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draft-ietf-mobike-protocol-01.txt --> draft-ietf-mobike-protocol-02.txt

MOBIKE Working Group P. Eronen, Ed.
Internet-Draft Nokia
Expires: January March 16, 2006 July 15, September 12, 2005

IKEv2 Mobility and Multihoming Protocol (MOBIKE)
draft-ietf-mobike-protocol-01.txt
draft-ietf-mobike-protocol-02.txt

Status of this Memo

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This Internet-Draft will expire on January March 16, 2006.

Abstract

This document describes the MOBIKE protocol, a mobility and
multihoming extension to IKEv2. MOBIKE allows mobile and/or
multihomed hosts to update the
(outer) IP addresses associated with IKE and IPsec Security
Associations (SAs). The main scenario for
MOBIKE is making it possible for a remote access A mobile VPN user to move
from one address client could use MOBIKE to another while keeping keep the
connection with the VPN gateway active. active while moving from one address
to another. Similarly, a multihomed host could use MOBIKE to move
the traffic to a different interface if, for instance, the currently
used one stops working.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 MOBIKE protocol overview Protocol Overview . . . . . . . . . . . . . . . . . 4
1.3 Terminology and notations Notations . . . . . . . . . . . . . . . . 5
2. MOBIKE protocol exchanges Protocol Exchanges . . . . . . . . . . . . . . . . . . 6
2.1 Signaling support Support for MOBIKE . . . . . . . . . . . . . . . 6
2.2 Additional addresses Addresses . . . . . . . . . . . . . . . . . . . 6
2.3 Changing addresses Addresses in IPsec SAs . . . . . . . . . . . . . 7
2.4 Updating additional addresses Additional Addresses . . . . . . . . . . . . . . 9 10
2.5 Path testing Return Routability Check . . . . . . . . . . . . . . . . . 11
2.6 Changes in NAT Mappings . . . . . . 10
2.6 Return routability check . . . . . . . . . . . 12
2.7 NAT Prohibition . . . . . . . 11
2.7 NAT prevention . . . . . . . . . . . . . . 12
2.8 Failure Recovery and Timeouts . . . . . . . . 11 . . . . . . 14
3. Payload formats Formats . . . . . . . . . . . . . . . . . . . . . . . 13 15
3.1 MOBIKE_SUPPORTED notification payload Notification Payload . . . . . . . . . . 13 15
3.2 ADDITIONAL_IP4/6_ADDRESS notification payloads Notification Payloads . . . . . . 13 15
3.3 UPDATE_SA_ADDRESSES notification payload NO_ADDITIONAL_ADDRESSES Notification Payload . . . . . . . 15
3.4 UPDATE_SA_ADDRESSES Notification Payload . . 13
3.4 UNACCEPTABLE_ADDRESSES notification payload . . . . . . . 13 15
3.5 COOKIE2 notification payload UNACCEPTABLE_ADDRESSES Notification Payload . . . . . . . 16
3.6 COOKIE2 Notification Payload . . . . . . . . 14
3.6 NAT_PREVENTION notification payload . . . . . . . 16
3.7 NO_NATS_ALLOWED Notification Payload . . . . . 14
3.7 NAT_PREVENTED notification payload . . . . . . 16
3.8 UNEXPECTED_NAT_DETECTED Notification Payload . . . . . . 14 . 16
4. Security considerations Considerations . . . . . . . . . . . . . . . . . . . 15 17
4.1 Traffic Redirection and Hijacking . . . . . . . . . . . . 17
4.2 IPsec Payload Protection . . . . . . . . . . . . . . . . . 17
4.3 Denial-of-Service Attacks Against Third Parties . . . . . 18
4.4 Spoofing Network Connectivity Indications . . . . . . . . 19
5. IANA considerations Considerations . . . . . . . . . . . . . . . . . . . . . 18 19
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 20
7.1 Normative references References . . . . . . . . . . . . . . . . . . . 19 20
7.2 Informative references References . . . . . . . . . . . . . . . . . . 19 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 20
1. 21
A. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 20 22
Intellectual Property and Copyright Statements . . . . . . . . 22 23

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1. Introduction

1.1 Motivation

IKEv2 is used for performing mutual authentication and establishing
and maintaining IPsec security associations (SAs). In the current
specifications, the IPsec and IKE SAs are created implicitly between
the IP addresses that are used when the IKE_SA is established. These
IP addresses are then used as the outer (tunnel header) addresses for
tunnel mode IPsec packets. Currently, it is not possible to change
these addresses after the IKE_SA has been created.

There are scenarios where these IP addresses might change. One
example is mobility: a host changes its point of network attachment,
and receives a new IP address. Another example is a multihoming host
that would like to change to a different interface if, for instance,
the currently used address stops working for some reason.

Although the problem can be solved by creating new IKE and IPsec SAs
when the addresses need to be changed, this may not be optimal for
several reasons. In some cases, creating a new IKE_SA may require
user interaction for authentication (entering a code from a token
card, for instance). Creating new SAs often also involves expensive
calculations and possibly a large number of roundtrips. Due to these
reasons, a mechanism for updating the IP addresses of existing IKE
and IPsec SAs is needed. The MOBIKE protocol described in this
document provides such a mechanism.

The main scenario for MOBIKE is making it possible for a remote
access VPN user to move from one address to another without re-
establishing all security associations with the VPN gateway. For
instance, a user could start from fixed Ethernet in the office, and
then disconnect the laptop and move to office wireless LAN. When
leaving the office the laptop could start using GPRS, and switch to a
different wireless LAN when the user arrives home. MOBIKE updates
only the outer (tunnel header) addresses of IPsec SAs, and the
addresses and others traffic selectors used inside the tunnel stay
unchanged. Thus, mobility can be (mostly) invisible to applications
and their connections using the VPN.

MOBIKE also supports more complex scenarios where the VPN gateway
also has several network interfaces: these interfaces could be
connected to different networks or ISPs, they may have may be a mix
of IPv4 and IPv6 addresses, and the addresses may change over time.
Furthermore, both parties could be VPN gateways relaying traffic for
other parties.

