attack
   is not as serious as the ones described earlier, but applications
   that rely on Route Optimization could still be affected. For
   instance, conversational multimedia sessions can suffer drastically
   from the additional delays caused by triangle routing.

4.4  Reflection and Amplification (*)

   Attackers sometimes try to hide the source of a packet flooding
   attack by reflecting the traffic from other hosts [Sav02]. That is,
   instead of sending the flood of packets directly to the target, the
   attacker sends data to other hosts, tricking them to send the same
   number, or more, packets to the target. Such reflection can hide
   the attacker's address even when ingress filtering prevents source
   address spoofing. Reflection is particularly dangerous if the
   packets can be reflected multiple times, if they can be sent into a
   looping path, or if the hosts can be tricked into sending many more
   packets than they receive from the attacker, because such features
   can be used to amplify the traffic by a significant factor. When
   designing protocols, one should avoid creating services that can be
   used for reflection and amplification.

   Triangle routing easily creates opportunities for reflection:
   Correspondent receives packets (e.g. TCP SYN) from the mobile and


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   replies to the home address given by the mobile (in a HAO). The
   mobile might not really be a mobile and the HoA could actually be
   the target address. The target at the HoA only sees the packets
   sent by the correspondent and cannot see the attacker's address
   (even if ingress filtering prevents the attacker from spoofing its
   source address).

   The BU protocol could also be used for reflection: the
   correspondent responds to a data packet or to an unauthenticated BU
   by initiating the BU authentication protocol, which usually
   involves sending a packet to HoA. In that case, the reflection
   attack can be discouraged by copying the mobile's address into the
   messages sent by the mobile to the correspondent. (The mobile's
   source address is usually the same as the CoA but an Alternative
   CoA suboption can specify a different CoA.)

   In some proposed BU authentication protocols (e.g. one we co-
   authored [Roe02]), the correspondent responds to an initial message
   from the mobile with two packets (one to HoA, one to CoA). This can
   be used to amplify a flooding attack by a factor of two.
   Furthermore, with public-key authentication, the packets sent by
   the correspondent may be significantly larger than the one that
   triggers them.

   These types of reflection and amplification could be avoided by
   ensuring that the correspondent only responds to the same address
   from which it received a packet, and only with a single packet of
   the same size. Depending on the BU protocol, this might require an
   increased number of messages and a reverse tunneling mechanism for
   sending packets from the mobile through the Home Agent to the
   correspondent. The question that needs to be decided is whether the
   cost of these protections is more acceptable than threat created by
   a small constant factor of amplification. Padding could be used to
   equalize the packet sizes although we believe this would be
   unnecessary in practice. The mobile nodes usually have low-
   bandwidth access links, which makes them vulnerable to flooding
   attack, but high-value targets, such as public servers, rarely are
   mobile.

5.  Preventing Resource Exhaustion

   In this section, we describe defenses against denial-of-service
   attacks that exploit features of the BU protocol to exhaust the
   target host's resources. As it usually is impossible to completely
   prevent DoS attacks, the right approach is to increase the cost and
   difficulty of the attacks and to mitigate their effects.

5.1  Delaying Commitment



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   A standard protection against DoS attacks is to delay committing
   one's resources to the protocol until the other party has provided
   some assurance about its honesty. This assurance could, for
   example, be authentication or expensive computation of resources by
   the other party.

   Expensive public-key operations can be delayed by starting with a
   weak but inexpensive authentication mechanism and by proceeding
   with the public-key operations only after the weak authentication
   succeeds [Mea99]. This either limits the number of attackers who
   can get to the public-key stage or increases the cost of the attack
   by forcing the attacker to break the weak mechanism. For example, a
   BU authentication protocol could start with a cookie exchange or,
   preferably, with a RR test for both HoA and CoA (see Sec. 3.3 ),
   and continue with a public-key authentication only if the RR test
   succeeds.

   Memory space for storing protocol state is another resource that
   attackers can exhaust. The conventional way to defeat attacks that
   consume state storage is to reserve enough memory for storing the
   state data and to manage the storage efficiently. Another method is
   to remain stateless until the authentication is complete [AN97a].
   Hosts with little memory and implementations aiming for simplicity
   are particularly likely to find the stateless approach easier. We
   therefore recommend the that BU authentication protocol should
   allow stateless implementation of the correspondent. On the other
   hand, there is no compelling reason to require statelessness for
   hosts that can manage the state data in other ways.

