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Internet Engineering Task Force                         Francis Dupont
INTERNET DRAFT                                           ENST Bretagne
Expires in September 2003                                Wassim Haddad
                                     Helsinki University of Technology
                                                           March 2003


              How to make IPsec more mobile IPv6 friendly

                  <draft-dupont-ipsec-mipv6-03.txt>


Status of this Memo

   This document is an Internet Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups.  Note that other groups may also
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   Distribution of this memo is unlimited.

Abstract

   IPsec specifications [1-6] do not work well with any mobility
   device based on addresses [7]. Mobile IPv6 interaction with IPsec
   is still far from being well achieved. This is mainly due to bad
   interpretations of IPsec specifications. HIP (Host Identity
   Payload) [9] should change this regrettable situation.

   This document specifies some points where improvements can be
   made in many current implementations, on the way of making IPsec
   more suitable for Mobile IPv6.

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1. Introduction

   This document assumes thar the reader knows IPsec (i.e., [1-6])
   but not Mobile IPv6. This section explains how Mobile IPv6 works.

   In Mobile IPv6, each Mobile Node (MN) is always identified by
   its Home Address (H@), regardless of its current point of
   attachment to the Internet. While situated away from its home,
   a MN is also associated with a Care-of Address (Co@), which
   provides information about the mobile node's current location.
   IPv6 packets from a Correspondent Node (CN) addressed to a MN's
   H@ are transparently routed to its Co@.

   The MN has a special CN, the Home Agent (HA), which is used to
   intercept packets addressed to the MN's H@ on the home link and
   to forward them to the MN at its current Co@ through a tunnel.

   End-to-end communications between a MN and a CN can use one of
   these three modes:
    - bidirectional tunneling with the HA:
       MN => HA -> CN (=> denotes an encapsulation)
       CN -> HA => MN
      The CN knows only the MN's H@ regardless whether the MN is
      mobile or not. This mode is very safe but not optimized at all.
    - triangular routing:
       MN ~> CN (~> denotes the use of an extension header)
       CN -> HA => MN
      The MN uses a Home Address Option (HAO): it puts its Co@ which
      is topologically correct into the source address field of the
      IPv6 header, and puts its H@ in the HAO.
    - optimized routing:
       MN ~> CN
       CN ~> MN
      The MN uses a HAO for packets to the CN, the CN uses a Routing
      Header (RH) and puts the MN's Co@ in the destination address
      field of the IPv6 header, and the MN's H@ in the RH. This mode
      raises many security concerns, mainly about the mobility
      signaling, but is very efficient.

   These uses of HAO and RH are in fact degenerated tunnels as
   shown by the Deering/Zill tunneling document [10], i.e., they
   can be considered as IPv6 in IPv6 tunnels where two addresses
   in the outer and inner IPv6 headers are redundant so one instance
   is removed.

   The association between a Co@ and the H@ is named "binding" and
   is cached by CNs. Bindings are managed (i.e., created, changed
   and deleted) by signaling messages named Binding Updates (BUs).

   The term node in MN is a bit misleading: mobile routers are not
   considered by current Mobile IPv6 specifications and communications
   in this document are always end-to-end.

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2. IPsec Architecture and Mobile IPv6

   IPsec defines two modes, Transport and Tunnel modes.
   As mobile IPv6 itself is based on real or degenerated tunneling,
   there are three possible basic interactions:
    - Transport mode after Mobile IPv6 tunneling,
    - Transport mode before Mobile IPv6 tunneling
    - A combination between Tunnel mode and Mobile IPv6 tunneling.

2.1 Transport Mode After Mobile IPv6

   This case is defined only when Mobile IPv6 uses real tunneling,
   i.e., in current specifications, between the MN and its HA.
   As the IPv6 header (and addresses!) viewed by IPsec is the outer
   one, in general this case has no interest (the MN's outer address
   is a transient Co@, the interesting one, the H@, is in the inner
   header).

2.2 Transport Mode Before Mobile IPv6

   In this case, the IPsec transform is applied to the payload of
   an ordinary packet between the MN and the CN. The MN will use
   its static/long term H@.
   After Mobile IPv6 includes its extension header corresponding to
   the mode used, the following cases can be depicted:

    - the bidirectional tunneling is perfectly transparent: IPsec and
      Mobile IPv6 do not interfere. The same consideration applies to
      the HA => MN tunnel.