Note that this document does not claim to solve all the problems IETF

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1.2 MOBIKE protocol overview

Since

MOBIKE allows both parties working group has been chartered to have several work on. It is assumed
that issues such as transport mode (updating traffic selectors),
PFKEY extensions, and tunnel overhead reduction will be handled in
separate documents.

1.2 MOBIKE Protocol Overview

Since MOBIKE allows both parties to have several addresses, this
leads us to an important question: there are up to N*M pairs of IP
addresses that could potentially be used. How to decide which of
these pairs should be used? The decision has to take into account
several factors. First, the parties have may preferences about which
interface should be used, due to performance and cost reasons, for
instance. Second, the decision is constrained by the fact that some
of the pairs may not work at all due to incompatible IP versions,
outages somewhere in the network, problems at the local link at
either end, and so on.

MOBIKE solves this problem by taking a simple approach: the party
that initiated the IKE_SA (the "client" in remote access VPN
scenario) is responsible for deciding which address pair is used for
the IPsec SAs, and collecting the information it needs to make this
decision (such as determining which address pairs work or do not
work). The other party (the "gateway" in remote access VPN scenario)
simply tells the initiator what addresses it has, but does not update
the IPsec SAs until it receives a message from the initiator to do
so.

Making the decision at the initiator is consistent with how normal
IKEv2 works: the initiator decides which addresses it uses when
contacting the responder. It also makes sense especially when the
initiator is the mobile node: it is in a better position to decide
which of its network interfaces should be used for both upstream and
downstream traffic.

The details of exactly how the initiator makes the decision, what
information is used in making it, how the information is collected,
how preferences affect the decision, and when a decision needs to be
changed, are largely beyond the scope of MOBIKE. This does not mean
that these details are unimportant: on the contrary, they are likely
to be crucial in any real system. However, MOBIKE is concerned with
these details only to the extent that they are visible in IKEv2/IPsec
messages exchanged between the peers (and thus need to be
standardized to ensure interoperability). Issues such as mobility
detection and local policies are also not specific to MOBIKE, but
apply to existing mobility protocols such as Mobile IPv4 [MIP4] as
well.

One important aspect of this information gathering that has to be
visible in the messages is determining whether a certain pair of
addresses can be used. IKEv2 Dead Peer Detection (DPD) feature can
provide information that the currently used pair does or does not
work. There are, however, some complications in using it for other

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addresses, and thus MOBIKE adds a new IKEv2 message that can be used
to "test" whether some particular pair of addresses works or not,
without yet committing to changing the addresses currently in use.

MOBIKE also has to deal with situations where the network contains
NATs or stateful packet filters (for brevity, the rest of this
document talks simply about NATs). When the addresses used for IPsec
SAs are changed, MOBIKE can enable or disable IKEv2 NAT Traversal as
needed. However, if the party "outside" the NAT changes its IP
address, it may no longer be able to send packets to the party
"behind" the NAT, since the packets may not (depending on the exact
type of NAT) match the NAT mapping state. Here MOBIKE assumes that
the initiator is the party "behind" the NAT, and does not fully
support the case where the responder's addresses change when NATs are
present.

Updating the addresses of IPsec SAs naturally has to take into
account several security considerations. MOBIKE includes two
features designed to address these considerations. First, a "return
routability" check can be used to verify the addresses provided by
the peer. This makes it more difficult to flood third parties with
large amounts of traffic. Second, a "NAT prevention" prohibition" feature
ensures that IP addresses have not been modified by NATs, IPv4/IPv6
translation agents, or other similar devices. This feature is mainly
intended for site-to-site VPNs where the administrators may know
beforehand that NATs are not present, and thus any modification to
the packet can be considered to be an attack.

1.3 Terminology and notations Notations

When messages containing IKEv2 payloads are shown, optional payloads
are shown in brackets (for instance, "[FOO]"), and a plus sign
indicates that a payload can be repeated one or more times (for
instance, "FOO+").

When this document talks about updating the source/destination
addresses of an IPsec SA, it means updating IPsec-related state so
that outgoing ESP/AH packets use those addresses in the tunnel
header. Depending on how the nominal division between Security
Association Database (SAD), Security Policy Database (SPD), and Peer
Authorization Database (PAD) described in [IPsecArch] is actually
implemented, an implementation can have several different places that
have to be updated.

In this document, the term "initiator" means the party who originally
initiated the first IKE_SA (in a series of possibly several rekeyed
IKE_SAs); "responder" is the other peer. During the lifetime of the
IKE_SA, both parties may initiate INFORMATIONAL or CREATE_CHILD_SA
exchanges; in this case, the terms "exchange initiator" and "exchange
responder" are used. The term "original initiator" (which in [IKEv2]
refers to the party who started the latest IKE_SA rekeying) is not
used in this document.

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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].

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2. MOBIKE protocol exchanges Protocol Exchanges

2.1 Signaling support Support for MOBIKE

Implementations that wish to use MOBIKE for a particular IKE_SA MUST
include a MOBIKE_SUPPORTED notification in the IKE_SA_INIT request
and response messages.

Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni,
N(MOBIKE_SUPPORTED),
[N(NAT_DETECTION_SOURCE_IP)+,
N(NAT_DETECTION_DESTINATION_IP)] -->

<-- HDR, SAr1, KEr, Nr,
[N(NAT_DETECTION_SOURCE_IP)+,
N(NAT_DETECTION_DESTINATION_IP)]
[CERTREQ],
N(MOBIKE_SUPPORTED)

The IKE_AUTH exchange (in
case of multiple IKE_AUTH exchanges, in the message containing the SA
payload).

The MOBIKE_SUPPORTED notification payload is described in Section 3.

2.2 Additional addresses Addresses

Both the initiator and responder MAY include one or more
ADDITIONAL_IP4_ADDRESS and/or ADDITIONAL_IP6_ADDRESS notification
payloads in the IKE_AUTH exchange (in case of multiple IKE_AUTH
exchanges, in the message containing the SA payload).

