Internet DRAFT - draft-hong-mobileip-applicability
draft-hong-mobileip-applicability
Mobile-IP Working Group Yong-Geun Hong
Internet Draft Myung-Ki Shin
draft-hong-mobileip-applicability-00.txt Hyoung-Jun Kim
Expires: December 2003 ETRI
Woo-Suck Cha
Gi-Hwan Cho
Chonbuk University
Considerations of FMIPv6 in 802.11 networks
draft-hong-mobileip-applicability-00.txt
Status of this Memo
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Abstract
This document describes the applicability of Fast Handovers for
Mobile IPv6 in 802.11 networks. Fast Handovers for Mobile IPv6
proposes a set of protocol enhancements to reduce handover latency
due to IP protocol operations as small as possible by the help of L2
information. 802.11 networks are considered as a popular wireless
infrastructure to meet broadband network service requirement in the
future. If Fast Handovers for Mobile IPv6 is applied to wireless
network, 802.11 might be a practicable target for deployment. At
this point some Fast Handover methods for Mobile IPv6 are thought to
be directly applicable to 802.11 networks and some others are not.
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Considerations of FMIPv6 in 802.11 networks June 2003
Table of Contents
1. Introduction...................................................2
2. Terminology....................................................3
3. Applicability of FMIPv6 in 802.11 networks.....................4
3.1 Layer 2 Handover in 802.11 networks........................4
3.1.1 Scan procedure.......................................5
3.1.2 Reassociation procedure..............................5
3.2 L2 triggers................................................5
3.2.1 L2 triggers applicable to FMIPv6.....................5
3.2.2 L2 triggers applicable to 802.11 networks............6
3.3 Fast handover mechanisms in FMIPv6.........................6
3.4 Considerations of the fast handovers in 802.11 networks....7
4. Security Considerations........................................8
References........................................................8
Acknowledgments...................................................8
Author's Addresses................................................9
1. Introduction
Fast Handovers for Mobile IPv6 (FMIPv6) [1] describes a set of
protocol enhancements to reduce handover latency due to IP protocol
operations as small as possible by the help of L2 handover
information. FMIPv6 allows a Mobile Node (MN) to keep using its Care
of Address (CoA) until it establishes itself as a Mobile IPv6 end-
point of its New Access Router (NAR) and expeditiously establish a
new CoA. 802.11 networks is a popular wireless network nowadays. If
the FMIPv6 is applied to wireless network, 802.11 might be a
practicable target for deployment.
The basic idea of FMIPv6 is to set up a routing path (tunnel)
between two access routers (so, PAR and NAR) to enable a MN to
send and receive IP packets while the MN establishes itself as a
Mobile IPv6 end-point. In FMIPv6, L2 triggers by the MN or the
network (i.e., PAR or NAR) could be used to establish this tunnel.
The anticipation handover initiation of FMIPv6 is significant only
whenever these triggers are invoked before a new wireless link is
established. If not, the triggers are used for reducing the L3
movement detection overhead. Triggers which are used for initiating a
handover must have some information to assist the fast handover
procedure thereafter. The information could include a link-layer
identifier, such as a base station id or BSSID in 802.11 networks.
In the previous draft of Mobile IPv6 Fast Handover for 802.11
Networks [2], the 802.11 handover operations are divided into 6
steps : scan, join, authentication, association or reassociation,
IAPP operations and sending L2 update frame. In the paper of
Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process [3],
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the scan (probe) phase is the dominating component of the overall L2
handover delay (the scan phase accounts for more than 90% delay of L2
handover). For the efficiency of FMIPv6, it is clear that triggers
must be made before or during the scan phase. In 802.11 network
environments, if triggers are invoked after the scan phase, so 90% of
L2 handover time passed away, these triggers may be few effective to
FMIPv6. Even if the MN can invoke triggers during or before the scan
phase with a prospective AP, it might be impossible to guarantee that
the MN will move to the AP actually. In 802.11 networks, it therefore
can not be guaranteed to go ahead the L3 handover before the
completion of L2 handover.