    - the triangular routing is more interesting and gives different
      results for the Authentication Header (AH [2]) and the
      Encapsulating Security Payload (ESP [3]):
       * AH authenticates both Co@ (in the IPv6 header) and the H@
         (in the HAO)
       * ESP with authentication will reject fake packets because
         the attacker may not know the authentication shared secret.

      So with authentication the only possible attack is a "Denial
      of Services" launched by an attacker who knows the SPI and is
      likely able to inject fake traffic without HAO. So this is an
      intrinsic IPsec thread.

      *RECOMMENDATION A: Packets with a HAO matching an IPsec SA
      *providing authentication (i.e., AH or ESP with non-null
      *authentication) MUST be accepted (i.e., the HAO considered
      *as verifiable) and the HAO MUST be considered as verified [11]
      *after successful IPsec processing.


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    - the optimized routing is like triangular routing for the MN ~>
      CN way with, in addition to recommendation A, the common way to
      verify HAO: the "binding cache entry check". Symmetrically IPsec
      adds nothing to the RH verification because the MN has already
      all important informations.

    - the optimized routing between two MNs is an interesting case
      not yet fully addressed by Mobile IPv6 specifications [8]:
      RH and HAO have more overhead than a plain tunnel so a
      combined Tunnel mode with Mobile IPv6 becomes very
      interesting in this special case...

2.3 Tunnel Mode not combined with Mobile IPv6

   "Not combined" means there are two overhead, one for IPsec
   tunneling and another one for Mobile IPv6 extra headers, when
   obviously one encapsulation provides enough addresses (two sources
   and two destinations) for Mobile IPv6!

   *RECOMMENDATION B: IPsec in Tunnel mode and Mobile IPv6 SHOULD
   *be combined in order to avoid to add their overheads.

2.4 Addresses of SAs

   SAs can be characterized by:
    - zero address (proposed inbound processing for unicast)
    - one address (destination in IPsec inbound processing [1])
    - two addresses (source and destination selectors [1])
    - three addresses (source, destination and proxy in PF_KEY [12])
    - four addresses (in Tunnel mode)

   This is a bit confusing and gives security holes and/or extra
   checks on addresses which are highly unfriendly with Mobile IPv6.

   The Transport mode is easy because there are always exactly two
   addresses. For instance in inbound processing, the destination
   address is used for the SA lookup and the source address MUST
   be checked ([1], section 5.2.1). So this document will assume
   the Tunnel mode is used.

   In general, the SADB is not designed to be managed directly
   and/or by itself, i.e., without the SPD. Addresses are handled
   by the SPD with a pair of selectors which characterized the
   source and the destination of the traffic which receives IPsec
   protection. SPD entries specify the type of IPsec processing
   (for instance one type is the bypassing of IPsec: this is needed
   by IKE for its own messages [1]) and the parameters of SAs
   to use or to build (using SADB_ACQUIRE PF_KEY messages [12]).


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   According to [1] the traffic selection is divided between the
   SPD and the SADB (a SPD entry points to many matching SAs) but
   this cannot be realized using IKE so this point is very
   confusing, and some implementations are nearly nonconform.

   IKE (and its successor(s)) is designed to build SAs per pairs,
   so IPsec implementations, using PF_KEY, should comply for
   Tunnel mode SAs with the following interpretation:
    - the source and the destination addresses are plain addresses
      in the general case and designate the end points of the tunnel
      (i.e., there are the outer header addresses).
    - the inner source address is the proxy address (and exists only
      in the Tunnel mode). This can be something else than a plain
      address (i.e., for instance a prefix) but not in the Mobile IPv6
      case. The check may be performed after each IPsec inbound
      processing [1] or at the SPD check (not the specified way but
      the final result is the same).

      *RECOMMENDATION C1: The source address checked after each
      *IPsec inbound processing against the SA selector MUST be
      *the inner header source address.

    - the selection of the traffic to be processed is handled
      by SPD entries. This includes the future inner addresses
      in outbound processing. Guidelines are to use the inbound
      processing rules for SADB design, the outbound processing
      rules for SPD design and to complete by symmetry (with the
      funny (?) side-effect that source and destination roles can
      be reversed).

   RFC 2401 [1] has detailed rules about the outer source address but
   they are commonly misunderstood: checking it gives no extra
   security because once a attacker can get the SPI, he can inject
   fake traffic too. But this check harm nearly all mobility
   mechanisms based on addresses, even nomadism a.k.a.  the "road
   warrior" case.

   *RECOMMENDATION C2: The outer source address in Tunnel mode
   *MUST NOT be checked after or before IPsec inbound processing.