Initiator Responder
----------- -----------
HDR, SK { IDi, [CERT], [IDr], AUTH,
[CP(CFG_REQUEST)]
SAi2, TSi, TSr,
N(MOBIKE_SUPPORTED),
[N(ADDITIONAL_*_ADDRESS)+] -->

<-- HDR, SK { IDr, [CERT], AUTH,
[CP(CFG_REPLY)],
SAr2, TSi, TSr,
N(MOBIKE_SUPPORTED)
[N(ADDITIONAL_*_ADDRESS)+] }

The recipient stores this information, but no other action is taken
at this time.

Although both the initiator and responder maintain a set of peer
addresses (logically associated with the IKE_SA), it is important to
note that they use this information for slightly different purposes.

The initiator uses the set of responder addresses as an input to its
address selection policy; it may at some later point decide to move
the IPsec traffic to one of these addresses using the procedure
described in Section 2.3. The responder normally does not use the
set of initiator addresses for anything: the addresses are used only

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when the responder's own addresses change.

The set of addresses available to the peers can change during the
lifetime of the IKE_SA. The procedure for updating this information
is described in Section 2.4.

Note that if some of the initiator's interfaces are behind a NAT
(from the responder's point of view), the addresses received by the
responder will be incorrect. This means the procedure for changing
responder addresses described in Section 2.4 does not fully work when
the initiator is behind a NAT. For the same reason, the peers also
SHOULD NOT use this information for any other purposes than what is
explicitly described in this document.

2.3 Changing addresses Addresses in IPsec SAs

In MOBIKE, the initiator of the IKE_SA decides what addresses are used in the IPsec
SAs. That is, the responder usually never updates any IPsec SAs
without receiving an explicit UPDATE_SA_ADDRESSES request from the
initiator. (As described below, the responder can, however, update
the IKE_SA in some circumstances.)

The description in this section assumes that reasons why the initiator has
already decided what wishes to change the new addresses should be. How this decision
is made is are
largely beyond the scope of this specification. When this
decision has been made, the initiator

o Updates the IKE_SA and IPsec SAs with the new addresses, and sets
the "pending_update" flag MOBIKE. Typically triggers include
information received from lower layers, such as changes in the IKE_SA. IP
addresses or link-down indications. Some of this information can be
unreliable: for instance, ICMP messages could be spoofed by an
attacker. Such information itself MUST NOT be used to conclude than
an update is needed: instead, the initiator SHOULD trigger dead peer
detection.

Changing addresses can also be triggered by events within IKEv2. At
least the following events can cause the initiator to re-evaluate its
local address selection policy, possibly leading to changing the
addresses.

o An IKEv2 request has been re-transmitted several times, but no
valid reply has been received. This suggests the current path is
no longer working.

o An INFORMATIONAL request containing ADDITIONAL_IP4/6_ADDRESS
payloads is received. This means the peer's addresses may have
changed.

o An UNACCEPTABLE_ADDRESSES notification is received as a response
to address update request (described below).

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o The initiator receives a NAT_DETECTION_DESTINATION_IP payload that
does not match the previous UPDATE_SA_ADDRESSES response (see
Section 2.6 for a more detailed description).

The description in the rest of this section assumes that the
initiator has already decided what the new addresses should be. When
this decision has been made, the initiator

o Updates the IKE_SA and the IPsec SAs associated with this IKE_SA
with the new addresses, and sets the "pending_update" flag in the
IKE_SA.

o If NAT Traversal is not enabled, and the responder supports NAT
Traversal (as indicated by NAT detection payloads in the
IKE_SA_INIT exchange), and the initiator either suspects or knows
that a NAT is likely to be present, enables NAT Traversal.

o If there are outstanding IKEv2 requests, requests (requests for which the
initiator has not yet received a reply), continues retransmitting
them using the addresses in the IKE_SA (the new addresses).

o When the window size allows, sends an INFORMATIONAL request
containing the UPDATE_SA_ADDRESSES notification payload (which
does not contain any data), and clears the "pending_update" flag.
(See Section 2.6 for description of the COOKIE2 notification.)

Initiator Responder
----------- -----------
HDR, SK { N(UPDATE_SA_ADDRESSES),
N(COOKIE2),
[N(NAT_DETECTION_*_IP)],
[N(NAT_PREVENTION)]
[N(NO_NATS_ALLOWED)],
[N(COOKIE2)] } -->

o If a new address change occurs while waiting for the response,
starts again from the first step (and ignores responses to this
UPDATE_SA_ADDRESSES request).

Note that if the responder has NAT Traversal enabled, it can update
the addresses in both the IKE_SA and IPsec SAs as usual (if it
implements the "SHOULD" from [IKEv2] Section 2.23).

When processing an INFORMATIONAL request containing the
UPDATE_SA_ADDRESSES notification, the responder

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o Determines whether it has already received a newer
UPDATE_SA_ADDRESSES request than this one (if the responder uses a
window size greater than one, it is possible that requests are
received out of order). If it has, a response message is sent,
but no other action is taken.

o If the NAT_PREVENTION NO_NATS_ALLOWED payload is present, processes it as
described in Section 2.7.

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o Checks that the (source IP address, destination IP address) pair
in the IP header is acceptable according to local policy. If it
is not, replies with "HDR, SK {N(COOKIE2),
N(UNACCEPTABLE_ADDRESSES)}".

o Updates the a message containing the
UNACCEPTABLE_ADDRESSES notification (and possibly COOKIE2).

o Updates the IP addresses in the IKE_SA with the values from the IP
header. (Using the address from the IP header is consistent with
normal IKEv2, and allows IKEv2 to work with NATs without needing
unilateral self-address fixing [UNSAF].)

o Replies with an INFORMATIONAL response:

Initiator Responder
----------- -----------
<-- HDR, SK { N(COOKIE2),
[N(NAT_DETECTION_*_IP)] [N(NAT_DETECTION_*_IP)],
[N(COOKIE2)], }

o If necessary, initiates a return routability check for the new
initiator address (see Section 2.6) 2.5) and waits for until the check to
finish.. is
completed.

o Updates the IPsec SAs associated with this IKE_SA with the new
addresses.

o If NAT Traversal is supported and NAT detection payloads were
included, enables or disables NAT Traversal.

