Network Working Group
Request for Comments: 4225
Category: Informational
P. Nikander
J. Arkko
Ericsson Research NomadicLab
T. Aura
Microsoft Research
G. Montenegro
Microsoft Corporation
E. Nordmark
Sun Microsystems
December 2005

Mobile IP Version 6 Route Optimization Security Design Background

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 © The Internet Society (2005).


This document is an account of the rationale behind the Mobile IPv6 (MIPv6) Route Optimization security design. The purpose of this document is to present the thinking and to preserve the reasoning behind the Mobile IPv6 security design in 2001 - 2002.

The document has two target audiences: (1) helping MIPv6 implementors to better understand the design choices in MIPv6 security procedures, and (2) allowing people dealing with mobility or multi-homing to avoid a number of potential security pitfalls in their designs.

Table of Contents

   1. Introduction ....................................................3
      1.1. Assumptions about the Existing IP Infrastructure ...........4
      1.2. The Mobility Problem and the Mobile IPv6 Solution ..........6
      1.3. Design Principles and Goals ................................8
         1.3.1. End-to-End Principle ..................................8
         1.3.2. Trust Assumptions .....................................8
         1.3.3. Protection Level ......................................8
      1.4. About Mobile IPv6 Mobility and its Variations ..............9
   2. Avenues of Attack ...............................................9
      2.1. Target ....................................................10
      2.2. Timing ....................................................10
      2.3. Location ..................................................11
   3. Threats and Limitations ........................................11
      3.1. Attacks Against Address 'Owners' ("Address Stealing").. ...12
         3.1.1. Basic Address Stealing ...............................12
         3.1.2. Stealing Addresses of Stationary Nodes ...............13
         3.1.3. Future Address Sealing ...............................14
         3.1.4. Attacks against Secrecy and Integrity ................15
         3.1.5. Basic Denial-of-Service Attacks ......................16
         3.1.6. Replaying and Blocking Binding Updates ...............16
      3.2. Attacks Against Other Nodes and Networks (Flooding) .......16
         3.2.1. Basic Flooding .......................................17
         3.2.2. Return-to-Home Flooding ..............................18
      3.3. Attacks against Binding Update Protocols ..................18
         3.3.1. Inducing Unnecessary Binding Updates .................19
         3.3.2. Forcing Non-Optimized Routing ........................20
         3.3.3. Reflection and Amplification .........................21
      3.4. Classification of Attacks .................................22
      3.5. Problems with Infrastructure-Based Authorization ..........23
   4. Solution Selected for Mobile IPv6 ..............................24
      4.1. Return Routability ........................................24
         4.1.1. Home Address Check ...................................26
         4.1.2. Care-of-Address Check ................................27
         4.1.3. Forming the First Binding Update .....................27
      4.2. Creating State Safely .....................................28
         4.2.1. Retransmissions and State Machine ....................29
      4.3. Quick expiration of the Binding Cache Entries .............29
   5. Security Considerations ........................................30
      5.1. Residual Threats as Compared to IPv4 ......................31
      5.2. Interaction with IPsec ....................................31
      5.3. Pretending to Be One's Neighbor ...........................32
      5.4. Two Mobile Nodes Talking to Each Other ....................33
   6. Conclusions ....................................................33
   7. Acknowledgements ...............................................34
   8. Informative References .........................................34

1. Introduction

Mobile IPv4 is based on the idea of supporting mobility on top of existing IP infrastructure, without requiring any modifications to the routers, the applications, or the stationary end hosts. However, in Mobile IPv6 [6] (as opposed to Mobile IPv4), the stationary end hosts may provide support for mobility, i.e., route optimization. In route optimization, a correspondent node (CN) (i.e., a peer for a mobile node) learns a binding between the mobile node's stationary home address and its current temporary care-of address. This binding is then used to modify the handling of outgoing (as well as the processing of incoming) packets, leading to security risks. The purpose of this document is to provide a relatively compact source for the background assumptions, design choices, and other information needed to understand the route optimization security design. This document does not seek to compare the relative security of Mobile IPv6 and other mobility protocols, or to list all the alternative security mechanisms that were discussed during the Mobile IPv6 design process. For a summary of the latter, we refer the reader to [1]. Even though incidental implementation suggestions are included for illustrative purposes, the goal of this document is not to provide a guide to implementors. Instead, it is to explain the design choices and rationale behind the current route optimization design. The authors participated in the design team that produced the design and hope, via this note, to capture some of the lessons and reasoning behind that effort.

The authors' intent is to document the thinking behind that design effort as it was. Even though this note may incorporate more recent developments in order to illustrate the issues, it is not our intent to present a new design. Rather, along with the lessons learned, there is some effort to clarify differing opinions, questionable assumptions, or newly discovered vulnerabilities, should such new information be available today. This is also very important, because it may benefit the working group's hindsight as it revises or improves the Mobile IPv6 specification.

To fully understand the security implications of the relevant design constraints, it is necessary to explore briefly the nature of the existing IP infrastructure, the problems Mobile IP aims to solve, and the design principles applied. In the light of this background, we can then explore IP-based mobility in more detail and have a brief look at the security problems. The background is given in the rest of this section, starting from Section 1.1.

Although the introduction in Section 1.1 may appear redundant to readers who are already familiar with Mobile IPv6, it may be valuable to read it anyway. The approach taken in this document is very different from that in the Mobile IPv6 specification. That is, we have explicitly aimed to expose the implicit assumptions and design choices made in the base Mobile IPv6 design, while the Mobile IPv6 specification aims to state the result of the design. By understanding the background, it is much easier to understand the source of some of the related security problems, and to understand the limitations intrinsic to the provided solutions.

In particular, this document explains how the adopted design for "Return Routability" (RR) protects against the identified threats (Section 3). This is true except for attacks on the RR protocol itself, which require other countermeasures based on heuristics and judicious implementation (Section 3.3).

The rest of this document is organized as follows: after this introductory section, we start by considering the avenues of attack in Section 2. The security problems and countermeasures are studied in detail in Section 3. Section 4 explains the overall operation and design choices behind the current security design. Section 5 analyzes the design and discuss the remaining threats. Finally, Section 6 concludes this document.

1.1. Assumptions about the Existing IP Infrastructure

One of the design goals in the Mobile IP design was to make mobility possible without changing too much. This was especially important for IPv4, with its large installed base, but the same design goals were inherited by Mobile IPv6. Some alternative proposals take a different approach and propose larger modifications to the Internet architecture (see Section 1.4).

To understand Mobile IPv6, it is important to understand the MIPv6 design view of the base IPv6 protocol and infrastructure. The most important base assumptions can be expressed as follows:

  1. The routing prefixes available to a node are determined by its current location, and therefore the node must change its IP address as it moves.
  1. The routing infrastructure is assumed to be secure and well functioning, delivering packets to their intended destinations as identified by destination address.

Although these assumptions may appear to be trivial, let us explore them a little further. First, in current IPv6 operational practice the IP address prefixes are distributed in a hierarchical manner. This limits the number of routing table entries each individual router needs to handle. An important implication is that the topology determines what globally routable IP addresses are available at a given location. That is, the nodes cannot freely decide what globally routable IP address to use; they must rely on the routing prefixes served by the local routers via Router Advertisements or by a DHCP server. In other words, IP addresses are just what the name says, addresses (i.e., locators).

Second, in the current Internet structure, the routers collectively maintain a distributed database of the network topology and forward each packet towards the location determined by the destination address carried in the packet. To maintain the topology information, the routers must trust each other, at least to a certain extent. The routers learn the topology information from the other routers, and they have no option but to trust their neighbor routers about distant topology. At the borders of administrative domains, policy rules are used to limit the amount of perhaps faulty routing table information received from the peer domains. While this is mostly used to weed out administrative mistakes, it also helps with security. The aim is to maintain a reasonably accurate idea of the network topology even if someone is feeding faulty information to the routing system.

