IKEv2 Mobility and Multihoming T. Kivinen
(mobike) Safenet, Inc.
Internet-Draft H. Tschofenig
Expires: April 24, 2006 Siemens
October 21, 2005
Design of the MOBIKE Protocol
draft-ietf-mobike-design-04.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The MOBIKE (IKEv2 Mobility and Multihoming) working group is
developing extensions for the Internet Key Exchange Protocol version
2 (IKEv2). These extensions should enable an efficient management of
IKE and IPsec Security Associations when a host possesses multiple IP
addresses and/or where IP addresses of an IPsec host change over time
(for example, due to mobility).
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This document discusses the involved network entities, and the
relationship between IKEv2 signaling and information provided by
other protocols. Design decisions for the MOBIKE protocol,
background information and discussions within the working group are
recorded.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Mobility Scenario . . . . . . . . . . . . . . . . . . . . 8
3.2. Multihoming Scenario . . . . . . . . . . . . . . . . . . . 9
3.3. Multihomed Laptop Scenario . . . . . . . . . . . . . . . . 10
4. Scope of MOBIKE . . . . . . . . . . . . . . . . . . . . . . . 11
5. Design Considerations . . . . . . . . . . . . . . . . . . . . 14
5.1. Choosing addresses . . . . . . . . . . . . . . . . . . . . 14
5.1.1. Inputs and triggers . . . . . . . . . . . . . . . . . 14
5.1.2. Connectivity . . . . . . . . . . . . . . . . . . . . . 14
5.1.3. Discovering connectivity . . . . . . . . . . . . . . . 15
5.1.4. Decision making . . . . . . . . . . . . . . . . . . . 15
5.1.5. Suggested approach . . . . . . . . . . . . . . . . . . 15
5.2. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 16
5.2.1. Background and constraints . . . . . . . . . . . . . . 16
5.2.2. Fundamental restrictions . . . . . . . . . . . . . . . 16
5.2.3. Moving to behind NAT and back . . . . . . . . . . . . 16
5.2.4. Responder behind NAT . . . . . . . . . . . . . . . . . 17
5.2.5. NAT Prevention . . . . . . . . . . . . . . . . . . . . 17
5.2.6. Suggested approach . . . . . . . . . . . . . . . . . . 17
5.3. Scope of SA changes . . . . . . . . . . . . . . . . . . . 18
5.4. Zero address set functionality . . . . . . . . . . . . . . 19
5.5. Return routability test . . . . . . . . . . . . . . . . . 19
5.5.1. Employing MOBIKE results in other protocols . . . . . 22
5.5.2. Suggested approach . . . . . . . . . . . . . . . . . . 23
5.6. IPsec Tunnel or Transport Mode . . . . . . . . . . . . . . 23
6. Protocol detail issues . . . . . . . . . . . . . . . . . . . . 24
6.1. Indicating support for mobike . . . . . . . . . . . . . . 24
6.2. Path Testing and Window size . . . . . . . . . . . . . . . 25
6.3. Message presentation . . . . . . . . . . . . . . . . . . . 26
6.4. Updating address list . . . . . . . . . . . . . . . . . . 27
7. Security Considerations . . . . . . . . . . . . . . . . . . . 28
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.1. Normative references . . . . . . . . . . . . . . . . . . . 31
10.2. Informative References . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
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Intellectual Property and Copyright Statements . . . . . . . . . . 35
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1. Introduction
The purpose of IKEv2 is to mutually authenticate two hosts, establish
one or more IPsec Security Associations (SAs) between them, and
subsequently manage these SAs (for example, by rekeying or deleting).
IKEv2 enables the hosts to share information that is relevant to both
the usage of the cryptographic algorithms that should be employed
(e.g., parameters required by cryptographic algorithms and session
keys) and to the usage of local security policies, such as
information about the traffic that should experience protection.
IKEv2 assumes that an IKE SA is created implicitly between the IP
address pair that is used during the protocol execution when
establishing the IKEv2 SA. This means that, in each host, only one
IP address pair is stored for the IKEv2 SA as part of a single IKEv2
protocol session, and, for tunnel mode SAs, the hosts places this
single pair in the outer IP headers. Existing documents make no
provision to change this pair after an IKE SA is created.
There are scenarios where one or both of the IP addresses of this
pair may change during an IPsec session. In principle, the IKE SA
and all corresponding IPsec SAs could be re-established after the IP
address has changed. However, this can be problematic, as the device
might be too slow for this task. Moreover, manual user interaction
(for example when using SecurID cards) might be required as part of
the IKEv2 authentication procedure. Therefore, an automatic
mechanism is need that updates the IP addresses associated with the
IKE SA and the IPsec SAs. MOBIKE provides such a mechanism.
The work of the MOBIKE working group and therefore this document is
based on the assumption that the mobility and multi-homing extensions
are developed for IKEv2 [I-D.ietf-ipsec-ikev2]. As IKEv2 is built on
the architecture described in RFC2401bis [I-D.ietf-ipsec-rfc2401bis],
all protocols developed within the MOBIKE working group must be
compatible with both IKEv2 and the architecture described in
RFC2401bis. The document does not aim to neither provide support
IKEv1 [RFC2409] nor the architecture described in RFC2401 [RFC2401].
This document is structured as follows. After introducing some
important terms in Section 2 a number of relevant usage scenarios are
discussed in Section 3. The next section Section 4 will describe the
scope of the MOBIKE protocol. Finally, Section 5 discusses design
considerations affecting the MOBIKE protocol. The document concludes
in Section 7 with security considerations.
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2. Terminology
This section introduces the terminology that is used in this
document.
Peer:
A peer is an IKEv2 endpoint. In addition, a peer implements the
MOBIKE extensions, as defined in this and related documents.
Available address:
An address is said to be available if the following conditions are
met:
* The address has been assigned to an interface.
