MIP6                                                            J. Kempf
Internet-Draft                            DoCoMo Communications Labs USA
Expires: August 13, 2004                                        J. Arkko
                                                       February 13, 2004

                 The Mobile IPv6 Bootstrapping Problem

Status of this Memo

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

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document discusses the creation of a security association
   between a mobile node and a home agent that is previously unknown to
   it.  This problem is called the bootstrapping problem.  The document
   discusses several different usage scenarios, as well as related
   issues involving informing the mobile node of changes in the home
   network topology and mobility management service.  Limitations of the
   base Mobile IPv6 protocol in dealing with the scenarios are outlined.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
         1.1   Requirements language  . . . . . . . . . . . . . . . .  4
   2.    Mobile IPv6 Configuration  . . . . . . . . . . . . . . . . .  5
   3.    Issues in Bootstrapping  . . . . . . . . . . . . . . . . . .  6
         3.1   Addressing . . . . . . . . . . . . . . . . . . . . . .  6
               3.1.1 Dynamic Home Address Assignment  . . . . . . . .  6
               3.1.2 Dynamic Home Agent Assignment  . . . . . . . . .  7
               3.1.3 Management requirements  . . . . . . . . . . . .  7
         3.2   Security Infrastructure  . . . . . . . . . . . . . . .  8
               3.2.1 Integration with AAA Infrastructure  . . . . . .  8
               3.2.2 "Opportunistic" or "Local" Discovery . . . . . .  8
         3.3   Topology Change  . . . . . . . . . . . . . . . . . . .  8
               3.3.1 Dormant Mode Mobile Nodes  . . . . . . . . . . .  8
               3.3.2 Use of ICMP  . . . . . . . . . . . . . . . . . .  9
   4.    Bootstrapping Scenarios  . . . . . . . . . . . . . . . . . . 10
   5.    Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.    Security Considerations  . . . . . . . . . . . . . . . . . . 12
         Normative References . . . . . . . . . . . . . . . . . . . . 13
         Informative References . . . . . . . . . . . . . . . . . . . 14
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
   A.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
         Intellectual Property and Copyright Statements . . . . . . . 16

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1. Introduction

   The bootstrapping problem for Mobile IPv6 [4, 5] is one of the issues
   the MIP6 WG is chartered to solve.  The text in the charter says:

      "A bootstrap mechanism for setting up security associations
      between the mobile node and home agent that would enable easier
      deployment of Mobile IPv6.  This bootstrap mechanism is intended
      to be used when the device is turned on the very first time and
      activates MIPv6.  The WG should investigate and define the scope
      before solving the problem."

   In addition to easier deployment, reasons for bootstrapping include
   the following:

   o  Resilience to network renumbering, provisioning of new home
      agents, and other network management operations.

   o  Increasing the efficiency of communications through the selection
      of an appropriate home agent.

   o  Load balancing.

   o  Hiding the topological location of the mobile node.

   The current Mobile IPv6 procedure for establishing an IPsec security
   association between the Mobile Node and home agent requires either
   manual keying or IKEv1 [2].  With manual keying, the security
   associations have to be bound to specific home addresses and home
   agent addresses.  With IKEv1, the mobile node needs to have a
   statically defined home address in order that the home agent can make
   an authorization decision and identify the credentials during the IKE
   Phase 1 ISAKMP exchange.  In this document, we discuss why these
   constraints may be problematic in some deployment scenarios.

   In addition, Mobile IPv6 defines mechanisms for dynamic home agent
   and home prefix discovery.  While these mechanisms are not
   specifically related to security, changes in the home agent address
   and mobile node home address also imply changes in authorization
   information related to security policies.  As a result, the discovery
   mechanisms might not be as easily deployed as would be desirable.

   This document defines the bootstrap problem.  In Section 2 we review
   the Mobile IPv6 mechanisms for configuration and security association
   establishment; bootstrapping issues related to addressing,
   infrastructure, and topology changes are discussed in Section 3.
   Some scenarios for possible use of a bootstrapping mechanism are
   outlined in Section 4.  Finally, some conclusions are presented in

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   Section 5.

1.1 Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [1].

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2. Mobile IPv6 Configuration

   The Mobile IPv6 protocol needs no configuration or security set-up
   mechanisms for its route optimization functionality.  However, it
   requires that mobile nodes and home agents have been assigned to each
   other via configuration.