In the current Mobile IPv6 design, it is explicitly assumed that the routers and the policy rules are configured in a reasonable way, and that the resulting routing infrastructure is trustworthy enough. That is, it is assumed that the routing system maintains accurate information of the network topology, and that it is therefore able to route packets to their destination locations. If this assumption is broken, the Internet itself is broken in the sense that packets go to wrong locations. Such a fundamental malfunction of the Internet would render hopeless any other effort to assure correct packet delivery (e.g., any efforts due to Mobile IP security considerations).

1.1.1. A Note on Source Addresses and Ingress Filtering

Some of the threats and attacks discussed in this document take advantage of the ease of source address spoofing. That is, in the current Internet it is possible to send packets with a false source IP address. The eventual introduction of ingress filtering is assumed to prevent this. When ingress filtering is used, traffic with spoofed addresses is not forwarded. This filtering can be applied at different network borders, such as those between an Internet service provider (ISP) and its customers, between downstream and upstream ISPs, or between peer ISPs [5]. Obviously, the granularity of ingress filters specifies how much you can "spoof inside a prefix". For example, if an ISP ingress filters a customer's link but the customer does nothing, anything inside the customer's /48 prefix could be spoofed. If the customer does filtering at LAN subnets, anything inside the /64 prefixes could be spoofed. Despite the limitations imposed by such "in-prefix spoofing", in general, ingress filtering enables traffic to be traceable to its real source network [5].

However, ingress filtering helps if and only if a large part of the Internet uses it. Unfortunately, there are still some issues (e.g., in the presence of site multi-homing) that, although not insurmountable, do require careful handling, and that are likely to limit or delay its usefulness [5].

1.2. The Mobility Problem and the Mobile IPv6 Solution

The Mobile IP design aims to solve two problems at the same time. First, it allows transport layer sessions (TCP connections, UDP- based transactions) to continue even if the underlying host(s) move and change their IP addresses. Second, it allows a node to be reached through a static IP address, a home address (HoA).

The latter design choice can also be stated in other words: Mobile IPv6 aims to preserve the identifier nature of IP addresses. That is, Mobile IPv6 takes the view that IP addresses can be used as natural identifiers of nodes, as they have been used since the beginning of the Internet. This must be contrasted to proposed and existing alternative designs where the identifier and locator natures of the IP addresses have been separated (see Section 1.4).

The basic idea in Mobile IP is to allow a home agent (HA) to work as a stationary proxy for a mobile node (MN). Whenever the mobile node is away from its home network, the home agent intercepts packets destined to the node and forwards the packets by tunneling them to the node's current address, the care-of address (CoA). The transport layer (e.g., TCP, UDP) uses the home address as a stationary identifier for the mobile node. Figure 1 illustrates this basic arrangement.

The basic solution requires tunneling through the home agent, thereby leading to longer paths and degraded performance. This tunneling is sometimes called triangular routing since it was originally planned that the packets from the mobile node to its peer could still traverse directly, bypassing the home agent.

    +----+                                       +----+
    | MN |=#=#=#=#=#=#=#=#=tunnel=#=#=#=#=#=#=#=#|#HA |
    +----+         ____________                  +-#--+
      | CoA    ___/            \_____              # Home Link
     -+-------/      Internet    * * *-*-*-*-#-#-#-#-----
             |               * *      |    * Home Address
              \___       * *    _____/   + * -+
                  \_____*______/         | MN |
                        *                + - -+
                      | CN |    Data path as     * * * *
                      +----+    it appears to correspondent node
                                Real data path   # # # #

Figure 1. Basic Mode of Operation in Mobile IPv6

To alleviate the performance penalty, Mobile IPv6 includes a mode of operation that allows the mobile node and its peer, a correspondent node (CN), to exchange packets directly, bypassing the home agent completely after the initial setup phase. This mode of operation is called route optimization (RO). When route optimization is used, the mobile node sends its current care-of address to the correspondent node, using binding update (BU) messages. The correspondent node stores the binding between the home address and care-of address into its Binding Cache.

Whenever MIPv6 route optimization is used, the correspondent node effectively functions in two roles. Firstly, it is the source of the packets it sends, as usual. Secondly, it acts as the first router for the packets, effectively performing source routing. That is, when the correspondent node is sending out packets, it consults its MIPv6 route optimization data structures and reroutes the packets, if necessary. A Binding Cache Entry (BCE) contains the home address and the care-of address of the mobile node, and records the fact that packets destined to the home address should now be sent to the destination address. Thus, it represents a local routing exception.

The packets leaving the correspondent node are source routed to the care-of address. Each packet includes a routing header that contains the home address of the mobile node. Thus, logically, the packet is first routed to the care-of address and then, virtually, from the care-of address to the home address. In practice, of course, the packet is consumed by the mobile node at the care-of address; the header just allows the mobile node to select a socket associated with the home address instead of one with the care-of address. However, the mechanism resembles source routing, as there is routing state involved at the correspondent node, and a routing header is used.

Nevertheless, this routing header is special (type 2) to avoid the risks associated with using the more general (type 0) variant.

1.3. Design Principles and Goals

The MIPv6 design and security design aimed to follow the end-to-end principle, to notice the differences in trust relationships between the nodes, and to be explicit about delivering a practical (instead of an over-ambitious) level of protection.

1.3.1. End-to-End Principle

Perhaps the leading design principle for Internet protocols is the so-called end-to-end principle [4][11]. According to this principle, it is beneficial to avoid polluting the network with state, and to limit new state creation to the involved end nodes.

In the case of Mobile IPv6, the end-to-end principle is applied by restricting mobility-related state primarily to the home agent. Additionally, if route optimization is used, the correspondent nodes also maintain a soft state relating to the mobile nodes' current care-of addresses, the Binding Cache. This can be contrasted to an approach that would use individual host routes within the basic routing system. Such an approach would create state on a huge number of routers around the network. In Mobile IPv6, only the home agent and the communicating nodes need to create state.

1.3.2. Trust Assumptions

In the Mobile IPv6 security design, different approaches were chosen for securing the communication between the mobile node and its home agent and between the mobile node and its correspondent nodes. In the home agent case, it was assumed that the mobile node and the home agent know each other through a prior arrangement, e.g., due to a business relationship. In contrast, it was strictly assumed that the mobile node and the correspondent node do not need to have any prior arrangement, thereby allowing Mobile IPv6 to function in a scalable manner, without requiring any configuration at the correspondent nodes.

1.3.3. Protection Level

As a security goal, Mobile IPv6 design aimed to be "as secure as the (non-mobile) IPv4 Internet" was at the time of the design, in the period 2001 - 2002. In particular, that means that there is little protection against attackers that are able to attach themselves between a correspondent node and a home agent. The rationale is simple: in the 2001 Internet, if a node was able to attach itself to the communication path between two arbitrary nodes, it was able to disrupt, modify, and eavesdrop all the traffic between the two nodes, unless IPsec protection was used. Even when IPsec was used, the attacker was still able to block communication selectively by simply dropping the packets. The attacker in control of a router between the two nodes could also mount a flooding attack by redirecting the data flows between the two nodes (or, more practically, an equivalent flow of bogus data) to a third party.

1.4. About Mobile IPv6 Mobility and its Variations

Taking a more abstract angle, IPv6 mobility can be defined as a mechanism for managing local exceptions to routing information in order to direct packets that are sent to one address (the home address) to another address (the care-of address). It is managing in the sense that the local routing exceptions (source routes) are created and deleted dynamically, according to instructions sent by the mobile node. It is local in the sense that the routing exceptions are valid only at the home agent, and in the correspondent nodes if route optimization is used. The created pieces of state are exceptions in the sense that they override the normal topological routing information carried collectively by the routers.

