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Versions: 00 01 02                                                      
Network Working Group                                     F. Dupont, Ed.
Internet-Draft                                                     CELAR
Expires: August 4, 2007                                       K. Weniger
                                                        January 31, 2007

  IKEv2-based Home Agent Assignment in Mobile IPv6/NEMO Bootstrapping

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   have been or will be disclosed, and any of which he or she becomes
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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document specifies how to use IKEv2 for Home Agent assignment in
   Mobile IPv6 or NEMO bootstrapping.  It uses IPv6 anycast addresses
   and should not introduce new security issues.

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1.  Introduction

   Home Agent (HA) assignment is an improvement over HA discovery: in
   place of giving a list of possible HA addresses, this procedure gives
   the HA to use in a way controlled by the Mobile Service Provider, for
   (initial) load balancing, fail-over or DoS avoidance.

   IKEv2 [RFC4306] is the tool of choice in bootstrapping scenarios
   because not only it established the needed Security Associations but
   it can allocate home Addresses, authenticate the Mobile Node (MN)
   using EAP [RFC2284], etc [IKEv2-MIPv6].  The only function it did not
   support is the HA assignment.

   This document addresses both Mobile IPv6 [RFC3775] and NEMO [RFC3963]
   cases, the second likely with another IPv6 subnet anycast address.

   The goal of the document is to provide HA assignment support using an
   IKEv2 initial message sent to an IPv6 anycast destination address
   without introduction of new security issues.

   This document could use the standard keywords [BCP14] to indicate
   requirement levels.

2.  The proposal

   In the IKEv2 exchanges, the MN takes the initiator role and the HA
   the responder role.  The MN takes parameters from its configuration,
   followed or not by a discovery phase.  The HA address is a
   recognizable IPv6 anycast address.  So:
   1.  the MN sends an IKE_SA_INIT request to the HA anycast address
   2.  the anycast receiver forwards the request to the "best" HA
   3.  the HA answers using its own address as the source address and
       includes an "under attack" cookie in its replies
   4.  the MN notes the HA own address and use it in subsequent
       messages, the MN retries the IKE_SA_INIT request with the cookie
       to the HA own address
   The standard IKEv2 procedure follows as usual.  The following figure
   illustrates the initial exchanges:

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           Initiator                          Responder ("best" HA)
           -----------                        ---------------------
           (IP_I:500 -> ANYCAST:500)
           HDR(A,0), SAi1, KEi, Ni   -->

                                     (IP_R:500 -> IP_I:500)
                                 <-- HDR(A,0), N(COOKIE)

           (IP_I:500 -> IP_R:500)
           HDR(A,0), N(COOKIE), SAi1, KEi, Ni   -->

                                     (IP_R:500 -> IP_I:500)
                                 <-- HDR(A,B), SAr1, KEr, Nr, [CERTREQ]

           (IP_I:500 -> IP_R:500)
           HDR(A,B), SK {IDi, [CERT,] [CERTREQ,] [IDr,]
               AUTH, SAi2, TSi, TSr} -->

                                     (IP_R:500 -> IP_I:500)
                                 <-- HDR(A,B), SK {IDr, [CERT,] AUTH,
                                                    SAr2, TSi, TSr}

                        Figure 1: Initial exchanges

   The proposal can be used in any environment, including Mobile IPv6
   and NEMO, as soon as a recognizable IPv6 anycast address is assigned
   to the provided service (cf. IANA (Section 3)).

   The term "recognizable IPv6 anycast address" means an IPv6 anycast
   address [RFC3513] which is recognizable as an IPv6 anycast address by
   the initiator.  This includes, but not exclusively, addresses in the
   subnet anycast address format defined by [RFC2526].

3.  IANA Considerations

   Mobile IPv6 defines an IPv6 subnet anycast address [RFC2526] (value
   decimal=126 / hexa=7E).  In the case Mobile IPv6 and NEMO services
   are not provided by the same HAs, a second IPv6 subnet anycast
   address has to be assigned by IANA for NEMO HAs.

4.  Security Considerations

   As the anycast address can be well known, the cookie-based defense
   against DoS ([RFC4306] section 2.6) is used by default.  Another
   advantage is the whole IKE_SA_INIT and IKE_AUTH exchanges are

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   performed using the "right" addresses so the impact of the proposal
   on IKEv2 implementations can be kept minimal.

   As in the standard IKEv2 ([RFC4306] section 2.4 4th paragraph) the
   initiator has to reject cryptographically invalid fake IKE_SA_INIT
   replies so there is no new attack against the initiator side.

5.  Acknowledgments

   The initial idea was in a Kilian Weniger's message about HA
   assignment.  Jean-Michel Combes, speaking for a Mobile Service
   Provider, insisted to improve the current HA discovery to HA
   assignment.  Kero Kivinen checked whether the proposal introduces new
   security issue and whether it is reasonably easy to implement, and
   proposed the figure for initial exchanges.

6.  References

6.1.  Normative References

   [BCP14]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, BCP 14, March 1997.

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, March 1999.

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, January 2005.

   [RFC4306]  Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
              Protocol", RFC 4306, December 2005.

6.2.  Informative References

              Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with
              IKEv2 and the revised IPsec Architecture",
              draft-ietf-mip6-ikev2-ipsec-08.txt (work in progress),
              December 2006.

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   [RFC2284]  Blunk, L. and J. Vollbrecht, "PPP Extensible
              Authentication Protocol (EAP)", RFC 2284, March 1998.

Appendix A.  Possible Enhancements

   The cookie phase is not strictly necessary so it is only RECOMMENDED.
   Note that one of the main reasons to provide assignment in place of
   discovery, i.e., perform the server selection by the network in place
   of the client, is to protect servers against denial of service
   attacks, so the cost of always using a cookie is in fact low.

   The mechanism can be used for other contexts than Mobile IPv6 and
   NEMO, for instance as a general Security Gateway assignment
   mechanism.  Instead of pre-configuring a well-known anycast addresses
   on the client, the anycast address can also be discovered prior to
   sending the IKE_SA_INIT, e.g., with DNS.  This allows for more
   flexibility, i.e., the operator can change the anycast address
   anytime and it can allocate different anycast addresses to different
   services.  And this simplifies the IANA considerations.

   The protocols used between the Home Agents, both to forward the
   initial request and to select the real server, are not described in
   this document.  They are currently left to the implementor's choice
   and can become the object of another document if needed.

Authors' Addresses

   Francis Dupont (editor)

   Email: Francis.Dupont@fdupont.fr

   Kilian Weniger
   Panasonic R&D Center Germany
   Monzastr. 4c
   Langen  63225

   Phone: +49 6103 766 137
   Email: kilian.weniger@eu.panasonic.com

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