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Internet Engineering Task Force                         Francis Dupont
INTERNET DRAFT                                           ENST Bretagne
Expires in December 2002                                     June 2002


              How to make IPsec more mobile IPv6 friendly

                  <draft-dupont-ipsec-mipv6-01.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
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   Distribution of this memo is unlimited.

Abstract

   IPsec specifications [1-6] does not work well with Mobile IPv6 [7]
   and with any mobility device based on addresses [8] even if HIP [9]
   should change this regrettable situation.

   But many problems come directly from bad or dubious interpretations
   of IPsec specifications, so this document presents many points
   where many current implementations can be improved (i.e., can
   become more Mobile IPv6 friendly).


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1. Introduction

   This document assumes the reader knows IPsec (i.e., [1-6]) but
   not Mobile IPv6, so 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@.

   A 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 3 modes:
    - bidirectional tunneling with the HA:
       MN => HA -> CN (=> denotes an encapsulation)
       CN -> HA => MN
      The CN knows only the MN's H@ and in fact even does not know
      the MN is mobile. 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 Home Address Option 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.


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   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.

   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).

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, 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, using for the MN
   its static/long term H@, and *after* Mobile IPv6 may add its
   extension header corresponding to its mode:
    - bidirectional tunneling is perfectly transparent: IPsec and
      Mobile IPv6 don't interfere. The same consideration
      applies to the HA => MN tunnel.
    - 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 from an attacker who knows the SPI and is likely
      be able to inject fake traffic without HAO. So this is an
      intrinsic IPsec thread.


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      *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.
    - optimized routing is like triangular routing for the MN ~> CN
      way with, in addition to recommendation A, the common way
      to verify HAO: binding cache entry check. Symmetrically
      IPsec adds nothing for the RH verification because the MN
      has already all important informations.
    - optimized routing between two MNs is an interesting case
      not yet fully addressed by Mobile IPv6 specifications [7]:
      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,
   another 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 Mobile IPv6 unfriend.

   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.


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   In general, the SADB is not designed to be managed directly
   and/or 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]).

   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 nearly nonconform.

   IKE (and its successor(s)) is designed to build SAs per pairs
   so IPsec implementations using PF_KEY follow (or should follow)
   for tunnel mode SAs this interpretation:
    - the source and the destination addresses are plain addresses
      in the general case and designate the end nodes of the tunnel
      (i.e., there are the outer header addresses).
    - the inner source address is the proxy address (and exists
      only in 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: to check it gives no
   extra security because when (where!) a attacker can get the SPI
   he can inject fake traffic too. But this check harm near 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 doesn't apply to the SPD checking, i.e.,
   step 4 of RFC 2401 [1] section 5.2.1.


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   This recommendation by itself doesn't 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.

   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, 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. [7]).

   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 (they are if a MN is not at home): this 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, for instance using an alternate Co@ suboption.

   The CN could like to get a proof 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 send 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 doesn't consider traffic from the HA itself
   which is handled by routing optimization as for a standard CN,
   but is about the MN <=> HA tunnels. This document assumes that
   the tunnel is bidirectional even the Mobile IPv6 specifications
   are not (yet) very clear about this.

   The addresses used for these tunnels 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 [7].
   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 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 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 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 for transport of IKE messages.
    - same for addresses used as (alternative) subjects:
      *RECOMMENDATION I: if an address is used as the subject or
      *an alternative subject of the certificate associated to the
      *signature, the address used for transport of IKE messages
      *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 cases for a MN's peer 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).

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.


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    - 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 addresses to transport messages,
      *identity payloads SHOULD be used in phases two.
    - in tunnel mode there is an ambiguity about the peer addresses:
      *RECOMMENDATION L2: when the phase one and phases two are
      *allowed to use different addresses, the peer addresses used
      *in a phase 2 context MUST be the addresses used for the
      *transport of IKE messages of this phase 2.
    - junk identities are not useful:
      *RECOMMENDATION M: identity payloads used in phase two SHOULD
      *denote clearly 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 doesn't match the address used for transport of 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:
    - to use the Co@ is more complex and should require a new phase
      one with each peer after a movement.
    - when the CN is the initiator it always use the H@.
   Note the IKE software should not even know 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
   *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!