Using the terminology introduced by J. Noel Chiappa [14], we can say that the home address functions in the dual role of being an end- point identifier (EID) and a permanent locator. The care-of address is a pure, temporary locator, which identifies the current location of the mobile node. The correspondent nodes effectively perform source routing, redirecting traffic destined to the home address to the care-of address. This is even reflected in the packet structure: the packets carry an explicit routing header.

The relationship between EIDs and permanent locators has been exploited by other proposals. Their technical merits and security problems, however, are beyond the scope of this document.

2. Avenues of Attack

From the discussion above, it should now be clear that the dangers that Mobile IPv6 must protect from lie in creation (or deletion) of the local routing exceptions. In Mobile IPv6 terms, the danger is in the possibility of unauthorized creation of Binding Cache Entries (BCE). The effects of an attack differ depending on the target of the attack, the timing of the attack, and the location of the attacker.

2.1. Target

   Basically, the target of an attack can be any node or network in the
   Internet (stationary or mobile).  The basic differences lie in the
   goals of the attack: does the attacker aim to divert (steal) the
   traffic destined to and/or sourced at the target node, or does it aim
   to cause denial-of-service to the target node or network?  The target
   does not typically play much of an active role attack.  As an
   example, an attacker may launch a denial-of-service attack on a given
   node, A, by contacting a large number of nodes, claiming to be A, and
   subsequently diverting the traffic at these other nodes so that A is
   no longer able to receive packets from those nodes.  A itself need
   not be involved at all before its communications start to break.
   Furthermore, A is not necessarily a mobile node; it may well be

Mobile IPv6 uses the same class of IP addresses for both mobile nodes (i.e., home and care-of addresses) and stationary nodes. That is, mobile and stationary addresses are indistinguishable from each other. Attackers can take advantage of this by taking any IP address and using it in a context where, normally, only mobile (home or care-of) addresses appear. This means that attacks that otherwise would only concern mobile nodes are, in fact, a threat to all IPv6 nodes.

In fact, a mobile node appears to be best protected, since a mobile node does not need to maintain state about the whereabouts of some remote nodes. Conversely, the role of being a correspondent node appears to be the weakest, since there are very few assumptions upon which it can base its state formation. That is, an attacker has a much easier task in fooling a correspondent node to believe that a presumably mobile node is somewhere it is not, than in fooling a mobile node itself into believing something similar. On the other hand, since it is possible to attack a node indirectly by first targeting its peers, all nodes are equally vulnerable in some sense. Furthermore, a (usually) mobile node often also plays the role of being a correspondent node, since it can exchange packets with other mobile nodes (see also Section 5.4).

2.2. Timing

An important aspect in understanding Mobile IPv6-related dangers is timing. In a stationary IPv4 network, an attacker must be between the communication nodes at the same time as the nodes communicate. With the Mobile IPv6 ability of creating binding cache entries, the situation changes. A new danger is created. Without proper protection, an attacker could attach itself between the home agent and a correspondent node for a while, create a BCE at the correspondent node, leave the position, and continuously update the correspondent node about the mobile node's whereabouts. This would make the correspondent node send packets destined to the mobile node to an incorrect address as long as the BCE remained valid, i.e., typically until the correspondent node is rebooted. The converse would also be possible: an attacker could also launch an attack by first creating a BCE and then letting it expire at a carefully selected time. If a large number of active BCEs carrying large amounts of traffic expired at the same time, the result might be an overload towards the home agent or the home network. (See Section 3.2.2 for a more detailed explanation.)

2.3. Location

In a static IPv4 Internet, an attacker can only receive packets destined to a given address if it is able to attach itself to, or to control, a node on the topological path between the sender and the recipient. On the other hand, an attacker can easily send spoofed packets from almost anywhere. If Mobile IPv6 allowed sending unprotected Binding Updates, an attacker could create a BCE on any correspondent node from anywhere in the Internet, simply by sending a fraudulent Binding Update to the correspondent node. Instead of being required to be between the two target nodes, the attacker could act from anywhere in the Internet.

In summary, by introducing the new routing exception (binding cache) at the correspondent nodes, Mobile IPv6 introduces the dangers of time and space shifting. Without proper protection, Mobile IPv6 would allow an attacker to act from anywhere in the Internet and well before the time of the actual attack. In contrast, in the static IPv4 Internet, the attacking nodes must be present at the time of the attack and they must be positioned in a suitable way, or the attack would not be possible in the first place.

3. Threats and Limitations

This section describes attacks against Mobile IPv6 Route Optimization and what protection mechanisms Mobile IPv6 applies against them. The goal of the attacker can be to corrupt the correspondent node's binding cache and to cause packets to be delivered 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 the Binding Update (BU) mechanism to exhaust the resources of the mobile node, the home agent, or the correspondent nodes. The aim of this section is to provide an overview of the various protocol mechanisms and their limitations. The details of the mechanisms are covered in Section 4.

It is essential to understand that some of the threats are more serious than others, that some can be mitigated but not removed, that some threats may represent acceptable risk, and that some threats may be considered too expensive to the attacker to be worth preventing.

We consider only active attackers. The rationale behind this is that in order to corrupt the binding cache, 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, some active attacks are easier, for the average attacker, to launch than a passive one would be. That is, in many active attacks the attacker can initiate binding update processing at any time, while most passive attacks require the attacker to wait for suitable messages to be sent by the target nodes.

   Nevertheless, an important class of passive attacks remains:  attacks
   on privacy.  It is well known that simply by examining packets,
   eavesdroppers can track the movements of individual nodes (and
   potentially, users) [3].  Mobile IPv6 exacerbates the problem by
   adding more potentially sensitive information into the packets (e.g.,
   Binding Updates, routing headers or home address options).  This
   document does not address these attacks.

We first consider attacks against nodes that are supposed to have a specified address (Section 3.1), continuing with flooding attacks (Section 3.2) and attacks against the basic Binding Update protocol (Section 3.3). After that, we present a classification of the attacks (Section 3.4). Finally, we consider the applicability of solutions relying on some kind of a global security infrastructure (Section 3.5).

3.1. Attacks Against Address 'Owners' ("Address Stealing")

The most obvious danger in Mobile IPv6 is address "stealing", when an attacker illegitimately claims to be a given node at a given address and tries to "steal" traffic destined to that address. We first describe the basic variant of this attack, follow with a description of how the situation is affected if the target is a stationary node, and continue with more complicated issues related to timing (so called "future" attacks), confidentiality and integrity, and DoS aspects.

3.1.1. Basic Address Stealing

If Binding Updates were not authenticated at all, an attacker could fabricate and send spoofed binding updates from anywhere in the Internet. All nodes that support the correspondent node functionality would become unwitting accomplices to this attack. As explained in Section 2.1, there is no way of telling which addresses belong to mobile nodes that really could send binding updates and which addresses belong to stationary nodes (see below), so potentially any node (including "static" nodes) is vulnerable.

        +---+  original       +---+ new packet   +---+
        | B |<----------------| A |- - - - - - ->| C |
        +---+  packet flow    +---+ flow         +---+
                                | False BU: B -> C
                            | Attacker |

Figure 2. Basic Address Stealing

Consider an IP node, A, sending IP packets to another IP node, B. The attacker could redirect the packets to an arbitrary address, C, by sending a Binding Update to A. The home address (HoA) in the binding update would be B and the care-of address (CoA) would be C. After receiving this binding update, A would send all packets intended for the node B to the address C. See Figure 2.

