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Internet Engineering Task Force                         Francis Dupont
INTERNET DRAFT                                       GET/ENST Bretagne
Expires in December 2004                            Jean-Michel Combes
                                                    France Telecom R&D
                                                             June 2004


       Using IPsec between Mobile and Correspondent IPv6 Nodes

                <draft-dupont-mipv6-cn-ipsec-01.txt>



   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

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

   Mobile IPv6 [1] uses IPsec [2] to protect signaling between the
   mobile node and the home agent [3]. This document defines how IPsec
   could be used between the mobile node and correspondent nodes,
   including home address option validation (aka. triangular routing),
   protection of mobility signaling for routing optimization and
   suitable configurations.


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1. Introduction

   Mobile IPv6 documents [1,3] specifies IPsec for the protection of
   the signaling between the mobile node (MN) and its home agent (HA),
   and the return routability procedure between the mobile node and
   its correspondent nodes (CN) for routing optimization. But any
   stronger mechanism (i.e., more secure than the return routability
   procedure) MAY be used, including of course IPsec.

   This document specifies which IPsec configurations can be useful
   in a Mobile IPv6 context and how they can validate home address
   options (enabling triangular routing) and protect mobility
   signaling (enabling routing optimization). It gives detailed
   IKE [4,5] configuration guidelines for common cases. An annex
   proposes an extension of mobility signaling for the safe support
   of alternate care-of addresses.

   This document uses the "MUST", "SHOULD", "MAY", ..., key words
   according to [6]. IKE terminology is copied from IKEv2 [5].


2, IPsec in a Mobile IPv6 context


   This document considers only suitable IPsec security associations,
   i.e., anything which does not fulfill the following requirements
   is out of scope:
    - IPsec security association pairs MUST be between the mobile
      node and one of its correspondent nodes.
    - authentication, integrity and anti-replay services MUST be
      selected.
    - the traffic selectors MUST match exclusively the home address
      of the mobile node and an address of the correspondent node
      (the address used for communication between peers).
    - the transport mode MUST be used.
    - for routing optimization, the mobility header "upper protocol"
      with at least binding update (BU) and acknowledgment (BA)
      message type MUST be accepted by the traffic selectors.

   The purpose of the first three requirements is to allow using
   IPsec as a proof of origin.


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3. Home address option validation


   This document amends the Mobile IPv6 [1] section 9.3.1 by adding
   a second way (other than binding cache entry check) to provide
   home address option validation.

   When a packet carrying a home address option is protected by a
   suitable IPsec security association, the home address option
   SHOULD be considered as validated.

   A way to implement this is to mark the home address option as
   "to be validated" when it is processed. When the upper protocol
   is reached, in order either:
    - an IPsec header was processed according to [2] section 5.2.1
      with a suitable IPsec security association, or
    - a binding cache entry check is successfully performed, or
    - the packet contains a binding update, or
    - the packet MUST be dropped.

   Note this enables triangular routing from any unicast routable
   care-of address, i.e., half optimization without any mobility
   signaling.


4. Routing Optimization


   A suitable IPsec security association can protect binding updates
   and acknowledgments. In binding updates the new requirements are:
    - the H (home registration) and K (key management mobility
      capability) bits MUST be cleared.
    - Nonce indices and binding authorization data options SHOULD
      NOT be sent by the mobile node and MUST be ignored by the
      correspondent node.
    - when an alternate care-of address option is present and the
      Annex is not in use, the alternate care-of address MUST
      match the source address in the IP header or the home address
      itself. Any binding update which does not fulfill this
      requirement MUST be rejected.
    - as ESP can only protect the payload, an alternate care-of
      address option MUST be used in conjunction with ESP
      (cf [1] section 11.7.1).


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   In binding acknowledgments the new requirements are:
    - the K (key management mobility capability) bit MUST be cleared.
    - Binding authorization data option SHOULD NOT be sent by the
      correspondent node and MUST be ignored by the mobile node.
    - "long" lifetime compatible with the IPsec policy (i.e., by
      default up to the IPsec security association lifetime) MAY
      be granted.