   All mobile node - home agent communications are protected by IPsec,
   as described in Section 5.1 of [4] and [5].  In order to protect
   messages exchanged between the mobile node and the home agent with
   IPsec, appropriate security policy database entries are needed.  A
   mobile node must be prevented from using its security association to
   send a message on the behalf of another mobile node using the same
   home agent.  This is achieved through the use of the security policy
   database, and via an authorization check in IKEv1 when dynamic keying
   is used.

   Sections 10.6 and 11.4 of [4] describe the operation of a protocol by
   which an mobile node that is away from home can discover it's home
   agent and by which the home agent can push prefix changes to the
   mobile node.  The protocol is an extension of ICMPv6.

   Prefix information propagation is envisioned as a way for network
   renumbering events in the home network to be propagated to the mobile
   node so that the mobile node can form a new home address.  Home agent
   discovery is envisioned as a way for the mobile node to find a home
   agent given the prefix of its home network.  While not strictly
   involved in the mobile node - home agent IPsec security relation,
   this protocol is required for bootstrapping the mobile node - home
   agent connection.

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3. Issues in Bootstrapping

3.1 Addressing

   In this section, we discuss the problems caused by the currently
   tight binding to home addresses and home agent addresses.

3.1.1 Dynamic Home Address Assignment

   Currently, the home agent uses the mobile node's home address for
   authorization.  When manual keying is used, this happens through the
   security policy database, which specifies that a certain security
   association may only use a specific home address.  When dynamic
   keying is used, the home agent ensures that the IKE Phase 1 identity
   is authorized to request security associations for the given home
   address.  Mobile IPv6 uses IKEv1, which is unable to update the
   security policy database based on a dynamically assigned home
   address.  As a result, static home address assignment is really the
   only home address configuration technique compatible with the current

   However, support for dynamic home address assignment would be
   desirable for the following reasons:

   Prefix changes in the home network

      The Mobile IPv6 specification contains support for a mobile node
      to autoconfigure a home address based on its discovery of prefix
      information on the home subnet [4].

   DHCP-based address assignment

      Some ISPs may want to use DHCPv6 from the home network to
      configure home addresses [7].

   Addressing privacy

      It may be desirable to establish randomly generated addresses as
      in RFC 3041 [3] and use them for a short period of time.
      Unfortunately, current protocols make it possible to use such
      addresses only from the visited network.  As a result, these
      addresses can not be used for communications lasting longer than
      the attachment to a particular visited network.

   Ease of deployment

      In order to make deployment of Mobile IPv6 easy, it would be
      desirable to free users and administrators from the task of

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      allocating home addresses and specifying them in the security
      policy database.

3.1.2 Dynamic Home Agent Assignment

   Currently, the address of the home agent is specified in the security
   policy database.  Support for multiple home agents requires the
   configuration of multiple security policy database entries.

   However, support for dynamic home agent assignment would be desirable
   for the following reasons:

   Home agent discovery

      The Mobile IPv6 specification contains support for a mobile node
      to autoconfigure a home agent address based on a discovery
      protocol [4].

   Independent network management

      An ISP may also want to dynamically assign home agents in
      different subnets, that is, not require that a roamed mobile node
      have a fixed home subnet.

   Local home agents

      The mobile node's home ISP may want to allow a local roaming
      partner ISP to assign a local home agent for the mobile node.
      This is useful both from the point of view of communications
      efficiency, and has also been mentioned as one approach to support
      location privacy.

   Ease of deployment

      ISP may want to allow "opportunistic" discovery and utilization of
      its mobility services without any prearranged contact.  These
      scenarios will require dynamic home address assignment.

3.1.3 Management requirements

   As described earlier, new addresses invalidate configured security
   policy databases and authorization tables.  Regardless of the
   specific protocols used, there is a need for either an automatic
   system for updating the security policy entries, or manual
   configuration.  These requirements apply to both home agents and
   mobile nodes, but it can not be expected that mobile node users are

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   capable of performing the required tasks.

3.2 Security Infrastructure

3.2.1 Integration with AAA Infrastructure

   The current IKEv1-based dynamic key exchange protocol described in
   [5] has no integration with backend authentication, authorization and
   accounting techniques unless the authentication credentials and trust
   relationships use certificates.

   Using certificates may require the ISP to deploy a PKI, which may not
   be possible or desirable in certain circumstances.  Where a
   traditional AAA infrastructure is used, the home agent is not able to
   leverage authentication and authorization information established
   between the mobile node, the foreign AAA server, and the home AAA
   server when the mobile node gains access to the foreign network, in
   order to authenticate the mobile node's identity and determine if the
   mobile node is authorized for mobility service.