When the initiator receives the reply, it

o If an address change has occured after the request was first sent,
no MOBIKE processing is done for the reply message, since a new
UPDATE_SA_ADDRESSES is going to be sent (or has already been sent,
if window size greater than one is in use).

o If the response contains the NAT_PREVENTED UNEXPECTED_NAT_DETECTED payload,
processes it as described in Section 2.7.

o If the response contains an UNACCEPTABLE_ADDRESSES notification
payload, the initiator MAY select another addresses and retry the
exchange, keep on using the current addresses, or disconnect.

o If NAT Traversal is supported and NAT detection payloads were
included, enables or disables NAT Traversal.

There is one exception to the rule that the responder never updates
any IPsec SAs without receiving an UPDATE_SA_ADDRESSES request. If
the source address the responder is currently using becomes
unavailable (i.e., sending packets using that source address is no

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longer possible), the responder is allowed to update the IPsec SAs to
use some other address (in addition to initiating the procedure
described in the next section).

2.4 Updating additional addresses Additional Addresses

As described in Section 2.2, both the initiator and responder can
send a list of additional addresses (in addition to the one used for
IKE_SA_INIT/IKE_AUTH exchange) to the initiator in the IKE_AUTH exchange. If this list of addresses changes, a new list This
information can be sent
in any INFORMATIONAL exchange request message.

When the responder (of the original IKE_SA) receives updated by sending an INFORMATIONAL exchange
request containing ADDITIONAL_*_ADDRESS payloads, it simply stores message that contains either one or more ADDITIONAL_IP4/
6_ADDRESS payloads or the information, but no other action is taken. NO_ADDITIONAL_ADDRESSES payload.

Initiator Responder
----------- -----------
HDR, SK { N(ADDITIONAL_*_ADDRESS)+,
N(COOKIE2) [N(ADDITIONAL_*_ADDRESS)+],
[N(NO_ADDITIONAL_ADDRESSES)],
[N(NO_NATS_ALLOWED)],
[N(COOKIE2)] } -->

<-- HDR, SK { N(COOKIE2) [N(COOKIE2)] }

When the initiator receives an INFORMATIONAL a request containing
ADDITIONAL_*_ADDRESS, it stores ADDITIONAL_*_ADDRESS or
NO_ADDITIONAL_ADDRESSES payloads is received, the information and also determines exchange responder

o Determines whether the currently used addresses need it has already received a newer request to be changed (for
instance, if
update the currently used address addresses (if a window size greater than one is no longer included in the
list); if used,
it does, is possible that the initiator proceeds as described in
Section 2.3.

Initiator Responder
----------- -----------
<-- HDR, SK { N(ADDITIONAL_*_ADDRESS)+,
N(COOKIE2) }

HDR, SK { N(COOKIE2) } --> requests are received out of order). If
it has, a response message is sent, but the implementation supports window sizes greater than one, address set is not
updated.

o If the NO_NATS_ALLOWED payload is present, processes it also
has to keep track of as
described in Section 2.7.

o Updates the Message ID set of peer addresses based on the latest update it has
received, IP header and
ADDITIONAL_IP4/6_ADDRESS or NO_ADDITIONAL_ADDRESS payloads.

o Sends a response.

The initiator MAY include these payloads in the same request as
UPDATE_SA_ADDRESSES.

If the request to avoid update the situation where new information addresses is overwritten
by older. retransmitted using several
different source addresses, a new INFORMATIONAL request MUST be sent.

There is one additional complication: when the responder wants to
send a new additional
update the address list, set, the currently used addresses may no longer
work. In this case, the responder uses the additional address list
received from the initiator, initiator and the list of its own
addresses, and, if necessary, the path testing feature (see
Section 2.5) addresses to

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determine a path that works, updates the which addresses in
the IKE_SA (but not IPsec SAs), and then sends to use for sending the INFORMATIONAL
request. This is the only time the responder uses the additional
address list received from the initiator.

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Note that both peers can have their own policies about what addresses
are acceptable to use. A minimal "mobile client" could have a policy
that says that only the responder's address specified in local
configuration is acceptable. This kind of client does not have to
send or process ADDITIONAL_*_ADDRESS notification payloads.
Similarly, a simple "VPN gateway" that has only a single address, and
is not going to change it, does not need to send or understand
ADDITIONAL_*_ADDRESS notification payloads.

2.5 Path testing

IKEv2 Dead Peer Detection allows Return Routability Check

Both parties can optionally verify that the peers to detect if other party can actually
receive packets at the currently
used path has stopped working. However, if either of claimed address. This "return routability
check" can be done before updating the peers has
several addresses, DPD alone does not indicate which of IPsec SAs, immediately after
updating them, or continuously during the other
paths might work. The path testing feature allows the parties to
determine whether a particular path (pair of addresses) works,
without yet committing to changing over to these addresses.

MOBIKE introduces a new IKEv2 exchange type, PATH_TEST, for testing
connectivity. This exchange is not part of any IKE_SA, so it is not
cryptographically protected. It also does not result in the
responder keeping any state.

Initiator Responder
----------- -----------
HDR(0,0), N(COOKIE2),
[N(NAT_DETECTION_*_IP)] -->

<-- HDR(0,0), N(COOKIE2),
[N(NAT_DETECTION_*_IP)]

The reason for introducing a new exchange type, instead of using
INFORMATIONAL exchanges, is to simplify implementations by allowing
MOBIKE to work with window size 1.

Performing path testing over several different paths is not required
if the node has other information that enables it to select which
path should be used. Also, responders do not perform path testing
unless they update their list of additional addresses (as described
in Section 2.4). Implementations MAY do path testing even if the
currently used path is working to e.g. detect when a better but
previously unavailable path becomes available, or to speed up
recovery in fault situations.

Implementations that perform path testing MUST take steps to avoid
causing unnecessary congestion. TBD: add some more details here.

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2.6 Return routability check

Both the initiator and the responder can optionally verify that the
other party can actually receive packets at the claimed address.
This "return routability check" MAY be done before updating the IPsec
SAs, immediately after updating them, or continuously during the
connection.

By default, return routability check SHOULD be done before updating connection.