   As explained in [9], ingress filtering either is not used and
   bombing attacks are possible without the "help" of any Mobile IPv6
   mechanism, or is used and provides protection against fake care-of
   addresses from a rogue mobile node. So the only constraint is to
   accept real alternate care-of addresses only with the Annex
   procedure.

5. IKE configurations

   This document considers only IKE where it is used for mobility
   purpose. Peer addresses (addresses IKE runs over) are the addresses
   seen at the transport or application layers, i.e., when the mobile
   node uses its home address as the source of an IKE message the
   source address in the IP header can (should!) be a care-of address.

   IKE MUST be run over the home address for the mobile node side
   when the home address is usable. In special circumstances where
   the home address can be unsable, IKE MUST be run over a care-of
   address but this has many known drawbacks:
    - a care-of address can not be used for authentication nor
      authorization.
    - security associations do not survive handoffs.
    - the establishment of transport mode IPsec security association
      using the home address as the mobile node traffic selector
      raises a policy / authorization issue.
   The home address MAY be used in (phase 1) mobile node Identity
   payload. But this does not work well with dynamic home addresses,
   so when it is acceptable by the correspondent node policy, name
   based Identity (i.e., of type ID_FQDN or ID_RFC822_ADDR, [5]
   section 3.5) payloads SHOULD be used by the mobile node

   When the home address is bound to a public key, for instance
   when the home address is a Cryptographically Generated Addresses
   (CGA) [10], the strong authentication MAY be replaced by an
   address ownership proof. In this case the public key MAY be
   transported by IKE from the mobile node to the correspondent
   node, for instance in a Certificate payload of type 11 ([5]
   section 3.6). Auxiliary parameters MAY be transported in
   an Identity payload of type ID_KEY_ID...

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   The IKE peer policy MAY restrict IPsec security associations to
   the protection of Mobile IPv6 signaling, i.e., restrict the traffic
   selectors to the mobility header "upper protocol" with at least
   binding update and acknowledgment message types. This SHOULD be
   the default policy when authentication or authorization can be
   considered as weak, for instance when the previous paragraph is
   applied.

6. Security Considerations

   IPsec is far more secure than the return routability procedure,
   thus it should be used where it is applicable. So this document
   could increase at least the overall security of Mobile IPv6.
   Note that some operators can not propose Mobile IPv6 based services
   knowing that the Mobile IPv6 security is based on assumptions.

   Two points are worthy of special considerations:
    - no care-of address test is required when ingress filtering
      can reject fake care-of addresses from a rogue mobile node
      but a correspondent node can use the Annex procedure to get
      extra insurance as well as support real alternate care-of
      addresses.
    - in order to avoid granting any extra privilege by a side effect
      of using IPsec, the peers (i.e., the mobile and correspondent
      nodes) may restrict the traffic selectors to the protection
      of mobility signaling only. This should be applied to any
      dubious cases, including by default when security administration
      is known to be too light.

7. Acknowledgments

   The authors would like to thank many people for believing in IPsec
   as a right way to secure Mobile IPv6. Special thanks to Wassim
   Haddad and Claude Castelluccia for keeping our attention to special
   cases where home addresses are derived from public keys.

   Thanks to the Mobile IPv6 IETF working group for discussions
   about the third party bombing issue, including for no convincing
   arguments in favor of a requirement for the care-of address test.
   No thanks to router vendors who do not support ingress filtering
   with reasonable performance on some models, and to Internet service
   provider managers who could enable ingress filtering but did not.


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8. Normative References

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

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

   [3] J. Arkko, V. Devarapalli, F. Dupont, "Using IPsec to Protect
   Mobile IPv6 Signaling between Mobile Nodes and Home Agents",
   RFC 3776, June 2004.

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

   [5] C. Kaufman, ed., "Internet Key Exchange (IKEv2) Protocol",
   draft-ietf-ipsec-ikev2-14.txt, May 2004.

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

   [7] R. Stewart & all, "Stream Control Transmission Protocol",
   RFC 2960, October 2000.

   [8] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
   Message Authentication", RFC 2104, March 1997.


9. Informative References


   [9] F. Dupont, "A note about 3rd party bombing in Mobile IPv6",
   draft-dupont-mipv6-3bombing-00.txt, February 2004.

