Movatterモバイル変換

Network Working Group                                        P. NikanderRequest for Comments: 4225                                      J. ArkkoCategory: Informational                     Ericsson Research NomadicLab                                                                 T. Aura                                                      Microsoft Research                                                           G. Montenegro                                                   Microsoft Corporation                                                             E. Nordmark                                                        Sun Microsystems                                                           December 2005Mobile IP Version 6 Route Optimization Security Design BackgroundStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2005).Abstract   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.Nikander, et al.             Informational                      [Page 1]

RFC 4225             Mobile IPv6 RO Security Design        December 2005Table of Contents1. Introduction ....................................................31.1. Assumptions about the Existing IP Infrastructure ...........41.2. The Mobility Problem and the Mobile IPv6 Solution ..........61.3. Design Principles and Goals ................................81.3.1. End-to-End Principle ..................................81.3.2. Trust Assumptions .....................................81.3.3. Protection Level ......................................81.4. About Mobile IPv6 Mobility and its Variations ..............92. Avenues of Attack ...............................................92.1. Target ....................................................102.2. Timing ....................................................102.3. Location ..................................................113. Threats and Limitations ........................................113.1. Attacks Against Address 'Owners' ("Address Stealing").. ...123.1.1. Basic Address Stealing ...............................123.1.2. Stealing Addresses of Stationary Nodes ...............133.1.3. Future Address Sealing ...............................143.1.4. Attacks against Secrecy and Integrity ................153.1.5. Basic Denial-of-Service Attacks ......................163.1.6. Replaying and Blocking Binding Updates ...............163.2. Attacks Against Other Nodes and Networks (Flooding) .......163.2.1. Basic Flooding .......................................173.2.2. Return-to-Home Flooding ..............................183.3. Attacks against Binding Update Protocols ..................183.3.1. Inducing Unnecessary Binding Updates .................193.3.2. Forcing Non-Optimized Routing ........................203.3.3. Reflection and Amplification .........................213.4. Classification of Attacks .................................223.5. Problems with Infrastructure-Based Authorization ..........234. Solution Selected for Mobile IPv6 ..............................244.1. Return Routability ........................................244.1.1. Home Address Check ...................................264.1.2. Care-of-Address Check ................................274.1.3. Forming the First Binding Update .....................274.2. Creating State Safely .....................................284.2.1. Retransmissions and State Machine ....................294.3. Quick expiration of the Binding Cache Entries .............295. Security Considerations ........................................305.1. Residual Threats as Compared to IPv4 ......................315.2. Interaction with IPsec ....................................315.3. Pretending to Be One's Neighbor ...........................325.4. Two Mobile Nodes Talking to Each Other ....................336. Conclusions ....................................................337. Acknowledgements ...............................................348. Informative References .........................................34Nikander, et al.             Informational                      [Page 2]

RFC 4225             Mobile IPv6 RO Security Design        December 20051.  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 fromSection 1.1.   Although the introduction inSection 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 veryNikander, et al.             Informational                      [Page 3]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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   inSection 2.  The security problems and countermeasures are studied   in detail inSection 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 (seeSection 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.   2.  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 theNikander, et al.             Informational                      [Page 4]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 doesNikander, et al.             Informational                      [Page 5]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 (seeSection 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.Nikander, et al.             Informational                      [Page 6]

RFC 4225             Mobile IPv6 RO Security Design        December 2005    +----+                                       +----+    | 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.Nikander, et al.             Informational                      [Page 7]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 toNikander, et al.             Informational                      [Page 8]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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.Nikander, et al.             Informational                      [Page 9]

RFC 4225             Mobile IPv6 RO Security Design        December 20052.1.  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   stationary.   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 alsoSection 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 theNikander, et al.             Informational                     [Page 10]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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.  (SeeSection3.2.2 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 inSection 4.Nikander, et al.             Informational                     [Page 11]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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.  AsNikander, et al.             Informational                     [Page 12]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   explained inSection 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.Nikander, et al.             Informational                     [Page 13]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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.Nikander, et al.             Informational                     [Page 14]

