Internet Draft P. McCann
Document: draft-ietf-mipshop-80211fh-02.txt Lucent Technologies
Expires: April 2005 October 2004
Mobile IPv6 Fast Handovers for 802.11 Networks
Status of this Memo
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Abstract
This document describes how a Mobile IPv6 Fast Handover could be
implemented on link layers conforming to the 802.11 suite of
specifications.
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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 [1].
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
5. FMIPv6 Message Exchanges.......................................7
6. Beacon Scanning and NAR Discovery..............................7
7. Scenarios......................................................9
7.1 Scenario 1abcdef23456g.....................................9
7.2 Scenario ab123456cdefg....................................10
7.3 Scenario 123456abcdefg....................................10
8. Security Considerations.......................................10
9. Conclusions...................................................11
10. References...................................................12
11. Acknowledgments..............................................13
12. 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.
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.
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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 trace the protocol steps for Mobile
IPv6 Fast Handover in this environment.
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 [7] at handover time that
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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).
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. Note that, as with any layer-2 technology,
handover from one layer-2 point of attachment (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 would not require IP-layer
mobility management, although some IP-layer signaling may be required
to determine that connectivity to the existing AR is still available.
However, a handover 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.
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Internet
/ | \
/ | \
/ | \
AR AR AR
AP1 AP2 AP3
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.
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5. If operating in accordance with the IAPP [7], the new AP may
contact the old AP to transfer some information about the
session and clean up the state at the old AP. As of this
writing, the IAPP is not widely implemented or used.
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.
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.
Note that in some existing 802.11 implementations, steps 1-4 are
performed by firmware. This might make it impossible for the host to
take any actions (including sending or receiving IP packets) before
the handover is complete. In other 802.11 implementations, it is
possible to invoke the scan (step 1) and join (step 2) operations
independently under host control. This would make it possible to,
e.g., perform step 1 far in advance of the handover and perhaps in
advance of any real-time traffic. This could substantially reduce
the handover latency, as one study has concluded that the 802.11
beacon scanning function may take several hundred milliseconds to
complete [8] during which time sending and receiving IP packets is
not possible. However, scanning too far in advance may make the
information out-of-date by the time of handover, which would cause
the subsequent join operation to fail if radio conditions have
changed so much in the interim that the target AP is no longer
reachable. So, a host may choose to do scanning based on, among
other considerations, the age of the previously scanned information.
In general, performing such subsequent scans is a policy issue that a
given implementation of FMIPv6 over 802.11 must consider carefully.
Even if steps 1 and 2 are performed in rapid succession, 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. Often, this decision is implemented in firmware and
the attached host would have no control over which AP is chosen.
However, tools such as the host AP driver [10] offer full control
over when and to which AP the host needs to associate. Operation as
an Independent BSS (IBSS) or "ad-hoc mode" [3] may also permit the
necessary control. Implementers can make use of such tools to obtain
the best combination of flexibility and performance.
The coverage area of a single AP is known as a Basic Service Set
(BSS). An Extended Service Set (ESS) is formed from a collection of
APs that all broadcast the same ESSID. Note that a STA would only
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send a re-association (which includes both the old and new AP
addresses) if the ESSID of the old and new APs are the same.
A change of BSS within an ESS may or may not require an IP-layer
handover, depending on whether the APs are connected to different or
the same IP subnets. If an IP-layer handover is required, then
FMIPv6 can decrease the overall latency of the handover. The main
goal of this document is to describe the most reasonable scenarios
for how the events of an 802.11 handover may interleave with the
message exchanges in FMIPv6.
5. FMIPv6 Message Exchanges
An FMIPv6 handover nominally consists of the following messages:
a. The MN sends a Router Solicitation for Proxy (RtSolPr) to find
out about neighboring ARs.
b. The MN receives a Proxy Router Advertisement (PrRtAdv)
containing one or more [AP-ID, AR-Info] tuples.
c. The MN sends a Fast Binding Update (FBU) to the Previous Access
Router (PAR).
d. The PAR sends a Handover Initiate (HI) message to the New
Access Router (NAR).
e. The NAR sends a Handover Acknowledge (HAck) message to the PAR.
f. The PAR sends a Fast Binding Acknowledgement (FBack) message to
the MS the new link. The FBack is also optionally sent on the
previous link if the FBU was sent from there.
g. The MN sends Fast Neighbor Advertisement (FNA) to the NAR after
attaching to it.
The MN may connect to NAR prior to sending the FBU if the handover is
un-anticipated. In this case, the FNA (step g) would contain the FBU
(listed as step c above) and then steps d, e, and f would take place
from there.