* If the address is an IPv6 address, we additionally require (a)
that the address is valid as defined in RFC 2461 [RFC2461], and
(b) that the address is not tentative as defined in RFC 2462
[RFC2462]. In other words, we require the address assignment
to be complete.
Note that this explicitly allows an address to be optimistic as
defined in [I-D.ietf-ipv6-optimistic-dad].
* If the address is an IPv6 address, it is a global unicast or
unique site-local address, as defined in [I-D.ietf-ipv6-unique-
local-addr]. That is, it is not an IPv6 link-local. Where
IPv4 is considered, it is not an RFC 1918 [RFC1918] address.
* The address and interface is acceptable for sending and
receiving traffic according to a local policy.
This definition is taken from [I-D.arkko-multi6dt-failure-
detection].
Locally Operational Address:
An address is said to be locally operational if it is available
and its use is locally known to be possible and permitted. This
definition is taken from [I-D.arkko-multi6dt-failure-detection].
Operational address pair:
A pair of operational addresses are said to be an operational
address pair, if and only if bidirectional connectivity can be
shown between the two addresses. Note that sometimes it is
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necessary to consider connectivity on a per-flow level between two
endpoints needs to be tested. This differentiation might be
necessary to address certain Network Address Translation types or
specific firewalls. This definition is taken from [I-D.arkko-
multi6dt-failure-detection] and adapted for the MOBIKE context.
Although it is possible to further differentiate unidirectional
and bidirectional operational address pairs, only bidirectional
connectivity is relevant to this document and unidirectional
connectivity is out of scope.
Path:
The sequence of routers traversed by the MOBIKE and IPsec packets
exchanged between the two peers. Note that this path may be
affected not only by the involved source and destination IP
addresses, but also by the transport protocol. Since MOBIKE and
IPsec packets have a different appearance on the wire they might
be routed along a different path, for example by load balancers.
This definition is taken from [RFC2960] and adapted to the MOBIKE
context.
Primary Path:
The sequence of routers traversed by an IP packet that carries the
default source and destination addresses is said to be the Primary
Path. This definition is taken from [RFC2960] and adapted to the
MOBIKE context.
Preferred Address:
The IP address of a peer to which MOBIKE and IPsec traffic should
be sent by default. A given peer has only one active preferred
address at a given point in time, except for the small time period
where it switches from an old to a new preferred address. This
definition is taken from [I-D.ietf-hip-mm] and adapted to the
MOBIKE context.
Peer Address Set:
We denote the two peers of a MOBIKE session by peer A and peer B.
A peer address set is the subset of locally operational addresses
of peer A that is sent to peer B. A policy available at peer A
indicates which addresses are included in the peer address set.
Such a policy might be created either manually or automatically
through interaction with other mechanisms that indicate new
available addresses.
Terminology regarding NAT types (e.g. Full Cone, Restricted Cone,
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Port Restricted Cone and Symmetric), can be found in Section 5 of
[RFC3489]. For mobility related terminology (e.g. Make-before-break
or Break-before-make) see [RFC3753].
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3. Scenarios
In this section we discuss three typical usage scenarios for the
MOBIKE protocol.
3.1. Mobility Scenario
Figure 1 shows a break-before-make mobility scenario where a mobile
node changes its point of network attachment. Prior to the change,
the mobile node had established an IPsec connection with a security
gateway which offered, for example, access to a corporate network.
The IKEv2 exchange that facilitated the set up of the IPsec SA(s)
took place over the path labeled as 'old path'. The involved packets
carried the MN's "old" IP address and were forwarded by the "old"
access router (OAR) to the security gateway (GW).
When the MN changes its point of network attachment, it obtains a new
IP address using stateful address configuration techniques or via the
stateless address autoconfiguration mechanism. The goal of MOBIKE,
in this scenario, is to enable the MN and the GW to continue using
the existing SAs and to avoid setting up a new IKE SA. A protocol
exchange, denoted by 'MOBIKE Address Update', enables the peers to
update their state as necessary.
Note that in a break-before-make scenario the MN obtains the new IP
address after it can no longer be reached at the old IP address. In
a make-before-break scenario, the MN is, for a given period of time,
reachable at both the old and the new IP address. MOBIKE should work
in both the above scenarios.
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(Initial IKEv2 Exchange)
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>v
Old IP +--+ +---+ v
address |MN|------> |OAR| -------------V v
+--+ +---+ Old path V v
. +----+ v>>>>> +--+
.move | R | -------> |GW|
. | | >>>>> | |
v +----+ ^ +--+
+--+ +---+ New path ^ ^
New IP |MN|------> |NAR|--------------^ ^
address +--+ +---+ ^
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>^
(MOBIKE Address Update)
---> = Path taken by data packets
>>>> = Signaling traffic (IKEv2 and MOBIKE)
...> = End host movement
Figure 1: Mobility Scenario
3.2. Multihoming Scenario
Another MOBIKE usage scenario is depicted in Figure 2. In this
scenario, the MOBIKE peers are equipped with multiple interfaces (and
multiple IP addresses). Peer A has two interface cards with two IP
addresses, IP_A1 and IP_A2, and peer B has two IP addresses, IP_B1
and IP_B2. Each peer selects one of its IP addresses as the
preferred address which is used for subsequent communication.
Various reasons, (e.g hardware or network link failures), may require
a peer to switch from one interface to another.
+------------+ +------------+
| Peer A | *~~~~~~~~~* | Peer B |
| |>>>>>>>>>> * Network *>>>>>>>>>>| |
| IP_A1 +-------->+ +--------->+ IP_B1 |
| | | | | |
| IP_A2 +********>+ +*********>+ IP_B2 |
| | * * | |
+------------+ *~~~~~~~~~* +------------+
---> = Path taken by data packets
>>>> = Signaling traffic (IKEv2 and MOBIKE)
***> = Potential future path through the network
(if Peer A and Peer B change their preferred
address)
Figure 2: Multihoming Scenario
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Note that MOBIKE does not aim to support load balancing between
multiple IP addresses. That is, each peer uses only one of the
available IP addresses at a given point in time.