RFC 4225             Mobile IPv6 RO Security Design        December 20053.1.4.  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   (seeSection 4.1).Nikander, et al.             Informational                     [Page 15]

RFC 4225             Mobile IPv6 RO Security Design        December 20053.1.5.  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 persection 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 aNikander, et al.             Informational                     [Page 16]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 BindingNikander, et al.             Informational                     [Page 17]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 (seeSection4.1.2).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; seeSection4.1.1.3.3.  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 theNikander, et al.             Informational                     [Page 18]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 bindingNikander, et al.             Informational                     [Page 19]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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, byNikander, et al.             Informational                     [Page 20]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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                                               V                                         +-----------+                                         | 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 usuallyNikander, et al.             Informational                     [Page 21]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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,                                                            MAC   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.Nikander, et al.             Informational                     [Page 22]

RFC 4225             Mobile IPv6 RO Security Design        December 20053.5.  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 veryNikander, et al.             Informational                     [Page 23]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 inSection 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 givenNikander, et al.             Informational                     [Page 24]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 (seeSection 4.1.1) and a care-of-address check (seeSection 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:   o  avoidance of reflection and amplification (seeSection 3.3.3), and   o  avoidance of state exhaustion DoS attacks (seeSection 4.2).   The reason for sending two Init packets instead of one is to avoid   amplification.  The correspondent node does not know anything aboutNikander, et al.             Informational                     [Page 25]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 inSection 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; seeSection 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 thenNikander, et al.             Informational                     [Page 26]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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.Nikander, et al.             Informational                     [Page 27]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 alsoSection 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.Nikander, et al.             Informational                     [Page 28]

RFC 4225             Mobile IPv6 RO Security Design        December 2005      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 (seeSection 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.Nikander, et al.             Informational                     [Page 29]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 inSection 5.1.  Considerations related to the application of IPsec to   authorize route optimization are discussed inSection 5.2.Section5.3 discusses an attack against neighboring nodes.  Finally,Section5.4 deals with the special case of two mobile nodes conversing and   performing the route optimization procedure with each other.Nikander, et al.             Informational                     [Page 30]

RFC 4225             Mobile IPv6 RO Security Design        December 20055.1.  Residual Threats as Compared to IPv4   As we mentioned inSection 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 (Section3.3).  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 entitledNikander, et al.             Informational                     [Page 31]

RFC 4225             Mobile IPv6 RO Security Design        December 2005      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 isNikander, et al.             Informational                     [Page 32]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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, inSection 4 we gave an   overview of the actual mechanisms employed, and the rational behind   them.Nikander, et al.             Informational                     [Page 33]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   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 inSection 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 inSection 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         2005.Nikander, et al.             Informational                     [Page 34]

RFC 4225             Mobile IPv6 RO Security Design        December 2005   [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",RFC4035, 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.Nikander, et al.             Informational                     [Page 35]

RFC 4225             Mobile IPv6 RO Security Design        December 2005Authors' Addresses   Pekka Nikander   Ericsson Research NomadicLab   JORVAS  FIN-02420   FINLAND   Phone: +358 9 299 1   EMail: pekka.nikander@nomadiclab.com   Jari Arkko   Ericsson Research NomadicLab   JORVAS  FIN-02420   FINLAND   EMail: jari.arkko@ericsson.com   Tuomas Aura   Microsoft Research Ltd.   Roger Needham Building   7  JJ Thomson Avenue   Cambridge CB3 0FB   United Kingdom   EMail: Tuomaura@microsoft.com   Gabriel Montenegro   Microsoft Corporation   One Microsoft Way   Redmond, WA  98052   USA   EMail: gabriel_montenegro_2000@yahoo.com   Erik Nordmark   Sun Microsystems   17 Network Circle   Menlo Park, CA 94025   USA   EMail: erik.nordmark@sun.comNikander, et al.             Informational                     [Page 36]

RFC 4225             Mobile IPv6 RO Security Design        December 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Nikander, et al.             Informational                     [Page 37]