MIPSHOP Working Group Heejin Jang
Internet-Draft Samsung AIT
Expires: January 11, 2006 Junghoon Jee
ETRI
Youn-Hee Han
Samsung AIT
Soohong Daniel Park
Samsung Electronics
Jaesun Cha
ETRI
July 10, 2005
Mobile IPv6 Fast Handovers over IEEE 802.16e Networks
draft-jang-mipshop-fh80216e-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes how a Mobile IPv6 Fast Handover could be
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implemented on link layers conforming to the 802.16e suite of
specifications.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Deployment Architectures for Mobile IPv6 on IEEE 802.16e . . . 6
4. IEEE 802.16e Handovers Overview . . . . . . . . . . . . . . . 8
5. Network Topology Acquisition, Cell Selection, and AR
Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Interaction between FMIPv6 and IEEE 802.16e . . . . . . . . . 10
6.1 Handover Preparation . . . . . . . . . . . . . . . . . . . 10
6.2 Handover Execution . . . . . . . . . . . . . . . . . . . . 10
6.3 802.16e Network Entry . . . . . . . . . . . . . . . . . . 11
6.4 Handover Completion . . . . . . . . . . . . . . . . . . . 11
7. The Examples of Handover Scenario . . . . . . . . . . . . . . 12
7.1 Predictive Mode . . . . . . . . . . . . . . . . . . . . . 12
7.2 Reactive Mode . . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 17
10. Normative References . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . 19
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1. Introduction
In order to provide the session continuity during handover, Mobile
IPv6 protocol [2] is currently available. It is capable of handling
IP handovers between different subnets in a transparent way for
higher-level connections. However, the handover latency resulting
from standard Mobile IPv6 is often unacceptable to real-time traffic
such as Voice over IP, and Mobile IPv6 Fast Handover protocol
(FMIPv6) [3] has been proposed as a mechanism to improve the handover
latency by predicting and preparing the impending handover in
advance.
As [4] pointed out, Mobile IPv6 Fast Handover assumes the support
from the link-layer technology, but 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 describes Mobile IPv6 Fast Handovers on 802.16
networks. There are three kinds of handover modes, hard handover,
fast BS switching and soft handover in IEEE 802.16e. In this version
of the draft, we consider the hard handover mode because this is the
default mode. We begin with a summary of a handover procedure on
802.16e [6], the amendment of 802.16 for mobility. Then the
interaction between 802.16e and FMIPv6 is presented with the
primitives proposed by IEEE 802.21 for the close interaction between
Layer 2 and Layer 3. Lastly, the examples of handover scenario are
described for both predictive mode and reactive mode.
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2. Terminology
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 RFC2119 [1].
Most of terms used in this draft are defined in Mobile IPv6 [2] and
FMIPv6 [3].
The following terms come from IEEE 802.16e specification [6].
MOB_NBR-ADV
IEEE 802.16e neighbor advertisement message sent periodically
by a base station.
MOB_MSHO-REQ
IEEE 802.16e handover request message sent by a mobile node.
MOB_BSHO-RSP
IEEE 802.16e handover response message sent by a base station.
MOB_BSHO-REQ
IEEE 802.16e handover request message sent by a base station.
MOB_HO-IND
IEEE 802.16e handover indication message sent by a mobile node.
BSID
IEEE 802.16e base station identifier.
Additionally, the following triggers are proposed by IEEE 802.21
[7][8] and the standardization is in progress. We also referred to
[5].
Link_Going_Down (LGD)
A trigger from the link layer to IP layer in the MN to report
that a link down event will be fired in the near future.
Link_Up (LUP)
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A trigger from the link layer to IP layer in the MN to report
that the MN completes L2 connection establishment with a new
BS.
Link_Switch (LSW)
A control command come from IP layer to the link layer in the
MN in order to force to switch to new BS.
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3. Deployment Architectures for Mobile IPv6 on IEEE 802.16e
In this section, we describe three possible deployment architectures
of 802.16e network and the mobile node's handover on it.
