IKEv2 Mobility and Multihoming T. Kivinen
(mobike) Safenet, Inc.
Internet-Draft June 24, 2004
Expires: December 23, 2004
Design of the MOBIKE protocol
draft-ietf-mobike-design-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document discusses the potential design decisions in the base
MOBIKE (IKEv2 Mobility and Multihoming) protocol. It also tries to
provide some background information about different choices and tries
to record the decisions made by the working group, so that we do not
need to repeat discussion later.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Roaming Laptop Scenario . . . . . . . . . . . . . . . . . 3
1.2 Multihoming SGW Scenario . . . . . . . . . . . . . . . . . 4
2. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Adopting a new address / multihoming support . . . . . . . . . 6
3.1 IP-address list or one IP-address . . . . . . . . . . . . 6
3.2 Indirect or direct indication (issue #1) . . . . . . . . . 7
3.3 Dead peer detection and IKEv2 (issue #11) . . . . . . . . 7
4. Simultaneous Movements (issue #2) . . . . . . . . . . . . . . 9
5. Interaction with NAT-T (issue #3) . . . . . . . . . . . . . . 10
6. Changing addresses or changing the paths (issue #10, #14) . . 11
7. How and When to do Return Routability Checks (issue #6,
#12, #15) . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Scope of SA changes (issue #8) . . . . . . . . . . . . . . . . 14
9. Zero Address Set (issue #5) . . . . . . . . . . . . . . . . . 15
10. What modes we support (issue #7) . . . . . . . . . . . . . . 16
11. Message representation . . . . . . . . . . . . . . . . . . . 17
12. Security Considerations . . . . . . . . . . . . . . . . . . 19
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . 20
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
14.1 Normative references . . . . . . . . . . . . . . . . . . . . 21
14.2 Non-normative references . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . 22
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1. Introduction
The current IKEv2 and IPsec documents explicitly say that the IPsec
and IKE SAs created implicitly between the IP-addresses used in the
IKEv2 SA. This means that there is only one IP-address pair attached
for the IKEv2 SA, and the only one IP-address pair used as a gateway
endpoint address for tunnel mode IPsec SAs. Also after the SA is
created there is no way to change those addresses.
There are scenarios which require that the IP address might change
rapidly. In some cases the problem could be solved by rekeying all
the IPsec and IKE SAs after the IP-address has changed. In some
scenarios this might be problematic, as the device might be too slow
to rekey the SAs that often, and other scenarios the rekeying and
required IKEv2 authentication might require user interaction (SecurID
cards etc). Due to these reasons, a mechanism to update the
IP-addresses tied to the IPsec and IKEv2 SAs is needed.
The charter of the MOBIKE working group requires IKEv2, and as IKEv2
assumes that the RFC2401bis architecture is used, all protocols
developed will use both IKEv2 and RFC2401bis (issue #9). No effort is
to be made to make protocols for IKEv1 or old RFC2401 architecture.
MOBIKE protocol provides solution to the problem of the updating the
IP-addresses. The MOBIKE protocol should take care following:
o Notifying the other end of IP-address(es) change
o Update the IKE SA endpoint addresses based on the notifications
o Switching to use new IP-address if old one does not work anymore
o Updating the tunnel mode IPsec SA tunnel endpoint addresses
o Ensuring that the given new addresses belong to the peer
The MOBIKE protocol can be used in different scenarios. Two such
scenarios are discussed below.
1.1 Roaming Laptop Scenario
In the roaming laptop scenario the device that moves around is
laptop, which might have several ways to connect to internet. It
might for example have fixed ethernet, WLAN and GPRS access to net,
and some of those can be used in different times. It tries to use the
most efficient connection it has all the time, but that connection
might change. For example user can disconnect himself from the fixed
ethernet, and then use the office WLAN, and then later leave the
office and start using GPRS during the trip to home. In home he might
again use again WLAN (but with different IP-addresses) etc.
The device does not use Mobile IP or anything similar, it simply
wants to keep the VPN connection to the corporate security gateway
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(SGW) up and running all the time. Even if the interface or the
IP-addresses change, the internal addresses used inside the IPsec
tunnel remains same (allocated from the SGW), i.e. the applications
might not detect the changes at all.