By default, return routability check SHOULD be done before updating
the IPsec SAs. In environments where the peer is expected to be
well-behaving (many corporate VPNs, for instance), or the address can
be verified by some other means (e.g., the address is included in the
peer's certificate), the return routability check MAY be skipped or
postponed until after the IPsec SAs have been updated.

Any INFORMATIONAL exchange can be used for return routability
purposes (with one exception, described below): when a valid response
is received, we know the other party can receive packets at the
claimed address.

To ensure that the peer cannot generate the correct INFORMATIONAL
response without seeing the request, a new payload is added to all
INFORMATIONAL messages. The sender of an INFORMATIONAL request MUST MAY
include a COOKIE2 notification payload, and if included, the
recipient of an INFORMATIONAL request MUST copy the payload as-is to
the response. When processing the response, the original sender MUST
verify that the value is the same one as sent. If the values do not
match, the IKE_SA MUST be closed.

There is one additional issue that must be taken into account. If
the INFORMATIONAL request has been sent to several different
addresses (i.e., the destination address in the IKE_SA has been
updated after the request was first sent), receiving the
INFORMATIONAL response does not tell which address is the working
one. In this case, a new INFORMATIONAL request needs to be sent to
check return routability.

2.7 NAT prevention

IKEv2/IPsec implementations that do not support NAT Traversal can, in
fact, work across some types of one-to-one "basic" NATs and IPv4/IPv6
translation agents

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2.6 Changes in NAT Mappings

IKEv2 performs Dead Peer Detection (DPD) if there has recently been
only outgoing traffic on all of the SAs associated with the IKE_SA.

In MOBIKE, these messages can also be used to detect if NAT mappings
have changed (for example, if the keepalive internal is too long, or
the NAT box is rebooted). More specifically, if both peers support
both this specification and NAT Traversal, NAT_DETECTION_*_IP
payloads MAY be included in any INFORMATIONAL request; if the request
includes them, the responder MUST also include them in the response
(but no other action is taken, unless otherwise specified).

When the initiator is behind a NAT, it SHOULD include these payloads
in DPD messages, and compare the received
NAT_DETECTION_DESTINATION_IP payload with the value from the previous
UPDATE_SA_ADDRESSES response (or the IKE_SA_INIT response). If the
values do not match, the IP address and/or port seen by the responder
has changed, and the initiator SHOULD send UPDATE_SA_ADDRESSES as
described in Section 2.3.

When MOBIKE is in use, the host not behind a NAT SHOULD NOT use the
dynamic updates specified in [IKEv2] Section 2.23 (where the peer
address and port are updated from the last valid authenticated
packet). This ensures that both peers have a consistent view of when
addresses are to be changed, and prevents conflicts between MOBIKE-
originated updates and NAT-T dynamic updates. It also means that an
INFORMATIONAL exchange that does not contain UPDATE_SA_ADDRESSES does
not cause any changes, allowing it to be used for, e.g., testing
whether a particular path works.

2.7 NAT Prohibition

IKEv2/IPsec implementations that do not support NAT Traversal can, in
fact, work across some types of one-to-one "basic" NATs and IPv4/IPv6
translation agents in tunnel mode. This may be considered a problem
in some circumstances, since in some sense any modification of the IP
addresses can be considered to be an attack.

The "NAT prevention" prohibition" feature allows both the initiator and responder to have a policy
that prevents the use of paths that contain NATs, IPv4/IPv6
translation agents, or other nodes that modify the

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IP header. This feature is mainly intended for site-to-site VPN
cases, where the administrators may know beforehand that NATs are not
present, and thus any modification to the packet can be considered to
be an attack.

This specification addresses the issue as follows. When an IPsec SA
is created, the tunnel

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is created, the tunnel header IP addresses (and port if doing UDP
encapsulation) are taken from the IKE_SA, not the message IP header.
The NAT_PREVENTION NO_NATS_ALLOWED payload is used to guarantee that NATs have not
modified the address used in IKE_SA. However, all response messages
are still sent to the address and port the corresponding request came
from.

The

An initiator MAY include who wishes to use this feature includes a NAT_PREVENTION
NO_NATS_ALLOWED payload in an the IKE_SA_INIT request. The responder
then MUST calculate the expected value based on the values from the
IP header, and compare this with the value in the NAT_PREVENTION NO_NATS_ALLOWED
payload. If they do not match, the responder replies with "HDR(A,0), N(NAT_PREVENTED)"
N(UNEXPECTED_NAT_DETECTED)" and does not create any state.

Initiator Responder
----------- -----------
HDR, [N(COOKIE)],
SAi1, KEi, Ni,
[N(NO_NATS_ALLOWED)] -->

<-- HDR, SAr1, KEr, Nr,
[CERTREQ]

If the values do match, the responder initializes (local_address,
local_port, peer_address, peer_port) in the to-be-created IKE_SA with
values from the IP header. The same applies if neither
NAT_PREVENTION
NO_NATS_ALLOWED nor NAT_DETECTION_*_IP payloads were included, or if
the responder does not support NAT Traversal.

If the IKE_SA_INIT request included NAT_DETECTION_*_IP payloads but
no NAT_PREVENTION NO_NATS_ALLOWED payload, the situation is different since the
initiator may at this point change from port 500 to 4500. In this
case, the responder initializes (local_address, local_port,
peer_address, peer_port) from the first IKE_AUTH request.

IKEv2 requires that if an IPsec endpoint discovers a NAT between it
and its correspondent, it MUST send all subsequent traffic to and
from port 4500. To simplify things, implementations that support
both this specification and NAT Traversal MUST change to port 4500 if
the correspondent also supports both, even if no NAT was detected
between them.

NAT_PREVENTION them (this way, there is no need to change the ports later).

NO_NATS_ALLOWED payloads can also be included when changing the
addresses of IPsec SAs (see Section 2.3). TBD: add better
description. 2.3) and updating the additional
addresses (see Section 2.4). An initiator using this "NAT
prohibition" feature includes a NO_NATS_ALLOWED payload in all
address update messages.