The attacker might select the care-of address to be either its own current address, another address in its local network, or any other IP address. If the attacker selected a local care-of address allowing it to receive the packets, it would be able to send replies to the correspondent node. Ingress filtering at the attacker's local+ network does not prevent the spoofing of Binding Updates but forces the attacker either to choose a care-of address from inside its own network or to use the Alternate care-of address sub-option.

The binding update authorization mechanism used in the MIPv6 security design is primarily intended to mitigate this threat, and to limit the location of attackers to the path between a correspondent node and the home agent.

3.1.2. Stealing Addresses of Stationary Nodes

The attacker needs to know or guess the IP addresses of both the source of the packets to be diverted (A in the example above) and the destination of the packets (B, above). This means that it is difficult to redirect all packets to or from a specific node because the attacker would need to know the IP addresses of all the nodes with which it is communicating.

Nodes with well-known addresses, such as servers and those using stateful configuration, are most vulnerable. Nodes that are a part of the network infrastructure, such as DNS servers, are particularly interesting targets for attackers and particularly easy to identify.

Nodes that frequently change their address and use random addresses are relatively safe. However, if they register their address into Dynamic DNS, they become more exposed. Similarly, nodes that visit publicly accessible networks such as airport wireless LANs risk revealing their addresses. IPv6 addressing privacy features [3] mitigate these risks to an extent, but note that addresses cannot be completely recycled while there are still open sessions that use those addresses.

Thus, it is not the mobile nodes that are most vulnerable to address stealing attacks; it is the well-known static servers. Furthermore, the servers often run old or heavily optimized operating systems and may not have any mobility related code at all. Thus, the security design cannot be based on the idea that mobile nodes might somehow be able to detect whether someone has stolen their address, and reset the state at the correspondent node. Instead, the security design must make reasonable measures to prevent the creation of fraudulent binding cache entries in the first place.

3.1.3. Future Address Sealing

If an attacker knows an address that a node is likely to select in the future, it can launch a "future" address stealing attack. The attacker creates a Binding Cache Entry with the home address that it anticipates the target node will use. If the Home Agent allows dynamic home addresses, the attacker may be able to do this legitimately. That is, if the attacker is a client of the Home Agent and is able to acquire the home address temporarily, it may be able to do so and then to return the home address to the Home Agent once the BCE is in place.

Now, if the BCE state had a long expiration time, the target node would acquire the same home address while the BCE is still effective, and the attacker would be able to launch a successful man-in-the- middle or denial-of-service attack. The mechanism applied in the MIPv6 security design is to limit the lifetime of Binding Cache Entries to a few minutes.

Note that this attack applies only to fairly specific conditions. There are also some variations of this attack that are theoretically possible under some other conditions. However, all of these attacks are limited by the Binding Cache Entry lifetime, and therefore they are not a real concern with the current design.

3.1.4. Attacks against Secrecy and Integrity

By spoofing Binding Updates, an attacker could redirect all packets between two IP nodes to itself. By sending a spoofed binding update to A, it could capture the data intended to B. That is, it could pretend to be B and highjack A's connections with B, or it could establish new spoofed connections. The attacker could also send spoofed binding updates to both A and B and insert itself in the middle of all connections between them (man-in-the-middle attack). Consequently, the attacker would be able to see and modify the packets sent between A and B. See Figure 3.

     Original data path, before man-in-the-middle attack
          +---+                               +---+
          | A |                               | B |
          +---+                               +---+

Modified data path, after the falsified binding updates

          +---+                               +---+
          | A |                               | B |
          +---+                               +---+
            \                                  /
             \                                /
              \          +----------+        /
               \---------| Attacker |-------/
                       Figure 3.  Man-in-the-Middle Attack

Strong end-to-end 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 binding updates could result in denial of service (see below) but not in disclosure or corruption of sensitive data beyond revealing the existence of the traffic flows. Two fixed nodes could also protect communication between themselves by refusing to accept binding updates from each other. Ingress filtering, on the other hand, does not help, as the attacker is using its own address as the care-of address and is not spoofing source IP addresses.

The protection adopted in MIPv6 Security Design is to authenticate (albeit weakly) the addresses by return routability (RR), which limits the topological locations from which the attack is possible (see Section 4.1).

3.1.5. Basic Denial-of-Service Attacks

By sending spoofed binding updates, the attacker could redirect all packets sent between two IP nodes to a random or nonexistent address (or addresses). As a result, it might be able to stop or disrupt communication between the nodes. This attack is serious because any Internet node could be targeted, including fixed nodes belonging to the infrastructure (e.g., DNS servers) that are also vulnerable. Again, the selected protection mechanism is return routability (RR).

3.1.6. Replaying and Blocking Binding Updates

Any protocol for authenticating binding updates has to consider replay attacks. That is, an attacker may be able to replay recently authenticated binding updates to the correspondent and, consequently, to direct packets to the mobile node's previous location. As with spoofed binding updates, this could be used both for capturing packets and for DoS. The attacker could capture the packets and impersonate the mobile node if it reserved the mobile's previous address after the mobile node has moved away and then replayed the previous binding update to redirect packets back to the previous location.

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. The attacker then takes over the mobile's 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 that the attacker 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. The limited lifetime of the Binding Cache entry and the associated nonces limit the time frame within which the replay attacks are possible. Replay protection is provided by the sequence number and MAC in the Binding Update. To not undermine this protection, correspondent nodes must exercise care upon deleting a binding cache entry, as per section 5.2.8 ("Preventing Replay Attacks") in [6].

3.2. Attacks Against Other Nodes and Networks (Flooding)

By sending spoofed binding updates, an attacker could redirect traffic to an arbitrary IP address. This could be used to overload an arbitrary Internet address with an excessive volume of packets (known as a 'bombing attack'). The attacker could also target a network by redirecting data to one or more IP addresses within the network. There are two main variations of flooding: basic flooding and return-to-home flooding. We consider them separately.

3.2.1. Basic Flooding

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

        (B is attacker)
        +---+  original       +---+ flooding packet   +---+
        | B |<================| A |==================>| C |
        +---+  packet flow    +---+ flow              +---+
         |                      ^
          \                    /
          False binding update + false acknowledgements

Figure 4. Basic Flooding Attack

A more sophisticated attacker would act itself as B; see Figure 4. It would first subscribe to a data stream (e.g., a video stream) and redirect this stream to the target address C. The attacker would even be able to spoof the acknowledgements. For example, consider a TCP stream. The attacker would perform the TCP handshake itself and thus know the initial sequence numbers. After redirecting the data to C, the attacker would continue to send spoofed acknowledgements. It would even be able to accelerate the data rate by simulating a fatter pipe [12].

This attack might be even easier with UDP/RTP. The attacker could create spoofed RTCP acknowledgements. Either way, the attacker would be able to redirect an increasing stream of unwanted data to the target address without doing much work itself. It could carry on opening more streams and refreshing the Binding Cache entries by sending a new binding update every few minutes. Thus, the limitation of BCE lifetime to a few minutes does not help here without additional measures.

During the Mobile IPv6 design process, the effectiveness of this attack was debated. It was mistakenly assumed that the target node would send a TCP Reset to the source of the unwanted data stream, which would then stop sending. In reality, all practical TCP/IP implementations fail to send the Reset. The target node drops the unwanted packets at the IP layer because it does not have a Binding

Update List entry corresponding to the Routing Header on the incoming packet. Thus, the flooding data is never processed at the TCP layer of the target node, and no Reset is sent. This means that the attack using TCP streams is more effective than was originally believed.