3.3. Multihomed Laptop Scenario
The third scenario we consider is about a laptop, which has multiple
interface cards and therefore several ways to connect to the network.
It may for example have a fixed Ethernet card, a WLAN interface, a
GPRS adaptor, a Bluetooth interface or USB hardware. Not all
interfaces are connected to the network at all times for a number of
reasons (e.g., cost, availability of certain link layer technologies,
user convenience). The mechanism that determines which interfaces
are connected to the network at any given point in time is outside
the scope of the MOBIKE protocol and, as such, this document.
However, as the laptop changes its point of attachment to the
network, the set of IP addresses under which the laptop is reachable,
changes too.
Even if IP addresses change due to interface switching or mobility,
the IP address obtained via the configuration payloads within IKEv2
remain unaffected. The IP address obtained via the IKEv2
configuration payloads allow the configuration of the inner IP
address of the IPsec tunnel. As such, applications might not detect
any change at all.
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4. Scope of MOBIKE
Getting mobility and multihoming actually working requires lots of
different components working together, including coordinating things
and making consistent decisions in several link layers, the IP
layers, different mobility mechanisms in those layers, and IPsec/IKE.
Most of those aspects are beyond the scope of MOBIKE: The MOBIKE
focuses on what two peers need to agree in IKEv2 level (like new
message formats and some aspects of their processing) for
interoperability.
The MOBIKE is not trying to be full mobility protocol; there is no
support for simultaneous movement or rendezvous mechanism, and there
is no support for route optimization etc. This current design
document focuses mainly on the tunnel mode, everything going inside
the tunnel is unaffected by the changes in the tunnel header IP
address, and this is the mobility feature provided by the MOBIKE.
I.e. the applications running through the MOBIKE IPsec tunnel cannot
even detect the movement, their IP address etc stay constant.
A MOBIKE protocol should be able to perform the following operations:
o inform the other peer about the peer address set
o inform the other peer about the preferred address
o test connectivity along a path and thereby to detect an outage
situation
o change the preferred address
o change the peer address set
o Ability to deal with Network Address Translation devices
Figure 3 shows an example protocol interaction between a pair of
MOBIKE peers. MOBIKE interacts with the IPsec engine using the
PF_KEY API [RFC2367]. Using this API, the MOBIKE daemon can create
entries in the Security Association (SAD) and Security Policy
Databases (SPD). The IPsec engine may also interact with IKEv2 and
MOBIKE daemon using this API. The content of the Security Policy and
Security Association Databases determines what traffic is protected
with IPsec in which fashion. MOBIKE, on the other hand, receives
information from a number of sources that may run both in kernel-mode
and in user-mode. Information relevant for MOBIKE might be stored in
a database. The contents of such a database, along with the
occurrence of events of which the MOBIKE process is notified, form
the basis on which MOBIKE decides regarding the set of available
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addresses, the peer address set, and the preferred address. Policies
may also affect the selection process.
The a peer address set and the preferred address needs to be
available to the other peer. In order to address certain failure
cases, MOBIKE should perform connectivity tests between the peers
(potentially over a number of different paths). Although a number of
address pairs may be available for such tests, the most important is
the pair (source address, destination address) of the primary path.
This is because this pair is selected for sending and receiving
MOBIKE signaling and IPsec traffic. If a problem along this primary
path is detected (e.g., due to a router failure) it is necessary to
switch to a new primary path. In order to be able to do so quickly,
it may be helpful to perform connectivity tests of other paths
periodically. Such a technique would also help in identifying
previously disconnected paths that become operational.
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+-------------+ +---------+
|User-space | | MOBIKE |
|Protocols | +-->>| Daemon |
|relevant for | | | |
|MOBIKE | | +---------+
+-------------+ | ^
User Space ^ | ^
++++++++++++++++++++++++++++ API ++++++ API ++++ PF_KEY ++++++++
Kernel Space v | v
_______ | v
+-------------+ / \ | +--------------+
|Routing | / Trigger \ | | IPsec |
|Protocols |<<-->>| Database |<<-+ +>+ Engine |
| | \ / | | (+Databases) |
+-----+---+---+ \_______/ | +------+-------+
^ ^ ^ | ^
| +---------------+-------------+--------+-----+
| v | | |
| +-------------+ | | |
I | |Kernel-space | | | | I
n | +-------->+Protocols +<----+-----+ | | n
t v v |relevant for | | v v v t
e +----+---+-+ |MOBIKE | | +-+--+-----+-+ e
r | Input | +-------------+ | | Outgoing | r
f | Packet +<--------------------------+ | Interface | f
==a>|Processing|===============================| Processing |=a>
c | | | | c
e +----------+ +------------+ e
s s
===> = IP packets arriving/leaving a MOBIKE node
<-> = control and configuration operations
Figure 3: Framework
Please note that Figure 3 illustrates an example of how a MOBIKE
implementation could work. Hence, it serves illustrative purposes
only.
Extensions of the PF_KEY interface required by MOBIKE are also within
the scope of the working group. Finally, certain optimizations for
wireless environments are also covered.
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5. Design Considerations
This section discusses aspects affecting the design of the MOBIKE
protocol.
5.1. Choosing addresses
One of the core aspects of the MOBIKE protocol is the selection of
the address for the IPsec packets we send. Choosing addresses for
the IKEv2 request is somewhat separate problem: in many cases, they
will be the same (and in some design choice they will always be the
same).
5.1.1. Inputs and triggers
How the address changes are triggered are largerly beyond the scope
of MOBIKE. The triggers can include e.g. changes in the set of
addresses, various link-layer indications, failing dead peer
detection, and changes in preferences and policies. Furthermore,
there may be less reliable sources of information (such as lack of
IPsec packets and ICMP packets) that do not trigger any changes
directly, but rather cause DPD to be performed sooner than it
otherwise would have been (and if that DPD fails, that may trigger
changing of addresses).