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If the initiator receives an UNEXPECTED_NAT_DETECTION notification in
response to its request, it SHOULD retry the operation several times
using new IKE_SA_INIT/INFORMATIONAL requests. This ensures that an
attacker who is able to modify only a single packet does not
unnecessarily cause a path to remain unused.

2.8 Failure Recovery and Timeouts

In MOBIKE, the initiator is responsible for detecting and recovering
from most failures.

To give the initiator enough time to detect the error, the responder
SHOULD use relatively long timeout intervals when, for instance,
retransmitting IKEv2 requests or deciding whether to initiate dead
peer detection.

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3. Payload formats Formats

3.1 MOBIKE_SUPPORTED notification payload Notification Payload

The MOBIKE_SUPPORTED notification payload is included in the
IKE_SA_INIT messages IKE_AUTH
exchange to indicate that the implementation supports this
specification.

The Notify Message Type for MOBIKE_SUPPORTED is TBD-BY-IANA
(16396..40959). TBD-BY-
IANA(16396..40959). The Protocol ID and SPI Size fields are set to
zero. There is no data associated with this Notify type.

3.2 ADDITIONAL_IP4/6_ADDRESS notification payloads Notification Payloads

Both initiator and responder parties can include ADDITIONAL_IP4_ADDRESS and/or
ADDITIONAL_IP6_ADDRESS payloads in the IKE_AUTH exchange and
INFORMATIONAL exchange request messages; see Section 2.2 and
Section 2.4 for more detailed description.

The Notify Message Types for ADDITIONAL_IP4_ADDRESS and
ADDITIONAL_IP6_ADDRESS are TBD-BY-IANA (16396..40959) TBD-BY-IANA(16396..40959) and TBD-BY-IANA
(16396..40959), TBD-BY-
IANA(16396..40959), respectively. The Protocol ID and SPI Size
fields are set to zero. The data associated with these Notify types
is either a four-octet IPv4 address or a 16-octet IPv6 address.

3.3 UPDATE_SA_ADDRESSES notification NO_ADDITIONAL_ADDRESSES Notification Payload

The NO_ADDITIONAL_ADDRESSES payload can be included in an
INFORMATIONAL exchange request messages to indicate that the exchange
initiator does not have addresses beyond the one used in the exchange
(see Section 2.4 for more detailed description).

The Notify Message Type for NO_ADDITIONAL_ADDRESSES is TBD-BY-
IANA(16396..40959). The Protocol ID and SPI Size fields are set to
zero. There is no data associated with this Notify type.

3.4 UPDATE_SA_ADDRESSES Notification Payload

This payload is included in INFORMATIONAL exchange requests sent by
the initiator of the IKE_SA to update addresses of the IKE_SA and IPsec SAs (see
Section 2.3).

The Notify Message Type for UPDATE_SA_ADDRESSES is TBD-BY-IANA
(16396..40959). TBD-BY-
IANA(16396..40959). The Protocol ID and SPI Size fields are set to
zero. There is no data associated with this Notify type.

3.4

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3.5 UNACCEPTABLE_ADDRESSES notification payload Notification Payload

The responder can include this notification payload in an
INFORMATIONAL exchange response to indicate that the address change
in the corresponding request message (which contained an
UPDATE_SA_ADDRESSES notification payload) was not carried out.

The Notify Message Type for UNACCEPTABLE_ADDRESSES is TBD-BY-IANA
(40..8191). TBD-BY-
IANA(40..8191). The Protocol ID and SPI Size fields are set to zero.
There is no data associated with this Notify type.

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3.5

3.6 COOKIE2 notification payload Notification Payload

This payload is MAY be included in all any INFORMATIONAL exchange messages request for return
routability check purposes (see Section 2.6). It is also used
in PATH_TEST messages to match requests and responses (see
Section 2.5). If the INFORMATIONAL
request includes COOKIE2, the exchange responder MUST copy the
payload to the response message.

The data associated with this notification MUST be between 8 and 64
octets in length (inclusive), and MUST be chosen by the exchange
initiator in a way that is unpredictable to the recipient. exchange responder.
The Notify Message Type for this message is TBD-BY-IANA (16396..40959). TBD-BY-
IANA(16396..40959). The Protocol ID and SPI Size fields are set to
zero.

3.6 NAT_PREVENTION notification payload

3.7 NO_NATS_ALLOWED Notification Payload

See Section 2.7 for a description of this payload.

The data associated with this notification is the SHA-1 hash
[FIPS180-2] of the following data: IKE SPIs (in the order they appear
in the header), the IP address and port from which the packet was
sent, and the IP address and port to which the packet was sent. The
Notify Message Type for this message is TBD-BY-IANA (16396..40959). TBD-BY-IANA(16396..40959).
The Protocol ID and SPI Size fields are set to zero.

3.7 NAT_PREVENTED notification payload

3.8 UNEXPECTED_NAT_DETECTED Notification Payload

See Section 2.7 for a description of this payload.

The Notify Message Type for NAT_PREVENTED UNEXPECTED_NAT_DETECTED is TBD-BY-IANA (40..8191). TBD-BY-
IANA(40..8191). The Protocol ID and SPI Size fields are set to zero.
There is no data associated with this Notify type.

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4. Security considerations Considerations

The main goals of this specification are to not reduce the security
offered by usual IKEv2 procedures and to counter mobility related
threats in an appropriate manner. In some specific cases MOBIKE is
also capable of protecting address changes better than existing NAT
Traversal procedures.

The threats arising in scenarios targeted This section describes new
security considerations introduced by MOBIKE are: MOBIKE. See [IKEv2] for
security considerations for IKEv2 in general.

4.1 Traffic redirection Redirection and hijacking

Insecure mobility management mechanisms may allow inappropriate
redirection of traffic. Hijacking

MOBIKE payload relating to updating addresses are encrypted,
integrity protected, and replay protected using the IKE_SA. This may allow inspection of
assures that no one except the
traffic as well participants can, for instance, give a
control message to change the addresses.