1.2 Multihoming SGW Scenario
Another possible scenario which might use MOBIKE is the SGW of the
other end of the roaming laptop scenario. The SGW might have multiple
interfaces to different ISPs, and wants to provide connection even
when some of those connections are broken. One of the interface might
also be the WLAN access point in the office. The SGW will know
beforehand what set of IP-addresses it will use, but it might need to
dynamically send update notifications the clients to tell them which
addresses to use. It might also use this to do some sort of load
balancing, i.e. giving different clients different preferred address,
to utilize all the connections. This kind of load balancing is
completely internal to the SGW (i.e. the clients will simply see that
the preferred IP-address to be used for tunnel endpoint changes, but
they do not know why or how the SGW decided to do that), and the
actual algorithms how to do that is outside the scope of MOBIKE
protocol (i.e. the whole issues is that MOBIKE does not disallow the
SGW to give different sets of IP-addresses in different preference
order to different clients).
Note, that the load-balancing inside the one IKE SA (i.e. one client)
is not handled in the MOBIKE protocol. Each client uses only one of
the IP-addresses given by the SGW at one time.
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2. Issues
The base protocol needs to perform the following things:
o Ability to inform the peer about the current or changed
address(es) of the sender
o Ability to inform the peer about the preferred address
o Ability to detect an outage situation and fall back to the use of
another address
o Ability to prevent flooding attacks based on announcing someone
else's address
o Ability to affect both the IKE and IPsec SAs
One of the key issues affecting the MOBIKE protocol is, whether
MOBIKE protocol needs to recover from the case where packets simply
dont get through. If the node can locally detect some problems with
the interfaces (IP-address change, interface disappearing, link going
down), it can act based on that and fix the situation. If the packets
are simply disappearing somewhere in the net, the detection of the
problem requires noticing that we cannot get packets through. If the
protocol only need to fix problems appearing in the local interfaces,
then the protocol is much simpler.
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3. Adopting a new address / multihoming support
From the MOBIKE's point of view the multihoming support is the set of
rules how and when to change to use new IP-address for the other end.
3.1 IP-address list or one IP-address
One option is that the other end can provide a list of addresses
which can be used as destination addresses, and the local end needs
to decide which of them to use. The MOBIKE does not include
load-balancing, i.e. the local end only uses one IP-address at time,
and it only changes to use new IP-address after some kind of
indication.
Another option is to only communicate one address for each end, and
both ends only use that address when communicating. When the
something changes, the end whose situation changes, sends update
notification to the other end, changing that one address.
If the other end provides the full list of possible IP-addresses,
then the other end can recover from the movements on its own, meaning
that when it detects it cannot get packets through it can try another
IP-address. If the other end only provides one IP-address to be used,
then the other end has to wait for the new IP-address before the
situation is fixed. The good thing about only one IP-address for the
remote host is that it makes retransmission easy, and it also makes
it clear which end should do the recovery (i.e. the end, whose
IP-address changed, MUST start recovery process and send the new
IP-address to the other end).
The one IP-address approach will not work if both ends happen to
loose their IP-address at the same time (routing problems, which
causes the one link between the hosts to go down, thus either end
cannot get recovery packets through as the link is down). It also may
cause the requirement for the IKEv2 window size larger than 1,
especially if only direct indications are used. This is because the
host needs to be able to send the IP-address change notifications
before it can switch to another address, and depending on the return
routability checks, retransmissions policies etc, it might be hard to
make the protocol such that it works with window size of 1 too (issue
#11). Also one IP-address approach does not really benefit much from
the indirect indications as the end getting those indirect
indications cannot often fix the situation by itself (i.e. even if
the host gets ICMP host unreachable for the old IP-address, it cannot
try other IP-addresses, as it does not know them).
The problems with IP-address list are mostly in its complexity.
Notification and recovery processes are more complex, as both end can
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recover from the IP-address changes. There is also possibilities that
both ends tries to recover at the same time and this must be taken
care in the protocol.