This attack is serious because the target can be any node or network, not only a 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 compounded if these techniques are used to amplify the effect of other distributed denial-of-service (DDoS) attacks. Ingress filtering in the attacker's local network prevents the spoofing of source addresses but the attack would still be possible by setting the Alternate care-of address sub-option to the target address.

Again, the protection mechanism adopted for MIPv6 is return routability. This time it is necessary to check that there is indeed a node at the new care-of address, and that the node is the one that requested redirecting packets to that very address (see Section 4.1.2).

3.2.2. Return-to-Home Flooding

A variation of the bombing attack would target the home address or the home network instead of the care-of address or a visited network. The attacker would claim to be a mobile with the home address equal to the target address. While claiming to be away from home, the attacker would start downloading a data stream. The attacker would then send a binding update cancellation (i.e., a request to delete the binding from the Binding Cache) or just allow the cache entry to expire. Either would redirect the data stream to the home network. As when bombing a care-of address, the attacker can keep the stream alive and even increase the data rate by spoofing acknowledgements. When successful, the bombing attack against the home network is just as serious as that against a care-of address.

The basic protection mechanism adopted is return routability. However, it is hard to fully protect against this attack; see Section 4.1.1.

3.3. Attacks against Binding Update Protocols

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.

3.3.1. Inducing Unnecessary Binding Updates

When a mobile node receives an IP packet from a new correspondent via the home agent, it may initiate the binding update protocol. An attacker can exploit this by sending the mobile node a spoofed IP packet (e.g., ping or TCP SYN packet) that appears to come from a new correspondent node. Since the packet arrives via the home agent, the mobile node may start the binding update protocol with the correspondent node. The decision as to whether to initiate the binding update procedure may depend on several factors (including heuristics, cross layer information, and configuration options) and is not specified by Mobile IPv6. Not initiating the binding update procedure automatically may alleviate these attacks, but it will not, in general, prevent them completely.

In a real attack the attacker would induce the mobile node to initiate binding update protocols with a large number of correspondent nodes at the same time. If the correspondent addresses are real addresses of existing IP nodes, then most instances of the binding update protocol might even complete successfully. The entries created in the Binding Cache are correct but useless. In this way, the attacker can induce the mobile to execute the binding update protocol unnecessarily, which can drain the mobile's resources.

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

This attack is possible against any binding update authentication protocol. The more resources the binding update protocol consumes, the more serious the attack. Therefore, strong cryptographic authentication protocol is more vulnerable to the attack than a weak one or unauthenticated binding updates. Ingress filtering helps a little, since it makes it harder to forge the source address of the spoofed packets, but it does not completely eliminate this threat.

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

The node may also recognize addresses with it had meaningful communication in the past and only send binding updates to, or accept them from, those addresses. Since it may be impossible for the IP layer to know about the protocol state in higher protocol layers, a good measure of the meaningfulness of the past communication is probably per-address packet counts. Alternatively, Neighbor Discovery [2] (Section 5.1, Conceptual Data Structures) defines the Destination Cache as a set of entries about destinations to which traffic has been sent recently. Thus, implementors may wish to use the information in the Destination Cache.

Section 11.7.2 ("Correspondent Registration") in [6] does not specify when such a route optimization procedure should be initiated. It does indicate when it may justifiable to do so, but these hints are not enough. This remains an area where more work is needed. Obviously, given that route optimization is optional, any node that finds the processing load excessive or unjustified may simply turn it off (either selectively or completely).

3.3.2. Forcing Non-Optimized Routing

As a variant of the previous attack, the attacker can prevent a correspondent node from using route optimization by filling its Binding Cache with unnecessary entries so that most entries for real mobiles are dropped.

Any successful DoS attack against a mobile or correspondent node can also prevent the processing of binding updates. We have previously 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 home agent-based 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 node 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, or 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.

3.3.3. Reflection and Amplification

Attackers sometimes try to hide the source of a packet-flooding attack by reflecting the traffic from other nodes [1]. That is, instead of sending the flood of packets directly to the target, the attacker sends data to other nodes, 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 nodes 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 would easily create opportunities for reflection: a correspondent node receives packets (e.g., TCP SYN) from the mobile node and replies to the home address given by the mobile node in the Home Address Option (HAO). The mobile might not really be a mobile and the home address could actually be the target address. The target would only see the packets sent by the correspondent and could not see the attacker's address (even if ingress filtering prevents the attacker from spoofing its source address).

        +----------+ TCP SYN with HAO    +-----------+
        | Attacker |-------------------->| Reflector |
        +----------+                     +-----------+
                                               | TCP SYN-ACK to HoA
                                         | Flooding  |
                                         | target    |

Figure 5. Reflection Attack

A badly designed binding update protocol could also be used for reflection: the correspondent would respond to a data packet by initiating the binding update authentication protocol, which usually involves sending a packet to the home address. 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 care-of address, but an Alternative Care-of Address sub-option can specify a different care-of address.) Some of the early proposals for MIPv6 security used this approach and were prone to reflection attacks.

In some of the proposals for binding update authentication protocols, the correspondent node responded to an initial message from the mobile with two packets (one to the home address, one to the care-of address). It would have been possible to use this to amplify a flooding attack by a factor of two. Furthermore, with public-key authentication, the packets sent by the correspondent might have been significantly larger than the one that triggers them.

These types of reflection and amplification can 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. These principles have been applied to MIPv6 security design.

3.4. Classification of Attacks

   Sect. Attack name                            Target Sev. Mitigation
   3.1.1 Basic address stealing                 MN     Med. RR
   3.1.2 Stealing addresses of stationary nodes Any    High RR
   3.1.3 Future address stealing                MN     Low  RR, lifetime
   3.1.4 Attacks against secrecy and integrity  MN     Low  RR, IPsec
   3.1.5 Basic denial-of-service attacks        Any    Med. RR
   3.1.6 Replaying and blocking binding updates MN     Low  lifetime,
                                                            seq number,
   3.2.1 Basic flooding                         Any    High RR
   3.2.2 Return-to-home flooding                Any    High RR
   3.3.1 Inducing unnecessary binding updates   MN, CN Med. heuristics
   3.3.2 Forcing non-optimized routing          MN     Low  heuristics
   3.3.3 Reflection and amplification           N/A    Med. BU design

Figure 6. Summary of Discussed Attacks

Figure 6 gives a summary of the attacks discussed. As it stands at the time of writing, the return-to-the-home flooding and the induction of unnecessary binding updates look like the threats against which we have the least amount of protection, compared to their severity.

3.5. Problems with Infrastructure-Based Authorization

Early in the MIPv6 design process, it was assumed that plain IPsec could be the default way to secure Binding Updates with arbitrary correspondent nodes. However, this turned out to be impossible. Plain IPsec relies on an infrastructure for key management, which, to be usable with any arbitrary pair of nodes, would need to be global in scope. Such a "global PKI" does not exist, nor is it expected to come into existence any time soon.

More minor issues that also surfaced at the time were: (1) insufficient filtering granularity for the state of IPsec at the time, (2) cost to establish a security association (in terms of CPU and round trip times), and (3) expressing the proper authorization (as opposed to just authentication) for binding updates [13]. These issues are solvable, and, in particular, (1) and (3) have been addressed for IPsec usage with binding updates between the mobile node and the home agent [7].

However, the lack of a global PKI remains unsolved.

One way to provide a global key infrastructure for mobile IP could be DNSSEC. Such a scheme is not completely supported by the existing specifications, as it constitutes a new application of the KEY RR, something explicitly limited to DNSSEC [8] [9] [10]. Nevertheless, if one were to define it, one could proceed along the following lines: A secure reverse DNS that provided a public key for each IP address could be used to verify that a binding update is indeed signed by an authorized party. However, in order to be secure, each link in such a system must be secure. That is, there must be a chain of keys and signatures all the way down from the root (or at least starting from a trust anchor common to the mobile node and the correspondent node) to the given IP address. Furthermore, it is not enough that each key be signed by the key above it in the chain. It is also necessary that each signature explicitly authorize the lower key to manage the corresponding address block below.