   [10] T. Aura, "Cryptographically Generated Addresses (CGA)",
   draft-ietf-send-cga-06.txt, April 2004.


10. Authors' Addresses


   Francis Dupont
   ENST Bretagne
   Campus de Rennes
   2, rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex
   FRANCE
   Fax: +33 2 99 12 70 30
   EMail: Francis.Dupont@enst-bretagne.fr


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   Jean-Michel Combes
   France Telecom R&D - DTL/SSR
   38/40, rue du General Leclerc
   92794 Issy-les-Moulineaux Cedex 9
   FRANCE
   Fax: +33 1 45 29 65 19
   EMail: jeanmichel.combes@francetelecom.com

11. Changes from the previous version

   Front IPR statement (cf new I-D guidelines).
   Peer address clarification (thanks to Mohan Parthasarathy).
   Change SHOULD/MAY to MUST/MUST for mobile node peer address.
   Reference updates.

A1. Signaling extension for alternate care-of address support

   This Annex defines a procedure which performs a "care-of address
   test". This procedure MAY be used in order to check whether the
   mobile node can really receive packets sent to the care-of address
   of a new binding update. It SHOULD NOT be used for entry deletion,
   i.e., when the care-of address is the home address. It MUST be used
   for real alternate care-of address, i.e., when the address carried
   by an alternate care-of address option is not the source address
   of the IP header nor the home address of the mobile node (following
   the recommendation of [9]).

   The procedure is based on the state cookie used in SCTP [7] which
   can be found again in IKEv2 proposal [5]. The binding update is
   in a first time (1) rejected by a binding acknowledgment with a
   new dedicated status and a cookie option sent to the tested care-of
   address. Unpon the reception (2) of this binding acknowledgment,
   the mobile node retransmits (3) the binding update with the exact
   received cookie placed in a cookie option. When the correspondent
   node receives (4) the augmented binding update, it can check by
   recomputing the cookie and comparing it to the cookie option that
   the binding update is from the same mobile node and for the same
   care-of address (so it can infer the mobile node is reachable at
   this care-of address, i.e., a "care-of address test" has been
   successfully performed).

   The cookie MUST reflect the mobile node identity or the binding
   cache entry or an equivalent, and MUST reflect the tested care-of
   address. It MUST not be easy to infer by the mobile node, including
   with the knowledge of previous cookies from the same node.


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   Two methods of generating cookies are proposed, the first one uses
   a secret per binding cache entry, the second uses a global secret.
   The first method uses in sequence:
    - a 16 bit timestamp on when the cookie is created
    - the tested care-of address
    - the truncated HMAC [8] keyed by the binding cache entry secret
      key of the preceding two fields and the correspondent address.
   The second method uses in sequence:
    - a 16 bit timestamp on when the cookie is created
    - the tested care-of address
    - the truncated HMAC [8] keyed by the global secret key of the
      preceding two fields, the home address and the correspondent
      address.
   The secret key SHOULD be random or pseudo-random and SHOULD be
   changed reasonably frequently. The timestamp MAY be used to
   determine which key was used. The HMAC has to be truncated in
   order to keep the cookie option length less than the maximum,
   the higher 96 bits of the HMAC should be enough.

   The last point is what to do waiting the retransmitted and augmented
   binding update. Possibilities are:
    - apply the binding update with the new care-of address. This
      defeats the purpose of the care-of address test so it SHOULD NOT
      be done, and it MUST NOT be done for a real alternate care-of
      address.
    - keep the previous care-of address. As it is not possible to know
      whether the previous care-of address is usable, i.e., whether
      the mobile node is still reachable at this previous care-of
      address, the default policy SHOULD NOT be to keep the previous
      care-of address. The correspondent node MAY keep the previous
      care-of address in special cases where this is known to be
      the best solution.
    - temporarily disable the binding cache entry, i.e., by using
      the home address for communication to the mobile node until
      another binding update is received. This SHOULD be the default
      policy.

A2. IANA Considerations

   This Annex requires:
    - a new status for binding acknowledgment.
    - a new option for mobility messages used in binding update
      and acknowledgment messages.


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