Mobile IP
Internet Draft P. McCann
Document: draft-mccann-mobileip-80211fh-01.txt Lucent Technologies
Expires: April 2003 October 2002
Mobile IPv6 Fast Handovers for 802.11 Networks
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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Abstract
This document describes how a Mobile IPv6 Fast Handover [2] could be
implemented on a link layers conforming to the 802.11 suite of
specifications [3].
Conventions used in this document
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 RFC-2119 [4].
Table of Contents
1. Introduction...................................................2
2. Terminology....................................................3
3. Deployment Architectures for Mobile IPv6 on 802.11.............4
4. 802.11 Handovers in Detail.....................................5
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5. Anticipated Handover...........................................7
6. Tunnel-based Handover..........................................8
7. Security Considerations........................................9
8. Conclusions...................................................10
References.......................................................12
Acknowledgments..................................................13
Author's Address.................................................13
1. Introduction
The Mobile IPv6 Fast Handover protocol [2] has been proposed as a way
to minimize the interruption in service experienced by a Mobile IPv6
node as it changes its point of attachment to the Internet. Without
such a mechanism, a mobile node cannot send or receive packets from
the time that it disconnects from one point of attachment in one
subnet to the time it registers a new care-of address from the new
point of attachment in a new subnet. Such an interruption would be
unacceptable for real-time services such as Voice-over-IP.
Note that there may be other sources of service interruption that may
be "built-in" to the link-layer handoff. For example, a recent study
has concluded that the 802.11 beacon scanning function may take
several hundred milliseconds to complete [5] during which time
sending and receiving IP packets is not possible. This sort of
interruption may present an obstacle to real-time service deployment
that needs further optimization; however, such optimization is
outside the scope of this document.
The basic idea behind a Mobile IPv6 fast handover is to leverage
information from the link-layer technology to either predict or
rapidly respond to a handover event. This allows IP connectivity to
be restored at the new point of attachment sooner than would
otherwise be possible. By tunneling data between the old and new
access routers, it is possible to provide IP connectivity in advance
of actual Mobile IP registration with the home agent or correspondent
node. This removes such Mobile IP registration, which may require
time-consuming Internet round-trips, from the critical path before
real-time service is re-established.
The particular link-layer information available, as well as the
timing of its availability (before, during, or after a handover has
occurred), differs according to the particular link-layer technology
in use. This document gives a set of deployment examples for Mobile
IPv6 Fast Handovers on 802.11 networks. We begin with a brief
overview of relevant aspects of basic 802.11 [3]. We examine how and
when handover information might become available to the IP layers
that implement Fast Handover, both in the network infrastructure and
on the mobile node. Finally, we give details on how the proposed
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Mobile IPv6 Fast Handover protocol would work in this environment and
evaluate the feasibility of the different IP-layer fast handover
mechanisms available.
2. Terminology
This document borrows all of the terminology from Mobile IPv6 Fast
Handovers [2], with the following additional terms from the 802.11
specification [3] (some definitions slightly modified for clarity):
Access Point (AP): Any entity that has station functionality and
provides access to the distribution services, via the
wireless medium (WM) for associated stations.
Association: The service used to establish access point/station
(AP/STA) mapping and enable STA access to the
Distribution System.
Basic Service Set (BSS): A set of stations controlled by a single
coordination function, where the coordination
function may be centralized (e.g., in a single AP) or
distributed (e.g., for an ad-hoc network). The BSS
can be thought of as the coverage area of a single
AP.
Distribution System (DS): A system used to interconnect a set of
basic service sets (BSSs) and integrated local area
networks (LANs) to create an extended service set
(ESS).
Extended Service Set (ESS): A set of one or more interconnected
basic service sets (BSSs) and integrated local area
networks (LANs) that appears as a single BSS to the
logical link control layer at any station associated
with one of those BSSs. The ESS can be thought of as
the coverage area provided by a collection of APs all
interconnected by the Distribution System. It may
consist of one or more IP subnets.