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3.4 IKE and Mobile IPv6 (Home Agent Case)

   Recommendation O doesn't 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 tunnels.

   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, establishment of this SA
   *pair MUST be allowed using a Co@ for the transport of all IKE
   *messages.
   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@.
    - phase one identity is not an address if no transient
      certificate for the 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 favorite is AH transport mode
      (enough security and best efficiency).
   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
   *still valid phase one state from a phase two message using
   *different addresses for transport, for instance using the
   *ISAKMP SA SPI a.k.a. cookies [5].


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   The SA pair for the tunnel is more easy: the only problem is
   about policies:
    - a solution is 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-define 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.

   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.


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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] D. Johnson, C. Perkins, J. Arkko, "Mobility Support in IPv6",
   draft-ietf-mobileip-ipv6-17.txt, May 2002.

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

   [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] D. Arkins and all, "Proposal for the IKEv2 Protocol",
   draft-ietf-ipsec-ikev2-02.txt, April 2002.

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

   [15] F. Dupont, "Transient pseudo-NAT attacks or
   how NATs are even more evil than you believed",
   draft-dupont-transient-pseudonat-00.txt, June 2002.

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

10. Changes from the Previous Draft

   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).


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.


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   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, for instance
      using 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).

   G: the Identity payload presented by the peer MUST be verified,
      for instance 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 for transport of
      IKE messages.

   I: if an address is used as the subject or an alternative subject
      of the certificate associated to the signature, the address
      used for transport of IKE messages 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.


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

   L1: when the phase one and phases two are allowed to use different
      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 peer addresses used in a phase 2 context MUST be
      the addresses used for the transport of 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 doesn't match
      the address used for transport of 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 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, establishment of this SA pair MUST be allowed
      using a Co@ for the transport of all IKE messages.

   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 addresses
      for transport, for instance using the ISAKMP SA SPI
      a.k.a. cookies [5].


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Appendix B: Proposals for IKEv2

   Many recommendations are directly applicable to IKEv2 [13]:
    - recommendations G, O, P, Q 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".

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

   Identities should not include addresses as recommended in [14]
   so recommendations H to K are obsolete in the IKEv2 context.

   *PROPOSAL 2: IKEv2 identity payloads MUST only use abstract
   *identities as recommended in [14] by the IAB.

   But this and section 2.11 "Address and Port Agility" of [13]
   remove any check of transport addresses which are still part
   of established SAs, opening the door to attacks as described
   in [15]. But mobility really needs address agility so:

   *PROPOSAL 3: section 2.11 should specify full address agility.

   The first counter-measure against abuse of this address agility
   is to protect the integrity of transport headers. There are two
   ways to provide this protection:
    - the protection of IKE data is based on ESP, if it is extended
      to a AH+ESP scheme (i.e., the authentication data in appendix
      B of [13] covering headers in an AH similar way) then the
      integrity of transport headers will be protected.
    - a new payload can be defined: it includes the transport
      parameters (addresses, ports and protocol). A format based
      on two IKEv1 ID_IPVx_ADDR identity payloads should fulfill
      the need.

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

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


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   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).
   Unfortunately this is not enough detailed in [13] to be
   clearly reusable outside the init case, for instance the
   exchange type value is not yet defined.

   *PROPOSAL 6: a mechanism similar to IKE_SA_init_reject
   *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. [15]), 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 transport 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.


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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
      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.

   The return routability check doesn't give a proof the peer is at
   the given address, it only proves the peer is on the path. Look
   at [7] for more details about return routability check theory.


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 addresses used for the transport of messages have an
   indirect indication to their zones.

   The IPv6 scoped address architecture [16] 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 addresses
   *for the transport of IKE/IKEv2/etc messages which are in
   *fully included zones.


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