   This recommendation does not apply to the SPD checking, i.e.,
   step 4 of RFC 2401 [1] section 5.2.1.

   This recommendation by itself does not solve the problem of the
   other SA of the pair: the MN may change its Co@ and continue
   to use the SA to the CN. But the other way/SA will work only
   when the other SA will be updated or rebuilt with the new Co@
   as the destination.


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   BTW, RFC 2401 [1] specifies IPv6 in IPv4 and IPv4 in IPv6 tunnels
   and these tunnels are taken into account in PF_KEY [12] so:

   *RECOMMENDATION D: Dual stack (i.e., IPv4 and IPv6) IPsec
   *implementations MUST support IPvX in IPvY Tunnel modes with
   *any X and Y, including cases where X != Y.

2.5 Combined Tunnel Mode with Mobile IPv6 (Standard Case)

   When IPsec in Tunnel mode is combined with Mobile IPv6, there
   is one encapsulation with the fixed H@ in the inner header
   and a transient Co@ in the outer header. Such configuration is the
   opposite of the common Tunnel mode usage between two security
   gateways.  The CN can be itself a MN: only one detail not directly
   related to IPsec matters: mobility has to support the simultaneous
   movement of both peers (handled by fallback through a HA for
   instance in Mobile IPv6, c.f. [8]).

   Between two movements the IPsec tunnels are not very special,
   they look like end-to-end IPsec tunnels between two peers.
   The only unusual detail is that the outer and inner addresses
   can be different (when the MN is not at home), which is an issue
   for IKE.

   The interesting case is what happens when a Co@ changes:
   the MN should send a BU to the CN which, according to
   recommendation C, must not be filtered out because the
   Co@ is not the same.

   *RECOMMENDATION E1: BU protected by an IPsec SA providing
   *authentication MUST be considered as authenticated.

   *RECOMMENDATION E2: In the E1 case, all BU parameters MUST
   *be covered by the authentication. Specially when the
   *authentication is provided by an ESP transform, the new Co@
   *MUST be covered by using, for instance, an alternate Co@
   *suboption.

   The CN could ask for a proof that the new Co@ is not a fake one,
   i.e., the default policy may be to check the new Co@ in order
   to avoid reflection attacks. This check is a return routability
   check: the CN sends a question to the MN at its new Co@ with
   a predictable answer. In a thread on the mobile-ip mailing list,
   we proposed to reject the first BU with a "sequence number
   out of the window" error.

   *RECOMMENDATION E3: The CN SHOULD have the possibility to
   *perform a return routability check on a new Co@ before
   *recommendations E1 and E2 are applied.


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   As explained before, to deal with the BU is not enough for
   the CN ~> MN way. For instance, the Binding Acknowledgment (BA)
   can be protected only when the reversed SA is updated or rebuilt.

   *RECOMMENDATION F: Mobility signaling and IPsec SA management
   *direct cooperation SHOULD be considered (i.e., development of
   *this kind of mechanisms encouraged).

2.6 Combined Tunnel Mode with Mobile IPv6 (Home Agent Case)

   This section does not consider traffic from the HA itself
   which is handled by routing optimization as for a standard CN.
   It is only about the MN <=> HA bidirectional tunnel.

   The addresses used for this tunnel are very simple:
    - on the MN side, inner header address is the H@, outer a Co@.
    - on the HA side, inner header address is any valid address
     (unicast address when used as a source address) and the
     outer address is the HA address [8].
   IPsec considerations are near the same than for standard CNs.
   In fact, things are simpler because one can assume the HA is
   never mobile. Recommendation F applies but is not useful when
   no IPsec SA exists, i.e., when a MN boots in visit: this will
   be the special case in IKE considerations (next section).


3. IKE and Mobile IPv6

   There are three basic issues:
    - how to handle the multiple addresses of a MN? In the phase
      one? In a phase two?
    - how to establish SAs between a MN and a standard CN?
    - how to establish SAs between a MN from a foreign link
      and its HA the first time?
   This document uses the name IKE [6] for the IPsec DOI [4]
   and ISAKMP [5] framework too. Some proposals for IKEv2 [13]
   (used as an instance of a Son-of-Ike with two phases) can be
   found in the appendix B.

3.1 IKE and Identities (Phase One)

   In the phase one, identities (IDii and IDir) designate the peers
   so subnet and range identities may not be used. This document
   assumes a phase one with digital signatures using a X.509 style
   of certificates, but most of the considerations applies to
   public key authentications too.