Figure 1 and 2 show the deployment with two IP subnets. In Figure 1,
an access router (AR) and several base stations (BS) forms a subnet
and the AR plays a role of a centralized controller for handover. In
Figure 2, a subnet consists of three ARs and several BSs. In this
case, one or more ARs may serve as a controller for handover. In
both cases, the movement among BSs does not always require IP
mobility. The handover from BS1 to BS2, or within same subnet, may
be carried out using link layer mobility without IP mobility.
However, the handover from BS5 to BS6 may require the change of AR
since BS5 and BS6 belong to the different subnets.
/-------------------------------------\
| IP Backbone |
\-------------------------------------/
| |
/-----------\ /-----------\
| AR1 | | AR2 |
\-----------/ \-----------/
/ / | \ \ / / | \ \
/ / | \ \ / / | \ \
/ | | | \ / | | | \
BS1 BS2 BS3 BS4 BS5 BS6 BS7 BS8 BS9 BS10
Figure 1. 802.16e deployment architecture
in a centralized manner
/------------------------------------------------\
| IP Backbone |
\------------------------------------------------/
| | | | | |
AR1 AR2 AR3 AR4 AR5 AR6
\ | / \ | /
\ | / \ | /
BS1 --(Subnet 1)--- BS5 BS6 --(Subnet 2)--- BS10
/ | \ / | \
/ | \ / | \
BS2 BS3 BS4 BS7 BS8 BS9
Figure 2. 802.16e deployment architecture
in a distributed manner
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Figure 3 represents an alternative 802.16e deployment where a subnet
consists of only single AR and single BS. In this case, a BS may be
integrated with an AR, composing one box in view of implementation.
The handover in this architecture means a change of subnet, resulting
in IP handovers.
/------------------\
| IP Backbone |
\------------------/
/ | \
/ | \
/ | \
----- ----- -----
| AR1 | | AR2 | | AR3 |
| BS1 | | BS2 | | BS3 |
----- ----- -----
Figure 3. 802.16e deployment architecture
with the integrated BS & AR
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4. IEEE 802.16e Handovers Overview
Compared with the handover in the wireless LAN, the 802.16e handover
mechanism consists of more steps since 802.16e embraces the
functionality for elaborate parameter adjustments and procedural
flexibility.
When an mobile node (MN) stays in a link, it listens to L2 neighbor
advertisement message, named MOB_NBR-ADV, from its serving BS. A BS
broadcasts it periodically to identify the network and announces the
characteristics of neighbor BSs. Once an MN receives this, it may
decode this message to find out information about the parameters of
neighbors for its future handover. With the provided information in
this message, the MN may minimize the handover latency by decoding
the channel number of neighbors and reducing the scanning time, or
may select the target BS tailored for its taste.
In 802.16e, the handover may be initiated by either MN or BS, and
regardless of whoever initiates, the handover procedure is
conceptually divided into two steps: ``handover preparation'' and
``handover execution'' [9]. The handover preparation begins with a
decision of MN or BS. During the handover preparation, neighbors are
compared by the metrics such as signal strength or QoS parameters and
the target BS is selected among them. If necessary, an MN may
associate (initial ranging) with candidate BSs to expedite a
potential future handover. Once the MN decides handover, it may
notify its intent by sending MOB_MSHO-REQ message to the serving BS.
The serving BS then replies with MOB_BSHO-RSP containing the
recommended BSs after negotiation with candidates. Also, the BS can
trigger handover with MOB_BSHO-REQ message. In both cases, BS may
confirm the handover to target BS over backbone.