   There are some difficulties in making the BU authentication
   protocol stateless, which should be understood. The main difficulty
   is that usually only the responder can be stateless, and it is not
   clear which party initiates the Binding Update process and which
   one responds. While the mobile normally initiates the
   authentication, this may be triggered by a packet belonging to
   another protocol that arrived from the correspondent. Moreover, if
   a packet from the correspondent triggers the BU, the correspondent
   may not know that this was the case. A further complication is that
   once an upper layer protocol has created a protocol state, it is of
   little value to refrain from doing so in the lower layers, but the
   IP layer cannot know when this is happening. For simplicity, we
   recommend making the correspondent stateless and advice against
   trying to guess which party initiated the protocol and whether the
   statelessness is necessary or not. We choose protection of the
   correspondent over the mobile for two reasons. First, any Internet
   host can be a correspondent. Second, it is less practical to make
   the mobile stateless because the mobile usually does not
   authenticate the correspondent and, thus, there is no clear point
   after which it is safe to create a state. We acknowledge that the


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   mobile is likely to have more stringent memory constraints than the
   correspondent and it may be left vulnerable to the state storage
   exhaustion attacks.

   Cryptographic puzzles [JB99][ANL00] are another proposed protection
   against resource-exhaustion attacks. A server requires its clients
   to solve a cryptographic puzzle (e.g. brute-force search for some
   input bits of a one-way function) before committing its own
   resources to the protocol. The server can adjust the difficulty of
   the puzzles according to its load. Solving the puzzle creates a
   small cost for each protocol invocation, which makes flooding
   attacks expensive but has little effect on honest hosts.
   Unfortunately, there are several drawbacks to this strategy in the
   Mobile IPv6 protocol. First, the IP layer does not know which one
   of the nodes, the mobile or the correspondent, is the server (i.e.
   the respondent). Second, mobile nodes can have extremely limited
   processor and battery capacity, which makes the cost of solving
   puzzles too high for them, while an attacker pretending to be a
   mobile is likely have much more computational capacity.  The puzzle
   protocols work well only when all clients have approximately equal
   computational capacity.

5.2  Controlling Damage

   As we suggested in Sec. 4.1 , a host can protect itself from
   flooding attacks by limiting the amount of resources it uses for BU
   authentication. It can set limits on the processor time, memory and
   communications capacity used for this purpose. When this limit is
   exceeded, the host should stop all further participation in BU
   authentication protocols. It may stop processing BUs and let all
   Binding Cache entries expire. Alternatively, it may continue to
   update entries already in the Binding Cache if there is a light-
   weight mechanism (e.g. a session key) for re-authenticating them.
   The effect of this is to stop Route Optimization for all
   communication, or only for new connections. The host may return to
   normal operation when it believes the attack is over.

   Since authentication cannot prevent all the attacks, every host
   SHOULD implement the limit on resource usage. A host that does not
   implement the limit will be vulnerable to flooding attacks but it
   will not cause any damage to other hosts.

5.3  Favoring Regular Customers

   A correspondent can give priority to mobiles with which it has a
   long-term relationship or recent meaningful communication in the
   upper protocols layers. The mobile may similarly favor selected
   correspondent addresses. The best way to secure Binding Updates
   when the hosts have a long-term relationship is to use an


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   authenticated IPSec tunnel. The following techniques should only be
   used when it is not possible to configure an IPSec Security
   Association.

   A host's local policy may have a list of addresses or address
   ranges, to and from which BUs are processed even when the host is
   under stress from attacks. These should not include addresses for
   which it is feasible to establish long-term IPSec Security
   Associations. The list of addresses should not be large or public
   because otherwise the attacker might be able to mount the attack by
   using only these addresses. Taking into account authentication and
   other displays of commitment in the upper protocol layers can be
   difficult to implement because it violates the stateless nature of
   the IP protocol layer and creates dependences between protocol
   layers. In some common situations, it may be worth while to violate
   the layering principle. For example, a Web server could accept BUs
   from its clients after it has successfully executed the TCP
   handshake.

   It may also help to keep updating the existing entries in the
   Binding Cache so that existing optimized routes can be maintained
   during the attack, although it is not certain that the existing
   cache entries belong to the most important mobiles or even to
   authentic ones. Some indication of this may be inferred from the
   packet counts associated with the traffic flowing through the
   entry.

6.  The Right Level of Protection

   We will conclude this document by discussing the criteria that
   should be used for selecting and comparing BU authentication
   protocols and issues that arise when there are several alternative
   protocols.