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   In phase one, a peer is identified by its address used for the
   transport of IKE messages (aka the "peer address") and its
   identity payload. Identities, in the general meaning, may be
   present in certificates too but all cases are not equivalent:

    - the Identity must be related to the certificate:

      *RECOMMENDATION G: The Identity payload presented by the peer
      *MUST be verified. For instance, when certificates are used,
      *the Identity and the subject or an alternative subject of
      *the certificate associated to the signature MUST match.

    - if the Identity is an address, it must be the right address:

      *RECOMMENDATION H: If the Identity payload presented by the
      *peer is an address, it MUST be the same address than the one
      *used to transport IKE messages (aka the "peer address").

    - same for addresses used as (alternative) subjects:

      *RECOMMENDATION I: If an address is used as the subject or as
      *an alternative subject of the certificate associated to the
      *signature, then the address used to transport IKE messages
      *(aka the "peer address") SHOULD match the subject or an
      *alternative subject.

      "SHOULD match" means the default policy is to perform the check.

    - last about Identity/address checks:

      *RECOMMENDATION J: The case where the certificate associated
      *to the signature has no address subject or alternative subject
      *SHOULD be considered as a special case by policies. For
      *instance, the policy MAY specify particular constraints for
      *this case.

    - of course, this can be a bit annoying for MNs which must use
      in some cases their Co@ for transport of IKE messages so:

      *RECOMMENDATION K: The addition of a Home Address Identity,
      *which should allow strict application of previous I and J
      *recommendations in all MN's peer cases, SHOULD be considered
      *in IKE.

   Today he possibility to include addresses in identities is not
   considered to be a good idea. The appendix B about IKEv2 will
   propose a very different way to solve concerns addressed by
   recommendations of this section (H-K).


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3.2 IKE and Identities (Phase Two)

   In the phase two (a.k.a. quick mode) identities (IDci and IDcr)
   are optional and designate the policy rule (in the SPD or in the
   IKE software configuration) to apply:

    - in Tunnel mode the peer addresses will be the outer header
      addresses, and identities can denote the traffic selector part
      of the policy rule.

    - without identities the SA pairs will be applied to all traffic
      not matched by a more specific policy between the peers using
      the same addresses than in IKE messages. This is ambiguous so:

      *RECOMMENDATION L1: When the phase one and phases two are
      *allowed to use different (peer) addresses to transport
      *messages, identity payloads SHOULD be used in phases two.

    - in Tunnel mode there is an ambiguity about the endpoint
      addresses:

      *RECOMMENDATION L2: When the phase one and phases two are
      *allowed to use different addresses, the endpoint addresses used
      *in a phase 2 context MUST be the (peer) addresses used to
      *transport IKE messages of this phase 2.

    - junk identities are not useful:

      *RECOMMENDATION M: Identity payloads used in phase two SHOULD
      *clearly denote address sets.

    - IKE [6] explicitly provides a "proxy case" usable for mobility:

      *RECOMMENDATION N: The policy MAY authorize the establishment of
      *a Transport mode SA pair using an address identity payload
      *which does not match the (peer) address used to transport
      *IKE messages. This authorization SHOULD be based on parameters
      *provided in the phase one authentication, for instance the
      *phase one peer identity and certificate.

3.3 IKE and Mobile IPv6 (Standard Case)

   If the MN has the choice between using its H@ or a Co@ for IKE
   exchanges, only the first choice makes sense with a standard CN.
   Actually, when the initiator is the CN, it always use the H@.
   Using the Co@ is more complex and should require a new phase
   one with each peer after a movement.


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   Note that the IKE software should not even notice that the node is
   mobile...  For the same reason, the SA pairs should use the H@ as
   the MN address, giving an IPsec transform before Mobility case.

   *RECOMMENDATION O: With a standard CN, the MN SHOULD ignore the
   *fact that it is a mobile node and SHOULD use its H@ for all IKE
   *exchanges and for its own address in the SA pairs. To avoid both
   *IPsec and Mobile IPv6 overheads, it SHOULD negotiate the Transport
   *mode.

   Transport mode was designed for end-to-end communications, IMHO
   this recommendation should be written with MUSTs!

3.4 IKE and Mobile IPv6 (Home Agent Case)

   Recommendation O does not work with the HA because, when the MN
   boots in visit, it can use its H@ only after processing of the
   first BU by the HA. This BU MUST be protected so it can not
   be protected by IPsec (trivial bootstrap problem).