3.2 Indirect or direct indication (issue #1)
The indication that the situation regarding the IP-address has
changed might be either direct or indirect. The direct indication
means that the other end will send specific indication that now
something changed. The indirect indication is something which can be
observed from infrastructure or lack of packets, not directly from
the other end.
The direct indication can be for example the other end IKEv2 sending
authenticated address update notification, which have different
IP-address(es) than used earlier.
The indirect indication can be many things. One example might be that
the local end notices that suddenly the other ends start using
different source address for the packets than what it used before, or
ICMP message or routing information change.
Another type of indirect information might that there has been no
traffic from the other end for some time (i.e. the current connection
might be broken).
This kind of indirect information should not directly cause any
changes to the IP-addresses, but they should be used as indication
that there might be need to do dead-peer-detection for the currently
used address. I.e. when the local end detects that the other end
started to use different source IP-address than which was used
before, it should initiate dead-peer-detection for the address
currently in use. If that dead-peer-detection tells that the
connection is alive, then there is no need to do anything. If local
end does not receive any reply to the dead-peer-detection, then it
should do dead-peer-detection for the other addresses in the list (if
available, in the preferred order). If it can find an address which
works, it will switch to that.
3.3 Dead peer detection and IKEv2 (issue #11)
The IKEv2 dead-peer-detection is done by sending empty informational
exchange packet to the other end, in which case the other end will
acknowledge that. If no acknowledge is received after certain timeout
(and after couple of retransmissions), the local end should try other
IP-addresses (if available). The packets to other IP-addresses should
use the same message-id as the original dead-peer-detection (i.e.
they are simply retransmissions of the dead-peer-detection packet
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using different destination IP-address). If different message-id is
used that violates the IKEv2 constraints on the mandatory ACK for
each message-id, causing the IKEv2 SA to be teared down.
If the local end does not receive acknowledge message back from any
of the IP-addresses, it should mark the IKE SA dead, and delete it
(as mandated by the IKEv2 specification).
Note, that as IKEv2 implementations might have window size of 1, it
means that while we are doing some other exchange, we cannot initiate
dead-peer-detection. This means that all other exchanges should also
receive identical retransmission policy than what is used for the
dead-peer-detection (issue #11).
The dead-peer-detection for the other IP-addresses can also be done
simultaneously, meaning that after the initial timeout of the
preferred address expires, we send packets simultaneously to all
other IP-addresses. The problem here is that we need to distinguish
from the acknowledge packets which IP-address actually works now
(i.e. we will check the acknowledge packets source IP-address, as it
should match the destination IP we sent out).
Also the other end is most likely going to reply only to the first
packet it receives, and that first packet might not be the most
preferred IP-address. The reason the other end is only responding to
the first packet it receives, is that implementations should not send
retransmissions if they have just sent out identical retransmissions.
This is to protect the packet multiplication problem, which can
happen if some node in the network queues up packets and then send
them to the destination. If destination will reply to all of them
then the other end will again see multiple packets, and will reply to
all of them etc.
The protocol should also be nice to the network, meaning, that when
some core router link goes down, and all those MOBIKE clients notice
that, they should not start sending lots of messages while trying to
recover from the problem. This might be especially bad if this
happens because packets are dropped because of the congested network.
If MOBIKE clients will try simultaneously test all IP-addresses
sending lots of packets to the net, because they lost one packet
because of the congestion, it simply make problem worse.
Also note, that IKE dead-peer-detection is not sufficient for the
return routability check. See Section 7 for more information.
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4. Simultaneous Movements (issue #2)
We do not need to solve the simultaneous movement recovery problem,
as we are not creating full mobility solution (charter forbids that),
but are instead concentrating on the VPN style scenarios. In the
scenarios we assume that the one end (SGW) will have fixed set of
addresses (from which some subset might be in use), thus it cannot
move to the address not known by the other end. This means that the
solutions how to recover from cases where both ends move and the
movement notifications do not reach other ends, is outside the scope
of the MOBIKE WG.
Note, that if we use only one address per each end, instead of
address list, we might end up in the case where it seems that both
ends changed their addresses at the same time. This is something that
the protocol must take care of.