Even though it would be theoretically possible to build a secure reverse DNS infrastructure along the lines shown above, the practical problems would be daunting. Whereas the delegation and key signing might work close to the root of the tree, it would probably break down somewhere along the path to the individual nodes. Note that a similar delegation tree is currently being proposed for Secure Neighbor Discovery [15], although in this case only routers (not necessarily every single potential mobile node) need to secure such a certificate. Furthermore, checking all the signatures on the tree would place a considerable burden on the correspondent nodes, making route optimization prohibitive, or at least justifiable only in very particular circumstances. Finally, it is not enough simply to check whether the mobile node is authorized to send binding updates containing a given home address, because to protect against flooding attacks, the care-of address must also be verified.

Relying on this same secure DNS infrastructure to verify care-of addresses would be even harder than verifying home addresses. Instead, a different method would be required, e.g., a return routability procedure. If so, the obvious question is whether the gargantuan cost of deploying the global secure DNS infrastructure is worth the additional protection it affords, as compared to simply using return routability for both home address and care-of address verification.

4. Solution Selected for Mobile IPv6

The current Mobile IPv6 route optimization security has been carefully designed to prevent or mitigate the threats that were discussed in Section 3. The goal has been to produce a design with a level of security close to that of a static IPv4-based Internet, and with an acceptable cost in terms of packets, delay, and processing. The result is not what one would expect: it is definitely not a traditional cryptographic protocol. Instead, the result relies heavily on the assumption of an uncorrupted routing infrastructure and builds upon the idea of checking that an alleged mobile node is indeed reachable through both its home address and its care-of address. Furthermore, the lifetime of the state created at the corresponded nodes is deliberately restricted to a few minutes, in order to limit the potential threat from time shifting.

This section describes the solution in reasonable detail (for further details see the specification), starting from Return Routability (Section 4.1), continuing with a discussion about state creation at the correspondent node (Section 4.2), and completing the description with a discussion about the lifetime of Binding Cache Entries (Section 4.3).

4.1. Return Routability

Return Routability (RR) is the name of the basic mechanism deployed by Mobile IPv6 route optimization security design. RR is based on the idea that a node should be able to verify that there is a node that is able to respond to packets sent to a given address. The check yields false positives if the routing infrastructure is compromised or if there is an attacker between the verifier and the address to be verified. With these exceptions, it is assumed that a successful reply indicates that there is indeed a node at the given address, and that the node is willing to reply to the probes sent to it.

The basic return routability mechanism consists of two checks, a Home Address check (see Section 4.1.1) and a care-of-address check (see Section 4.1.2). The packet flow is depicted in Figure 7. First, the mobile node sends two packets to the correspondent node: a Home Test Init (HoTI) packet is sent through the home agent, and a Care-of Test Init (CoTI) directly. The correspondent node replies to both of these independently by sending a Home Test (HoT) in response to the Home Test Init and a Care-of Test (CoT) in response to the Care-of Test Init. Finally, once the mobile node has received both the Home Test and Care-of Test packets, it sends a Binding Update to the correspondent node.

           +------+   1a) HoTI            +------+
           |      |---------------------->|      |
           |  MN  |   2a) HoT             |  HA  |
           |      |<----------------------|      |
           +------+                       +------+
   1b) CoTI | ^  |                        /  ^
            | |2b| CoT                   /  /
            | |  |                      /  /
            | |  | 3) BU               /  /
            V |  V                    /  /
           +------+   1a) HoTI       /  /
           |      |<----------------/  /
           |  CN  |   2a) HoT         /
           |      |------------------/

Figure 7. Return Routability Packet Flow

It might appear that the actual design was somewhat convoluted. That is, the real return routability checks are the message pairs < Home Test, Binding Update > and < Care-of Test, Binding Update >. The Home Test Init and Care-of Test Init packets are only needed to trigger the test packets, and the Binding Update acts as a combined routability response to both of the tests.

There are two main reasons behind this design:

  • avoidance of reflection and amplification (see Section 3.3.3), and
  • avoidance of state exhaustion DoS attacks (see Section 4.2).

The reason for sending two Init packets instead of one is to avoid amplification. The correspondent node does not know anything about the mobile node, and therefore it just receives an unsolicited IP packet from some arbitrary IP address. In a way, this is similar to a server receiving a TCP SYN from a previously unknown client. If the correspondent node were to send two packets in response to an initial trigger, that would provide the potential for a DoS amplification effect, as discussed in Section 3.3.3.

This scheme also avoids providing for a potential reflection attack. If the correspondent node were to reply to an address other than the source address of the packet, that would create a reflection effect. Thus, the only safe mechanism possible for a naive correspondent is to reply to each received packet with just one packet, and to send the reply to the source address of the received packet. Hence, two initial triggers are needed instead of just one.

Let us now consider the two return routability tests separately. In the following sections, the derivation of cryptographic material from each of these is shown in a simplified manner. For the real formulas and more detail, please refer to [6].

4.1.1. Home Address Check

The Home Address check consists of a Home Test (HoT) packet and a subsequent Binding Update (BU). It is triggered by the arrival of a Home Test Init (HoTI). A correspondent node replies to a Home Test Init by sending a Home Test to the source address of the Home Test Init. The source address is assumed to be the home address of a mobile node, and therefore the Home Test is assumed to be tunneled by the Home Agent to the mobile node. The Home Test contains a cryptographically generated token, home keygen token, which is formed by calculating a hash function over the concatenation of a secret key, Kcn, known only by the correspondent node, the source address of the Home Test Init packet, and a nonce.

home keygen token = hash(Kcn | home address | nonce | 0)

An index to the nonce is also included in the Home Test packet, allowing the correspondent node to find the appropriate nonce more easily.

The token allows the correspondent node to make sure that any binding update received subsequently has been created by a node that has seen the Home Test packet; see Section 4.2.

In most cases, the Home Test packet is forwarded over two different segments of the Internet. It first traverses from the correspondent node to the Home Agent. On this trip, it is not protected and any eavesdropper on the path can learn its contents. The Home Agent then forwards the packet to the mobile node. This path is taken inside an IPsec ESP protected tunnel, making it impossible for the outsiders to learn the contents of the packet.

At first, it may sound unnecessary to protect the packet between the home agent and the mobile node, since it travelled unprotected between the correspondent node and the mobile node. If all links in the Internet were equally insecure, the additional protection would be unnecessary. However, in most practical settings the network is likely to be more secure near the home agent than near the mobile node. For example, if the home agent hosts a virtual home link and the mobile nodes are never actually at home, an eavesdropper should be close to the correspondent node or on the path between the correspondent node and the home agent, since it could not eavesdrop at the home agent. If the correspondent node is a major server, all the links on the path between it and the home agent are likely to be fairly secure. On the other hand, the Mobile Node is probably using wireless access technology, making it sometimes trivial to eavesdrop on its access link. Thus, it is fairly easy to eavesdrop on packets that arrive at the mobile node. Consequently, protecting the HA-MN path is likely to provide real security benefits even when the CN-HA path remains unprotected.

4.1.2. Care-of-Address Check

From the correspondent node's point of view, the Care-of-Address check is very similar to the home check. The only difference is that now the source address of the received Care-of Test Init packet is assumed to be the care-of address of the mobile node. Furthermore, the token is created in a slightly different manner in order to make it impossible to use home tokens for care-of tokens or vice versa.

      care-of keygen token = hash(Kcn | care-of address | nonce | 1)

The Care-of Test traverses only one leg, directly from the correspondent node to the mobile node. It remains unprotected all along the way, making it vulnerable to eavesdroppers near the correspondent node, on the path from the correspondent node to the mobile node, or near the mobile node.