   In fact there are two possible MN - HA SA pairs:
    - a SA pair for BU/BA exchange protection.
    - a SA pair for the MN <=> HA tunnel.

   The first SA pair is a bit hairy to establish, because the MN
   can only use its Co@ in some circumstances:

   *RECOMMENDATION P: If BU/BA exchanges between the MN and the HA
   *are protected by an IPsec SA pair, the establishment of this SA
   *pair MUST be allowed using a Co@ for the transport of all IKE
   *messages (i.e., the MN peer address is a Co@).

   The detailed requirements are:
    - an API should give a suitable Co@ for communications with
      the HA (i.e., something like getsockname() which returns
      the Co@ for a connected socket to the HA address in place
      of the Ho@).
    - phase one and phase two messages are sent using this Co@.
    - the phase one identity is not an address if no transient
      certificate for a Co@ is available.
    - the authentication with this identity must be allowed
      (including when recommendation J is enforced).
    - until the home address identity is defined and implemented,
      the phase two identity must be an address identity using the
      H@, and this must be allowed (according to recommendation O).
    - the result is a SA pair between the MN with its H@ and
      the HA with its HA Address. The most adequat is AH Transport
      mode (enough security and best efficiency).


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   This SA pair establishment stresses the issue of relative lifetimes
   of the phase one and the SA pair so:

   *RECOMMENDATION Q: IKE implementations MUST support lifetimes
   *for the phase one, which are far longer or far shorter than
   *the lifetime of SA pairs established by phases two.

   and with very long phase one lifetimes:

   *RECOMMENDATION R: IKE implementations SHOULD be able to lookup
   *a still valid phase one state from a phase two message using
   *different (peer) addresses for transport. For instance using the
   *ISAKMP SA SPI a.k.a. cookies [5].

   The SA pair for the tunnel is more easy: the only problem is
   about policies:
    - A solution is to dynamically create the proper policy from
      mobility information (i.e., BUs) and to establish the SA
      pair described in section 2.6 (combined Tunnel mode with HA).
      One can make things a bit faster reusing the phase one
      (c.f. recommendations R and L2).
    - Another solution is to create a SA pair using only the MN's H@
      (this can be done as soon as the first BU is processed because
      the HA can use the standard routing optimization mode for its
      own traffic. In fact this is the default behavior), and to
      use a to-be-defined direct cooperation mechanism between IPsec
      and mobility to update the outer MN address in the SA pair
      (a good use of the HIP readdress [9]).


4. Security Considerations

   At the exception of recommendations F and K, all recommendations
   made in this document are about interpretation of IPsec
   specification details. In fact, we are convinced these
   recommendations shall improve the security of both IPsec and
   Mobile IPv6.


5. Acknowledgments

   Some of the MN - HA ideas were developed in the authentication vs.
   authorization brainstorming, for instance the home address identity
   (unfortunately I don't remember who proposed this).

   Of course the current terrible interaction between IPsec and
   Mobile IPv6 was and still is discussed in the mobile-ip WG list
   and from time to time in the ipsec WG list too. So many thanks
   to the little number of persons who participate to both lists.


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   The return routability check for new Co@ was proposed by Alper
   Yegin and makes IPsec a very good candidate in the MN - CN case
   when it is applicable.

   I finish with authors of "open source" IKE implementations,
   particularly Shoichi Sakane who has written the IPsec
   implementation (including an IKE daemon, racoon) I use.


6. Normative References

   [1] S. Kent, R. Atkinson, "Security Architecture for the Internet
   Protocol", RFC 2401, November 1998.

   [2] S. Kent, R. Atkinson, "IP Authentication Header", RFC 2402,
   November 1998.

   [3] S. Kent, R. Atkinson, "IP Encapsulating Security Payload (ESP)",
   RFC 2406, November 1998.

   [4] D. Piper, "The Internet IP Security Domain of Interpretation
   for ISAKMP", RFC 2407, November 1998.

   [5] D. Maughan, M. Schertler, M. Schneider, J. Turner, "Internet
   Security Association and Key Management Protocol (ISAKMP)",
   RFC 2408, November 1998.

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


7. Informative References

   [7] F. Dupont, "Mobility-aware IPsec ESP tunnels",
   draft-dupont-movesptun-00.txt, February 2001.

   [8] D. Johnson, C. Perkins, J. Arkko, "Mobility Support in IPv6",
   draft-ietf-mobileip-ipv6-20.txt, January 2002.

   [9] R. Moskowitz, "Host Identity Payload And Protocol",
   draft-moskowitz-hip-05.txt, November 2001.

