[Docs] [txt|pdf] [Tracker] [Email] [Nits]

Versions: 00 01 02

Network Working Group                                        P. Nikander
Internet-Draft                             Ericsson Research Nomadic Lab
Expires: October 6, 2003                                         T. Aura
                                                      Microsoft Research
                                                                J. Arkko
                                           Ericsson Research Nomadic Lab
                                                           G. Montenegro
                                                        Sun Microsystems
                                                           April 7, 2003


   Mobile IP version 6 Route Optimization Security Design Background
                  draft-nikander-mobileip-v6-ro-sec-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

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

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

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on October 6, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   In this document we present the design rationale behind the Mobile
   IPv6 (MIPv6) Route Optimization Security Design. The purpose of this
   document is to assemble together the collective wisdom and
   understanding obtained during the Mobile IPv6 Security Design in
   2001-2002. The main body of the document is intentionally kept
   relatively short: the details of a number of specific issues are
   explored in appendices and elsewhere.



Nikander, et al.        Expires October 6, 2003                 [Page 1]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   The document has two target audiences. Firstly, it is intended for
   MIPv6 implementors so that they could better understand the reasons
   behind the design choices in MIPv6 security procedures. Secondly, it
   is aimed to help other people dealing with mobility or multi-homing
   to avoid a number of potential security pitfalls in their design.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1   Assumptions about the Existing IP Infrastructure . . . . . .  5
   1.1.1 A note on source addresses and ingress filtering . . . . . .  6
   1.2   The Mobility Problem and the Mobile IPv6 Solution  . . . . .  6
   1.3   Design Principles and Goals  . . . . . . . . . . . . . . . .  8
   1.3.1 End-to-end principle . . . . . . . . . . . . . . . . . . . .  8
   1.3.2 Trust assumptions  . . . . . . . . . . . . . . . . . . . . .  8
   1.3.3 Protection level . . . . . . . . . . . . . . . . . . . . . .  9
   1.4   About Mobile IPv6 Mobility and its Variations  . . . . . . .  9
   1.4.1 Mobility variations  . . . . . . . . . . . . . . . . . . . .  9
   1.4.2 Relationship between mobility and multi-homing . . . . . . . 10
   2.    Dimensions of Danger . . . . . . . . . . . . . . . . . . . . 11
   2.1   Target . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   2.2   Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   2.3   Location . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   3.    Threats and limitations  . . . . . . . . . . . . . . . . . . 13
   3.1   Attacks against address 'owners' aka. address 'stealing' . . 13
   3.1.1 Basic address stealing . . . . . . . . . . . . . . . . . . . 14
   3.1.2 Stealing addresses of stationary nodes . . . . . . . . . . . 15
   3.1.3 Future address stealing  . . . . . . . . . . . . . . . . . . 15
   3.1.4 Attacks against Secrecy and Integrity  . . . . . . . . . . . 16
   3.1.5 Basic Denial of Service Attacks  . . . . . . . . . . . . . . 17
   3.1.6 Replaying and Blocking Binding Updates . . . . . . . . . . . 17
   3.2   Attacks against other nodes and networks (flooding)  . . . . 18
   3.2.1 Basic flooding . . . . . . . . . . . . . . . . . . . . . . . 18
   3.2.2 Return-to-home flooding  . . . . . . . . . . . . . . . . . . 19
   3.3   Attacks against BU protocols . . . . . . . . . . . . . . . . 20
   3.3.1 Inducing Unnecessary Binding Updates . . . . . . . . . . . . 20
   3.3.2 Forcing Non-Optimized Routing  . . . . . . . . . . . . . . . 21
   3.3.3 Reflection and Amplification . . . . . . . . . . . . . . . . 21
   3.4   Classification of attacks  . . . . . . . . . . . . . . . . . 23
   3.5   Problems with infrastructure based authorization . . . . . . 23
   4.    The solution selected for Mobile IPv6  . . . . . . . . . . . 25
   4.1   Return Routability . . . . . . . . . . . . . . . . . . . . . 25
   4.1.1 Home Address check . . . . . . . . . . . . . . . . . . . . . 27
   4.1.2 Care-of-Address check  . . . . . . . . . . . . . . . . . . . 28
   4.1.3 Forming the first Binding Update . . . . . . . . . . . . . . 28
   4.2   Creating state safely  . . . . . . . . . . . . . . . . . . . 28
   4.2.1 Retransmissions and state machine  . . . . . . . . . . . . . 30
   4.3   Quick expiration of the Binding Cache Entries  . . . . . . . 30



Nikander, et al.        Expires October 6, 2003                 [Page 2]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   5.    Security considerations  . . . . . . . . . . . . . . . . . . 32
   5.1   Time shifting attacks  . . . . . . . . . . . . . . . . . . . 32
   5.2   Interaction with IPsec . . . . . . . . . . . . . . . . . . . 32
   5.3   Pretending to be your neighbor . . . . . . . . . . . . . . . 33
   5.4   Two mobile nodes talking to each other . . . . . . . . . . . 34
   6.    Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
         References (informative) . . . . . . . . . . . . . . . . . . 37
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 37
         Intellectual Property and Copyright Statements . . . . . . . 39









