   The lack of connection to the AAA infrastructure also means the home
   agent does not know where to issue accounting records at appropriate
   times during the mobile node's session, as determined by the business
   relationship between the home ISP and the mobile node's owner.
   Presumably, some backend AAA protocol between the home agent and home
   AAA could be utilized but IKEv1 does not contain support for
   exchanging the right kind of information, primarily the NAI [6], with
   the mobile node.

3.2.2 "Opportunistic" or "Local" Discovery

   The home agent discovery protocol does not support "opportunistic" or
   local discovery mechanisms in a roaming partner's local access
   network.  It is expected that the mobile node must know the prefix of
   the home subnet in order to be able to discover a home agent, it must
   either obtain that information through prefix update or have it
   statically configured.  A more typical pattern for interdomain
   service discovery in the Internet is that the client (mobile node in
   this case) knows the domain name of the service, and uses DNS in some
   manner to find the server in the other domain.  For local service
   discovery, DHCP is typically used.

3.3 Topology Change

3.3.1 Dormant Mode Mobile Nodes

   The description of the protocol to push prefix information to mobile
   nodes in Section 10.6 has an implicit assumption that the mobile node

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   is active and taking IP traffic.  In fact, many, if not most, mobile
   devices will be in a low power "dormant mode" to save battery power,
   or even switched off, so they will miss any propagation of prefix
   information.  As a practical matter, if this protocol is used, an ISP
   will need to keep the old prefix around and handle any queries to the
   old home agent anycast address on the old subnet, whereby the mobile
   node asks for a new home agent as described in Section 11.4, until
   all mobile nodes are accounted for.  Even then, since some mobile
   nodes are likely to be turned off for long periods, some owners would
   need to be contacted by other means, reducing the utility of the

3.3.2 Use of ICMP

   Many ISPs now routinely block ICMP at firewalls as a blanket security
   measure, to remove the possibility of ping attacks, etc.  Requiring
   them to pass the Mobile IPv6 prefix update and home agent discovery
   messages is likely to meet with a skeptical response.

   While the ICMP messages associated with the prefix update are
   required to be sent within the mobile node - home agent IPsec
   security association, the home agent discovery message is sent to an
   anycast address.  Securing anycast messages is, however, difficult
   with IPsec.  As a result, at least some of the ICMP messages have to
   be processed in the clear.  While the specific threats relating to
   the discovery of home agent addresses are not that significant, it is
   at least necessary for the ICMP messages to pass firewalls.

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4. Bootstrapping Scenarios

   In this section, we discuss four different scenarios involving

   The simplest bootstrapping scenario involves the creation of the
   security association "from thin air", i.e., without any pre-existing
   relationship.  This could be achieved using, for instance, SSH-style
   leap-of-faith or other weak authentication mechanisms [9].
   Unfortunately, as discussed in Section 6, some of the assumptions of
   the base Mobile IPv6 protocol rely on there being at least some
   administrative relationship between the mobile node and its home
   agent.  As a result, either this approach should be ruled out, or the
   assumptions of the base protocol removed through extensions.

   Another scenario involves turning an existing security association in
   the user's home network for a different purpose into a new security
   association suitable for protecting Mobile IPv6.  For instance, an
   existing security association for a VPN service could be used to
   generate suitable Mobile IPv6 security associations, on a
   first-come-first-served home address basis.

   The third scenario is similar to the second one, but utilizes a
   security association from one of the access networks to which the
   node is connected, if available, rather than from the home network.
   The network access security relationship is used in order to create a
   security association suitable for Mobile IPv6.  For instance, when
   the mobile node boots and connects to the network for the first time,
   it could create a security association with the access operator's
   home agent.  This home agent could then be used as the mobile node
   moves into a different position or even into a different access

   The fourth scenario involves turning an existing security association
   with a home agent into a new one.  For instance, the existing
   security association for one home address can be used to communicate
   changed addresses and home agents.  Based on this the parties can
   modify their security policy entries and authorization tables.

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5. Conclusions

   The ability to bootstrap security associations for Mobile IPv6 is
   necessary for many purposes, and can be expected to have a
   significant impact on the speed with which the protocol can be
   deployed.  New protocol mechanisms are required for this bootstrap to
   become possible, however, as the base Mobile IPv6 protocol does not
   accommodate for it.