There is three different cases here:
Two mobile nodes getting a new address at the same time, and then
being unable to tell each other where they are. This problem is
called the rendevouz problem, and is traditionally solved using
home agents (Mobile IPv6) or forwarding agents (Host Identity
Protocol). Essentially, solving this problem requires the
existence of a stable infrastructure node somewhere. Example:
roaming laptop to another roaming laptop, no SGW involved.
Simultaneous changes to addresses such that at least one of the
new addresses was known by both peers before the change occurred.
The primary problem in first case was not knowing the new
addresses beforehand. Here we know the address so there is no
problem. Example 1: two SGWs failover to another path. Example 2:
roaming laptop gets a new address at the same time as its SGW's
primary interface goes down.
No simultaneous changes at all.
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5. Interaction with NAT-T (issue #3)
In some way the MOBIKE and NAT-T are not compatible. The NAT-T tries
to work regardless of the IP-addresses, i.e. regardless whether
someone modifies its IP-address or not. One of the goals in the
MOBIKE is to AUTHENTICATE the change of the IP-address, i.e. when the
IP-address changes we want to verify that this change is actually
legitimate change done by the other end, not something done by the
attacker along the path.
There is no way to distinguish the cases where there is NAT along the
path which modifies the packets or if it is an attacker doing that.
If NAT is detected in the IKE SA creation, that should automatically
disable the MOBIKE extensions and use NAT-T.
If MOBIKE is enabled for the IKE SA (i.e. no NAT along the path when
the IKE SA was created), then if NAT is later added then MOBIKE can
detect that, but it cannot securely do anything for the issue. We can
disable MOBIKE extensions completely at that time and move to use
NAT-T, but as we loose all the security offered by the MOBIKE, it
might be better to rekey the IKE SA (if policy allows that) so that
we do not use MOBIKE at all and start using normal NAT-T.
If we start using NAT-T, then there is no defined way to detect that
we moved away from the inside of the NAT. Thus we need to modify
NAT-T and add that kind of detection capability there, if we want to
start using MOBIKE at that point.
As a summary, if the policy accepts the risks caused by enabling
NAT-T, then it can switch to NAT-T when it detects we are behind NAT.
Easiest way to do it is to create new IKE SA, as NAT-T can only be
enabled by the initial IKE SA creation, and it cannot be enabled by
rekeys. Moving back from NAT-T to MOBIKE is harder as it requires
changes to NAT-T. On the other hand keeping NAT-T enabled simply adds
few bytes of extra overhead.
If we define some additions and extensions to NAT-T we can probably
make it work better with MOBIKE, but there are quite a lot open
issues. One way of seeing this is, that we have few other parameters
we might want to turn on during the address update, i.e. the NAT-T
parameters. Those include turning on or off keepalives, UDP
encapsulation, or automatic address updating.
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6. Changing addresses or changing the paths (issue #10, #14)
The question here is, if it is enough for the MOBIKE to detect the
dead-address, or do it need to detect also dead-paths. Dead-address
detection means that we only detect that we cannot get packets
through to that remote address by using the local IP-address given by
the local IP-stack (i.e. local address selected normally by the
routing information). Dead-path detection means that we need to try
all possible local interfaces/IP-addresses for each remote addresses,
i.e. find all possible paths between the hosts and try them all to
see which of them work (or at least find one working path).
Doing the dead-address detection is simpler, and there is less probe
packets to be sent, thus it does not cause that much stress to the
network. It also is enough for the scenarios where the connection
problems are local (i.e. interfaces going down, WLAN access
disappearing etc). It does not help if some router somewhere along
the path breaks down, in which case rerouting the packets along
another path might get around that broken router. The question is,
whether rerouting around that problem inside the core network is a
problem that MOBIKE needs to solve.
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7. How and When to do Return Routability Checks (issue #6, #12, #15)
One of the decisions that needs to be done is when to do return
routability checks. The simple approach is to do it always. Another
option is to do it every time new IP-address is taken in to use. The
basic format of the return routability check could be similar than
dead-peer-detection, but the problem is that if that fails then the
IKEv2 specification requires the IKE SA to be deleted. Because of
this we might need to do some kind of other exchange.