Inter-Access Point Protocol (IAPP): A protocol defined for use
between access points [6] at handover time that
allows for the old association with the old AP to be
deleted, and for context to be transferred to the new
AP.
Station (STA): Any device that contains an IEEE 802.11 conformant
medium access control (MAC) and physical layer (PHY)
interface to the wireless medium (WM).
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3. Deployment Architectures for Mobile IPv6 on 802.11
In this section we describe the two most likely relationships between
Access Points (APs), Access Routers (ARs), and IP subnets that are
possible in an 802.11 network deployment. Here we consider only the
infrastructure mode [3] of 802.11. A given STA may be associated
with one and only one AP at any given point in time; when a STA moves
out of the coverage area of a given AP it must handover (re-
associate) with a new AP. It is important to understand that 802.11
offers great flexibility, and that handover from one AP to another
does not necessarily mean a change of AR or subnet.
AR AR
AR | AR AR | AR
\ | / \ | /
Subnet 1 Subnet 2
/ / | \ \ / / | \ \
/ / | \ \ / / | \ \
/ | | | \ / | | | \
AP1 AP2 AP3 AP4 AP5 AP6 AP7 AP8 AP9 AP10
Figure 1: An 802.11 deployment with relay APs.
Figure 1 depicts a typical 802.11 deployment with two IP subnets,
each with three Access Routers and five Access Points. Note that the
APs in this figure are acting as link-layer relays, which means that
they transport Ethernet-layer frames between the wireless medium and
the subnet. Each subnet is implemented as a single LAN or VLAN.
Note that a handover from AP1 to AP2 does not require a change of AR
because all three ARs are link-layer reachable from a STA connected
to any AP1-5. Therefore, such handoffs are outside the scope of IP-
layer handover mechanisms. However, a handoff from AP5 to AP6 would
require a change of AR, because the STA would be attaching to a
different subnet. An IP-layer handover mechanism would need to be
invoked in order to provide low-interruption handover between the two
ARs.
Internet
/ | \
/ | \
/ | \
AR AR AR
AP1 AP2 AP3
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Figure 2. An 802.11 deployment with integrated APs/ARs.
Figure 2 depicts an alternative 802.11 deployment where each AP is
integrated with exactly one AR. In this case, every change of AP
would result in a necessary change of AR, which would require some
IP-layer handover mechanism to provide for low-interruption handover
between the ARs. Also, the AR shares a MAC-layer identifier with its
attached AP.
In the next section, we examine the steps involved in any 802.11
handover. Subsequent sections discuss how these steps could be
integrated with an IP-layer handover mechanism in each of the above
deployment scenarios.
4. 802.11 Handovers in Detail
An 802.11 handover takes place when a STA changes its association
from one AP to another ("re-association"). This process consists of
the following steps:
1. The STA performs a scan to see what APs are available. The
result of the scan is a list of APs together with physical layer
information, such as signal strength.
2. The STA chooses one of the APs and performs a join to
synchronize its physical and MAC layer timing parameters with
the selected AP.
3. The STA requests authentication with the new AP. For an "Open
System", such authentication is a single round-trip message
exchange with null authentication.
4. The STA requests association or re-association with the new AP.
A re-association request contains the MAC-layer address of the
old AP, while a plain association request does not.
5. If operating in accordance with the IAPP [6], the new AP
performs a lookup based on MAC-layer address to obtain the IP
address of the old AP by consulting a local table or RADIUS
server. It opens a UDP or TCP connection, protected by IPSec
encryption, to the old AP. Via the secure connection, it
informs the old AP of the re-association so that information
about the STA is deleted from the old AP. Note that IAPP can
only be invoked based on a re-association message, as the plain
association message does not contain the old AP's MAC-layer
address.
6. The new AP sends a Layer 2 Update frame on the local LAN segment
to update the learning tables of any connected Ethernet bridges.
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Note that in most existing 802.11 implementations, steps 1-4 are
performed by firmware that is on-board the 802.11 PCMCIA card. This
might make it impossible for the host to take any actions (including
sending or receiving IP packets) before the handoff is complete.
