MOBIKE Working Group P. Eronen, Ed.
Internet-Draft Nokia
Expires: January 16, 2006 July 15, 2005
IKEv2 Mobility and Multihoming Protocol (MOBIKE)
draft-ietf-mobike-protocol-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
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 VPN user to move
from one address to another while keeping the connection with the VPN
gateway active.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 MOBIKE protocol overview . . . . . . . . . . . . . . . . . 4
1.3 Terminology and notations . . . . . . . . . . . . . . . . 5
2. MOBIKE protocol exchanges . . . . . . . . . . . . . . . . . . 6
2.1 Signaling support for MOBIKE . . . . . . . . . . . . . . . 6
2.2 Additional addresses . . . . . . . . . . . . . . . . . . . 6
2.3 Changing addresses in IPsec SAs . . . . . . . . . . . . . 7
2.4 Updating additional addresses . . . . . . . . . . . . . . 9
2.5 Path testing . . . . . . . . . . . . . . . . . . . . . . . 10
2.6 Return routability check . . . . . . . . . . . . . . . . . 11
2.7 NAT prevention . . . . . . . . . . . . . . . . . . . . . . 11
3. Payload formats . . . . . . . . . . . . . . . . . . . . . . . 13
3.1 MOBIKE_SUPPORTED notification payload . . . . . . . . . . 13
3.2 ADDITIONAL_IP4/6_ADDRESS notification payloads . . . . . . 13
3.3 UPDATE_SA_ADDRESSES notification payload . . . . . . . . . 13
3.4 UNACCEPTABLE_ADDRESSES notification payload . . . . . . . 13
3.5 COOKIE2 notification payload . . . . . . . . . . . . . . . 14
3.6 NAT_PREVENTION notification payload . . . . . . . . . . . 14
3.7 NAT_PREVENTED notification payload . . . . . . . . . . . . 14
4. Security considerations . . . . . . . . . . . . . . . . . . . 15
5. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1 Normative references . . . . . . . . . . . . . . . . . . . 19
7.2 Informative references . . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 20
1. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . 22
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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.
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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" 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
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+").
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
2.1 Signaling 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 MOBIKE_SUPPORTED notification payload is described in Section 3.
2.2 Additional 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(ADDITIONAL_*_ADDRESS)+] -->
<-- HDR, SK { IDr, [CERT], AUTH,
[CP(CFG_REPLY)],
SAr2, TSi, TSr,
[N(ADDITIONAL_*_ADDRESS)+] }
The recipient stores this information, but no other action is taken
at this time.
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2.3 Changing 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 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 the initiator has
already decided what the new addresses should be. How this decision
is made is 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 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, 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)] } -->
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 payload is present, processes it as
described in Section 2.7.
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 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)] }
o If necessary, initiates a return routability check for the new
initiator address (see Section 2.6) and waits for the check to
finish..
o Updates the IPsec SAs 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 the response contains the NAT_PREVENTED 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.
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2.4 Updating 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 can be sent
in any INFORMATIONAL exchange request message.
When the responder (of the original IKE_SA) receives an INFORMATIONAL
request containing ADDITIONAL_*_ADDRESS payloads, it simply stores
the information, but no other action is taken.
Initiator Responder
----------- -----------
HDR, SK { N(ADDITIONAL_*_ADDRESS)+,
N(COOKIE2) } -->
<-- HDR, SK { N(COOKIE2) }
When the initiator receives an INFORMATIONAL request containing
ADDITIONAL_*_ADDRESS, it stores the information and also determines
whether the currently used addresses need to be changed (for
instance, if the currently used address is no longer included in the
list); if it does, the initiator proceeds as described in
Section 2.3.
Initiator Responder
----------- -----------
<-- HDR, SK { N(ADDITIONAL_*_ADDRESS)+,
N(COOKIE2) }
HDR, SK { N(COOKIE2) } -->
If the implementation supports window sizes greater than one, it also
has to keep track of the Message ID of the latest update it has
received, to avoid the situation where new information is overwritten
by older.