After handover preparation, handover execution occurs. When the MN
sets the target BS finally and is about to move to the link, it sends
MOB_HO-IND to the serving BS as a final indication for handover and
for resource release for it, then conducting handover. Once the MN
switches the link, it shall conduct 802.16e ranging through which it
can acquire physical parameters from target BS, tuning its parameters
to the target BS. After ranging with the target BS successfully, the
MN negotiates basic capabilities and performs authentication, finally
registering with the target BS. If the target BS has already learned
some contexts such as authentication or capability parameters through
backbone, the MN may omit the corresponding procedures. Since this
point, the target BS starts to serve the MN and communication via
target BS is available. However, when the MN moves to different
subnet, it should re-configure new IP address and re-establish IP
connection based on it. To resume the active session of previous
link, the MN should perform IP handover additionally.
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5. Network Topology Acquisition, Cell Selection, and AR Discovery
An MN can learn the network topology and acquire L2 information in
two ways. One method is via L2 neighbor advertisement. A BS
supporting mobile functionality shall broadcast MOB_NBR-ADV message
including the network topology at a periodic interval (maximum
interval, 1sec.). This message includes the BSID and channel
information of neighbor BSs and is used to facilitate the MN's
synchronization with neighbor BSs by removing the need to monitor
transmission from the target BS for L2 broadcast.
Another method for acquisition of network topology is scanning, which
is the process to seek and monitor available BS suitability as
targets for handover. While the MOB_NBR-ADV message includes static
information about neighbor BSs, scanning provides rather dynamic
parameters such as link quality parameters. Since the MOB_NBR-ADV
message delivers a list of neighbor BSIDs periodically and scanning
provides a way to sort out some adequate BSs, it is recommended that
when new BSs are found in the advertisement, an MN identifies them
via scanning and resolves their BSIDs to associated network
information. The association, optional initial ranging procedure
occurring during scanning, is one of the helpful method to facilitate
the impending handover. An MN is able to get ranging parameters and
service availability information for the purpose of proper selection
of the target BS and expediting a potential future handover to it.
After learning about neighbors, the MN may compare them to find
another BS which can serve better than the serving BS. This is
called Cell selection. The target BS may be determined considering
various criteria such as required QoS, cost, user preference, policy,
etc. Once the target is determined, the MN should learn the
associated AR information, which is the AR discovery. With BSID in
MOB_NBR-ADV message, the MN requests the associated AR information to
the PAR (Previous AR). The result of resolving BSIDs is a list of
[BSID, AR-Info] tuples. AR-Info consists of the corresponding new
router's information including its prefix, IP address and L2 address.
The RtSolPr (Router Solicitation for Proxy Advertisement) and PrRtAdv
(Proxy Router Advertisement) messages of FMIPv6 are used for the
resolution. If IEEE 802.16e network is able to provide the MN with
AR information, AR discovery of FMIPv6 may be skipped. Note that
network topology acquisition, cell selection, and AR discovery are
not necessarily involved with any specific handover procedure and the
MN may perform them at any convenient time.
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6. Interaction between FMIPv6 and IEEE 802.16e
In this section, we introduce three primitives for the close
interaction between FMIPv6 and 802.16e, and the interaction is
presented with it.
6.1 Handover Preparation
As mentioned in Section 4, an MN may initiate handover by sending
MOB_MSHO-REQ to the serving BS and receive MOB_BSHO-RSP from it.
Also, the BS can initiate handover by sending MOB_BSHO-REQ to the MN.
After receiving either MOB_BSHO-RSP or MOB_BSHO-REQ message, the MN
may sends FBU (Fast Binding Update) to the PAR. At this time, the
Link_Going_Down (LGD) is introduced to signal L3 of L2 reception of
MOB_BSHO-REQ/MOB_BSHO-RSP as soon as possible. The MN may be
notified of the target BS at the same time, but how to learn target
BS is implementation-specific. On receiving LGD, the MN L3 sends FBU
to the PAR. Before sending FBAck (Fast Binding Acknowledgement) to
the MN, the PAR sets up tunnel between PCoA (Previous CoA) and NCoA
(New CoA) by exchange of HI (Handover Initiate) and HAck (Handover
Acknowledge) messages, and forwards the packets destined for the MN
to NCoA. During this time, an available NCoA is confirmed with HAck
message.