During step 5, IAPP is used to communicate with the old AP. The
IPSec tunnel between the two APs is originally established with key
distribution via RADIUS, but can be subsequently re-used for
different MNs that may need to handover between the same pair of APs.
Note that the SA is between the pair of APs and has nothing to do
with any security association that might be in place between the MN
and either of the APs. During IAPP operation, link-layer context may
be transferred from the old AP to the new AP. The IAPP defines a
container for context information. However, no such context has
currently been defined or standardized by IEEE.
Also note that there is no guarantee that an AP found during step 1
will be available during step 2 because radio conditions can change
dramatically from moment to moment. The STA may then decide to
associate with a completely different AP. Usually, this decision is
implemented in firmware and the attached host would have no control
over which AP is chosen.
There is no standardized trigger for step 1. It may be performed as
a result of decaying radio conditions on the current AP or at other
times as determined by local implementation decisions. Usually this
step will be performed autonomously by firmware in the NIC using
proprietary scanning algorithms.
The coverage area of a single AP is known as a Basic Service Set
(BSS). Note that both APs in the above description are considered to
belong to the same Extended Service Set (ESS). This is to trigger a
re-association (rather than plain association) from the STA, which
contains information about both the old and new AP. All APs should
therefore broadcast the same ESSID. If two APs belong to different
administrative domains, this may require some inter-domain
coordination of the ESSID. Otherwise, there may not be sufficient
information to construct the link-layer triggers required by some
handover mechanisms.
A change of BSS within an ESS may or may not require an IP-layer
handover, depending on whether the APs provide STAs access to
different or the same IP subnets. The next two sections detail how
each mechanism from the Mobile IPv6 Fast Handover specification might
accomplish the necessary IP-layer reconfiguration. First we consider
Anticipated handover and then move on to Tunnel-based handover.
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5. Anticipated Handover
Because all 802.11 handovers are mobile initiated, the network-
initiated Anticipated Handover is not applicable to 802.11.
In mobile initiated Anticipated Handover, the MN first sends a Router
Solicitation for Proxy (RtSolPr) to the oAR containing the link-layer
address of the new Access Point. This would happen between steps 1
and 2 from Section 4. Note that for this to be possible, the MLME-
SCAN.request primitive (See Section 10.3.2.1 of the 802.11
specification [3]) must be available to the host, and the card
firmware must not make autonomous handover decisions. The oAR maps
the new AP's link-layer address into the IP address of the nAR that
should be used by the MN on the new link. Note that this requires a
mapping table to be maintained at oAR, either by manual configuration
or with the use of unspecified discovery protocols. Then, the oAR
determines whether stateful or stateless addressing is used by nAR.
For stateless addresses, the oAR picks an nCoA on the new subnet
(using the MN's interface identifier) and proposes it to the nAR
using HI/HACK. For stateful addresses, the oAR must request an
address from nAR with the HI/HACK exchange. The oAR returns a Proxy
Router Advertisement (PrRtAdv) to the MN. This PrRtAdv may be sent
in parallel with HI/HACK, in the case of stateless address
configuration, but must be serialized after HI/HACK in the case of
stateful address configuration. The MN then sends a Fast Binding
Update (F-BU) to the oAR with a binding to the new care-of address
(nCoA).
At this point the MN should move to nAR (steps 2-6 from Section 4).
Note that here we assume the host can send IP layer messages such as
F-BU prior to step 2, which implies that the interface firmware did
not autonomously skip to step 2 without permission from the host.
Once re-associated with the new AP, the MN will hopefully receive the
F-BACK indicating that the oAR received its F-BU and also that the
nCoA is valid. This message is sent on both the old and new links
because the MN is in transit between them. Packets from the oAR will
be forwarded to nAR based on the F-BU. If it doesn't receive the F-
BACK right away, the MN retransmits the F-BU and indicates its
presence to the nAR with a Fast Neighbor Advertisement (F-NA). The
nAR should return a Router Advertisement containing a Neighbor
Advertisement Acknowledgement (NAACK) indicating whether the nCoA is
valid. If not, the MN can continue to use oCoA as a source address
for packets while it obtains a valid nCoA. Either the F-BACK or the
Router Advertisement informs the MN which link-layer address to use
as its default router on subsequent outbound packets.