Nikander, et al.        Expires October 6, 2003                 [Page 3]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


1. Introduction

   Mobile IP is based on the idea of providing mobility support on the
   top of existing IP infrastructure, without requiring any
   modifications to the routers, the applications or the stationary end
   hosts.  However, in Mobile IPv6 (as opposed to Mobile IPv4) also the
   stationary end hosts are supposed (though not absolutely required) to
   provide additional support for mobility, i.e., to support 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 packets,
   leading to security risks. The purpose of this document is the
   provide a relatively compact source of the background assumptions,
   design choices, and other information needed to understand the route
   optimization security design.  It is not a goal of this document 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].

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

   While the introduction in Section 1.1 (Section 1.1) may appear
   redundant to those readers who are already familiar with Mobile IPv6,
   it may be valuable to read it anyway. The approach taken in this
   document is very different from the one 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.

   The rest of this document is organized as follows. After this
   introductory section, we start by considering the dimensions of the
   danger in Section 2 (Section 2).  The security problems and
   countermeasures are studied in detail in Section 3 (Section 3).
   Section 4 (Section 4) explains the overall operation and design
   choices behind the current security design. In Section 5 (Section 5)
   we analyze the design and discuss the remaining threats. Finally
   Section 6 (Section 6) concludes this document.



Nikander, et al.        Expires October 6, 2003                 [Page 4]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


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 was
   inherited by Mobile IPv6. (Some alternative proposals, such as the
   Host Identity Protocol (HIP), take a different route and propose
   larger modifications to the Internet architecture; see Section 1.4.1
   (Section 1.4.1).)

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

      The routing prefixes available to a node are determined by its
      current location, and therefore the node must change its IP
      address as its moves.

      The routing infrastructure is assumed to be secure and well
      functioning, delivering packets to their intended destinations as
      identified by the destination address.

   While these may appear as trivial, let us explore them a little more
   for a moment. Firstly, in the current IPv6 operational practise the
   IP address prefixes are distributed in a hierarchical manner. This
   limits the amount of routing table entries each single router needs
   to handle. An important implication is that the topology determines
   what globally routable IP addresses are available at a given
   location. That is, the nodes cannot freely decide what globally
   routable IP address to use, but 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 they name
   says, addresses,or locators, i.e., names of locations.

   Secondly, 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 musttrust each other, at least to an extend. 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.




Nikander, et al.        Expires October 6, 2003                 [Page 5]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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 an accurate idea
   of the network topology and that it is therefore able to route
   packets to their destination locations, if at all. If this assumption
   is broken, the Internet is broken in the sense that packets go to
   wrong locations. Under such a circumstance it does not matter however
   hard the mechanism above try to make sure that packets are not
   delivered to wrong addresses, e.g., due to Mobile IP security
   problems.

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 false source IP
   address. Ingress filtering is assumed to eventually prevent this.
   When ingress filtering is used, the source address of all packets are
   screened by the internet service provider, and if the source address
   has a routing prefix that is a that should not be used by the
   customer, the packets are dropped.

   It should be noted that ingress filtering is relatively easy to apply
   at the edges of the network, but almost impossible in the core
   network. Basically, ingress filtering is easy only when the network
   topology and prefix assignment do follow the same hierarchical
   structure. Secondly, ingress filtering helps if and only if a large
   part of the internet uses it. Thirdly, ingress filtering has its own
   technical problems, e.g. w.r.t. site multi-homing, and these problems
   are likely to limit its usefulness.

1.2 The Mobility Problem and the Mobile IPv6 Solution

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

   The latter design choice can also be stated in other words: Mobile
   IPv6 aims to preserve the identifier nature of IP addresses. That is,
   Mobile IPv6 takes the view that IP addresses can be used as natural
   identifiers of nodes, as they have been used since the beginning of
   the Internet. This must be contrasted to proposed and existing
   alternative designs where the identifier and locator natures of the
   IP addresses have been separated (see Section 1.4.1 (Section 1.4.1))




Nikander, et al.        Expires October 6, 2003                 [Page 6]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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 current address, the care-of-address (CoA). The transport
   layer (TCP, UDP) uses the home address as a stationary identifier for
   the mobile node. Figure 1 (Figure 1) illustrates this basic
   arrangement.



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

                                Real data path   # # # #

                                Figure 1

   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 originally it was
   originally planned that the packets from the mobile node to its peer
   could still traverse directly, bypassing the home agent.

   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 converse 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



Nikander, et al.        Expires October 6, 2003                 [Page 7]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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 practise, of course, the
   packet is consumed by the mobile node at the care-of-address, and 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 since there is routing state
   involved at the correspondent node, and a routing header is used.