These triggers are largerly the same as for, e.g. Mobile IP, and are
beyond the scope of MOBIKE.
5.1.2. Connectivity
There can be two kind of "failures" in the connectivity; local or
middle. Local failure is a property of an address (interface), while
the failures in the middle is property of address pair. MOBIKE does
not assume full connectivity, but it does not try to use
unidirectional address pairs (multi6 has discussed about how to deal
with unidirectional paths). Unidirectional address pairs is
complicated issue, and supporting it would require abandoning the
principle that you always send the IKEv2 reply to the address the
request came from. Because of that MOBIKE decided to deal only with
bidirectional address pairs. It does consider unidirectional address
pairs as broken, and not use them, but the connection will not break
even if unidirectional address pairs are present, provided there is
at least one bidirectional address pair it can use.
Note, that MOBIKE is not really concerned about the actual path used,
it cannot even detect if some path is unidirectional, if the same
address pair has some other unidirectional path back. Ingress
filters might still cause such path pairs to be unusable, and in that
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case MOBIKE will detect that there is no connection between address
pair.
In a sense having both IPv4 and IPv6 address is basically a case of
partial connectivity (putting both IPv4 and IPv6 address in the same
IP header does not work). The main difference is that it is known
beforehand, and there is no need to discover that IPv4/IPv6
combination does not work.
5.1.3. Discovering connectivity
To detect connectivity, i.e failures in the middle, MOBIKE needs to
have some kind of probe which it can send to the other end and get a
reply back to that. If it will see the reply it knows the connection
works, if it does not see the reply after multiple retransmissions it
may assume that the address pair tested is broken.
The connectivity tests do require to take in to account the
congestion problems, because the connection failure might be because
of congestion, and the MOBIKE should not make it worse by sending
lots of probe packets.
5.1.4. Decision making
One of the core decisions to the MOBIKE protocol is to who makes the
decisions to fix situations, i.e. symmetry in decision making vs.
asymmetry in decisions. Symmetric decision making may cause the
different peers to make different decisions, thus causing asymmetric
upstream/downstream traffic. In mobility case it is desirable that
the mobile peer can move both upstream and downstream traffic to some
particular interface, and this requires asymmetric decision making.
Working with stateful packet filters and NATs is easier if same
address pair is used in both upstream and downstream directions.
Also in common cases only the peer behind NAT can actually do actions
to recover from the connectivity problems, as it might be that the
other peer is not able to initiate any connections to the peer behind
NAT.
5.1.5. Suggested approach
Because of those issues listed above, the MOBIKE protocol decided to
select method where the initiator will decide which addresses are
used. This has the benefits that it makes one peer to decide, thus
we cannot end up in the asymmetric decisions, and it also works best
with NATs, as the initiator is the most common peer to be behind NAT,
and thus is the only peer which can recover in those cases.
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5.2. NAT Traversal
5.2.1. Background and constraints
Another core aspect of the MOBIKE was the co-operation and working
with NATs. In IKEv2 the tunnel header IP addresses are not sent
inside the IKEv2 payloads, and thus there is no need to do unilateral
self-address fixing (UNSAF). The tunnel header IP addresses are
taken from the outer IP header of the IKE packets, thus they are
already processed by the NAT.
The NAT detection payloads are used to detect if the addresses in the
IP header were modified by a NAT between the peers, and that enables
UDP encapsulation of ESP packets if needed. MOBIKE is not to change
how IKEv2 NAT-T works, in particular, any kind of UNSAF or explicit
interaction with NATs (e.g. midcom or nsis natfw) are beyond the
scope. The MOBIKE will need to define how MOBIKE and NAT-T are used
together.
The NAT-T support should also be optional, i.e. if the IKEv2
implementation does not implement NAT-T, since it is not required in
some particular environment, implementing MOBIKE should not require
adding support for NAT-T as well.
The property of being behind NAT is actually property of the address
pair, thus one peer can have multiple IP-addresses and some of those
might be behind NAT and some might not be behind NAT.
5.2.2. Fundamental restrictions
There are some cases which cannot be made work with the restrictions
provided by the MOBIKE charter. One of those cases is the case where
the party "outside" a symmetric NAT changes its address to something
not known by the the other peer (and old address has stopped
working). It cannot send a packet containing the new addresses to
the peer, because the NAT does not contain the necessary state.
Furthermore, since the party behind the NAT does not know the new IP
address, it cannot cause the NAT state to be created.
This case could be solved using some rendezvous mechanism outside
IKEv2, but that is beyond the scope of MOBIKE.
5.2.3. Moving to behind NAT and back
MOBIKE provides mechanism where peer not initially behind the NAT,
can move to behind NAT, when new address pair is selected. MOBIKE
does not need to detect when someone attach NAT to the currently
working address pair, i.e. the NAT detection is only done when MOBIKE
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changes the address pair used.
Similarly the MOBIKE provides mechanism to move from the address pair
having NAT to the address pair not having NAT.
As we only use one address pair at time, effectively MOBIKE peer is
either behind NAT or not behind NAT, but each address change can
change the situation. Because of this and because initiator always
chooses the addresses it is enough to send keepalive packets only to
that one address pair.
5.2.4. Responder behind NAT
MOBIKE can work in cases where the responder is behind static NAT,
but in that case the initiator needs to know all possible addresses
where the responder can move to, i.e. responder cannot move to the
address which is not known by the initiator.
If the responder is behind NAPT then it might need to communicate
with the NAT to create mapping so initiator can connect to it. Those
external hole punching mechanisms are beyond the scope of MOBIKE.
In case the responder is behind NAPT then also finding the port
numbers used by the responder, is outside the scope of MOBIKE.