There is one additional complication: when the responder wants to
send a new additional address list, the currently used addresses may
no longer work. In this case, the responder uses the additional
address list received from the initiator, the list of its own
addresses, and, if necessary, the path testing feature (see
Section 2.5) to determine a path that works, updates the addresses in
the IKE_SA (but not IPsec SAs), and then sends 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 the peers to detect if the currently
used path has stopped working. However, if either of the peers has
several addresses, DPD alone does not indicate which of 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
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
include a COOKIE2 notification payload, and 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 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" 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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addresses in the 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 header IP addresses (and port if doing UDP
encapsulation) are taken from the IKE_SA, not the message IP header.
The NAT_PREVENTION 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 initiator MAY include a NAT_PREVENTION payload in an 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 payload. If they do not match, the responder
replies with "HDR(A,0), N(NAT_PREVENTED)" and does not create any
state.
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 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 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 payloads can also be included when changing the
addresses of IPsec SAs (see Section 2.3). TBD: add better
description.
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3. Payload formats
3.1 MOBIKE_SUPPORTED notification payload
The MOBIKE_SUPPORTED notification payload is included in the
IKE_SA_INIT messages to indicate that the implementation supports
this specification.
The Notify Message Type for MOBIKE_SUPPORTED 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.
3.2 ADDITIONAL_IP4/6_ADDRESS notification payloads
Both initiator and responder 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) and 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 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). The Protocol ID and SPI Size fields are set to zero.
There is no data associated with this Notify type.
3.4 UNACCEPTABLE_ADDRESSES 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). 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 COOKIE2 notification payload
This payload is included in all INFORMATIONAL exchange messages 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).
The data associated with this notification MUST be between 8 and 64
octets in length (inclusive), and MUST be chosen in a way that is
unpredictable to the recipient. The Notify Message Type for this
message is TBD-BY-IANA (16396..40959). The Protocol ID and SPI Size
fields are set to zero.
3.6 NAT_PREVENTION 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).
The Protocol ID and SPI Size fields are set to zero.
3.7 NAT_PREVENTED notification payload
See Section 2.7 for a description of this payload.
The Notify Message Type for NAT_PREVENTED is 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
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 by MOBIKE are:
Traffic redirection and hijacking
Insecure mobility management mechanisms may allow inappropriate
redirection of traffic. This may allow inspection of the
traffic as well as man-in-the-middle and session hijacking
attacks.
The scope of these attacks in the MOBIKE case is limited, as
all traffic is protected using IPsec. However, it should be
observed that 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 through flooding
Traffic redirection may be performed not just to gain access to
the traffic, 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 attacks
by participants can be easily dealt with. However, this
requires that strong authentication was performed in the
initial IKEv2 negotiation. 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. 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 the flooding attack exists in IKEv2 NAT
Traversal functionality [PseudoNAT]. In this variant, the
attacker has to be on the path between the participants,
changing the addresses in the packets that pass by. This
attack is possible because the addresses in the outer headers
are not protected. When the attacker leaves the path, the
correct situation is restored after the exchange of the next
packets between the participants.
Spoofing indications related to network connectivity
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 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 of the participants through MOBIKE
Inappropriate MOBIKE protocol mechanisms might make it possible
for attackers to disconnect the participants, or to move them
to non-operational addresses.
MOBIKE addresses these threats using the following countermeasures:
Payload traffic protection
The use of IPsec protection on payload traffic protects the
participants against disclosure of the contents of the traffic,
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should 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 the use of techniques
specific to the other parts of the stack, such as properly dealing
with ICMP errors [ICMPAttacks], link layer security, or the use of
[SEND] to protect IPv6 Router and Neighbor Discovery.
5. IANA 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,
including [AddrMgmt], [Kivinen], [MOPO], and [SMOBIKE], and the
MOBIKE design document [Design].
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7. References
7.1 Normative 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
[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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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
1. Changelog
(This section should be removed by the RFC editor.)
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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