1.3 Design Principles and Goals

   The MIPv6 design and security design aimed to follow the end-to-end
   principle, to duly notice the differences in trust relationships
   between the nodes, and to establish an explicit goal in the provided
   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 [3][4]. 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 about 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 crate state to 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 MN and the HA know each other



Nikander, et al.        Expires October 6, 2003                 [Page 8]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   through a prior arrangement, e.g., due to a business relationships.
   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 scalablemanner, 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 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 rational is simple: in the
   2001 Internet, if a node was able to attach itself to the
   communication path between two arbitrary nodes, it was able to
   disrupt, modify, and eavesdrop all the traffic between the two nodes,
   unless IPsec protection was used. Even when IPsec was used, the
   attacker was still able to selectively block communication 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 technical 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, based on the 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 semantically override the normal
   topological routing information carried collectively by the routers.

   Using the terminology introduced by J. Noel Chiappa [8], 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, identifying 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.

1.4.1 Mobility variations




Nikander, et al.        Expires October 6, 2003                 [Page 9]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   Even though Mobile IP is currently the standard IP mobility solution,
   the astute reader should notice that it is by no means the only
   possible approach. For example, the Host Identity Payload (HIP) [9]
   approach is based on using a separate cryptographic name space for
   end-point identifiers, and using IP addresses only as locators. On
   the other hand, many micro mobility solutions [2] use IP addresses as
   local end-point identifiers, and maintain host-based routes in their
   internal routing tables.  Mobility support can also be implemented at
   the transport layer, in middleware, or within an application.  Since
   such approaches are structurally different than Mobile IP, their
   security problems are also different, and beyond the scope of this
   document.

1.4.2 Relationship between mobility and multi-homing

   Another aspect worth noticing is the relationship between end-host
   mobility and end-host multi-homing. A mobile node has several IP
   addresses, one after each other. A multi-homed host, on the other
   hand, also has several IP addresses, but all of them at the same
   time. Thus, they may be considered as semantical duals of each other.
   Furthermore, many of the mobility related security problems are also
   present in multi-homing, at least if one wants to allow a multi-homed
   host to use its parallel IP addresses in an interchangeable way.
   Again, the details fall beyond the scope of this document.



























Nikander, et al.        Expires October 6, 2003                [Page 10]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


2. Dimensions of Danger

   Based on the discussion above it should now be clear that the dangers
   in Mobile IPv6 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 affects of
   an attack differ depending on the target of the attack, the timing of
   the attack,and the location of the attacker.

2.1 Target

   Basically, the target of an attack can be any node or network in the
   Internet, stationary or mobile. The basic differences lie in the
   nature of the attack goals: does the attacker aim to divert (steal)
   the traffic destined and/or sourced at the target node, or does it
   aim to cause denial-of-service to the target node or network. Whether
   the target is actually, in real life, a mobile node or not does not
   typically pay much of a role since the actual target node may not be
   an active part in the attack scheme at all. As an example, consider a
   case where an attacker targets a given node A by contacting a large
   number of other nodes, claiming itself to be A, and diverting the
   traffic at these other nodes so that A is harmed. A itself need not
   be involved at all before its communications start to break. Note
   that A does not need to be a mobile node, it may well be a stationary
   node.

   Mobile IPv6 uses the same class of IP addresses for both mobile home
   and care-of addresses and for stationary node addresses.  Thus, it is
   impossible to distinguish a mobile address from a stationary one.
   Attackers can take advantage of this by taking any IP address and
   using it in a context where normally only home or care-of addresses
   appear.  This means that attacks that otherwise would only concern
   mobiles are, in fact, a threat to all IPv6 nodes.

   In fact, the role of being a mobile node appears to be most
   protected, since in that role a 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 point since
   there are very few assumptions upon which it can base its state
   formation. That is, an attacker has much easier task to fool a
   correspondent node to believe that an assumably mobile node is
   somewhere where it is not than to fool a mobile node to believe
   something similar. On the other hand, since it is possible to attack
   against a node by fooling around with its peers, all nodes are
   equally vulnerable in some sense. Furthermore, a mobile node often
   usually to also play the role of being a correspondent node, since it
   often talks to other mobile nodes; see also Section 5.4 (Section
   5.4).



Nikander, et al.        Expires October 6, 2003                [Page 11]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


2.2 Timing

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

2.3 Location

   In a static IPv4 internet, an attacker can only receive packets
   destined to a given address if it is able to attach itself to or
   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 CN. 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 source routing state (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.










Nikander, et al.        Expires October 6, 2003                [Page 12]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


3. Threats and limitations

   This section describes attacks against Mobile IPv6 Route Optimization
   and related protection mechanisms. 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) protocol to exhaust the resources of the mobile
   node, the home agent, or the correspondent nodes. The aim of this
   section is to describe the major attacks and to overview various
   protocol mechanisms and their limitations. The details of the
   mechanisms are covered in Section 4 (Section 4).

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

   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 the BU protocol execution at any time,
   while most passive attacks require the attacker to wait for suitable
   messages to be sent by the targets nodes.