In this draft, when FMIPv6 is applied to 802.11 networks, its
applicability is dealt with. With considering the deployment of
802.11 networks, some L2 triggers might not be always applicable to
FMIPv6 handover. L2-ST and L2-TT might not be applicable, since L2
handover in 802.11 networks must be initiated by the MN. L2-MT can be
selectively applicable under the particular conditions. On the other
hand, L2-LU and L2-LD might be applicable to FMIPv6 in 802.11
networks. These are meaningful since they eliminate the L3 movement
detection time.
2. Terminology
L2 handover Layer 2 handover
L3 handover Layer 3 handover
AP Access Point
PAP Previous Access Point
NAP New Access Point
MN Mobile Node
PAR Previous Access Router
NAR New Access Router
RtSolPr Router Solicitation Proxy
PrRtAdv Proxy Router Advertisement
F-BU Fast Binding Update
CoA Care of Address
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AAR Anchor Access Router
HI Handover Initiate
HACK Handover Acknowledge
HTT Handover To Third
3. Applicability of FMIPv6 in 802.11 networks
3.1 Layer 2 Handover in 802.11 networks
L2 handover refers to the mechanism of messages exchanged by AP and
MN resulting in a transfer of physical point of network connectivity.
Thus L2 handover is a link layer function carried out by at least
three participating entries, MN, PAP, and NAP. When a MN moves from
its current AP to another AP, there is no standardized method which
gets the MN to initiate L2 handover operations. Usually, it is
implemented as a result of decaying the signal quality condition on
the current AP. That is, when a MN approaches to the border of its
current AP's coverage area, the MN repeatedly monitors the link
signal quality to the current AP. The signal values are reiteratively
compared with a predefined threshold. If the value becomes smaller
than the threshold, the MN will initiate L2 handover procedure.
Figure 1 depicts the message sequence in 802.11 handover procedure [4,
5].
MN neighbor APs
-------------------|----Probe Request ----------->+-----+
^ |<------------ Probe Reply-----| AP1 |
| | +-----+
| Scan |----Probe Request ----------->+-----+
| Procedure |<------------ Probe Reply-----| AP2 |
| | +-----+
| | : :
v | : :
-------------------| (Choose a NAP)
^ |---Authentication Request --->+-----+
| Reassociation |<--- Authentication Reply-----| |
| Procedure | | New |
| |---Reassociation Request ---->| AP |
v |<--- Reassociation Reply------+-----+
-------------------|
Figure 1: 802.11 L2 handover operation
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L2 handover in 802.11 can be divided into two distinct procedures in
the time consuming point of view : scan procedure and reassociation
procedure as described below.
3.1.1 Scan procedure
Whenever L2 handover is initiated, the MN needs to find out a
potential AP to be associated with. This is accomplished by a scan
procedure which is a MAC layer function. The Scan Procedure consists
of the following steps. The MN sends "Probe Request" frames to an
assigned channel and listens for "Probe Response" frames from all APs
on the channel. These are iterated until all range of channels are
scanned. The MN creates a list of APs prioritized by the signal
strength of received of "Probe Response" frames. The MN selects an AP
as a prospective NAP that has the strongest signal strength among
them.
3.1.2 Reassociation procedure
The MN now attempts to reassociate with the prospective NAP selected
during the scan procedure was performed. The reassociation procedure
typically involves an authentication and a reassociation with the NAP
and requires the following steps to be taken.
The MN sends an "Authentication Request" frame to the NAP. If the MN
has been correctly authenticated by the NAP, the NAP replies with an
"Authentication Response" frame to the MN. Upon successful
authentication process, the MN sends a "Reassociation Request" frame
to the NAP. If the MN has been correctly reassociated, the NAP
replies with a "Reassociation Response" frame to the MN. When the MN
receives a "Reassociation Response" frame, L2 handover procedure
completes.
Based on L2 handover procedure, L2 triggers are classified and
identified whether they are applicable to FMIPv6 and to IEEE
802.11 networks respectively.