If the other end is SGW with limited set of fixed IP-addresses, then
the SGW may have a certificate having all the IP-addresses in the
certificate. If the certificate includes all the IP-addresses, it is
no point to do weaker return routability check, the data in the
certificate is already properly authenticated after the IKE SA is
created, so the peer might simply use that and ignore return
routability checks for the addresses of the SGW.
Another option is to use draft-dupont-mipv6-3bombing
[I.D.dupont-mipv6-3bombing] approach: do it only if you had to send
the update from some other address than indicated preferred address.
Final option would simply not to do return routability checks at all.
If we use indirect change notifications then we only move to the new
IP address after successful dead-peer-detection on the new address,
which is already return routability check. In the direct change
notifications the authenticated peer have given out authenticated
IP-address, thus we could simply trust the other end. There is no way
external attacker can cause any attacks, but we are not protected by
the internal attacker, i.e. the authenticated peer forwarding its
traffic to the new address. On the other hand we do know the identity
of the peer in that case.
There are some attacks which can be launched unless the return
routability checks include some kind of nonce (issue #15). In this
attack the valid end points sends address update notification for the
third party, trying to get all the traffic to be sent there, causing
denial of service attack. If the return routability checks does not
contain any cookies or other random information not known by the
other end, then that valid node could reply to the return routability
checks even when it cannot see the request. This might cause the
other end to turn the traffic to there, even when the real original
recipient isn't at that address.
The IKEv2 NAT-T does not do any return routability checks. It simply
uses the last address used by the other end, as and address where to
send return packets back. The attacker can change those IP-addresses,
and can cause the return packets to be sent to wrong IP-address. The
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situation is fixed immediately when the attacker no longer changes
the packets, and the first packet with real IP-address reaches the
other end. In IKEv2 NAT-T the valid client can cause third party
bombing by redirecting all the traffic pointed to him to third party.
As the MOBIKE tries to provide better security than IKEv2 NAT-T the
MOBIKE protocol should protect against those attacks.
There might be also return routability information available from the
other parts of the system too (IP-stack, Mobile IP or so), but the
checks done might be different (issue #12). There are multiple
different levels for the return routability checks:
o None, no tests
o A party willing to answer is on the path to the claimed address.
This is the basic form of return routability test.
o There is an answer from the tested address, and that answer was
authenticated (including the address) to be from our peer.
o There was an authenticated answer from the peer, but it is not
guaranteed to be from the tested address or path to it (because
the peer can construct a response without seeing the request).
The basic return routability checks do not protect against 3rd party
bombing, if the attacker is along the path, as the attacker can
forward the return routability checks to the real peer (even if those
packets are cryptographically authenticated)
If the address to be tested is inside the packet too, then attacker
cannot forward packets, thus it prevents 3rd party bombings.
If the reply packet can be constructed without seeing the request
packet (i.e. there is no nonce or similar), then the real peer can
cause 3rd party bombing, by replying to those requests without seeing
them at all.
Other levels might only return information saying that yes, there is
someone there in the IP-address which replied to my request. Or say
that I sent request to IP-address and got reply back, but I am not
sure whether that reply was freshly generated or repeated, or sent
from different address. The MOBIKE requirements for the return
routability checks could be to verify that there is same
(cryptographically) authenticated node in the other end and it can
now receive packets from the IP-address it claims to have.
MOBIKE might also want to export the information it has done the
return routability checks to the other modules, like Mobile IP, so
Mobile IP does not need to do return routability checks again, if it
is satisfied with the level of checks done by the MOBIKE.
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8. Scope of SA changes (issue #8)
The basic question is that how the IPsec SAs are changed to use new
address. One option is that when the IKE SA address is changed then
automatically all IPsec SAs associated with it are moved along with
it to new address. Another option is to have separate exchange to
move the IPsec SAs separately.