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

3.1 Attacks against address 'owners' aka. address 'stealing'

   The most obvious danger in Mobile IPv6 is address "stealing", i.e.,
   an attacker illegitimately claiming to be a given node at a given
   address, and then trying to "steal" traffic destined to that address.
   There are several variants of this attack. We first describe the
   basic variant, followed by a description how the situation is
   affected if the target is a stationary node, and continuing more
   complicated issues related to timing (the so called "future"



Nikander, et al.        Expires October 6, 2003                [Page 13]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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 BUs from anywhere in the Internet. All
   nodes that support the correspondent node functionality would be
   vulnerable to this attack.  As explained in Section 2.1 (Section
   2.1), there is no way of telling which addresses belong to mobile
   nodes that really could send BUs and which addresses belong to
   stationary nodes (see below).


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

                                Figure 2

   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 BU would
   be B and the care-of address (CoA) would be C. After receiving this
   BU, A would send all packets intended for the node B to the address
   C. See Figure 2 (Figure 2).

   The attacker might select the CoA to be either its own current
   address (or another address in its local network) or any other IP
   address. If the attacker selected a local CoA 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 CoA from inside its own network or to use the
   Alternate CoA sub-option.

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





Nikander, et al.        Expires October 6, 2003                [Page 14]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


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). This means that it is difficult to
   redirect all packets to or from a specific node because the attacker
   would need to know the IP addresses of all the nodes with which it is
   communicating.

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

   Nodes that frequently change their address and use random addresses
   are relatively safe. However, if they register their address into
   DynDNS, they become more exposed. Similarly, nodes that visit
   publicly accessible networks such as airport wireless LANs risk
   revealing their addresses. IPv6 addressing privacy features [ND01]
   mitigate these risks to an extent but it should be noted that
   addresses cannot be completely recycled while there are still open
   sessions that use those addresses.

   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 if 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 stealing

   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, using the home address that
   it anticipates the target node to 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 able to acquire the home address temporarily, it may be able to
   do so, and then return the home address back 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



Nikander, et al.        Expires October 6, 2003                [Page 15]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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 not a
   real concern under the current design.

3.1.4 Attacks against Secrecy and Integrity

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

     Original data path, before man-in-the-middle attack

          +---+                               +---+
          | A |                               | B |
          +---+                               +---+
            \___________________________________/

     Modified data path, after the falsified BUs

          +---+                               +---+
          | A |                               | B |
          +---+                               +---+
            \                                  /
             \                                /
              \          +----------+        /
               \---------| Attacker |-------/
                         +----------+


                                Figure 3

   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
   BUs could result in denial of service (see below) but not in
   disclosure or corruption of sensitive data beyond revealing the



Nikander, et al.        Expires October 6, 2003                [Page 16]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   existence of the traffic flows. Two fixed nodes could also protect
   communication between themselves by refusing to accept BUs from each
   other. Ingress filtering, on the other hand, does not help because
   the attacker is using its own address as the CoA and is not spoofing
   source IP addresses.

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

3.1.5 Basic Denial of Service Attacks

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

3.1.6 Replaying and Blocking Binding Updates

   Any protocol for authenticating BUs has to consider replay attacks.
   That is, an attacker may be able to replay recent authenticated BUs
   to the correspondent and, that way, direct packets to the mobile
   node's previous location. Like spoofed BUs, 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 BU 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, and takes over its connections at the old
   location. The attacker will be able to capture the packets sent to
   the mobile and to impersonate the mobile until the correspondent's
   Binding Cache entry expires.

   Both of the above attacks require the attacker to be on the same
   local network with the mobile, where it can relatively easily observe
   packets and block them even if the mobile does not move to a new
   location. Therefore, we believe that these attacks are not as serious
   as ones that can be mounted from remote locations.The limited
   lifetime of the Binding Cache entry and the associated nonces limit
   the time frame within which the replay attacks are possible.




Nikander, et al.        Expires October 6, 2003                [Page 17]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


3.2 Attacks against other nodes and networks (flooding)

   By sending spoofed BUs, an attacker could redirect traffic to an
   arbitrary IP address. This could be used to bomb an arbitrary
   Internet address with excessive amounts of packets. The attacker
   could also target a network by redirecting data to one or more IP
   addresses within the network. There are two main variations of
   flooding: basic flooding and return-to-the-home flooding. We consider
   them separate.

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 acknowledgments from B.


   (B is attacker)

        +---+  original       +---+ flooding packet   +---+
        | B |<================| A |==================>| C |
        +---+  packet flow    +---+ flow              +---+
         |                      ^
          \                    /
           \__________________/
          False BU + false acknowledgements


                                Figure 4

   A more sophisticated attacker would act itself as B; see Figure 4
   (Figure 4). It would first subscribe to a data stream (e.g. a video
   stream) and then redirects 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 one
   spoofed acknowledgments. It would even be able to accelerate the data
   rate by simulating a fatter pipe [5].

   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 BUs every few minutes. Thus, the limitation of BCE
   lifetime to a few minutes does not help here alone.