3.2 L2 triggers
3.2.1 L2 triggers applicable to FMIPv6
FMIPv6 relies on the existence of generic L2 triggers. Fast handover
mechanisms in FMIPv6 make use of the following L2 triggers : L2-MT,
L2-ST, L2-TT, L2-LU, L2-LD. These triggers can be classified into
two types based on the time of their occurrence. L2-MT, L2-ST and
L2-TT may be taken place in the link of the current AP before the
new link of a NAP is configured. L2-LU and L2-LD occur after the new
link of a NAP is connected and represent the completion of L2
handover. These triggers contain the L2 handover information causing
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the trigger to be fired, what entities will receive the trigger and
some parameters like as the L2 address of entities.
3.2.2 L2 triggers applicable to 802.11 networks
Note that L2 triggers have to contain the information about the next
attachment point where the MN will move to. Particularly, this
information is very important to L2-MT, L2-ST, and L2-TT which make
use of it to initiate L3 handover procedure before the link of a NAP
is connected. Again, 802.11 handover is divided into two procedures :
scan procedure, reassociation procedure. Since the information such
as the NAP's L2 address is determined after the scan procedure is
successfully finished. L2-MT, L2-ST, and L2-TT may be initiated after
the scan procedure. But, L2-LU and L-LD may be naturally initiated
when the reassociation procedure is successfully finished.
As a result of an analysis of 802.11 handover procedure, L2-LU and
L2-LD can be naturally applicable to 802.11 networks. These triggers
are useful since they eliminate the L3 move detection time. However,
L2-MT, L2-ST, and L2-TT can not be applicable to 802.11 networks.
There are basically two main reasons why they are.
- First, according to the resulting of an empirical analysis
of the IEEE 802.11 MAC Layer Handoff Process described in
[3], the scan procedure delay is more than 90% of the
overall L2 handover delay. The NAP's L2 information involved
by L2 triggers may be acquired after the scan procedure, L2-
MT, L2-ST, and L2-TT containing the NAP's L2 information can
be taken place after scan procedure is performed. Since the
scan procedure consumes most all L2 handover delay, these
triggers might be few effective to FMIPv6.
- Seconds, the MN's movement might be influenced by the ping-
pong effect and the radio conditions, even the
authentication or the reassociation between the MN and the
prospective NAP may be failed. Therefore, there is no
guarantee that the MN will actually move to the NAP selected
during the scan procedure.
Nevertheless, some considerations for applicability of L2-MT are
discussed to improve the fast handover in 802.11 networks in section
3.4
3.3 Fast handover mechanisms in FMIPv6
In the anticipation handover initiation described in FMIPv6 [1],
the MN or the PAR may initiate a fast handover by sending the
protocol messages such as RtSolPr or PrRtAdv. For the mobile-
initiated anticipated handover, the MN sends a RtSolPr message to the
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PAR, but on the other hand, in the case of network-initiated
anticipated handover the PAR sends a PrRtAdv message to the MN. The
RtSolPr message sent by the MN requests some information the IP
address, L2 address and network prefix of the NAR which the MN
will move to. The PrRtAdv message sent by the PAR then supplies the
information requested by the RtSolPr message to the MN. Once the MN
receives the PrRtAdv message from the PAR, the MN responds to the
PAR with a F-BU message which is binding to the new CoA.
In optimization using link-layer assisted features, it describes how
the tunnel between the PAR and the NAR could be set up by using L2
triggers, without IP signaling messages, so RtSolPr or PrRtAdv. On
the other hand, the anticipation handover initiation method is not
considering a specific L2 technology. Either the PAR that receives
L2-ST or NAR that receives L2-TT initiates a tunnel establishment
between PAR and NAR. The tunnel establishment is performed by
exchanging HI/HACK messages between them.