5.2.5. NAT Prevention
One new feature created by the MOBIKE, is the NAT prevention, i.e. if
we detect NAT between the peers, we do not allow that address pair to
be used. This can be used to protect IP-addresses in cases where it
is known by the configuration that there is no NAT between the nodes
(for example IPv6, or fixed site-to-site VPN). This gives extra
protection against 3rd party bombing attacks (attacker cannot divert
the traffic to some 3rd party). This feature means that we
authenticate the IP-address and detect if they were changed. As this
is done on purpose to break the connectivity if NAT is detect, and
decided by the configuration, there is no need to do UNSAF
processing.
5.2.6. Suggested approach
The working group decided that MOBIKE uses NAT-T mechanisms from the
IKEv2 protocol as much as possible, but decided to change the dynamic
address update for IKEv2 packets to MUST NOT (it would break path
testing using IKEv2 packets). Working group also decided to only
send keepalives to the current address pair.
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5.3. Scope of SA changes
Most sections of this document discuss design considerations for
updating and maintaining addresses in the database entries that
relate to an IKE-SA. However, changing the preferred address also
affects the entries of the IPsec SA database. The outer tunnel
header addresses (source and destination IP addresses) need to be
modified according to the primary path to allow the IPsec protected
data traffic to travel along the same path as the MOBIKE packets (if
we only consider the IP header information). If the MOBIKE messages
and the IPsec protected data traffic travel along a different path
then NAT handling is severely complicated.
The basic question is then how the IPsec SAs are changed to use the
new address pair (the same address pair as the MOBIKE signaling
traffic). One option is that when the IKE SA address is changed then
automatically all IPsec SAs associated with it are moved along with
it to new address pair. Another option is to have a separate
exchange to move the IPsec SAs separately.
If IPsec SAs should be updated separately then a more efficient
format than the notification payload is needed to preserve bandwidth.
A notification payload can only store one SPI per payload. A
separate payload could have list of IPsec SA SPIs and new preferred
address. If there is a large number of IPsec SAs, those payloads can
be quite large unless ranges of SPI values are supported. If we
automatically move all IPsec SAs when the IKE SA moves, then we only
need to keep track which IKE SA was used to create the IPsec SA, and
fetch the IP addresses from IKE SA, i.e. no need to store IP
addresses per IPsec SA. Note that IKEv2 [I-D.ietf-ipsec-ikev2]
already requires implementations to keep track which IPsec SAs are
created using which IKE SA.
If we do allow each IPsec SA address set to be updated separately,
then we can support scenarios, where the machine has fast and/or
cheap connections and slow and/or expensive connections, and it wants
to allow moving some of the SAs to the slower and/or more expensive
connection, and prevent the move, for example, of the news video
stream from the WLAN to the GPRS link.
On the other hand, even if we tie the IKE SA update to the IPsec SA
update, then we can create separate IKE SAs for this scenario, e.g.,
we create one IKE SA which have both links as endpoints, and it is
used for important traffic, and then we create another IKE SA which
have only the fast and/or cheap connection, which is then used for
that kind of bulk traffic.
MOBIKE protocol decided to move all IPsec SAs implicitly when the IKE
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SA address pair changes. If more granular handling of the IPsec SA
is required, then multiple IKE SAs can be created one for each set of
IPsec SAs needed.
5.4. Zero address set functionality
One of the features which is potentially useful is for the peer to
announce that it will now disconnect for some time, i.e. it will not
be reachable at all. For instance, a laptop might go to suspend
mode. In this case the it could send address notification with zero
new addresses, which means that it will not have any valid addresses
anymore. The responder of that kind of notification would then
acknowledge that, and could then temporarily disable all SAs and
therefore stop sending traffic. If any of the SAs gets any packets
they are simply dropped. This could also include some kind of ACK
spoofing to keep the TCP/IP sessions alive (or simply set the TCP/IP
keepalives and timeouts large enough not to cause problems), or it
could simply be left to the applications, e.g. allow TCP/IP sessions
to notice the link is broken.
The local policy could then indicate how long the peer should allow
remote peers to remain disconnected.
From a technical point of view this feature addresses two aspects:
o There is no need to transmit IPsec data traffic. IPsec protected
data can be dropped which saves bandwidth. This does not provide
a functional benefit, i.e., nothing breaks if this feature is not
provided.
o MOBIKE signaling messages are also ignored. The IKE-SA must not
be deleted and the suspend functionality (realized with the zero
address set) may require the IKE-SA to be tagged with a lifetime
value since the IKE-SA should not be kept in alive for an
undefined period of time. Note that IKEv2 does not require that
the IKE-SA has a lifetime associated with it. In order to prevent
the IKE-SA from being deleted the dead-peer detection mechanism
needs to be suspended as well.
Due to the fact that this extension could be complicated and there
was no clear need for it, a first version of the MOBIKE protocol will
not provide this feature.
5.5. Return routability test
Changing the preferred address and subsequently using it for
communication is associated with an authorization decision: Is a peer
allowed to use this address? Does this peer own this address? Two
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mechanisms have been proposed in the past to allow a peer to
determine the answer to this question:
o The addresses a peer is using are part of a certificate.
[RFC3554] introduced this approach. If the other peer is, for
example, a security gateway with a limited set of fixed IP
addresses, then the security gateway may have a certificate with
all the IP addresses appear in the certificate.
o A return routability check is performed by the remote peer before
the address is updated in that peer's Security Association
Database. This is done in order provide a certain degree of
confidence to the remote peer that local peer is reachable at the
indicated address.
Without taking an authorization decision a malicious peer can
redirect traffic towards a third party or a blackhole.
A MOBIKE peer should not use an IP addressed provided by another
MOBIKE peer as a primary address without computing the authorization
decision. If the addresses are part of the certificate then it is
not necessary to execute the weaker return-routability test. The
return-routability test is a form of authorization check, although it
provides weaker guarantees then the inclusion of the IP address as
part of a certificate. If multiple addresses are communicated to the
remote peer then some of these addresses may be already verified even
if the primary address is still operational.