   We expect the bootstrapping mechanisms to focus on scenarios 2, 3,
   and 4 described in Section 4, i.e., bootstrapping based on an
   existing home network security association, network access security
   association, or the modification of an existing security association
   for Mobile IPv6.

   The ability of the mobile node to dynamically locate a home agent
   impacts whether the mobile node can set up the IPsec security
   association, and the constraints on the ability of the mobile node to
   dynamically configure the IPsec security association also constrain
   how dynamic home agent location can be.  The current mobile node -
   home agent IPsec SA bootstrapping procedure is constrained by the
   requirements of IKEv1 dynamic key exchange.  These constraints, in
   turn, make dynamic home address assignment, dynamic home agent
   assignment, and proper integration with AAA infrastructure difficult.
   The IKEv2 design [8] is much less constrained in many of these areas,
   and may be a good candidate for a more flexible bootstrapping

   The design intent of the prefix updating and home agent discovery
   protocols described in Sections 10.6 and 11.4 of [4] is to extend the
   same kind of subnet configuration service enjoyed by hosts and
   routers on a local subnet (address autoconfiguration, router
   discovery) to a remote mobile node, using a similar mechanism (home
   address autoconfiguration, home agent discovery).  This basic subnet
   configuration mechanism is not well suited to a loose collection of
   perhaps millions of roaming mobile nodes.  Some utilization of
   existing interdomain mechanisms for bootstrapping home network
   mobility service from a foreign domain and standard service
   configuration mechanisms for performing the same function within a
   roaming partner's network is more likely to be viable.  On the other
   hand, IP layer mechanisms for bootstrapping Mobile IPv6 should not be
   bound to mechanisms specific to a particular type of access network
   technology, wireless technology, or ISP, in order to ease the use of
   the mechanism across many different kinds of access networks and

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6. Security Considerations

   This document does not propose any new protocols, and therefore does
   not involve any security considerations in that sense.  However,
   throughout this document there are discussions of problems with
   Mobile IPv6 involving the mobile node - home agent IPsec security

   When considering different bootstrapping solutions, it is important
   to keep the security assumptions of the Mobile IPv6 protocol design
   in mind.  In particular, the protocol relies on the home agent's
   operator to have an administrative relationship with the mobile
   node's user.  Through this relationship, rogue mobile nodes can be
   tracked down.  Completely automatic bootstrapping without any
   pre-existing relationship is thus out of the question, unless
   additional defenses (such as new care-of address verification) are
   built into the Mobile IPv6 protocol.

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Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
        RFC 2409, November 1998.

   [3]  Narten, T. and R. Draves, "Privacy Extensions for Stateless
        Address Autoconfiguration in IPv6", RFC 3041, January 2001.

   [4]  Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
        IPv6", draft-ietf-mobileip-ipv6-24 (work in progress), July

   [5]  Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to
        Protect Mobile IPv6 Signaling between Mobile Nodes and Home
        Agents", draft-ietf-mobileip-mipv6-ha-ipsec-06 (work in
        progress), July 2003.

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Informative References

   [6]   Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
         2486, January 1999.

   [7]   Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
         Carney, "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.

   [8]   Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
         draft-ietf-ipsec-ikev2-11 (work in progress), October 2003.

   [9]   Arkko, J. and P. Nikander, "How to Authenticate Unknown
         Principals without Trusted Parties", To appear in Proceedings
         of Security Protocols Workshop 2002, Cambridge, UK, April 2002.

   [10]  Montenegro, G., Patil, B., Arkko, J. and J. Kempf, "Thoughts on
         Bootstrapping Mobility Securely", Presentation in MIP6 WG in
         IETF-57, July 2003.

   [11]  Arkko, J. and C. Perkins, "Alternative (Future) Proposals for
         MIPv6 Security", Presentation in MIP6 WG in IETF-57, July 2003.

Authors' Addresses

   James Kempf
   DoCoMo Communications Labs USA
   181 Metro Drive
   San Jose, CA  94043

   EMail: kempf@docomolabs-usa.com

   Jari Arkko

   Jorvas  02420

   EMail: jari.arkko@ericsson.com

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Appendix A. Acknowledgements

   The authors would like to thank Tom Hiller and Gabriel Montenegro for
   interesting discussions in this problem space.  Part of this draft is
   based on ideas presented in [10] and [11].

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