Nikander, et al.        Expires October 6, 2003                [Page 18]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   During the Mobile IPv6 design process, the effectiveness of this
   attack was debated.  It was mistakenly assumed that the target node
   would send a TCP Reset to the source of the unwanted data stream,
   which would then stop sending.  In reality, all practical TCP/IP
   implementations fail to send the Reset.  The target node drops the
   unwanted packets at the IP layer because it does not have a Binding
   Update List entry corresponding to the Routing Header on the incoming
   packet.  Thus, the flooding data is never processed at the TCP layer
   of the target node and no Reset is sent.  This means that the attack
   using TCP streams is more effective than was originally believed.

   This attack is serious because the target can be any node or network,
   not only a mobile one. What makes it particularly serious compared to
   the other attacks is that the target itself cannot do anything to
   prevent the attack. For example, it does not help if the target
   network stops using Route Optimization. The damage is the worst 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 CoA
   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 indeed to one
   that requested redirecting packets to that very address (see Section
   4.1.2 (Section 4.1.2)).

3.2.2 Return-to-home flooding

   A variation of the bombing attack targets 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
   BU 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. Just like when bombing
   a care-of-address, the attacker can keep the stream alive and even
   increase data rate by spoofing acknowledgments. When successful, the
   bombing attack against the home network is just as serious as the one
   against a care-of-address.

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





Nikander, et al.        Expires October 6, 2003                [Page 19]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


3.3 Attacks against BU protocols

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

3.3.1 Inducing Unnecessary Binding Updates

   When a mobile node receives an IP packet from a new correspondent via
   the home agent, it automatically initiates the BU 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 would automatically start the BU protocol with the
   correspondent node, thereby spending resources unnecessarily.

   In a real attack the attacker would induce the mobile node to
   initiate BU 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 BU protocol might even
   complete successfully. The entries created in the Binding Cache are
   correct but useless. This way, the attacker can induce the mobile to
   execute the BU 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 BU protocol with the target node.
   Again, most of the BU protocol executions will complete successfully.
   By inducing a large number of unnecessary BUs, the attacker is able
   to consume the target node's resources.

   This attack is possible against any BU authentication protocol. The
   more resources the BU protocol consumes, the more serious the attack.
   Hence, strong cryptographic authentication protocol is more
   vulnerable to the attack than a weak one or unauthenticated BUs.
   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, which it uses for processing BUs.When the
   limit is exceeded, the node can simply stop attempting route



Nikander, et al.        Expires October 6, 2003                [Page 20]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   optimization. Sometimes it is possible to process some BUs 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 BUs 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 BUs will be accepted
   even when the correspondent is under a BU DoS attack.

   The node may also recognize addresses with which they have had
   meaningful communication in the past and sent BUs 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.

3.3.2 Forcing Non-Optimized Routing

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

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

3.3.3 Reflection and Amplification

   Attackers sometimes try to hide the source of a packet flooding
   attack by reflecting the traffic from other nodes [Sav02]. 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



Nikander, et al.        Expires October 6, 2003                [Page 21]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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

   A badly designed BU protocol could also be used for reflection: the
   correspondent would respond to a data packet by initiating the BU
   authentication protocol, which usually involves sending a packet to
   the home address. In that case, the reflection attack can be
   discouraged by copying the mobile's address into the messages sent by
   the mobile to the correspondent. (The mobile's source address is
   usually the same as the CoA but an Alternative CoA suboption can
   specify a different CoA.) Some of the early proposals for MIPv6
   security used this approach, and were prone to the reflection
   attacks.

   In some of the proposals for BU authentication protocols, the
   correspondent node responded to an initial message from the mobile
   with two packets (one to HoA, one to CoA). 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



Nikander, et al.        Expires October 6, 2003                [Page 22]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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,
                                                            cookies
   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

   Table 1 (Figure 6) gives a summary of the discussed attacks. As it
   stands today, the return-to-the-home flooding and the induction of
   unnecessary BUs look like the threats that we have the least amount
   of protection, compared to their severity.

3.5 Problems with infrastructure based authorization

   Early in the MIPv6 design process it was assumed that plain IPsec
   could be used for securing Binding Updates. However, this turned out
   to be impossible for two reasons. The first reason can be inferred
   from the attack descriptions above: IPsec is not designed to protect
   against the kinds of DoS attacks that would be possible with MIPv6;
   especially, protecting against the flooding attacks would be very
   difficult or even impossible with plain vanilla IPsec. The second
   reason is scalability.

   Relying on IPsec requires key management, and key management requires
   infrastructure to distribute the keys. Furthermore, in MIPv6 it is
   important to show whom an IP address belongs to, i.e., who has the
   authorityto control where packets destined to the given address may



Nikander, et al.        Expires October 6, 2003                [Page 23]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   be redirected to. Only the "owner" of an address may send Binding
   Updates to redirect packets to a care-of-address. [6]

   On way of providing a global key infrastructure for mobile IP would
   be DNSSEC. If there was secure reverse DNS that provided a public key
   for each IP address, that could be used for verifying that a BU 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 to the
   given IP address. Furthermore, it is not enough that each key is
   signed by the key above, it is also necessary that each signature
   carries the meaning of authorizing the lower key to manage the
   address block below it.