Note that Anticipated Handover requires that the MN send a RtSolPr
and receive a PrRtAdv prior to executing the layer-two handover.
Otherwise, the MN will not have any information about the new subnet,
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and will need to begin neighbor discovery and care-of address
configuration from scratch once it has completed the layer-two
handover. There is no guarantee that the RtSolPr/PrRtAdv exchange
will complete especially in a radio environment where the connection
to the old AP is deteriorating rapidly. Also, there is no guarantee
that the MN will actually attach to the given new AP after it has
sent the F-BU to the oAR, because changing radio conditions may cause
nAR to be suddenly unreachable. The precise impact of these factors
in an Anticipated Handover can only be evaluated after
experimentation in a particular deployment.
6. Tunnel-based Handover
In a Tunnel-based Handover, the oAR and nAR collaborate to establish
a bi-directional edge tunnel (BET) in reaction to a layer-2 handover
event. In an 802.11 network, this event would be step 4 from
Section 4 (target trigger) or perhaps step 5 at the old AP (source
trigger). If the network looks like Figure 2, where the APs are
integrated with the ARs, then the L2-TT (or L2-ST) is available at
nAR (or oAR) through some internal interface. However, if the
network is deployed like Figure 1, then some network message will
need to be sent from the new AP (or old AP) to nAR (or oAR). This
message might be the object of future standardization efforts. Note
also that there may be several ARs present on the new subnet, and the
new AP must choose one to which to deliver the trigger, which becomes
nAR. The Layer 2 Update frame sent by the new AP might be of some
assistance in constructing L2-TT; however, this message is broadcast
to all ARs on the new subnet and does not indicate which one is to be
chosen as the endpoint of the tunnel. Also, it does not contain the
MAC address of the old AP that would enable discovery of oAR.
The AR that received the trigger sends a HI message to the other AR,
who in turn responds with a HACK. Note that this requires a mapping
table to be maintained, similar to the one for Anticipated handover,
which yields the IP address of an AR given the link-layer address of
an AP. This table must be maintained manually or with the aid of
some unspecified discovery protocol. The re-association provides L2-
LD and L2-LU triggers to oAR and nAR, respectively. At this point
the BET is established and traffic is tunneled between the two ARs so
that the MN continues to receive service, using oCoA, without the
need to exchange any messages immediately before, during, or
immediately after the handoff. At some future time, the MN may
obtain an nCoA and register from the new network, perhaps using
completely standard Mobile IPv6 mechanisms to do movement detection
and registration.
Note that the MN must somehow obtain the link-layer address of nAR
before service can resume, so that it has a link-layer destination
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address for outgoing packets (default router information). In the
deployment illustrated in Figure 2, this would be exactly the AP's
MAC layer address, which can be learned from the link-layer handoff
messages. However, in the case of Figure 1, this information must be
learned through other means currently unspecified. Also note that
even in the case of Figure 2, the MN must somehow be made aware that
it is in fact operating in a Figure 2 network and not a Figure 1
network. One option might be the Candidate Access Router (CAR)
discovery protocol [7] currently being worked in the Seamoby working
group. Interestingly, this information is also available from the
PrRtAdv message, although its use is currently prohibited in tunnel-
based handover. A MN could conceivably obtain advertisements from
all neighboring APs well in advance of the handover, even if it
intended to use a Tunnel-Based instead of Anticipated handover.
Note that the BET is established at the behest of layer-2 messages.
Because this is a redirection of the MN's traffic, care must be taken
to ensure that the layer-2 messages are secure. This issue is
discussed in more detail in Section 7.