6.1  Prioritizing the Goals (*)

   The strength of the protection against DoS attacks that do not
   corrupt routing tables is independent of the strength of the BU
   authentication. As we have observed, strong authentication
   mechanisms may even enable DoS attacks that would not otherwise be
   possible. Nevertheless, the following principles apply to all
   attacks.

   - A protection mechanism MUST be implemented and used if security
     of other hosts or communication between other hosts depends on
     it (except in cases covered by the next bullet).

   - A protection mechanism SHOULD be implemented if communication
     between the host itself and others depends on it.


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   - A protection mechanism SHOULD be implemented if only the host's
     own security depends on it.

   - Some hosts, e.g. minimal implementations, MAY elect not to
     protect themselves and their own connections against all the
     attacks as long as that does not compromise the protection for
     others.

   Since Route Optimization is an optimization, a host MAY always
   protect itself and others by ignoring all Binding Updates, which
   means giving up Route Optimization. A slightly smarter host MAY
   implement only the mandatory mechanisms that help protect others
   and stop using Route Optimization when it detects a DoS attack
   against itself.

   The most serious attack is the bombing of third parties with
   unwanted data (Sec. 2.5 -2.6 ) because they cannot do anything to
   stop the flood of data. All mobiles and correspondents MUST either
   implement and use some kind of protection against this attack, or
   ignore all Binding Updates and give up Route Optimization.

   Attacks against correspondent hosts that accept Binding Updates
   form the second most serious class of attacks. This includes
   attacks on data secrecy and integrity as well as denial-of-service
   attacks. Every IP host is a potential correspondent and if the
   Mobile IP protocol makes them vulnerable, then every IP host is
   vulnerable. It is important that reliable protection mechanisms are
   available for the correspondents and every IP host SHOULD implement
   and use these mechanisms to protect itself. If the protection for
   the correspondents requires support from mobiles, all mobile hosts
   MUST implement and use the supporting functionality.

   Finally, there are the attacks against the mobiles. As with
   correspondents, it is important to have protection mechanisms for
   the mobiles and they SHOULD be implemented and used. Any necessary
   supporting functionality in the correspondents MUST be implemented
   and used. Although attacks against mobile hosts cannot bring down
   the Internet as we know it, the number and significance of mobiles
   will increase. If Mobile IPv6 is to become the primary mobility
   protocol in the Internet, it is essential to protect its
   reliability against malicious parties.

6.2  Multiple Levels of Authentication (*)

   The computational and communications capabilities of Internet hosts
   vary vastly, as does the level of security they require. It would,
   therefore, be desirable to have a range of BU authentication
   protocols with different cost and security trade-offs. For example,


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   closed high-security groups could use pre-established shared keys
   or a PKI, most hosts CCA authentication with return routability
   tests for DoS prevention, and low-end mobile devices a protocol
   based only on RR. However, care must be taken to accommodate the
   multiple levels of protection so that the attacker cannot bid down
   to the lowest level.

   In the end, the decision about accepting or not accepting a BU is
   made by the correspondent. Therefore, the correspondent will always
   make the final decision about the required level of authentication
   (e.g. CGA/RR or plain RR) for the particular HoA. Also, it makes
   little sense for the correspondent to allow multiple levels of
   authentication for the same HoA because the attacker could always
   tackle the weakest one. Thus, the mobile must either authenticate
   itself using the protocol chosen by the correspondent or give up
   Route Optimization. It makes little sense to negotiate the
   protocol.

   In order to conform to the principles of the previous section, any
   protocol chosen by the correspondent must provide whatever level of
   DoS protection is considered sufficient for the mobile and for
   third parties. Thus, the protocols will mostly differ in DoS
   protection for the correspondent itself, which the correspondent
   may freely tune, and in the strength of the authentication. It is
   worth noting that different correspondents can make their choices
   of authentication strength independently. This is because a weak
   mechanism accepted by one correspondent would not help the attacker
   to redirect packets to or from correspondents that use a stronger
   protocol.

   It is advisable that the default protocol, used for mobiles with
   which the correspondent has not prior relationship, be the
   strongest one which correspondent can afford under normal use.
   Typically, this could be CGA/RR. This default level of
   authentication MUST be statically configured at the correspondent
   and MUST NOT be adjusted according to the current load. Otherwise,
   the attacker could artificially induce the conditions under which a
   weaker protocol is chosen.

   The correspondent MAY have a local policy that mandates a higher
   level (e.g. shared key authentication or PKI) or lower level (e.g.
   plain RR) of authentication for a particular HoA or range of
   addresses. These addresses can be statically configured or
   dynamically determined by strong authentication or equivalent
   commitment shown by the mobile in upper protocol layers. For
   example, a correspondent could be configured to allow weaker
   authentication for some low-end mobile devices that benefit from
   Route Optimization but cannot afford to run the stronger
   authentication protocol.