After the MN sends a MOB_HO-IND to the serving BS, any packet data
transfer between MN and serving BS is not allowed even though the
resource retain timer does not expire in serving BS. Therefore, if
possible, the MN should exchange a FBU and FBAck messages with the
PAR before sending MOB_HO-IND to the serving BS so as to operate as
predictive mode.
6.2 Handover Execution
When the FBAck message arrives before the handover, the MN runs as
predictive mode. If the MN can not acquire the FBAck message on the
current link, it should run as reactive mode. One reason for this is
that the MN has not sent FBU. The other is that FBAck is lost during
the handover. Both cases means that the current link is not
available anymore and the handover is already in progress. Note that
when MOB_HO-IND is sent prior to the arrival of FBAck, the MN should
operate as reactive mode. In both cases, the MN should conduct the
handover right after sending a MOB_HO-IND to the serving BS. When
the serving BS receives this message, it releases MN's all
connections and resources. The serving BS may retain the resource
until the resource retain timer expires.
If applicable, the Link_Switch trigger (LSW) from IEEE 802.21
document [8] can be used to optimize the L2/L3 interaction. The LSW
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trigger can be issued from the MN's L3 to L2 on the reception of
FBAck message, thereby the MOB_HO-IND is sent immediately. Similar
concept has already introduced for the wireless LAN in [5].
6.3 802.16e Network Entry
After switching the link, the MN synchronizes with the target BS and
performs the 802.16e network entry procedure. The MN may exchange
the RNG-REQ/RSP, SBC-REQ/RSP, PKM-REQ/RSP, REG-REQ/RSP messages with
the target BS. Some of these messages may be omitted if the
(previously) serving BS transferred the context to the target BS over
the backbone before. As soon as completing the network entry, the MN
L2 informs its L3 of it with LinkUp (LUP) trigger, forcing L3 to send
FNA (Fast Neighbor Advertisement) to the NAR (New AR). In case of
reactive mode, the MN should include the FBU within the FNA message.
6.4 Handover Completion
Receiving the FNA, the NAR should verify the availability of NCoA.
If the NAR detects the NCoA is already in use, it MUST discard the
FBU and reply with Router Advertisement with Neighbor Advertisement
Acknowledge (NAACK) option to the MN. Otherwise, in predictive mode,
the NAR starts to flush the buffered packets to the MN. In reactive
mode, the NAR should forward the inner FBU to the PAR, establishing
the tunnel, finally forwarding the packets destined to the NCoA to
the MN. At this time, if the NAR is co-located with the target BS as
shown in figure 3, its L3 may notify the reception of FNA to its L2
(target BS) in order that the target BS can send the backbone message
to the previously serving BS for resource releases. When serving BS
receives this, it shall remove all resources for the MN regardless of
expiration of resource retain timer according to Section 6.3.21 of
[6].
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7. The Examples of Handover Scenario
In this section, the examples of handover procedure over 802.16e are
shown for both predictive mode and reactive mode. Note that there is
no need of IP mobility when the target BS is under same subnet.
Therefore FBU is sent conditionally depending on whether the target
BS is under different subnet or not. In following scenarios, the MN
is assumed to move to different subnet.
7.1 Predictive Mode
The procedure is described briefly as follows.
1. A BS broadcasts MOB_NBR-ADV periodically.
2. If the MN discovers new neighbor BSs in this message, it
may perform scanning for them.
3. Then the MN may try to resolve new neighbor's BSID to the
associated AR by exchange of RtSolPr and PrRtAdv with the
PAR.
4. The MN may initiate handover by sending MOB_MSHO-REQ to
the serving BS and receive MOB_BSHO-RSP from it. Also,
the serving BS can initiate handover by sending
MOB_BSHO-REQ to the MN.
5. When the MN receives either MOB_BSHO-RSP or MOB_BSHO-REQ
from BS, its L2 triggers Link_Going_Down to L3.