For now we do not discuss the Handoff to Third (HTT) mechanism of a
Tunnel-based handover. Its configuration and security implications
are similar to the basic scheme.
7. Security Considerations
As stated in the Mobile IPv6 fast handover specification, the
security considerations of Anticipated Handover are very similar to
those required of any Mobile IPv6 Binding Update message. The oAR
and MN are assumed to have a security association for the Binding
Updates, which also provides authentic PrRtAdv messages to the MN.
However, creating such a security association for a roaming MN is
still an open problem. Also, security must be established between
all possible (oAR, nAR) pairs so that PrRtAdv/HI/HACK messages may be
authenticated. This might be achieved through manual configuration
or automatic discovery, using whatever means was used to set up the
mapping table discussed in Section 5.
Similar to Anticipated handover, Tunnel-based handover also requires
a secure means to establish neighbor-mapping tables, so that tunnels
can be established securely between oAR and nAR based on the L2
triggers. In addition, the security of a Tunnel-based handover
depends on the link-layer security in place. This is because a BET
is established and MN traffic is redirected purely in reaction to
link-layer handoff messages. Note that step 3 from Section 4 could
potentially provide some security; however, due to the identified
weaknesses in WEP shared key security [8], there is currently no
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authentication algorithm for step 3 that is both standardized and
secure.
It may be the case that many deployments are configured as "Open
Systems", which will rely instead on higher-layer authentication such
as 802.1X Port-Based Network Access Control [9], or, ultimately, the
future output of the PANA working group [10]. According to published
standards, such authentication techniques would happen only after
association or re-association takes place, which leaves the re-
association messages unprotected. This would allow malicious nodes
to redirect traffic to a different subnet in a Tunnel-based handover
environment, or to a different AP on the same subnet even in an
Anticipated handover environment. Work is currently underway to
better integrate 802.1X with 802.11 [11] but it is not yet complete.
The 802.1X standard [9] defines a way to encapsulate EAP on 802
networks (EAPOL, for "EAP over LANs"). With this method, the client
and AP participate in an EAP exchange which itself can encapsulate
any of the various EAP authentication methods. The EAPOL exchange
can output a master key, which can then be used to derive transient
keys, which in turn can be used to encipher/authenticate subsequent
traffic. It is possible to use 802.1X pre-authentication [11]
between a STA and a target AP while the STA is associated with
another AP; this would enable authentication to be done in advance of
handover, which would both protect the re-association message and
allow fast resumption of service after roaming. However, because
EAPOL frames carry only MAC-layer instead of IP-layer addresses, this
is currently only specified to work within a single subnet, where IP
layer handoff mechanisms are not needed anyway. In our case (roaming
across subnet boundaries) the 802.1X exchange would need to be
performed after roaming to, but prior to re-association with, the new
AP. This would introduce additional handover delay while the 802.1X
exchange takes place, which may also involve round-trips to RADIUS or
Diameter servers.
Perhaps faster cross-subnet authentication could be achieved by
leveraging the context transfer features of the IAPP to carry
security credentials [12], or with the use of pre-authentication
using PANA. To our knowledge this sort of work is not currently
underway in the IEEE. The security considerations of these new
approaches would need to be carefully studied.
8. Conclusions
The Mobile IPv6 Fast Handoff specification presents two alternative
protocols for shortening the period of service interruption during a
change in link-layer point of attachment. This document has
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attempted to show how each may be applied in the context of 802.11
access networks.
There are currently serious security problems in the published
specifications that define the 802.11 handover process that must be
fixed before even intra-subnet mobility can be considered secure.
Tunnel-based handovers would depend on these mechanisms to secure
cross-subnet mobility. In-progress specifications may fix these
problems but may also introduce additional delay for handover across
different subnets. Usually, only the APs themselves are aware that
good link-layer security is in place. This information could be made
available to ARs with the use of a new protocol (e.g., [13]), but as
such mechanisms are prone to be link-layer specific, we recommend
that work on Tunnel-based handovers be progressed in the IEEE rather
than the IETF.
