Network Working Group M. Bagnulo
Internet-Draft A. Garcia-Martinez
Expires: December 1, 2003 I. Soto
UC3M
June 2, 2003
Preserving MIPv6 communications when the HoA becomes unreachable
draft-bagnulo-mobileip-unreachable-hoa-00
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This note proposes a modification to the MIPv6 specification in order
to allow the preservation of established communications when the path
between the MN and the CN through the HoA becomes unavailable. The
proposed modification essentially consists on allowing the extension
of BCE lifetime upon the reception of ICMP Destination Unreachable
message as reply to a Binding Refresh Request (BRR) message.
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1. Introduction
The MIPv6 [1] specification defines that Binding Cache Entries (BCE)
that have been authorized using the Return Routability (RR) procedure
have a maximum lifetime of MAX_RR_BINDING_LIFE (420 seconds). This
means that the BCE linking a Home Address (HoA) and a Care-of Address
(CoA) at the Correspondent Node (CN) will only remain valid for 7
minutes after the Binding Update (BU) reception. If this CoA is to
be used to reach the HoA after this period, a new BU message binding
the HoA and the CoA has to be sent. In order to be able to do this,
the Mobile Node (MN) has to acquire new BU authorization data using
the RR procedure, implying communication through both the CoA and the
HoA. This implies that if the HoA becomes unreachable from the CN,
the established communication will be interrupted because the BCE has
expired, even if the path that is actually being used for the
communication is still available. Summarizing, as currently defined,
MIPv6 communication is vulnerable not only to outages along the
communication path used to carry data packets, but also to outages
along the path between the MN and the Home Agent (HA), and along the
path between the HA and the CN. This behavior not only introduces
additional points of failure in MIPv6 communications but it also
limits the potential usage of MIPv6 to provide multi-homing support
as described in [2].
This note proposes a modification to the MIPv6 specification in order
to allow the preservation of established communications when the path
between the MN and the CN through the HoA becomes unavailable. The
proposed modification essentially consists on allowing the extension
of BCE lifetime upon the reception of ICMP Destination Unreachable
message as reply to a Binding Refresh Request (BRR) message.
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2. Security Concerns that lead to reduced BCE lifetime.
In order to propose a modification to the defined behavior, we must
first analyze the security concerns that lead to the current design.
BCE lifetime has been limited to a few minutes in order to limit the
possibility of time shifting attacks, as it is presented in [3].
The goal of MIPv6 security is to avoid the introduction of new
security hazards which are not present in non-MIPv6 enabled
environments. In particular, the RR procedure limits the set of
potential attackers to those who can intercept packets flowing
between the CN and the HA. This procedure forces the attacker to be
present somewhere along the path between the CN and the HA in order
to acquire the valid authorization data needed to generate forged BU
messages.
However, this mechanism by itself only imposes that the attacker has
to be present on the path the time needed to intercept the messages
that carry authorization information. Once that the attacker has
intercepted the valid authorization information, he can leave his
position along the path and still perform attacks using such
information. These are called time shifting attacks, since an
attacker that once was on-path intercepting packets can perform
attacks in the future when he is no longer on the communication path.
The limitation of the BCE lifetime to a few minutes limits the
effects of the following time shifting attack: the attacker placed
along the communication path intercepts authorization information and
generates a forged BU message. The attacker leaves the position but
the attack continues since the traffic is still diverted to the CoA
contained in the fake BU message. The effect of this attack is
limited by reducing BCE lifetime in the CN to 7 minutes, imposing the
generation of a new BU message in order to restore the BCE. Since
the attacker is no longer along the communication path, he will not
be able to generate new BU messages.
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3. Proposed modifications to the MIPv6 specification
Currently time shifting attacks are prevented by imposing periodical
message exchange which imply that the attacker has to be present
along the path between the CN and the MN's HoA in order to continue
with the attack. So, the currently available mechanism assumes that
an attack is being perpetrated when no information can be exchanged
with the other end through the HoA. However, it is not really
necessary to perform a message exchange with the MN to prevent a time
shifting attack. The only thing that is really needed is a mechanism
that requires the presence of the attacker along the path between the
CN and the MN's HoA in order to continue with the attack. This can
be achieved through a message exchange with any device along the path
which does not has to be the communicating end-points. This note
proposes the exchange of messages between the CN and the first router
with no route to the final destination address as a time shifting
attack prevention mechanism when the HoA is unreachable from the CN.
3.1 Proposed mechanism
3.1.1 Correspondent Node Part
When the remaining lifetime of an existent BCE reaches 32 seconds,
the CN sends a Binding Refresh Request (BRR) to the MN's HoA for this
binding. The timeout for this request is set to 1 second. If no
response is obtained within this interval, the CN retransmits the BRR
until a response is received or the BCE lifetime expires.