   For example, consider the reverse DNS entry e.f.f.3.ip6.arpa . It
   could be associated with a key, say K_3ffe. On order to be valid,
   that key should be signed by an upper level key, let's say K3ff,
   etc., up to the top level. Similarly, any subrange of addresses below
   3ff0::/16 would need to be signed by K3ffe. Additionally, when the
   human managing the K_3ffe key signs subkeys, he or she should make
   sure that the singed subkey really belongs to a party that is
   authorized to assign address blocks in the said address range. In
   other words, the keys and signatures should form a tree reflecting
   the actual address allocations.

   Even though it would be theoretically possible to build a secure
   reverse DNS infrastructure along the lines show above, the practical
   problems would be insurmountable. That is, while the delegation and
   key signing might work close to the root of the tree, it would
   probably break down somewhere between the root and the individual
   nodes. Furthermore, checking all the signatures up the tree would
   place a considerable burden to the correspondent nodes, making route
   optimization computationally very expensive. As the last nail on the
   coffin, checking just that the mobile node is authorized to send BUs
   containing a given Home Address would not be enough, since a
   malicious mobile node would still be able to launch flooding attacks.
   On the other hand, relying on such an infrastructure to assign and
   verify "ownership" of care-of-addresses would be even harder than
   verifying home address "ownership".












Nikander, et al.        Expires October 6, 2003                [Page 24]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


4. The solution selected for Mobile IPv6

   The current Mobile IPv6 route optimization security has been
   carefully designed to prevent or mitigate the threats that were
   discussed in Section 3 (Section 3). The goal has been to produce a
   design whose security is close to that of a static IPv4 based
   Internet, and whose cost in terms of packets, delay and processing is
   not excessive. The result is not what one would expect; the result 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 both through 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 ability of time shifting.

   In this section we describe the solution in reasonable detail (for
   the fine details see the specification), starting from Return
   Routability (Section 4.1 (Section 4.1)), continuing with a discussion
   about state creation at the correspondent node (Section 4.2 (Section
   4.2)), and completing the description with a discussion about the
   lifetime of Binding Cache Entries (Section 4.3 (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. Basically, it means
   that a node verifies that there is a node that is able to respond to
   packets sent to a given address. The check yields false positives if
   the routing infrastructure is compromised or if there is an attacker
   between the verifier and the address to be verified. With these
   exceptions, it is assumed that a successful reply indicates that
   there is indeed a node at the given address, and that the node is
   willing to reply to the probes sent to it.

   The basic return routability mechanism consist of two checks, a Home
   Address check (see Section 4.1.1 (Section 4.1.1)) and a
   care-of-address check (see Section 4.1.2 (Section 4.1.2)). The packet
   flow is depicted in Figure 7 (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 in response to the Home Test
   Init and a Care-of Test 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.




Nikander, et al.        Expires October 6, 2003                [Page 25]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


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


                                Figure 7

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

   There are two main reasons behind this design:

      avoidance of reflection and amplification (see Section 3.3.3
      (Section 3.3.3)), and

      avoidance of state exhaustion DoS attacks (see Section 4.2
      (Section 4.2)).

   The reason for sending two Init packets instead of one is the
   avoidance of amplication.The correspondent node is replying to
   packets that come out of the blue. It does not know anything about
   the mobile node, and therefore it just suddenly receives an IP packet
   from some arbitrarily looking IP address. In a way, this is similar
   to a server receiving a TCP SYN from a previously unknown client. If
   the correspondent node would send two packets in response to an
   initial trigger, that would create a DoS amplification effect, as
   discussed in Section 3.3.3 (Section 3.3.3).

   Reflection avoidance is directly related. If the correspondent node
   would reply to another address but the source address of the packet,
   that would create a reflection effect. Thus, since the correspondent
   node does not know better, the only safe way is to reply to the



Nikander, et al.        Expires October 6, 2003                [Page 26]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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.

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 HoTI by
   sending a HoT to the source address of the HoTI. The source address
   is assumed to be the home address of a mobile node, and therefore the
   HoT is assumed to be tunneled by the Home Agent to the mobile node.
   The HoT 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 HoTI packet, and a nonce.

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

   An index to the nonce is also included in the HoT packet, allowing
   the correspondent node to easier find the appropriate nonce.

   The token allows the correspondent node to make sure that the
   subsequently received BU is created by a node that has seen the HoT
   packet; see Section 4.2 (Section 4.2).

   In most cases the HoT packet is forwarded over two different segments
   of the Internet. It first traverses from the correspondent node to
   the Home Agent. On this trip, it is not protected and any
   eavesdropper on the path can learn its contents. The Home Agent then
   forwards the packet to the mobile node. This path is taken inside the
   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
   HA and the MN since it travelled unprotected between the CN and the
   MN. If all links in the Internet were equally insecure, the situation
   would indeed be so, that 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
   big 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



Nikander, et al.        Expires October 6, 2003                [Page 27]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   probably using wireless access technology, making it sometimes
   trivial to eavesdrop its access link. Thus, it is fairly easy to
   eavesdrop 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 check is
   very similar to the Home check. The only difference is that now the
   source address of the received CoTI 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 CoT 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 HoT and CoT messages, it
   creates a binding key Kbm by taking a hash function over the
   concatenation of the tokens received.