However, just like with normal IKEv2, the actual IP addresses in the
IP header are not covered by the integrity protection. This means
that a NAT between the parties (or an attacker acting as man-in-the-middle a NAT) can
modify the addresses and session hijacking
attacks. cause incorrect tunnel header (outer) IP
addresses to be used for IPsec SAs. The scope of these attacks in the MOBIKE case this attack is limited, as
limited mainly to denial-of-service, since all traffic is protected
using IPsec. However, it should be
observed that security associations originally created for

MOBIKE introduces the
protection of a specific flow between specific addresses may NO_NATS_ALLOWED payload that can be
moved through MOBIKE. The level used to
detect modification of required protection may the addresses in the IP header by outsiders
When this payload is used, communication through NATs and other
address translators is impossible. This feature is mainly intended
for site-to-site VPN cases, where the administrators may know
beforehand that valid NATs are not present, and thus any modification
to the packet can be considered to be an attack. However, this
feature SHOULD NOT be enabled by default, since it creates a denial-
of-service vulnerability even when no malicious attackers are
present: a misconfiguration or introduction of a (non-malicious) NAT
can cause the connection to fail.

4.2 IPsec Payload Protection

The use of IPsec protection on payload traffic protects the
participants against disclosure of the contents of the traffic,
should the traffic end up in an incorrect destination or be
eavesdropped along the way.

However, security associations originally created for the protection
of a specific flow between specific addresses may be moved through
MOBIKE. The level of required protection may be different in a new
location of a VPN client, for instance.

Third-party denial-of-service

It is recommended that security policies for peers that are allowed

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to use MOBIKE are configured in a manner that takes into account that
a single security association can be used through flooding different paths at
different times.

4.3 Denial-of-Service Attacks Against Third Parties

Traffic redirection may be performed not just to gain access to the traffic,
traffic (not very interesting since it is encrypted) or deny service
to the peers, but also to cause a denial-of-service attack for a
third party. For instance, a high-speed TCP session or a multimedia
stream may be redirected towards a victim host, causing its
communications capabilities to suffer.

The attackers in this threat can be either outsiders or even one of
the participants. In usual VPN usage scenarios scenarios, attacks by the
participants can be easily dealt with. However, this
requires that strong with if the authentication was performed
in the initial IKEv2 negotiation. negotiation can be traced to persons who can be
held responsible for the attack. This may not be the case in all
scenarios, particularly with opportunistic approaches to security.

Normally such attacks would expire in a short time frame due to the
lack of responses (such as transport layer acknowledgements) from the
victim. However, as described in [Aura02], malicious participants
would typically be able to spoof such acknowledgements and maintain
the traffic flow for an extended period of time. For instance, if
the attacker opened the TCP stream itself before redirecting it to
the victim, the attacker becomes aware of the sequence number space

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used in this particular session.

It should also be noted, as shown in [Bombing], that without ingress
filtering in the attacker's network such attacks are already possible
simply by sending spoofed packets from the attacker to the victim
directly. Furthermore, if the attacker's network has ingress
filtering, this attack is largely prevented for MOBIKE as well.
Consequently, it makes little sense to protect against attacks of
similar nature in MOBIKE. However, it still makes sense to limit the
amplification capabilities provided to attackers, so that they cannot
use stream redirection to send 1000 packets to the victim by sending
just a few packets themselves.

Note that a variant of

This specification requires the flooding attack exists in IKEv2 NAT
Traversal functionality [PseudoNAT]. In this variant, the
attacker has use of return routability tests
(under certain conditions) to be on the path between the participants,
changing limit the addresses in duration of any "third party
bombing" attacks by off-path (relative to the packets victim) attackers. The
tests are authenticated messages that pass by. This
attack is possible because the addresses in the outer headers
are not protected. When peer has to respond to, and
can be performed either before the attacker leaves address change takes effect,
immediately afterwards, or even periodically during the path, session. The
tests contain unpredictable data, and only someone who has the
correct situation is restored after keys
associated with the exchange of IKE SA and has seen the next
packets between request packet can

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properly respond to the participants. test.

4.4 Spoofing indications related to network connectivity Network Connectivity Indications

Attackers may also spoof various indications from lower layers and the
network in an effort to confuse the peers about which addresses are
or are not working. For example, attackers may spoof ICMP link-layer
error messages in an effort to cause the parties to move their
traffic elsewhere or even to disconnect. Attackers may also spoof
information related to network attachments, router discovery, and
address assignments in an effort to make the parties believe they
have Internet connectivity when in reality they do not.

This may cause use of non-preferred addresses or even denial-
of-service.

Denial-of-service denial-of-
service.

MOBIKE does not provide any protection of its own for indications
from other parts of the participants through MOBIKE

Inappropriate MOBIKE protocol mechanisms might make it possible
for attackers stack. These vulnerabilities can be
mitigated through the use of techniques specific to disconnect the participants, other parts
of the stack, such as properly dealing with ICMP errors
[ICMPAttacks], link layer security, or to move them
to non-operational addresses.

MOBIKE addresses these threats using the following countermeasures:

Payload traffic protection

The use of IPsec protection [SEND] to protect
IPv6 Router and Neighbor Discovery.

Ultimately MOBIKE depends on payload traffic protects the
participants against disclosure of the contents delivery of IKEv2 messages to
determine which paths can be used. If IKEv2 messages sent using a
particular source and destination addresses reach the traffic,

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reply is received, MOBIKE Protocol July 2005

should will usually consider the traffic end up in an incorrect destination. It is
recommended that security policies be configured in a manner
that takes into account that a single security association can
be used through different paths at different times.

Protection of MOBIKE payloads

The payloads used in MOBIKE are encrypted, integrity protected,
and replay protected. This assures that no one except the
participants can, for instance, give a control message to
change the addresses.

Note, however, that the actual IP address communicated in these
messages is in the outer IP header and not protected, just as
in IKEv2 NAT Traversal. MOBIKE adds the NAT_PREVENTION
payload, however, which can be used to prevent modifications by
outsiders. Where this payload is used, communication through
NATs and other address translators is impossible, however.
This feature is mainly intended for site-to-site VPN cases,
where the administrators may know beforehand that NATs are not
present, and thus any modification to the packet can be
considered to be an attack.

Explicit address change

MOBIKE allows only address changes that are explicitly
requested. This provides additional security beyond to what
IKEv2 NAT Traversal has, but it should be noted that the
benefits of this can only be realized when MOBIKE is used
without intervening NATs and NAT Traversal.