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   It would be possible to allocate one bit of the IP address, similar
   to the "u" and "g" bits [HD98] in the interface identifier, to
   indicate whether the address is a CGA address. The correspondent
   would then select the BU protocol based on this bit. A disadvantage
   of this scheme is that the length of the public-key hash would
   become one bit shorter. Also, there may be increasing pressure to
   allocate bits of the IPv6 address for various purposes and the 64-
   bit interface identifier may soon prove to be surprisingly short.
   The problem can be mitigated by requiring only mobile hosts (rather
   than all IP nodes) to set the CGA-bit correctly.

   Based on the above, it should be recommended (or possibly even
   mandated) that the mobiles implement all common BU protocols (e.g.
   both CGA/RR and plain RR). If one does not, it may be unable to use
   Route Optimization with many correspondents. There is also the risk
   that business reasons will force practically all IP nodes to use
   the weakest level of authentication that is mandatory to implement
   and use. For example, if many low-end mobiles only implement the
   weakest standardized protocol, virtually all correspondents will
   default to this mechanism, which would defeat the purpose of having
   any stronger protocol.

7.  Security Considerations

   This document discusses security of Mobile IPv6 Route Optimization
   but does not present any specific protocols or detailed solutions.
   A separate specification of the actual BU authentication protocol
   is needed.

   There are other security concerns with Mobile IPv6 that are not
   addresses in this document. The Home Agent Discovery needs to be
   secured so that the mobile can establish a secure tunnel to a
   reliable Home Agent. Also, some features of the Mobile IPv6
   protocol may reduce the effectiveness of ingress filtering. The
   Home Address Option, the Alternative CoA sub-option, and possibly
   IPv6 in IPv6 tunnels, may be used to spoof the origin of packets
   without spoofing the source IP address.

   We assume that the mobile has chosen a reliable Home Agent and that
   Binding Updates and Acknowledgments between the mobile and its Home
   Agent are strongly authenticated with techniques outside the scope
   of this document. For example, there could be a manually keyed
   IPSec tunnel to a router on the mobile's Home Network. There are
   situations where no secure tunnel exist, for example if the mobile
   automatically contracts the services of temporary Home Agents along
   its path, but we feel that the security in such situations is based
   on much weaker assumptions and should be considered separately.



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   It seems necessary to send the Binding Acknowledgments from other
   sources than the Home agent without authentication. In most BU
   authentication protocols, only the mobile is authenticated, and
   without authenticating the correspondent, there is no way of
   authenticating the acknowledgments. The consequence is that an
   attacker can send false acknowledgments and cause the mobile to
   send the next BU sooner or later than the correspondent expects. At
   worst, the effect is that the Binding Cache entry expires and Route
   Optimization is not used. Thus, an attacker anywhere on the network
   can prevent the use of Route Optimization by sending false Binding
   Acknowledgments.

   Finally, where an authenticated IPSec tunnel between the mobile and
   the correspondent exists, Binding Updates can be sent through the
   tunnel. However, the return routability test for DoS prevention
   might still be needed. We feel that currently the simplest and most
   effective solution is to design the BU protocol independently of
   IPSec.

8.  Conclusions

   We have described attacks against Mobile IPv6 Route Optimization
   and mechanisms for protecting the protocol participants and third
   parties. Some of the attacks may be new in the sense that they have
   not been considered in earlier BU authentication requirements and
   protocol drafts. It is our hope that this document will help in
   designing BU authentication protocols and in the process of
   choosing the protocol or protocols that will be part of the Mobile
   IPv6 standard. We are also working on a family of protocols that
   takes into account the points of this document [Roe02].

Acknowledgments

   Many of the ideas originate from Mike Roe, Greg O'Shea, Pekka
   Nikander, Erik Nordmark and various Internet Drafts.

References

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  [ANL00]   Tuomas Aura, Pekka Nikander, and Jussipekka Leiwo. DOS-
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            pages 170-181, Cambridge, UK, April 2000. Springer.




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Authors' Addresses

   Tuomas Aura
   7 J J Thompson Avenue
   Cambridge, CB3 0FB
   United Kingdom

   Phone: +44 1223 479708
   Email: tuomaura@microsoft.com

   Jari Arkko
   Oy LM Ericsson Ab
   02420 Jorvas
   Finland

   Phone: +358 40 5079256
   Email: Jari.Arkko@ericsson.com




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