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

   Note that the key Kbm is available to anyone that is able to receive
   both the CoT and HoT messages. However, they are normally router
   through different routes through the network, and the HoT is
   transmitted over an encrypted tunnel from the home agent to the
   mobile node; see also Section 5.4 (Section 5.4).

4.2 Creating state safely

   The correspondent node may remain stateless until it receives the
   first Binding Update.  That is, it does not need to record receiving
   and replying to the HoTI and CoTI messages.  The HoTI/HoT and CoTI/
   CoT exchanges take place in paraller but independetly 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 when
   processing HoTI and CoTI messages. Furthermore, HoTI and CoTI



Nikander, et al.        Expires October 6, 2003                [Page 28]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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 HoT and a CoT 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 BU
   arrives, the BU itself must contain enough information so that
   relevant state can be created. The BU contains the following pieces
   of information for that:

      The source address must be equal to the source address used in the
      CoTI message.

      This must be the same address that was used as the source address
      for the HoTI message and as the destination address for the HoT
      message.

      These are copied over from the HoT and CoT messages, and together
      with the other information they allow the correspondent node to
      re-create the tokens sent in the HoT and CoT messages and used for
      creating Kbm. Without them the correspondent node might need to
      try the 2-3 latest nonces, leading to unnecessary resource
      consumption.

      The BU 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 either the mobile node moves (and needs to get a new
   care-of-address token), the care-of token expires, or the home token
   expires.




Nikander, et al.        Expires October 6, 2003                [Page 29]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


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 HoTI / CoTI messages until it receives a HoT / CoT,
   respectively. Similarly, it may need to send the BU 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 the key Kbm, represents the return
   routability state of the network at the time when the HoT and CoT
   messages were sent out. Now, it is possible that a specific attacker
   is able to eavesdrop a HoT message at some point of time but not
   later. If the HoT had an infinite or a long lifetime, that would
   allow the attacker to perform a time shifting attack (see Section 2.2
   (Section 2.2)). That is, in the current IPv4 architecture an attacker
   at 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 HoT, 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 not any more able to eavesdrop the
   path.

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

   While short life times are necessary given the other aspects of the
   security design and the goals, they are clearly detrimental for
   efficiency and robustness. That is, a HoTI / HoT message pair must be
   exchanged through the home agent every few minutes. These messages
   are unnecessary from a pure 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 HoTI / HoT
   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 brief walkthrough of the selected security design.
   The cornerstones of the design were the employment of the return
   routability idea in the HoT, CoT and binding update messages, the
   ability to remain stateless until a valid binding update is received,
   and the limiting of the binding life times to a few minutes. Next we
   briefly discuss some of the remaining threats and other problems



Nikander, et al.        Expires October 6, 2003                [Page 30]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   inherent to the design.


















































Nikander, et al.        Expires October 6, 2003                [Page 31]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


5. Security considerations

   In this section we give a brief analysis of the security design,
   mostly in the light of what was know at the time the design was
   completed in fall 2002. It should be noted that this section does
   notpresent a proper security analysis of the protocol, but 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, to 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 to be
   insignificant in real life.

   The known difference to IPv4, a time shifting problem, is discussed
   in Section 5.4 (Section 5.4) discusses the special case of two mobile
   nodes conversing with each other.

5.1 Time shifting attacks

   As we mentioned in Section 4.2 (Section 4.2), the lifetime of a
   binding represents a potential time shift in an attack. That is, an
   attacker that is able to create a false binding is able to reap the
   benefits of the binding as long as the binding lasts, or,
   alternatively, is able to delay a return-to-the-home flooding attack
   (Section 3.2.2) (Section 3.2.2)) until the binding expires. This is a
   difference from IPv4 where an attacker may continue an attack only as
   long as it is at 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 respectively
   restricted.

5.2 Interaction with IPsec

   A major motivational aspect behind the current BU design was
   scalability, the ability to run the protocol without any existing
   security infrastructure. An alternatively would have been reliance on
   existing trust relationships, perhaps in the form of a special
   purpose PKI and IPsec. That would have limited scalability, making
   route optimization available in environments where it is possible to
   create appropriately authorized 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



Nikander, et al.        Expires October 6, 2003                [Page 32]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   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.  If one wants
   to replace the home address check with some cryptographic
   credentials, the credentials must carry proper authorization for the
   specific home address. For example, if the mobile nodes hands out a
   certificate to the correspondent node and they consequently create a
   pair of IPsec security associations, it is not necessarily clear that
   those security associations could be used to replace the home address
   check. Instead, if and only if the certificate explicitly states what
   the mobile node's home address is and that the mobile node is
   authorized to create bindings for its home address, home address
   checks may be dropped. Furthermore, care must be taken to make sure
   that the issuer of the certificate is entitled to express such
   authorization.