When NAT Traversal is supported, the peer's address may be
updated automatically to allow changes in NAT mappings. The
"continued return routability" feature, implemented by the
COOKIE2 payload, allows verification of the new address after
the change. This limits the duration of any "third party
bombing" attack by off-path (relative to the victim) attackers.

Return routability tests

This specification requires the use of return routability tests
(under certain conditions) to ensure that third party flooding
attacks are prevented. The tests are authenticated messages
that the peer has to respond to in order for the address change
to be committed to. The tests contain unpredictable data, and
can only be properly responded to by someone who has the keys
associated with the IKEv2 security association and who has seen
the request packet for the test.

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MOBIKE does not provide any protection of its own for indications
from other parts of the protocol stack. However, MOBIKE is resistant
to incorrect information from these sources in the sense that it
provides its own security for both the signaling of addressing
information as well as actual payload data transmission. Denial-of-
service vulnerabilities remain, however. Some aspects of these
vulnerabilities can be mitigated through path working;if
no reply is received even after retransmissions, MOBIKE will suspect
the use of techniques
specific to path is broken. An attacker who can consistently control the other parts
delivery or non-delivery of the stack, such as properly dealing
with ICMP errors [ICMPAttacks], link layer security, or IKEv2 messages in the use of
[SEND] to protect IPv6 Router and Neighbor Discovery. network can
thus influence which addresses actually get used.

5. IANA considerations Considerations

This document does not create any new namespaces to be maintained by
IANA, but it requires new values in namespaces that have been defined
in the IKEv2 base specification [IKEv2].

This document defines one new IKEv2 exchange, "PATH_TEST", whose
value is to be allocated from the "IKEv2 Exchange Types" namespace.
This exchange is described in Section 2.5.

This document also defines several new IKEv2 notification payloads whose values are
to be allocated from the "IKEv2 Notification Payload Types"
namespace. These notification payloads are described in Section 3.

6. Acknowledgements

This document is a collaborative effort of the entire MOBIKE WG. We
would particularly like to thank Jari Arkko, Francis Dupont, Paul
Hoffman, Tero Kivinen, and Hannes Tschofenig. This document also
incorporates ideas and text from earlier MOBIKE protocol proposals,

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including [AddrMgmt], [Kivinen], [MOPO], and [SMOBIKE], and the
MOBIKE design document [Design].

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7. References

7.1 Normative references References

[FIPS180-2]
National Institute of Standards and Technology,
"Specifications for the Secure Hash Standard", Federal
Information Processing Standard (FIPS) Publication 180-2,
August 2002.

[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-17 (work in progress),
October 2004.

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

[UDPEncap]
Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.

7.2 Informative references References

[AddrMgmt]
Dupont, F., "Address Management for IKE version 2",
draft-dupont-ikev2-addrmgmt-07 (work in progress),
May 2005.

[Aura02] Aura, T., Roe, M., and J. Arkko, "Security of Internet
Location Management", Proc. 18th Annual Computer Security
Applications Conference (ACSAC), December 2002.

[Bombing] Dupont, F., "A note about 3rd party bombing in Mobile
IPv6", draft-dupont-mipv6-3bombing-02 (work in progress),
June 2005.

[Design] Kivinen, T. and H. Tschofenig, "Design of the MOBIKE
protocol", draft-ietf-mobike-design-02 (work in progress),
February 2005.

[ICMPAttacks]
Gont, F., "ICMP attacks against TCP",
draft-gont-tcpm-icmp-attacks-03 (work in progress),
December 2004.

[Kivinen] Kivinen, T., "MOBIKE protocol",

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December 2004.

[IPsecArch]
Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work
in progress), March 2005.

[Kivinen] Kivinen, T., "MOBIKE protocol",
draft-kivinen-mobike-protocol-00 (work in progress),
February 2004.

[MIP4] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.

[MOPO] Eronen, P., "Mobility Protocol Options for IKEv2 (MOPO-
IKE)", draft-eronen-mobike-mopo-02 (work in progress),
February 2005.

[PseudoNAT]
Dupont, F. and J-J. Bernard, "Transient pseudo-NAT attacks
or how NATs are even more evil than you believed",
draft-dupont-transient-pseudonat-04 (expired) (work in
progress), June 2004.

[SEND] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.

[SMOBIKE] Eronen, P. and H. Tschofenig, "Simple Mobility and
Multihoming Extensions for IKEv2 (SMOBIKE)",
draft-eronen-mobike-simple-00 (work in progress),
March 2004.

[UNSAF] Daigle, L., "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.

Author's Address

Pasi Eronen (editor)
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland

Email: pasi.eronen@nokia.com

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Appendix A. Changelog

(This section should be removed by the RFC editor.)

Changes from -01 to -02:

o Moved MOBIKE_SUPPORTED from IKE_SA_INIT to IKE_AUTH (issues 35,
37).

o Changed terminology related to NAT prohibition (issues 22, 24).

o Rewrote much of the ADDITIONAL_*_ADDRESS text, added
NO_ADDITIONAL_ADDRESSES notification.

o Use NAT detection payloads to detect changes in NAT mappings
(issue 34).

o Removed separate PATH_TEST message (issue 34).

o Clarified processing of UNACCEPTABLE_ADDRESSES when request has
been sent using several different addresses (issue 36).

o Clarified changing of ports 500/4500 (issue 33).

o Updated security considerations (issues 27 and 28).

o No need to include COOKIE2 in non-RR messages (issue 32).

o Many editorial fixes and clarifications (issue 38, 40).

o Use the terms initiator and responder more consistently.

o Clarified that this document does not solve all problems in MOBIKE
WG charter (issue 40).

Changes from -00 to -01:

o Editorial fixes and small clarifications (issues 21, 25, 26, 29).

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o Use Protocol ID zero for notifications (issue 30).

o Separate ADDITIONAL_*_ADDRESS payloads for IPv4 and IPv6 (issue
23).

o Use the word "path" only in senses that include the route taken
(issue 29).

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Eronen Expires January March 16, 2006 [Page 22] 23]

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