4.1.3. Forming the First Binding Update

When the mobile node has received both the Home Test and Care-of Test messages, it creates a binding key, Kbm, by computing a hash function over the concatenation of the tokens received.

This key is used to protect the first and the subsequent binding updates, as long as the key remains valid.

Note that the key Kbm is available to anyone who is able to receive both the Care-of Test and Home Test messages. However, they are normally routed by different routes through the network, and the Home Test is transmitted over an encrypted tunnel from the home agent to the mobile node (see also Section 5.4).

4.2. Creating State Safely

The correspondent node may remain stateless until it receives the first Binding Update. That is, it does not need to record receiving and replying to the Home Test Init and Care-of Test Init messages. The Home Test Init/Home Test and Care-of Test Init/Care-of Test exchanges take place in parallel but independently of each other. Thus, the correspondent can respond to each message immediately, and it does not need to remember doing that. This helps in potential denial-of-service situations: no memory needs to be reserved for processing Home Test Init and Care-of Test Init messages. Furthermore, Home Test Init and Care-of Test Init processing is designed to be lightweight, and it can be rate limited if necessary.

When receiving a first binding update, the correspondent node goes through a rather complicated procedure. The purpose of this procedure is to ensure that there is indeed a mobile node that has recently received a Home Test and a Care-of Test that were sent to the claimed home and care-of addresses, respectively, and to make sure that the correspondent node does not unnecessarily spend CPU or other resources while performing this check.

Since the correspondent node does not have any state when the binding update arrives, the binding update itself must contain enough information so that relevant state can be created. To that end, the binding update contains the following pieces of information:

   Source address:  The care-of address specified in the Binding Update
      must be equal to the source address used in the Care-of Test Init
      message.  Notice that this applies to the effective Care-of
      Address of the Binding Update.  In particular, if the Binding
      Update includes an Alternate Care-of Address (AltCoA) [6], the
      effective CoA is, of course, this AltCoA.  Thus, the Care-of Test
      Init must have originated from the AltCoA.
   Home address:  The home address specified in the Binding Update must
      be equal to the source address used in the Home Test Init message.
   Two nonce indices:  These are copied over from the Home Test and
      Care-of Test messages, and together with the other information
      they allow the correspondent node to re-create the tokens sent in
      the Home Test and Care-of Test messages and used for creating Kbm.

Without them, the correspondent node might need to try the 2-3 latest nonces, leading to unnecessary resource consumption.

   Message Authentication Code (MAC):  The binding update is
      authenticated by computing a MAC function over the care-of
      address, the correspondent node's address and the binding update
      message itself.  The MAC is keyed with the key Kbm.

Given the addresses, the nonce indices (and thereby the nonces) and the key Kcn, the correspondent node can re-create the home and care- of tokens at the cost of a few memory lookups and computation of one MAC and one hash function.

Once the correspondent node has re-created the tokens, it hashes the tokens together, giving the key Kbm. If the Binding Update is authentic, Kbm is cached together with the binding. This key is then used to verify the MAC that protects integrity and origin of the actual Binding Update. Note that the same Kbm may be used for a while, until the mobile node moves (and needs to get a new care-of- address token), the care-of token expires, or the home token expires.

4.2.1. Retransmissions and State Machine

Note that since the correspondent node may remain stateless until it receives a valid binding update, the mobile node is solely responsible for retransmissions. That is, the mobile node should keep sending the Home Test Init / Care-of Test Init messages until it receives a Home Test / Care-of Test, respectively. Similarly, it may need to send the binding update a few times in the case it is lost while in transit.

4.3. Quick expiration of the Binding Cache Entries

A Binding Cache Entry, along with the key Kbm, represents the return routability state of the network at the time when the Home Test and Care-of Test messages were sent out. It is possible that a specific attacker is able to eavesdrop a Home Test message at some point of time, but not later. If the Home Test had an infinite or a long lifetime, that would allow the attacker to perform a time shifting attack (see Section 2.2). That is, in the current IPv4 architecture an attacker on the path between the correspondent node and the home agent is able to perform attacks only as long as the attacker is able to eavesdrop (and possibly disrupt) communications on that particular path. A long living Home Test, and consequently the ability to send valid binding updates for a long time, would allow the attacker to continue its attack even after the attacker is no longer able to eavesdrop on the path.

To limit the seriousness of this and other similar time shifting threats, the validity of the tokens is limited to a few minutes. This effectively limits the validity of the key Kbm and the lifetime of the resulting binding updates and binding cache entries.

Although short lifetimes are required by other aspects of the security design and the goals, they are clearly detrimental for efficiency and robustness. That is, a Home Test Init / Home Test message pair must be exchanged through the home agent every few minutes. These messages are unnecessary from a purely functional point of view, thereby representing overhead. What is worse, though, is that they make the home agent a single point of failure. That is, if the Home Test Init / Home Test messages were not needed, the existing connections from a mobile node to other nodes could continue even when the home agent fails, but the current design forces the bindings to expire after a few minutes.

This concludes our walk-through of the selected security design. The cornerstones of the design were the employment of the return routability idea in the Home Test, Care-of Test, and binding update messages, the ability to remain stateless until a valid binding update is received, and the limiting of the binding lifetimes to a few minutes. Next we briefly discuss some of the remaining threats and other problems inherent to the design.

5. Security Considerations

This section gives a brief analysis of the security design, mostly in the light of what was known when the design was completed in Fall 2002. It should be noted that this section does not present a proper security analysis of the protocol; it merely discusses a few issues that were known at the time the design was completed.

It should be kept in mind that the MIPv6 RO security design was never intended to be fully secure. Instead, as we stated earlier, the goal was to be roughly as secure as non-mobile IPv4 was known to be at the time of the design. As it turns out, the result is slightly less secure than IPv4, but the difference is small and most likely insignificant in real life.

The known residual threats as compared with IPv4 are discussed in Section 5.1. Considerations related to the application of IPsec to authorize route optimization are discussed in Section 5.2. Section 5.3 discusses an attack against neighboring nodes. Finally, Section 5.4 deals with the special case of two mobile nodes conversing and performing the route optimization procedure with each other.

5.1. Residual Threats as Compared to IPv4

As we mentioned in Section 4.2, the lifetime of a binding represents a potential time shift in an attack. That is, an attacker that is able to create a false binding is able to reap the benefits of the binding as long as the binding lasts. Alternatively, the attacker is able to delay a return-to-home flooding attack (Section 3.2.2) until the binding expires. This is different from IPv4, where an attacker may continue an attack only as long as it is on the path between the two hosts.

Since the binding lifetimes are severely restricted in the current design, the ability to do a time shifting attack is equivalently restricted.

Threats possible because of the introduction of route optimization are, of course, not present in a baseline IPv4 internet (Section 3.3). In particular, inducing unnecessary binding updates could potentially be a severe attack, but this would be most likely due to faulty implementations. As an extreme measure, a correspondent node can protect against these attacks by turning off route optimization. If so, it becomes obvious that the only residual attack against which there is no clear-cut prevention (other than its severe limitation as currently specified) is the time shifting attack mentioned above.

5.2. Interaction with IPsec

A major motivation behind the current binding update design was scalability, which implied the ability to run the protocol without any existing security infrastructure. An alternative would have been to rely on existing trust relationships, perhaps in the form of a special-purpose Public Key Infrastructure in conjunction with IPsec. That would have limited scalability, making route optimization available only in environments where it is possible to create appropriate IPsec security associations between the mobile nodes and the corresponding nodes.

