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.
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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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