Another option is to use the [I-D.dupont-mipv6-3bombing] approach
which suggests to perform a return routability test only when an
address update needs to be sent from some address other than the
indicated preferred address.
Finally it would be possible not to execute return routability checks
at all. In case of indirect change notifications we only move to the
new preferred address after successful dead-peer detection (i.e., a
response to a DPD test) on the new address, which is already a return
routability check. With a direct notification the authenticated peer
may have provided an authenticated IP address. Thus it is would be
possible to simply trust the MOBIKE peer to provide a proper IP
address. There is no way an adversary can successfully launch an
attack by injecting faked addresses since it does not know the IKE SA
and the corresponding keying material. A protection against an
internal attacker, i.e. the authenticated peer forwarding its traffic
to the new address, is not provided. This might be an issue when
extensions are added to IKEv2 that do not require authentication of
end points (e.g., opportunistic security using anonymous Diffie-
Hellman). On the other hand we know the identity of the peer in that
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case.
There is also a policy issue when to schedule a return routability
test. Before moving traffic? After moving traffic?
The basic format of the return routability check could be similar to
dead-peer detection. There are potential attacks if a return
routability check does not include some kind of nonce. The valid end
point could send an address update notification for a third party,
trying to get all the traffic to be sent there, causing a denial of
service attack. If the return routability checks does not contain
any cookies or other random information not known to the other end,
then that valid node could reply to the return routability checks
even when it cannot see the request. This might cause a peer to move
the traffic to a location where the original recipient cannot be
reached.
The IKEv2 NAT-T mechanism does not perform return routability checks.
It simply uses the last seen source IP address used by the other peer
as the destination address to send response packets. An adversary
can change those IP addresses, and can cause the response packets to
be sent to wrong IP address. The situation is self-fixing when the
adversary is no longer able to modify packets and the first packet
with an unmodified IP address reaches the other peer. Mobility
environments make this attack more difficult for an adversary since
it requires the adversary to be located somewhere on the individual
paths ({CoA1, ..., CoAn} towards the destination IP address) have a
shared path or if the adversary is located near the MOBIKE client
then it needs to follow the user mobility patterns. With IKEv2
NAT-T, the genuine client can cause third party bombing by
redirecting all the traffic pointed to him to third party. As the
MOBIKE protocol tries to provide equal or better security than IKEv2
NAT-T mechanism it should protect against these attacks.
There may be return routability information available from the other
parts of the system too (as shown in Figure 3), but the checks done
may have a different quality. There are multiple levels for return
routability checks:
o None, no tests
o A party willing to answer the return routability check is located
along the path to the claimed address. This is the basic form of
return routability test.
o There is an answer from the tested address, and that answer was
authenticated, integrity and replay protected.
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o There was an authenticated, integrity and replay protected answer
from the peer, but it is not guaranteed to originate at the tested
address or path to it (because the peer can construct a response
without seeing the request).
The return routability checks do not protect against 3rd party
bombing if the attacker is along the path, as the attacker can
forward the return routability checks to the real peer (even if those
packets are cryptographically authenticated).
If the address to be tested is carried inside the MOBIKE payload,
then the adversary cannot forward packets. Thus 3rd party bombings
are prevented.
If the reply packet can be constructed without seeing the request
packet (for example, if there is no nonce, challenge or similar
mechanism to show liveness), then the genuine peer can cause 3rd
party bombing, by replying to those requests without seeing them at
all.
Other levels might only provide a guarantee that there is a node at
the IP address which replied to the request. There is no indication
as to whether or not the reply is fresh, and whether or not the
request may have been transmitted from a different source address.
5.5.1. Employing MOBIKE results in other protocols
If MOBIKE has learned about new locations or verified the validity of
a remote address through a return routability check, can this
information be useful for other protocols?
When considering the basic MOBIKE VPN scenario, the answer is no.
Transport and application layer protocols running inside the VPN
tunnel are unaware of the outer addresses or their status.
Similarly, IP layer tunnel termination at a gateway rather than a
host endpoint limits the benefits for "other protocols" that could be
informed -- all application protocols at the other side are unaware
of IPsec, IKE, or MOBIKE.
However, it is conceivable that future uses or extensions of the
MOBIKE protocol make such information distribution useful. For
instance, if transport mode MOBIKE and SCTP were made to work
together, it would potentially be useful for SCTP to learn about the
new addresses at the same time as MOBIKE. Similarly, various IP
layer mechanisms may make use of the fact that a return routability
test of a specific type has been performed. However, care should be
exercised in all these situations.
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[I-D.crocker-celp] discusses the use of common locator information
pools in a IPv6 multi-homing context; it assumed that both transport
and IP layer solutions are be used in order to support multi-homing,
and that it would be beneficial for different protocols to coordinate
their results in some way, for instance by sharing throughput
information of address pairs. This may apply to MOBIKE as well,
assuming it co-exists with non-IPsec protocols that are faced with
the same or similar multi-homing choices.
Nevertheless, all of this is outside the scope of current MOBIKE base
protocol design and may be addressed in future work.
5.5.2. Suggested approach
MOBIKE protocol selected to use IKEv2 INFORMATIONAL exchanges as a
return routability tests, but added random cookie there to prevent
redirections done by authenticated attacker. Return routability
tests are done by default before moving the traffic. However these
tests are optional. Nodes MAY also perform these tests upon their
own initiative at other times.
It is worth noting that the return routability test in MOBIKE is not
he same as return routability test in MIP6: The MIP6 WG decided that
it is not necessary to do return routability tests between the mobile
node and the home agent at all.
5.6. IPsec Tunnel or Transport Mode
Current MOBIKE design is focused only on the VPN type usage and
tunnel mode. Transport mode behavior would also be useful, but will
be discussed in future documents.