   [10] S. Deering, B. Zill, "Redundant Address Deletion when
   Encapsulating IPv6 in IPv6",
   draft-deering-ipv6-encap-addr-deletion-00.txt, November 2001.

   [11] C. Perkins, "[mobile-ip] A new proposal for handling Home
   Address destination options", http://playground.sun.com/mobile-ip/,
   Message-ID: <3C6C7780.4CFAA7B4@iprg.nokia.com>, February 2002.

   [12] D. McDonald, C. Metz, B. Phan, "PF_KEY Key Management API,
   Version 2", RFC 2367, July 1998.


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8. References for Appendixes

   [13] C. Kaufman, ed., "Proposal for the IKEv2 Protocol",
   draft-ietf-ipsec-ikev2-05.txt, February 2003.

   [14] B. Korver, E. Rescorla, "The Internet IP Security PKI
   Profile of ISAKMP and PKIX",
   draft-ietf-ipsec-pki-profile-01.txt, October 2002.

   [15] P. Hoffman, "Adding revised identities to IKEv2",
   http://www.vpnc.org/ietf-ipsec/,
   Message-Id: <p05200f06b9edf48ac57b@[165.227.249.18]>,
   November 2002.

   [16] M. Kaat, "Overview of 1999 IAB Network Layer Workshop",
   RFC 2956, October 2000.

   [17] F. Dupont, J.-J. Bernard, "Transient pseudo-NAT attacks
   or how NATs are even more evil than you believed",
   draft-dupont-transient-pseudonat-01.txt, December 2002.

   [18] S. Deering and all, "IPv6 Scoped Address Architecture",
   draft-ietf-ipngwg-scoping-arch-04.txt, June 2002.

   [19] Franck Le and all, "Mobile IPv6 Authentication, Authorization,
   and Accounting Requirements",
   draft-le-aaa-mipv6-requirements-01.txt, November 2002.

9. Author's Address

   Francis Dupont
   ENST Bretagne
   Campus de Rennes
   2, rue de la Chataigneraie
   BP 78
   35512 Cesson-Sevigne Cedex
   FRANCE
   Fax: +33 2 99 12 70 30
   EMail: Francis.Dupont@enst-bretagne.fr

   Wassim Haddad
   Helsinki University of Technology
   Theoretical Computer Science Laboratory
   PO BOX 9201
   HUT 02015
   FINLAND
   EMail: whaddad@tcs.hut.fi


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10. Changes from Previous Drafts

   Addition of recommendation E3 about return routability check
   for new Co@.

   Addition of Appendix A (list of recommendations), B (proposals
   for IKEv2), C (return routability) and D (scoped addresses).

   Introduction of the "peer address" term.

   Some rewordings (from Wassim Haddad).

   New appendix about mobile IPsec VPN.


Appendix A: List of Recommendations

   A: Packets with a HAO matching an IPsec SA providing authentication
      (i.e., AH or ESP with non-null authentication) MUST be accepted
      (i.e., the HAO considered as verifiable) and the HAO MUST be
      considered as verified [11] after successful IPsec processing.

   B: IPsec in Tunnel mode and Mobile IPv6 SHOULD be combined in
      order to avoid to add their overheads.

   C1: The source address checked after each IPsec inbound processing
      against the SA selector MUST be the inner header source address.

   C2: The outer source address in Tunnel mode MUST NOT be checked
      after or before IPsec inbound processing.

   D: Dual stack (i.e., IPv4 and IPv6) IPsec implementations MUST
      support IPvX in IPvY Tunnel modes with any X and Y, including
      cases where X != Y.

   E1: BU protected by an IPsec SA providing authentication MUST
      be considered as authenticated.

   E2: In the E1 case, all BU parameters MUST be covered by the
      authentication. Specially when the authentication is provided
      by an ESP transform, the new Co@ MUST be covered by using, for
      instance, an alternate Co@ suboption.

   E3: The CN SHOULD have the possibility to perform a return
      routability check on a new Co@ before recommendations E1 and
      E2 are applied.

   F: Mobility signaling and IPsec SA management direct cooperation
      SHOULD be considered (i.e., development of this kind of
      mechanisms encouraged).


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   G: The Identity payload presented by the peer MUST be verified. For
      instance, when certificates are used, the Identity and the
      subject or an alternative subject of the certificate associated
      to the signature MUST match.

   H: If the Identity payload presented by the peer is an address,
      it MUST be the same address than the one used to transport
      IKE messages (aka the "peer address").