If we want to update each IPsec SA separately, we probably need more
efficient format than notification payload, as it can only store one
SPI per payload. I.e. we want separate payload which have list of
IPsec SA SPIs and new address set for them. If we have lots of IPsec
SAs, those payloads can be quite large unless we support ranges in
SPIs or at least have some kind of notation of move those SAs not
moved separately (i.e. rest of the SAs indication). The
implementations need also keep state of IP-addresses per IPsec SA,
not per IKE SA. If we automatically move all IPsec SAs when the IKE
SA moves, then we only need to keep track which IKE SA was used to
create the IPsec SA, and fetch the IP-addresses from that (Note, that
IKEv2 [I-D.ietf-ipsec-ikev2] already requires implementations to keep
track which IPsec SAs are created using which IKE SA).
If we do allow each IPsec SAs address sets to be updated separately,
then we can support scenarios, where the machine have fast and/or
cheap connection and slow and/or expensive connection, and it wants
to allow moving some of the SAs to the slower and/or more expensive
connection, and forbid some SAs to move. I.e. never move the news
video stream from the WLAN to the GPRS link.
On the other hand, even if we tie the IKE SA update to the IPsec SA
update, then we can create separate IKE SAs for this scenario, i.e.
we create one IKE SA which have both links as endpoints, and it is
used for important traffic, and then we create another IKE SA which
have only the fast and/or cheap connection, which is then used for
that kind of bulk traffic.
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9. Zero Address Set (issue #5)
One of the features which might be useful would be for the peer to
announce the other end that it will now disconnect for some time,
i.e. it will not be reachable at all. For instance, a laptop might go
to suspend mode. In this case the peer could send address
notification with zero new addresses, which means that it will not
have any valid addresses anymore. The responder of that kind of
notification would then acknowledge that, and could then temporarily
disable all SAs. If any of the SAs gets any packets they are simply
dropped. This could also include some kind of ACK spoofing to keep
the TCP/IP sessions alive (or simply set the TCP/IP keepalives and
timeouts large enough not to cause problems), or it could simply be
left to the applications, i.e. allow TCP/IP sessions to notice the
link is broken.
The local policy could then decide how long the peer would allow
other peers to be disconnected, i.e. whether this is only allowed for
few minutes, or do they allow users to disconnect Friday evening and
reconnect Monday morning (consuming resources during weekend too, but
on the other hand not more than is normally used during week days,
but we do not need lots of extra resources on the Monday morning to
support all those people connecting back to network).
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10. What modes we support (issue #7)
The charter mostly talks about VPN style usage, and all scenarios are
using tunnel mode, so that is where this document mostly
concentrates. For transport mode to be used we first need to define
the scenarios where it is needed. XXX someone needs to write more
text here.
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11. Message representation
One of the basic design choices that is needed for the MOBIKE is the
format of the messages. The IKEv2 offers some formatting alternatives
for the protocol. The basic IP-address change notifications can be
sent either via informational exchange already specified in the
IKEv2, or we could also have our own MOBIKE specific exchange. Using
informational exchange has the main advantage that it is already
specified in the IKEv2 and the implementations should already have
code for those.
One advantage of creation of the new exchange would be that we could
incorporate the return routability checks to the exchange in this
state (i.e. create 3-4 packet exchange). The problem here is that we
might need to do the return routability checks for each IP-address
separately, thus we might not be able to do it in this phase.
Another question is the basic format of the address update
notifications. The address update notifications can include multiple
addresses, which some can be IPv4 and some IPv6 addresses. The number
of addresses is most likely going to be quite small (less than 10).
The format might need to give out senders preference of the use of
the addresses, i.e. the sender will tell this is the preferred
address to be used. The format could either contain the preference
number, giving out the relative order of the addresses, or it could
simply be ordered list of IP-addresses in the order of the most
preferred first. Benefits of the ordered list is, that then we do not
need to define what happens if the preference numbers are identical,
and we do not need to reserve space for the numbers. Normally we do
not need any priority values, we simply need an ordered list.
Even when the load-balancing inside the one connection is outside the
scope of the MOBIKE, there might be future work to include that. The
format selected needs to be flexible enough to allow addition of some
kind of extra information for the load-balancing features in the
future. This might be something like one reserved field, which can
later be used to store that information. As there is other potential
information which might have to be tied to the address in the future,
a reserved field seems like a prudent design in any case.