6. Beacon Scanning and NAR Discovery
The RtSolPr message is used to request information about the
router(s) connected to one or more APs. The APs are specified by
link layer address in the RtSolPr and associated IP-layer information
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is returned in the PrRtAdv. In the case of an 802.11 link, the link
layer address is the BSSID of some AP.
Beacon scanning (step 1 from Section 4) produces a list of available
APs along with signal strength information for each. This list would
supply the necessary BSSIDs to fill into the RtSolPr messages. To
obtain this list the host needs to invoke the MLME-SCAN.request
primitive (See Section 10.3.2.1 of the 802.11 specification [3]).
Because beacon scanning takes on the order of a few hundred
milliseconds to complete, and because it is generally not possible to
send and receive IP packets during this time, the MN needs to
schedule these events with care so that they do not disrupt ongoing
real-time services. For example, the scan could be performed at the
time the MN attaches to the network prior to any real-time traffic.
However, if the interval between scanning and handover is too long,
the neighbor list may be out of date. For example, the signal
strengths of neighboring APs may have dramatically changed, and a
handover directed to the apparently best AP from the old list may
fail. If the handover is executed in firmware, the STA may even
choose a new target AP that is entirely missing from the old list
(after performing its own scan). Both cases would limit the ability
of the MN to choose the correct NAR for the FBU in step c during an
anticipated handover.
Note that, aside from physical layer parameters such as signal
strength, it may be possible to obtain all necessary information
about neighboring APs by using the wildcard form of the RtSolPr
message. This would cause the current access router to return a list
of neighboring APs, and would not interrupt ongoing communication
with the current AP. This request could be made at the time the MN
first attaches to the access router, and periodically thereafter.
This would enable the MN to cache the necessary [AP-ID, AR-Info]
tuples and might enable it to react more quickly when a handover
becomes necessary due to a changing radio environment. However,
because the information does not include up-to-date signal strength,
it would not enable the MN to predict accurately the next AP prior to
a handover. Also, if the scale of the network is such that a given
access router is attached to many APs, then it is possible that there
may not be room to list all APs in the PrRtAdv.
The time taken to scan for beacons is significant because it involves
iteration through all 802.11 channels and listening on each one for
active beacons. A more targeted approach would allow the STA to
scan, e.g., only one or two channels of interest, which would provide
for much shorter interruption of real-time traffic. However, such
optimizations are currently outside the scope of 802.11
specifications.
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7. Scenarios
In this section we look at a few of the possible scenarios for using
FMIPv6 in an 802.11 context. Each scenario is labeled by the
sequence of events that take place, where the numbered events are
from Section 4 and the lettered events are from Section 5. For
example, "1abcde23456fg" is the sequence where the MN performs a
scan, then the MN executes the FMIPv6 messaging to obtain NAR
information and send a binding update, then the PAR initiates
HI/HAck exchange, then the 802.11 handover completes, and finally
the HAck is received at the PAR and the MN sends an FNA.
Each scenario is followed by a brief description and discussion of
the benefits and drawbacks.
7.1 Scenario 1abcdef23456g
This scenario is the predictive mode of operation from the FMIPv6
specification. In this scenario, the host executes the scan sometime
prior to the handover, and is able to send the FBU prior to handover.
Only the FNA is sent after the handover. This mode of operation
requires that the scan and join operations (steps 1 and 2) can be
performed separately and under host control, so that steps a-f can be
inserted between 1 and 2. As mentioned previously, such control may
be possible in some implementations [10] but not in others.
Steps 1ab may be executed far in advance of the handover, which would
remove them from the critical path. This would minimize the service
interruption from beacon scanning, and allow at least one
RtSolPr/PrRtAdv exchange to complete so that the host has link layer
information about some NARs. Note that if steps ab were delayed
until handover is imminent, there would be no guarantee that the
RtSolPr/PrRtAdv exchange would complete especially in a radio
environment where the connection to the old AP is deteriorating
rapidly. However, if there were a long interval between the scan and
the handover, then the FBU (step c) would be created with out-of-date
information. There is no guarantee that the MN will actually attach
to the desired new AP after it has sent the FBU to the oAR, because
changing radio conditions may cause nAR to be suddenly unreachable.
If this were the case, then the handover would need to devolve into
one of the reactive cases given below.
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7.2 Scenario ab123456cdefg
This is the reactive mode of operation from the FMIPv6 specification.
This scenario does not require host intervention between steps 1 and
2.