The BRR message contains a Cookie Mobility option as defined in
section 3.2. This option contains a 64-bit randomly generated cookie
which will be copied to the response packets in order to verify that
the replying party has received (or intercepted) the BRR.
If the MN is reachable through the HoA, and it is interested in
preserving the BCE valid, it will send a BU message, extending the
BCE lifetime.
However, if an outage has occurred along the path between the CN and
the MN's HoA, an ICMP Destination Unreachable message containing a No
Route to Destination Code will be generated by a router along the
path according to [4]. The ICMP message contains the ICMP header and
it will will be completed with as much of the invoking packet as it
will fit within the MTU defined for IPv6 [5], which is 1280 bytes.
This means that the complete BRR message, including the newly defined
cookie option will be included within the ICMP message.
When the CN receives an ICMP Destination Unreachable message
containing a No Route to Destination Code, it verifies that the ICMP
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message was generated as a reply to the BRR. It does so by verifying
that the packet included in the ICMP message is a BRR message and
that the cookie included in the Cookie Mobility Option matches with
the one included in the initial BRR message. If the verification
succeeds, the CN detects that an outage has occurred and extends the
BCE lifetime for 180 seconds, preserving the established
communication through the CoA. After 150 seconds, a new BRR message
will be sent.
The BCE lifetime can only be renewed 60 times, limiting to 3 hours
the maximum time that an BCE entry can be valid without performing
the RR procedure.
3.1.2 Mobile Node Part
A similar mechanism is to be implemented in the MN in order to
preserve the state needed in the MN to maintain the established
communication, so that the MN continues to send packets directly to
the CN without using the HA. Such state is stored in the Binding
Update List (BUL) within the MN and it has a limited lifetime,
imposing its periodical refresh. So when a BUL entry is about to
expire, the RR procedure is to be performed so that the authorization
information needed to send a BU message is acquired. The RR
procedure consists on the exchange of the CoTI/CoT messages directly
between the CN and the MN and the exchange of the HoTI/HoT messages
through the HA. If the path between the CN and the MN's HoA is
working properly, the RR procedure will be completed successfully and
a new BU message will be issued, and the lifetime of the BUL entry
corresponding to that CN will be extended. If the path between the
CN and the MN's HoA is not working, the RR procedure will not be
completed, preventing the generation of the BU message, implying that
the BUL entry corresponding to that CN will expire. This means that
forthcoming packets will be sent from the MN to the CN through the HA
and since there is no path available, the communication will fail.
It is proposed that the BUL lifetime is extended upon the reception
of an ICMP Destination Unreachable message containing a No Route to
Destination Code as a reply to a HoTI message issued by the MN. The
resulting behavior is that when a BUL entry is about to expire, the
MN will initiate the RR procedure sending a HoTI and a CoTI message.
If there is no route available between the CN and the MN through the
HA, an ICMP Destination Unreachable message containing a No Route to
Destination Code is be sent back to the MN. Then, when the MN
receives such message, it verifies that the ICMP message was
generated as a reply to the HoTI message. It does so by verifying
that the packet included in the ICMP message is a HoTI message and
that the cookie included in the Home Init Cookie field matches with
the one included in the initial HoTI message. If the verification
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succeeds, the MN detects that an outage has occurred and extends the
BUL lifetime for a period equal to the initial value of the lifetime
(contained in the BUL entry), preserving the established
communication.
3.2 Cookie Mobility Option
The Cookie option has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This Mobility Option contains a 64 bit long randomly generated
cookie.
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4. Security Considerations
This note proposes changes to MIPv6 security. The reader is referred
to section 2 for the risks that the modified security features
prevent and to section 3 for an analysis of the proposed changes.
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5. Acknowledgments
Thanks to Pekka Nikander for suggesting a more general problem for
the solution proposed in this document and also for providing many
constructive comments.
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References
[1] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", Internet Draft, Work in progress
draft-ietf-mobileip-ipv6-21.txt, May 2002.
[2] Bagnulo, M., Garcia-Martinez, A. and I. Soto, "Application of
the MIPv6 protocol to the multi-homing problem", Internet Draft,
Work in progress draft-bagnulo-multi6-mnm-00, February 2003.
[3] Nikander, P., Aura, T., Arkko, J. and G. Montenegro, "Mobile IP
version 6 (MIPv6) Route Optimization Security Design
Background", Internet Draft, Work in progress
draft-nikander-mobileip-v6-ro-sec-00, March 2003.
[4] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6
(IPv6)Specification", RFC 2463, December 1998.
[5] Hinden, R. and S. Deering, "Internet Protocol, version 6(IPv6)
Specification", RFC 2460, December 1998.
Authors' Addresses
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6249500
EMail: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es/marcelo
Alberto Garcia-Martinez
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6249500
EMail: alberto@it.uc3m.es
URI: http://www.it.uc3m.es/alberto
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Ignacio Soto
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6249500
EMail: isoto@it.uc3m.es
URI: http://www.it.uc3m.es/isoto
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