   I: If an address is used as the subject or an alternative subject
      of the certificate associated to the signature, then the address
      used to transport IKE messages (aka the "peer address") SHOULD
      match the subject or an alternative subject.

   J: The case where the certificate associated to the signature has
      no address subject or alternative subject SHOULD be considered
      as a special case by policies. For instance, the policy MAY
      specify particular constraints for this case.

   K: The addition of a Home Address Identity, which should allow
      strict application of previous I and J recommendations in all
      MN's peer cases, SHOULD be considered in IKE.

   L1: When the phase one and phases two are allowed to use different
      (peer) addresses to transport messages, identity payloads SHOULD
      be used in phases two.

   L2: When the phase one and phases two are allowed to use different
       addresses, the endpoint addresses used in a phase 2 context
       MUST be the (peer) addresses used to transport IKE messages of
       this phase 2.

   M: Identity payloads used in phase two SHOULD denote clearly
      address sets.

   N: The policy MAY authorize the establishment of a Transport mode
      SA pair using an address identity payload which does not match
      the (peer) address used to transport IKE messages. This
      authorization SHOULD be based on parameters provided in the
      phase one authentication, for instance the phase one peer
      identity and certificate.

   O: With a standard CN, the MN SHOULD ignore the fact that it is a
      mobile node and SHOULD use its H@ for all IKE exchanges and for
      its own address in the SA pairs. To avoid both IPsec and Mobile
      IPv6 overheads, it SHOULD negotiate the Transport mode.

   P: If BU/BA exchanges between the MN and the HA are protected by
      an IPsec SA pair, the establishment of this SA pair MUST be
      allowed using a Co@ for the transport of all IKE messages (i.e.,
      the MN peer address is a Co@).


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   Q: IKE implementations MUST support lifetimes for the phase one
      which are far longer or far shorter than the lifetime of SA
      pairs established by phases two.

   R: IKE implementations SHOULD be able to lookup still valid phase
      one state from a phase two message using different (peer)
      addresses for transport. For instance using the ISAKMP SA SPI
      a.k.a. cookies [5].


Appendix B: Proposals for IKEv2

   Many recommendations are directly applicable to IKEv2 [13]:
    - recommendations G, O, P apply without modifications.
    - recommendations L1, L2 and N applies to traffic selectors
      in place of phase two identities.
    - recommendation M is integrated in IKEv2.
    - recommendation R is partially integrated in section 2.6
      of [13], but we propose to make very clear the IKE-SA lookup
      MUST be done using the cookies as a SPI *only*. Note that
      IKEv2 guarantees the uniqueness of these "SPIs".

   [14] introduces the term "peer addresses" for the addresses used
   for the transport of IKE messages and includes the recommendations
   G, H and I. [15] is not directly applicable to IKEv2 but [15] (not
   yet included in the IKEv2 draft) proposes a new form of identities
   without any kind of binding to addresses.

   In IKEv2 the phase one SA is named the IKE SA and when it is
   deleted all the IPsec SAs it negotiated have to be deleted too
   (so the recommendation Q does not stand). The idea is to solve
   the dead peer detection issue by keepalives over the IKE SA.

   *PROPOSAL 1: IKEv2 implementations MUST lookup IKE-SA using
   *only the SPI at the exclusion of peer addresses.

   Identities should not include addresses as recommended in [16]
   so recommendations H to K are obsolete in the IKEv2 context.
   (this is a call to adopt [15] ASAP)

   *PROPOSAL 2: IKEv2 identity payloads MUST only use abstract
   *identities as recommended in [16] by the IAB and proposed by [15].

   But this and the section 4.11 "Address and Port Agility" of [13]
   remove any check of peer addresses which are still part of
   established SAs, opening the door to attacks as described in
   [17]. But mobility really needs address agility so:

   *PROPOSAL 3: The section 4.11 should specify full address agility.


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   The first counter-measure against abuse of this address agility
   is to protect the integrity of transport headers. The new
   notifications NAT-DETECTION-SOURCE-IP and NAT-DETECTION-
   DESTINATION-IP are the beginning of a solution.

   *PROPOSAL 4: IKEv2 MUST provide a way to protect the integrity
   *of transport parameters (peer addresses, ports and protocol).

   *PROPOSAL 5: The default policy SHOULD be the protection of
   *the integrity of transport parameters for IPv6.