Anticipated handover places requirements that messages be exchanged
over the wireless link prior to handover, during a period that is
normally under the control of low-level firmware. The performance
impact of this requirement, and of the failure to meet it in certain
radio conditions, must be critically evaluated with experimental
data. Also, given a particular firmware implementation of handover,
it may be impossible for a host to send the required IP-layer
messages at the proper time.
Both schemes rely on unspecified mechanisms for mapping AP L2
addresses into AR IP addresses (Anticipated and Tunnel-based) or AR
L2 addresses (Tunnel-based). This problem is arguably more severe
with Tunnel-based handovers, especially on networks like Figure 1,
because the MN itself does the unspecified mapping and it cannot be
handled by manual configuration. In Anticipated handover, the oAR
must be configured with this information so that it can send the
proper PrRtAdv to the MN.
The relationship between the PrRtAdv and Candidate Access Router
discovery protocols needs further study. Some similar functionality
seems to be provided by each and it may not be necessary to
standardize both mechanisms independently.
For these reasons, we recommend that the draft be refocused to
concentrate on the specification and security considerations for the
F-BU and F-BACK messages only. This allows for updating the oAR with
the current MN location under any circumstance, whether the handover
is anticipated or not. The other mechanisms outlined in the draft
either need more support from the link layer, and should be moved
into the IEEE, or require further study to determine their
relationship with other work in the IETF.
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References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Dommety, G. (editor), Yegin, A., Perkins, C., Tsirtsis, G., El-
Malki, K., and Khalil, K., "Fast Handovers for Mobile IPv6",
draft-ietf-mobileip-fast-mipv6-04.txt, March 2002. Work In
Progress.
3 "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)
Specifications", ANSI/IEEE Std 802.11, 1999 Edition.
4 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
5 Mitra, A., Shin, M., and Arbaugh, W., "An Empirical Analysis of
the IEEE 802.11 MAC Layer Handoff Process", CS-TR-4395, University
of Maryland Department of Computer Science, September 2002.
6 "Recommended Practice for Multi-Vendor Access Point
Interoperability via an Inter-Access Point Protocol Across
Distribution Systems Supporting IEEE 802.11 Operation", IEEE Std
802.11f/D4, July 2002. Work In Progress.
7 Krishnamurthi, G. (editor), "Requirements for CAR Discovery
Protocols", draft-ietf-seamoby-card-requirements-01.txt, August,
2002. Work In Progress.
8 Borisov, N., Goldberg, I., and Wagner, D., "Intercepting Mobile
Communications: The Insecurity of 802.11", Proceedings of the
Seventh Annual International Conference on Mobile Computing and
Networking, July 2001, pp. 180-188.
9 "Port-Based Network Access Control", IEEE Std 802.1X-2001,
October, 2001.
10 Penno, R. (editor), Yegin, A., Ohba, Y., Tsirtsis, G, and Wang,
C., "Protocol for Carrying Authentication for Network Access
(PANA) Requriements and Terminology", draft-ietf-pana-
requirements-02.txt, June 2002. Work In Progress.
11 "Draft Supplement to IEEE 802.11: Specification for Enhanced
Security", IEEE Std 802.11i/D2.2, July 2002. Work In Progress.
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12 Aboba, B., and Moore, T., "A Model for Context Transfer in IEEE
802", draft-aboba-802-context-02.txt, April, 2002. Work In
Progress.
13 Yegin, A., "Link-layer Triggers Protocol", draft-yegin-l2-
triggers-00.txt, June 2002. Work In Progress.
Acknowledgments
Thanks to Bob O'Hara for providing explanation and insight on the
802.11 standards. Thanks to James Kempf and Erik Anderlind for
providing comments on an earlier draft.
Author's Address
Pete McCann
Lucent Technologies
Rm 9C-226R
1960 Lucent Lane
Naperville, IL 60563
Phone: +1 630 713 9359
Fax: +1 630 713 1921
Email: mccap@lucent.com
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McCann Expires - April 2003 [Page 14]