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6. Protocol detail issues
6.1. Indicating support for mobike
In order for MOBIKE to function, both peers must implement the MOBIKE
extension of IKEv2. If one or none of the peers supports MOBIKE,
then, whenever an IP address changes, IKEv2 will have to be re-run in
order to create a new IKE SA and the respective IPsec SAs. In
MOBIKE, a peer needs to be confident that its address change messages
are understood by the other peer. If these messages are not
understood, it is possible that connectivity between the peers is
lost.
One way to ensure that a peer receives feedback on whether or not its
messages are understood by the other peer, is by using IKEv2
messaging for MOBIKE and to mark some messages as "critical".
According to the IKEv2 specification, such messages either have to be
understood by the receiver, or an error message has to be returned to
the sender.
A second way to ensure receipt of the above-mentioned feedback is by
using Vendor ID payloads that are exchanged during the initial IKEv2
exchange. These payloads would then indicate whether or not a given
peer supports the MOBIKE protocol.
A third approach would use the Notify payload which is also used for
NAT detection (via NAT_DETECTION_SOURCE_IP and
NAT_DETECTION_DESTINATION_IP payloads).
Both a Vendor ID and a Notify payload may be used to indicate the
support of certain extensions.
Note that a MOBIKE peer could also attempt to execute MOBIKE
opportunistically with the critical bit set when an address change
has occurred. The drawback of this approach is, however, that an
unnecessary message exchange is introduced.
Although Vendor ID payloads and Notifications are technically
equivalent, Notifications are already used in IKEv2 as a capability
negotiation mechanism. Hence, notification payloads are used in the
MOBIKE to indicate support of it.
Also as the information of the support of the MOBIKE is not needed
during the IKE_SA_INIT exchange, the indication of the support is
done inside the IKE_AUTH exchange. The reason for this is to need to
keep the IKE_SA_INIT messages as small as possible, so they do not
get fragmented. The idea is that responder can do stateless
processing of the first IKE_SA_INIT packet, and request cookie from
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the other end if it is under attack. To mandate responder to be able
to reassemble initial IKE_SA_INIT packets would not allow fully
stateless processing of the initial IKE_SA_INIT packets.
6.2. Path Testing and Window size
As the IKEv2 has the window of outgoing messages, and the sender is
not allowed to violate that window (meaning, that if the window is
full, then he cannot send packets), it do cause some complications to
the path testing. The another complication created by IKEv2 is that
once the message is first time sent to the other end, it cannot be
modified in its future retransmissions. This makes it impossible to
know what packet actually reached first to the other end. We cannot
use IP headers to find out which packet reached the other end first,
as if responder gets retransmissions of the packet it has already
replied (and those replies might have been lost due unidirectional
address pair), it will retransmit the previous reply using the new
address pair of the request. Because of this it might be possible
that the responder has already used the IP-address information from
the header of the packet, and the reply packet ending up to the
initiator has different address pair.
Another complication comes from the NAT-T. The current IKEv2
document says that if NAT-T is enabled the node not behind NAT SHOULD
detect if the IP-address changes in the incoming authenticated
packets, and update the remote peers addresses accordingly. This
works fine with the NAT-T, but it causes some complications in the
MOBIKE, as it needs an ability to probe the another address pairs,
without breaking the old one.
One approach to fix those would be to add completely new protocol
that is outside the IKE SA message id limitations (window code),
outside identical retransmission requirements, and outside the
dynamic address updating of the NAT-T.
Another approach is to make the protocol so that it does not violate
window restrictions and does not require changing the packet is sent,
and change the dynamic address updating of NAT-T to MUST NOT in case
MOBIKE is used. To not to violate window restrictions, it means that
the addresses of the currently ongoing exchange needs to be changed,
to test different paths. To not to require changing the packet after
it is first sent, requires that the protocol needs to restart from
the beginning in case packet was retransmitted to different addresses
(so sender does not know which packet was the one that responder got
first, i.e. which IP-addresses it used).
MOBIKE protocol decided to use normal IKEv2 exchanges for the path
testing, and decided to change the dynamic address updating of NAT-T
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to MUST NOT.
6.3. Message presentation
The IP address change notifications can be sent either via an
informational exchange already specified in the IKEv2, or via a
MOBIKE specific message exchange. Using informational exchange has
the main advantage that it is already specified in the IKEv2 and
implementations incorporate the functionality already.
Another question is the format of the address update notifications.
The address update notifications can include multiple addresses, of
which some may be IPv4 and some IPv6 addresses. The number of
addresses is most likely going to be limited in typical environments
(with less than 10 addresses). The format may need to indicate a
preference value for each address. The format could either contain a
preference number that determines the relative order of the
addresses, or it could simply be ordered, according to preference,
list of IP addresses. While two addresses can have the same
preference value an ordered list avoids this situation.
Even if load balancing is currently outside the scope of MOBIKE,
future work might include support for it. The selected format needs
to be flexible enough to include additional information (e.g. to
enable load balancing). This may be realized with an reserved field,
which can later be used to store additional information. As there
may arise other information which may have to be tied to an address
in the future, a reserved field seems like a prudent design in any
case.
There are two formats that place IP address lists into a message.
One includes each IP address as separate payload (where the payload
order indicates the preference value, or the payload itself might
include the preference value), or we can put the IP address list as
one payload to the exchange, and that one payload will then have
internal format which includes the list of IP addresses.
Having multiple payloads with each one having carrying one IP address
makes the protocol probably easier to parse, as we can already use
the normal IKEv2 payload parsing procedures. It also offers an easy
way for the extensions, as the payload probably contains only the
type of the IP address (or the type is encoded to the payload type),
and the IP address itself, and as each payload already has length
associated to it, we can detect if there is any extra data after the
IP address. Some implementations might have problems parsing IKEv2
payloads that are longer than a certain threshold, but if the sender
sends them in the most preferred first, the receiver can only use the
first addresses.
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Having all IP addresses in one big MOBIKE specified internal format
provides more compact encoding, and keeps the MOBIKE implementation
more concentrated to one module. It also avoids problems of packets
arriving in an order different from what they were sent.