In three party handover, three party handover method occurs when the
MN is on the AAR, and moves to PAR, then moves to NAR before
completing MIP registration at PAR. Hence PAR that receives L2-ST (or
NAR that receives L2-TT) from Layer 2 must inform NAR to contact AAR
about changing the radio directed end of the tunnel. So the PAR and
NAR exchange a HTT/HI or HACK/HTT pair. Finally the NAR must perform
a new tunnel establishment with AAR. When the tunnel re-establishment
is successful, the previous tunnel between AAR and PAR is canceled.
Note that even if FMIPv6 was only considering L2-ST and L2-TT
triggers in the optimization using the link-layer assisted features
and the three party handover, it would be meaningful to make use of
L2-MT trigger if it is helpful to the fast handover in 802.11
networks.
3.4 Considerations of the fast handovers in 802.11 networks
As mentioned earlier, since L2 handover in 802.11 networks is always
initiated by MN, L2-ST and L2-TT are not basically applicable.
However, L2-MT might be selectively applicable to improve the fast
handover in 802.11 networks, in the particular environments that the
MN's movement pattern is known ahead, so anticipated, and/or that the
radio condition of 802.11 networks is stable, so determinant. The
method to anticipate the MN's move direction is out of scope in this
document.
To make use of L2-MT trigger, one possible way is to start the fast
handover procedure on deciding the prospective NAP, so finishing the
scan procedure. Assuming that the authentication or the reassociation
after the fast handover is few possible to be failed in the
particular environments, L2-MT must improve the fast handover in
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802.11 networks. More aggressively, L2-MT may be triggered on
deciding the prospective NAP even during the scan procedure. When the
MN meets an AP which has the signal strength over a pre-defined
threshold, it assigns the AP as the prospective NAP, and starts the
fast handover procedure without concerning the scan procedure
thereafter. To do this, the MN must terminate intentionally the scan
procedure in 802.11 handover, when it decides the prospective NAP.
As a result the anticipation handover initiation in FMIPv6 is now
applicable in 802.11 networks. A MN received L2-MT sends a RtSolPr
message to the PAR, then goes ahead the fast handover procedure.
4. Security Considerations
The security issues are not studied yet.
References
[1] Koodli, R., "Fast Handovers for Mobile IPv6",
draft-ietf-mobileip-fast-mipv6-06 (work in progress), March 2003.
[2] P. McCann, "Mobile IPv6 Fast Handovers for 802.11 Networks",
draft-mccann-mobileip-80211fh-01.txt (work in progress),
October 2002.
[3] 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
[4] "Wireless LAN Medium Access Control (MAC) and Physical Layer
(PHY) Specifications", ANSI/IEEE Std 802.11. 1999 Edition
[5] Lucent Technologies, "roaming with WaveLAN/IEEE 82.11 WaveLAN"
Technical ulletin 021.A, Dec. 1998
Acknowledgments
Thanks to Pete McCann for his pioneering Internet Draft "Mobile IPv6
Fast Handovers for 802.11 Networks". Thanks to the authors of "An
Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process" for
providing the detailed analysis of the 802.11 handoff process.
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Author's Addresses
Yong-Guen Hong
ETRI PEC
161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea
Tel : +82 42 860 6447
Fax : +82 42 861 5404
E-mail : yghong@etri.re.kr
Myung-Ki Shin
ETRI PEC
161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea
Tel : +82 42 860 4847
Fax : +82 42 861 5404
E-mail : mkshin@etri.re.kr
Hyoung-Jun Kim
ETRI PEC
161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea
Tel : +82 42 860 6576
Fax : +82 42 861 5404
E-mail : khj@etri.re.kr
Woo-Suck Cha
Chonbuk University
664-141 Duckjin-Dong Duckjin-Gu Jeonju Jeonbuk 561-756, Korea
Tel : +82 63 270 3437
Fax : +82 63 270 3403
E-mail : wscha@dcs.chonnuk.ac.kr
Gi-Hwan Cho
Chonbuk University
664-141 Duckjin-Dong Duckjin-Gu Jeonju Jeonbuk 561-756, Korea
Tel : +82 63 270 3437
Fax : +82 63 270 3403
E-mail : ghcho@dcs.chonnuk.ac.kr
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