   In practise, it seems highly unlikely that the nodes were ever able
   to replace the care-of address check with credentials. The care-of
   addresses are ephemeral, and it is highly unlikely that a mobile node
   would be able to present credentials that show it authorizedto use
   the care of address without any check.

   The current specification does not specify how to use IPsec together
   with the mobility procedures between the mobile node and
   correspondent node. Hence, currently there are no standard way of
   replacing the home address check. On the other hand, the
   specification is carefully written to allow the creation of the
   binding management key Kbm through some different means.

5.3 Pretending to be your neighbor

   One possible attack against the security design is to pretend to be a
   neighboring node. To launch this attack, the mobile nodes establishes
   route optimization with some arbitrary correspondent node. While
   performing the return routability tests and creating the binding
   management key Kbm, the attacker uses its real home address but a
   faked care-of address. Indeed, the care-of address would be the
   address of the neighboring node on the local link. The attacker is
   able to create the binding since it receives a valid HoT normally,
   and it is able to eavesdrop the CoT 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 practise. Firstly, it is
   limited in the terms of location, since it is only possible against
   neighbors. Secondly, the attack works also against the attacker,



Nikander, et al.        Expires October 6, 2003                [Page 33]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   since it is sharing the local link with the target. Thirdly, a
   similar attack can be worked out 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 HoT and CoT 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 CoT itself, combine it with the HoT, and the claim to the
   neighboring node that the other node has just arrived at the same
   link.

   There is an easy way to void this attack. In the correspondent node
   role, the mobile node should tunnel the sent HoT messages through its
   home agent. This prevents the co-located attacker from learning any
   valid HoT messages.





























Nikander, et al.        Expires October 6, 2003                [Page 34]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


6. Conclusions

   In this document we have 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 dimensions of the danger: the targets, the time shifting
   abilities, and the possible locations of an attacker. We outlined a
   number of identified threat scenarios, and discussed how they are
   mitigated in the current design. Finally, in Section 4 (Section 4) we
   gave an overview of the actual mechanisms employed, and the rational
   behind them.

   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 from the design.
   As a consequence, it is most likely that the design needs to be
   revised in the light of experience and insights.






























Nikander, et al.        Expires October 6, 2003                [Page 35]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


7. Acknowledgements

   Hesham Soliman for reminding us about the threat explained in Section
   5.3 (Section 5.3).  Francis Dupont for first discussing the case of
   two mobile nodes talking to each other Section 5.4.














































Nikander, et al.        Expires October 6, 2003                [Page 36]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


References (informative)

   [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]  Campbell, A., Gomez, J., Kim, S., Turanyi, Z., Wan, C-Y. and A.
        Valko, "Comparison of IP Micro-Mobility Protocols", IEEE
        Wireless Communications Magazine Vol. 9, No. 1, February 2002.

   [3]  Bush, R. and D. Meyer, "Some Internet Architectural Guidelines
        and Philosophy", RFC 3439, December 2002.

   [4]  Chiappa, J., "Will The Real "End-End Principle" Please Stand
        Up?", date unknown.

   [5]  Savage, S., Cardwell, N., Wetherall, D. and T. Anderson, "TCP
        Congestion Control with a Misbehaving Receiver", Computer
        Communication Review 29:5, 1999.

   [6]  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.

   [7]  Perlman, R., "Network Layer Protocols with Byzantine
        Robustness", PhD thesis Department of EECS, MIT, August 1988.

   [8]  Chiappa, J., "Endpoints and Endpoint Names: A Proposed
        Enhancement  to the Internet Architecture", date unknown.

   [9]  Nikander, P., Ylitalo, J. and J. Wall, "Integrating Security,
        Mobility, and Multi-Homing   in a HIP Way", Proceedings of
        Network and Distributed Systems Security Symposium (NDSS'03),
        February 6-7, 2003,  San Diego, CA, pages 87-99,  Internet
        Society, February 2003.














Nikander, et al.        Expires October 6, 2003                [Page 37]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


Authors' Addresses

   Pekka Nikander
   Ericsson Research Nomadic Lab

   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   EMail: pekka.nikander@nomadiclab.com


   Tuomas Aura
   Microsoft Research


   Jari Arkko
   Ericsson Research Nomadic Lab


   Gabriel Montenegro
   Sun Microsystems





























Nikander, et al.        Expires October 6, 2003                [Page 38]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property 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; neither does it represent that it
   has made any effort to identify any such rights. Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11. Copies of
   claims of rights made available for publication 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 implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard. Please address the information to the IETF Executive
   Director.


Full Copyright Statement

   Copyright (C) The Internet Society (2003). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION



Nikander, et al.        Expires October 6, 2003                [Page 39]


Internet-Draft       Mobile IPv6 RO Security Design           April 2003


   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































Nikander, et al.        Expires October 6, 2003                [Page 40]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/