There clearly are situations where there exists an appropriate relationship between a mobile node and the correspondent node. For example, if the correspondent node is a server that has pre- established keys with the mobile node, that would be the case. However, entity authentication or an authenticated session key is not necessarily sufficient for accepting Binding Updates.

   Home Address Check:  If one wants to replace the home address check
      with cryptographic credentials, these must carry proper
      authorization for the specific home address, and care must be
      taken to make sure that the issuer of the certificate is entitled

to express such authorization. At the time of the design work, the route optimization security design team was not aware of standardized certificate formats to do this, although more recent efforts within the IETF are addressing this issue. Note that there is plenty of motivation to do so, as any pre-existing relationship with a correspondent node would involve the mobile node's home address (instead of any of its possible care-of addresses). Accordingly, the IKE exchange would most naturally run between the correspondent node and the mobile node's home address. This still leaves open the issue of checking the mobile node's care-of address.

   Care-of Address Check:  As for the care-of-address check, in
      practice, it seems highly unlikely that nodes could completely
      replace the care-of-address check with credentials.  Since the
      care-of addresses are ephemeral, in general it is very difficult
      for a mobile node to present credentials that taken at face value
      (by an arbitrary correspondent node) guarantee no misuse for, say,
      flooding attacks (Section 3.2).  As discussed before, a
      reachability check goes a long way to alleviate such attacks.
      Notice that, as part of the normal protocol exchange, establishing
      IPsec security associations via IKE includes one such reachability
      test.  However, as per the previous section, the natural IKE
      protocol exchange runs between the correspondent node and the
      mobile node's home address.  Hence, another reachability check is
      needed to check the care-of address at which the node is currently
      reachable.  If this address changes, such a reachability test is
      likewise necessary, and it is included in ongoing work aimed at
      securely updating the node's current address.

Nevertheless, the Mobile IPv6 base specification [6] does not specify how to use IPsec together with the mobility procedures between the mobile node and correspondent node. On the other hand, the specification is carefully written to allow the creation of the binding management key Kbm through some different means. Accordingly, where an appropriate relationship exists between a mobile node and a correspondent node, the use of IPsec is possible, and is, in fact, being pursued in more recent work.

5.3. Pretending to Be One's Neighbor

One possible attack against the security design is to pretend to be a neighboring node. To launch this attack, the mobile node establishes route optimization with some arbitrary correspondent node. While performing the return routability tests and creating the binding management key Kbm, the attacker uses its real home address but a faked care-of address. Indeed, the care-of address would be the address of the neighboring node on the local link. The attacker is able to create the binding since it receives a valid Home Test normally, and it is able to eavesdrop on the Care-of Test, as it appears on the local link.

This attack would allow the mobile node to divert unwanted traffic towards the neighboring node, resulting in an flooding attack.

However, this attack is not very serious in practice. First, it is limited in the terms of location, since it is only possible against neighbors. Second, the attack works also against the attacker, since it shares the local link with the target. Third, a similar attack is possible with Neighbor Discovery spoofing.

5.4. Two Mobile Nodes Talking to Each Other

When two mobile nodes want to establish route optimization with each other, some care must be exercised in order not to reveal the reverse tokens to an attacker. In this situation, both mobile nodes act simultaneously in the mobile node and the correspondent node roles. In the correspondent node role, the nodes are vulnerable to attackers that are co-located at the same link. Such an attacker is able to learn both the Home Test and Care-of Test sent by the mobile node, and therefore it is able to spoof the location of the other mobile host to the neighboring one. What is worse is that the attacker can obtain a valid Care-of Test itself, combine it with the Home Test, and then claim to the neighboring node that the other node has just arrived at the same link.

There is an easy way to avoid this attack. In the correspondent node role, the mobile node should tunnel the Home Test messages that it sends through its home agent. This prevents the co-located attacker from learning any valid Home Test messages.

6. Conclusions

This document discussed the security design rationale for the Mobile IPv6 Route Optimization. We have tried to describe the dangers created by Mobile IP Route Optimization, the security goals and background of the design, and the actual mechanisms employed.

We started the discussion with a background tour to the IP routing architecture the definition of the mobility problem. After that, we covered the avenues of attack: the targets, the time shifting abilities, and the possible locations of an attacker. We outlined a number of identified threat scenarios, and discussed how they are mitigated in the current design. Finally, in Section 4 we gave an overview of the actual mechanisms employed, and the rational behind them.

As far as we know today, the only significant difference between the security of an IPv4 Internet and that of an Internet with Mobile IPv6 (and route optimization) concerns time shifting attacks. Nevertheless, these are severely restricted in the current design.

We have also briefly covered some of the known subtleties and shortcomings, but that discussion cannot be exhaustive. It is quite probable that new subtle problems will be discovered with the design. As a consequence, it is most likely that the design needs to be revised in the light of experience and insight.

7. Acknowledgements

We are grateful for: Hesham Soliman for reminding us about the threat explained in Section 5.3, Francis Dupont for first discussing the case of two mobile nodes talking to each other (Section 5.4) and for sundry other comments, Pekka Savola for his help in Section 1.1.1, and Elwyn Davies for his thorough editorial review.

8. Informative References

   [1]   Aura, T., Roe, M., and J. Arkko, "Security of Internet Location
         Management", Proc. 18th Annual Computer Security Applications
         Conference, pages 78-87, Las Vegas, NV, USA, IEEE Press,
         December 2002.
   [2]   Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
         for IP Version 6 (IPv6)", RFC 2461, December 1998.
   [3]   Narten, T. and R. Draves, "Privacy Extensions for Stateless
         Address Autoconfiguration in IPv6", RFC 3041, January 2001.
   [4]   Bush, R. and D. Meyer, "Some Internet Architectural Guidelines
         and Philosophy", RFC 3439, December 2002.
   [5]   Baker, F. and P. Savola, "Ingress Filtering for Multihomed
         Networks", BCP 84, RFC 3704, March 2004.
   [6]   Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
         IPv6", RFC 3775, June 2004.
   [7]   Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
         Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
         Agents", RFC 3776, June 2004.
   [8]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "DNS Security Introduction and Requirements", RFC 4033, March
   [9]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Resource Records for the DNS Security Extensions", RFC 4034,
         March 2005.
   [10]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Protocol Modifications for the DNS Security Extensions", RFC
         4035, March 2005.
   [11]  Chiappa, J., "Will The Real 'End-End Principle' Please Stand
         Up?", Private Communication, April 2002.
   [12]  Savage, S., Cardwell, N., Wetherall, D., and T. Anderson, "TCP
         Congestion Control with a Misbehaving Receiver", ACM Computer
         Communication Review, 29:5, October 1999.
   [13]  Nikander, P., "Denial-of-Service, Address Ownership, and Early
         Authentication in the IPv6 World", Security Protocols 9th
         International Workshop, Cambridge, UK, April 25-27 2001, LNCS
         2467, pages 12-26, Springer, 2002.
   [14]  Chiappa, J., "Endpoints and Endpoint Names: A Proposed
         Enhancement to the Internet Architecture", Private
         Communication, 1999.
   [15]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
         Neighbor Discovery (SEND)", RFC 3971, March 2005.

Authors' Addresses

   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   Phone: +358 9 299 1
   Jari Arkko
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   Tuomas Aura
   Microsoft Research Ltd.
   Roger Needham Building
   7  JJ Thomson Avenue
   Cambridge CB3 0FB
   United Kingdom


   Gabriel Montenegro
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


Erik Nordmark
Sun Microsystems
17 Network Circle
Menlo Park, CA 94025


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