Another choice is which type of payloads to use. IKEv2 already
specifies a notify payload. It includes some extra fields (SPI size,
SPI, protocol etc), which gives 4 bytes of the extra overhead, and
there is the notify data field, which could include the MOBIKE
specific data.
Another option would be to have a custom payload type, which then
includes the information needed for the MOBIKE protocol.
MOBIKE decided to use IKEv2 NOTIFY payloads, and put only one data
item per notify, i.e. there will be one NOTIFY payload for each item
to be sent.
6.4. Updating address list
Because of the initiator decides, the initiator needs to know all the
addresses used by the responder. The responder needs also that list
in case it happens move to the address unknown by the initiator, and
needs to send address update notify to the initiator, and it might
need to try different addresses for the initiator.
MOBIKE could send the full peer address list every time any of the IP
addresses changes (either addresses are added, removed, the order
changes or the preferred address is updated) or an incremental
update. Sending incremental updates provides more compact packets
(meaning we can support more IP addresses), but on the other hand
have more problems in the synchronization and packet reordering
cases, i.e., the incremental updates must be processed in order, but
for full updates we can simply use the most recent one, and ignore
old ones, even if they arrive after the most recent one (IKEv2
packets have message id which is incremented for each packet, thus we
know the sending order easily).
MOBIKE decided to use protocol format, where both ends can send full
list of their addresses to the other end, and that list overwrites
the previous list. To support NAT-T the IP-addresses of the received
packet is added to the list (and they are not present in the list).
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7. Security Considerations
As all the messages are already authenticated by the IKEv2 there is
no problem that any attackers would modify the contents of the
packets. The IP addresses in the IP header of the packets are not
authenticated, thus the protocol defined must take care that they are
only used as an indication that something might be different, and
that do not cause any direct actions.
An attacker can also spoof ICMP error messages in an effort to
confuse the peers about which addresses are not working. At worst
this causes denial of service and/or the use of non-preferred
addresses.
One type of attack that needs to be taken care of in the MOBIKE
protocol is the 'flooding attack' type. See [I-D.ietf-mip6-ro-sec]
and [Aur02] for more information about flooding attacks.
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8. IANA Considerations
This document does not introduce any IANA considerations.
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9. Acknowledgments
This document is the result of discussions in the MOBIKE working
group. The authors would like to thank Jari Arkko, Pasi Eronen,
Francis Dupont, Mohan Parthasarathy, Paul Hoffman, Bill Sommerfeld,
James Kempf, Vijay Devarapalli, Atul Sharma, Bora Akyol, Joe Touch,
Udo Schilcher, Tom Henderson, Andreas Pashalidis and Maureen Stillman
for their input.
We would like to particularly thank Pasi Eronen for tracking open
issues on the MOBIKE mailing list. He helped us to make good
progress on the document.
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10. References
10.1. Normative references
[I-D.ietf-ipsec-ikev2]
Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-17 (work in progress),
October 2004.
[I-D.ietf-ipsec-rfc2401bis]
Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work
in progress), April 2005.
10.2. Informative References
[I-D.arkko-multi6dt-failure-detection]
Arkko, J., "Failure Detection and Locator Selection in
Multi6", draft-arkko-multi6dt-failure-detection-00 (work
in progress), October 2004.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[I-D.dupont-mipv6-3bombing]
Dupont, F., "A note about 3rd party bombing in Mobile
IPv6", draft-dupont-mipv6-3bombing-02 (work in progress),
June 2005.
[I-D.ietf-mip6-ro-sec]
Nikander, P., "Mobile IP version 6 Route Optimization
Security Design Background", draft-ietf-mip6-ro-sec-03
(work in progress), May 2005.
[I-D.ietf-hip-mm]
Nikander, P., "End-Host Mobility and Multihoming with the
Host Identity Protocol", draft-ietf-hip-mm-02 (work in
progress), July 2005.
[I-D.crocker-celp]
Crocker, D., "Framework for Common Endpoint Locator
Pools", draft-crocker-celp-00 (work in progress),
February 2004.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
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"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC3753] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[I-D.ietf-tsvwg-addip-sctp]
Stewart, R., "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration",
draft-ietf-tsvwg-addip-sctp-12 (work in progress),
June 2005.
[I-D.dupont-ikev2-addrmgmt]
Dupont, F., "Address Management for IKE version 2",
draft-dupont-ikev2-addrmgmt-07 (work in progress),
May 2005.
[RFC3554] Bellovin, S., Ioannidis, J., Keromytis, A., and R.
Stewart, "On the Use of Stream Control Transmission
Protocol (SCTP) with IPsec", RFC 3554, July 2003.
[I-D.ietf-ipv6-optimistic-dad]
Moore, N., "Optimistic Duplicate Address Detection for
IPv6", draft-ietf-ipv6-optimistic-dad-06 (work in
progress), September 2005.
[I-D.ietf-ipv6-unique-local-addr]
Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", draft-ietf-ipv6-unique-local-addr-09 (work in
progress), January 2005.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2367] McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
Management API, Version 2", RFC 2367, July 1998.
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Kivinen & Tschofenig Expires April 24, 2006 [Page 32]
Internet-Draft Design of the MOBIKE Protocol October 2005
Discovery for IP Version 6 (IPv6)", RFC 2461,
December 1998.
[Aur02] Aura, T., Roe, M., and J. Arkko, "Security of Internet
Location Management", In Proc. 18th Annual Computer
Security Applications Conference, pages 78-87, Las Vegas,
NV USA, December 2002.
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Internet-Draft Design of the MOBIKE Protocol October 2005
Authors' Addresses
Tero Kivinen
Safenet, Inc.
Fredrikinkatu 47
HELSINKI FIN-00100
FI
Email: kivinen@safenet-inc.com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
URI: http://www.tschofenig.com
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Internet-Draft Design of the MOBIKE Protocol October 2005
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