There are two basic formats for putting IP-address list in to the
exchange, we can include each IP-address as separate payload (where
the payload order indicates the preference value, or the payload
itself might include the preference value), or we can put the
IP-address list as one payload to the exchange, and that one payload
will then have internal format which includes the list of
IP-addresses.
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Having multiple payloads each having one IP-address makes the
protocol probably easier to parse, as we can already use the normal
IKEv2 payload parsing procedures to get the list out. It also offers
easy way for the extensions, as the payload probably contains only
the type of the IP-address (or the type is encoded to the payload
type), and the IP-address itself, and as each payload already has
length associated to it, we can detect if there is any extra data
after the IP-address. Some implementations might have problems
parsing too long list of IKEv2 payloads, but if the sender sends them
in the most preferred first, the other end can simply only take n
first addresses and use them. It might loose connection in some cases
if all the n first address are not in use anymore, and the other end
hasn't sent new list, but in most cases everything will still work.
Having all IP-addresses in one big payload having MOBIKE specified
internal format, provides more compact encoding, and keeps the MOBIKE
implementation more concentrated to one module.
The next choice is which type of payloads to use. IKEv2 already
specifies a notify payload, which could be used for that. It includes
some extra fields (SPI size, SPI, protocol etc), which gives 4 bytes
of the extra overhead, and there is the notify data field, which
could include the MOBIKE specific data.
Another option would be to have our own payload type, which then
include the information needed for the MOBIKE protocol.
The basic protocol is most likely going to be something where we send
list of all IP-addresses every time the list changes (either
addresses are added, removed, or the preferred order changes).
Another option is that we send some kind of incremental updates to
the IP-address list. Sending incremental updates provides more
compact packets (meaning we can support more IP-addresses), but on
the other hand have more problems in the synchronization and packet
reordering cases i.e. the incremental updates must be processed in
order, but for full updates we can simply use the most recent one,
and ignore old ones, even if they arrive after the most recent one
(IKEv2 packets have message id which is incremented for each packet,
thus we know the sending order easily).
The address update notification protocol is not restricted to only
one way, i.e. both ends might have multiple IP-addresses and both
ends might send address updates. Example of that is when the roaming
laptop connects to the multihoming SGW.
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12. Security Considerations
As all the messages are already authenticated by the IKEv2 there is
no problem that any attackers would modify the actual contents of the
packets. The IP addresses in IP header of the packets are not
authenticated, thus the protocol defined must take care that they are
only used as an indication that something might be different, they
should not cause any direct actions.
Attacker can also spoof ICMP error messages in an effort to confuse
the peers about which addresses are not working. At worst this causes
denial-of-service and/or the use of non-preferred addresses, so it is
not that serious.
One type of attacks which needs to be taken care of the MOBIKE
protocol is also various flooding attacks. See
[I-D.nikander-mobileip-v6-ro-sec] and [Aur02] for more information
about flooding attacks.
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13. IANA Considerations
No IANA assignments are needed.
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14. References
14.1 Normative references
[I-D.ietf-ipsec-ikev2]
Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-14 (work in progress), June 2004.
[Kiv04] Kivinen, T., "MOBIKE protocol",
draft-kivinen-mobike-protocol-00 (work in progress),
February 2004.
14.2 Non-normative references
[I-D.nikander-mobileip-v6-ro-sec]
Nikander, P., "Mobile IP version 6 Route Optimization
Security Design Background",
draft-nikander-mobileip-v6-ro-sec-02 (work in progress),
December 2003.
[I-D.dupont-mipv6-3bombing]
Dupont, F., "A note about 3rd party bombing in Mobile
IPv6", draft-dupont-mipv6-3bombing-00 (work in progress),
February 2004.
[Aur02] Aura, T., Roe, M. and J. Arkko, "Security of Internet
Location Management", In Proc. 18th Annual Computer
Security Applications Conference, pages 78-87, Las Vegas,
NV USA, December 2002.
Author's Address
Tero Kivinen
Safenet, Inc.
Fredrikinkatu 47
HELSINKI FIN-00100
FI
EMail: kivinen@safenet-inc.com
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