6. On reception of LGD, the MN L3 exchanges FBU and FBAck
with the PAR. Before sending FBAck, the PAR establishes
tunnel with the NAR by exchange of HI and HAck messages.
During this time, the NAR confirms NCoA available in new
link via HAck.
7. When the FBAck arrives before the handover, the MN
operates as predictive mode. It sends MOB_HO-IND
as a final indication for handovers.
8. The MN conducts handover to the target BS and performs
802.16e network entry procedure.
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9. When the network entry is completed, the MN L2 signals
its L3 with Link_Up and then the MN issues FNA to the
NAR.
10. When the NAR receives FNA from the MN, it delivers the
buffered packets to the MN.
-------------- --------------
MN L3 MN L2 |serving BS PAR| |NAR target BS|
-------------- --------------
| | | | | |
| |<-----MOB_NBR-ADV-------| | | |
| |( Scanning )| | | |
|--------------(RtSolPr)-------------->| | |
|<--------------PrRtAdv----------------| | |
| | | | | |
| | [MN initiation] | | | |
| |------MOB_MNHO-REQ----->| | | |
|<-LGD-|<-----MOB_BSHO-RSP------| | | |
| | or | | | |
| | [BS initiation] | | | |
|<-LGD-|<-----MOB_BSHO-REQ------| | | |
| | | | | |
|------------------FBU---------------->| | |
| | | |-----HI---->| |
| | | |<---HACK----| |
|<-----------------FBACK---------------|--> | |
|(LSW)>|-------MOB_HO-IND------>| forward========>| |
disconnect | packets | |
| connect | | | |
|<-LUP-|<--------------802.16 network reentry------------->|
connect | | | |
|-------------------------FNA---------------------->| |
|<===============================================deliver |
| | | | packets |
Figure 4: Predictive Fast Handover in 802.16
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7.2 Reactive Mode
The procedure is described as follows.
1. A BS broadcasts MOB_NBR-ADV periodically.
2. If the MN discovers new neighbors BS in this message, it
may perform scanning for them.
3. Then the MN may try to resolve BSID to the associated AR
by exchange of RtSolPr and PrRtAdv with the PAR.
4. The MN may initiate handover by sending MOB_MSHO-REQ to
the BS and receive MOB_BSHO-RSP from the BS. Also, the BS
can initiate handover by sending MOB_BSHO-REQ to the MN.
5. When the MN receives either MOB_BSHO-RSP or MOB_BSHO-REQ
from the BS, its L2 triggers Link_Going_Down to L3,
thereby sending FBU if possible.
6. When the MN can not receive FBAck on the current link, it
runs as reactive mode. After conducting handover to the
target BS, the MN should perform the 802.16e network entry
procedure.
7. As soon as it is completed, the MN L2 signals L3 with
Link_Up and then the MN issues FNA encapsulating FBU to
the NAR.
8. Receiving FNA, the NAR verifies the availability of NCoA
and forwards the inner FBU to the PAR, establishing the
tunnel.
9. If the NAR detects the NCoA is already in use, it MUST
discard the FBU and reply with Router Advertisement with
NAACK option to the MN. Otherwise, it delivers the packets
destined to NCoA to the MN.