However, it does require that the MN obtain the link-layer address of
NAR prior to handover, so that it has a link-layer destination
address for outgoing packets (default router information). This
would then be used for sending the FNA (with encapsulated FBU) when
it reaches the new subnet.
7.3 Scenario 123456abcdefg
In this scenario the MN does not obtain any information about the NAR
prior to executing the handover. It is completely reactive, and
consists of soliciting a router advertisement after handover, and
then sending an FNA with encapsulated FBU immediately.
This scenario may be appropriate when it is difficult to learn the
link-layer address of the NAR prior to handover. This may be the
case, e.g., if the scan primitive is not available to the host and
the wildcard PrRtAdv form returns too many results. It may be
possible to skip the router advertisement/solicitation steps (ab) in
some cases, if it is possible to learn the NAR's link-layer address
through some other means. In the deployment illustrated in Figure 2,
this would be exactly the new AP's MAC layer address, which can be
learned from the link-layer handover messages. However, in the case
of Figure 1, this information must be learned through router
discovery of some form. 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.
8. Security Considerations
The security considerations applicable to FMIPv6 are described in the
base FMIPv6 specification [2]. In particular, the PAR must be
assured of the authenticity of the FBU before it begins to redirect
user traffic. However, if the association with the new AP is not
protected using mutual authentication, it may be possible for a rogue
AP to fool the MN into sending an FBU to the PAR when it is not in
its best interest to do so.
Note that steps 5 and 6 from Section 4 install layer-2 forwarding
state that can redirect user traffic and cause disruption of service
if they can be triggered by malicious nodes.
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Note that step 3 from Section 4 could potentially provide some
security; however, due to the identified weaknesses in WEP shared key
security [9] this should not be relied upon. Instead, the Robust
Security Network [5] will require the STA to undergo 802.1X Port-
Based Network Access Control [4] before proceeding to steps 5 or 6.
802.1X 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 encrypt/authenticate subsequent traffic. It is possible to
use 802.1X pre-authentication [5] 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 handover mechanisms
are not needed anyway. In the most interesting case for FMIPv6
(roaming across subnet boundaries) the 802.1X exchange would need to
be performed after handover to 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, or with the use of pre-authentication using an
IP-layer mechanism that could cross subnet boundaries. 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.
9. Conclusions
The Mobile IPv6 Fast Handover specification presents a protocol for
shortening the period of service interruption during a change in
link-layer point of attachment. This document attempts to show how
this protocol may be applied in the context of 802.11 access
networks.
Implementation of FMIPv6 must be done in the context of a particular
link layer implementation, which must provide the triggers for the
FMIPv6 message flows. For example, the host must be notified of such
events as degradation of signal strength or attachment to a new AP.
The particular implementation of the 802.11 hardware and firmware may
dictate how FMIPv6 is able to operate. For example, to execute a
predictive handover, the scan request primitive must be available to
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the host and the firmware must execute join operations only under
host control [10], not autonomously in response to its own handover
criteria. Obtaining the desired PrRtAdv and sending an FBU
immediately prior to handover requires that messages be exchanged
over the wireless link during a period when connectivity is
degrading. In some cases the scenario given in Section 7.1 may not
complete successfully or the FBU may redirect traffic to the wrong
NAR. However, in these cases the handover may devolve to the
scenario from Section 7.2 or the scenario from Section 7.3.
Ultimately, falling back to basic Mobile IPv6 operation [6] and
sending a Binding Update directly to the Home Agent can be used to
recover from any failure of the FMIPv6 protocol.
10. References
10.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Koodli, R. (editor), "Fast Handovers for Mobile IPv6", draft-
ietf-mipshop-fast-mipv6-02.txt, July 2004. Work In Progress.
[3] "Wireless LAN Medium Access Control (MAC) and Physical Layer
(PHY) Specifications", ANSI/IEEE Std 802.11, 1999 Edition.
[4] "Port-Based Network Access Control", IEEE Std 802.1X-2001,
October 2001.
[5] "Medium Access Control (MAC) Security Enhancements", IEEE Std
802.11i-2004, July 2004.
[6] Johnson, D., Perkins, C., and Arkko, J., "Mobility Support in
IPv6", RFC 3775, June 2004.
10.2 Informative References
[7] "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.
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[8] 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.
[9] 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.
[10] Malinen, J., "Host AP driver for Intersil Prism2/2.5/3 and WPA
Supplicant", http://hostap.epitest.fi/, July 2004.
11. Acknowledgments
Thanks to Bob O'Hara for providing explanation and insight on the
802.11 standards. Thanks to James Kempf, Erik Anderlind, and Rajeev
Koodli for providing comments on an earlier draft.
12. 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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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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