   These proposals defeat en-route modifications of messages,
   i.e., fulfill some mobility requirements, but not all of
   them because these proposals give no proof about the real
   origin of messages, i.e., one should trust its peer.
   The solution is of course a simple return routability check,
   and IKEv2 already uses this kind of mechanisms in the
   "Responder under attack" case (IKE_SA_init_reject).

   *PROPOSAL 6: A mechanism MUST be provided in order to make return
   *routability checks available on peer address changes.

   *PROPOSAL 7: The default policy SHOULD be to perform
   *return routability checks on peer address changes.

   Now that mobility can be securely handled (this is not the
   case for NAT traversal but we believe this issue can not be
   solved, c.f. [17]), we can look for some dedicated improvements.
   The fist special case to be dealt with is the MN - HA SA pair
   to protect the BU/BA exchange a.k.a. the "home registration".

   *PROPOSAL 8: When the policy authorizes it, a traffic selector
   *in Transport mode MAY override peer addresses as SA selectors.

   (this is a reformulation of recommendations N and P.)

   The other item is to instantiate the recommendation F:

   *PROPOSAL 9: A new mechanism MUST be defined for the update
   *of the peer address in the SA pair (the source outer address of
   *the inbound SA and the destination outer address of the outbound
   *SA) without mandatory rekeying.


Appendix C: Return Routability

   The proposed return routability check assumes these properties:
    - a secret is shared with the peer, i.e., there is a proof
      that the received packets are from the peer.
    - an anti-replay mechanism proves the received packets are fresh.
   If the exchange involved some hard state change (for instance the
   proposal 9), a sequencing mechanism should be provided too.


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   The return routability check does not give a proof that the peer is
   at the given address, it only proves the peer is on the path.
   For more details about return routability check theory, please
   refer to [8].


Appendix D: Scoped Addresses

   This topics is not really a Mobile IPv6 one, but in practice the
   "mobile VPN" case there is a heavy usage of limited scope or
   private addresses.

   The issue is that addresses carried in identity or traffic
   selector payloads are not clothed with zone identifiers.
   Only the peer addresses used to transport messages have an
   indirect indication of their zones.

   The IPv6 scoped address architecture [18] gives the properties
   of zones: at a given scope, zones formed a partition, i.e.,
   an interface belongs to one and only one zone. They have an
   inclusion property too, i.e., a zone of a given scope is
   fully included into a zone of any higher scope. This gives
   an inheritance property which is safe when it is used in
   the proper way: to establish SAs with global addresses with
   IKE running over link-local addresses is safe, the opposite
   is not.

   *RULE: The default policy SHOULD accept scoped addresses as
   *selectors of SAs only when they are established using peer
   *addresses (for the transport of IKE/IKEv2/etc messages) which
   *are in fully included zones.

Appendix E: mobile IPsec VPN

   Mobile IPsec Virtual Private Networks (VPNs) provides the same kind
   of functionality than mobile IP: the VPN client (the Mobile Node in
   the mobility context) opens an ESP tunnel with a Security Gateway
   (the equivalent of a Home Agent) located in the home site.

   Even if the style of mobile IPsec VPNs are more Mobile IPv6 than
   Mobile IPv4 (there is no equivalent of Foreign Agents for
   instance), they can be used for the two versions of IP so this
   appendix is about both.

   Current mobile IPsec VPNs have no Security Gateway detection,
   support for multiple inner addresses, prefix discovery, etc, but
   they can be connected to a remote network access control with an
   optional address allocation. Today they have no support for an
   extended AAA system where the AAA infrastructure connects the local
   and remote network access control with some assistance to the
   initial security setup (via credentials and/or piggy-backing of IKE
   initial exchanges [19]).


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   The layout of data packets of mobile IPsec VPNs are exactly the
   same than for Mobile IPv6 with an ESP protected MN-HA bidirectional
   tunnel (the outer header client address is a Care-of Address, the
   inner one is the Home Address) at one exception: in Mobile IPv6 the
   Home Agent is a correspondent node for its own address, for
   instance the Home Agent sends genuine packets to the Mobile Node
   using a Routing Header, not through the tunnel. If the
   corresponding rule of [8] (section 9.3.2 Sending Packets to a
   Mobile Node) is applied only to Mobile IPv6 signaling packets,
   mobile IPsec VPNs and Mobile IPv6 are indistinguishable.

   So mobile IPsec VPNs are a good replacement for unoptimized Mobile
   IPv6 or for Mobile IPv4 with secure reverse tunneling. Movements
   can be handled by peer address update mechanisms, including
   rekeying.

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