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-------------- --------------
MN L3 MN L2 |serving BS PAR| |NAR target BS|
-------------- --------------
| | | | | |
| |<-----MOB_NBR-ADV-------| | | |
| |( Scanning )| | | |
|--------------(RtSolPr)-------------->| | |
|<--------------PrRtAdv----------------| | |
| | | | | |
| | [MN initiation] | | | |
| |------MOB_MSHO-REQ----->| | | |
|<-LGD-|<-----MOB_BSHO-RSP------| | | |
| | or | | | |
| | [BS initiation] | | | |
|<-LGD-|<-----MOB_BSHO-REQ------| | | |
| | | | | |
|-----------------(FBU)--------------->| | |
| |-------MOB_HO-IND------>| | | |
disconnect| | | | |
| connect | | | |
|<-LUP-|<--------------802.16 network reentry------------->|
connect | | | |
|-------------------------FNA[FBU]----------------->| |
| | | |<---FBU-----| |
| | | |----FBACK-->| |
| | | forward | |
| | | packets=========>| |
|<================================================deliver |
| | | | packets |
Figure 5: Reactive Fast Handover in 802.16
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8. Security Considerations
The security consideration of the FMIPv6 specification [3] are
applicable to this document. Particularly, 802.16e architecture
supports a number of mandatory authorization mechanisms, for example,
EAP-TTLS, EAP-SIM and EAP-AKA, as well as, secure IP address
management between the MN and its network entity. That will allow
secure handover operation between the mobile node and the network
entity.
Further security considerations will be carefully studied along with
this document.
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9. Acknowledgment
Many thanks IETF Mobility Working Group members of KWISF (Korea
Wireless Internet Standardization Forum) for their efforts on this
work. In addition, we would like to thank Alper E. Yegin, Jinhyeock
Choi and Misun Do who have provided the technical advice.
10. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[3] Koodli, R., "Fast Handovers for Mobile IPv6",
draft-ietf-mipshop-fast-mipv6-03 (work in progress),
October 2004.
[4] McCann, P., "Mobile IPv6 Fast Handovers for 802.11 Networks",
draft-ietf-mipshop-80211fh-04 (work in progress), April 2005.
[5] Mitani, K., "Unified L2 Abstractions for L3-Driven Fast
Handover", draft-koki-mobopts-l2-abstractions-02 (work in
progress), February 2005.
[6] IEEE 802.16 TGe Working Document (Draft Standard), "Amendment
for Physical and Medium Access Control Layers for Combined Fixed
and Mobile Operation in Licensed Bands", 802.16e/D8, May 2005.
[7] Gupta, V. and D. Johnston, íIEEE 802.21, A Generalized Model for
Link Layer Triggersí, IEEE 802.21 Media Independent Handoff
Working Group, Mar. 2004 mtg. mins., http://www.ieee802.org/21/
march04_meeting_docs/21-04-0027-00-0000-Generalized_triggers.pdf,
March 2004.
[8] Liu, X. and Y.-H. Han, íInteraction between L2 and Upper Layers
in IEEE 802.21í, IEEE 802.21 Media Independent Handoff Working
Group, Mar. 2004 mtg. mins., http://www.ieee802.org/21/march04_
meeting_docs/21-04-0008-00-0000-L2_upper_layer_interaction.ppt,
March 2004.
[9] Kim, K., Kim, C., and T. Kim, "A Seamless Handover Mechanism
for IEEE 802.16e Broadband Wireless Access", International
Conference on Computational Science, vol. 2, pp. 527-534, 2005.
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Authors' Addresses
Heejin Jang
Samsung Advanced Institute of Technology
P.O. Box 111
Suwon 440-600
KOREA
Email: heejin.jang@samsung.com
Junghoon Jee
Electronics and Telecommunications Research Institute
161 Gajeong-dong, Yuseong-gu
Daejon 305-350
KOREA
Email: jhjee@etri.re.kr
Youn-Hee Han
Samsung Advanced Institute of Technology
P.O. Box 111
Suwon 440-600
KOREA
Email: yh21.han@samsung.com
Soohong Daniel Park
Samsung Electronics
416 Maetan-3dong, Yeongtong-gu
Suwon, Gyeonggi-do 442-742
KOREA
Email: soohong.park@samsung.com
Jaesun Cha
Electronics and Telecommunications Research Institute
161 Gajeong-dong, Yuseong-gu
Daejon 305-350
KOREA
Email: jscha@etri.re.kr
Jang, et al. Expires January 11, 2006 [Page 18]
Internet-Draft FMIPv6 over 802.16e July 2005
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