No Specific Working Group T. Ernst
Internet-Draft WIDE at Keio University
Expires: August 25, 2005 N. Montavont
LSIIT - ULP
R. Wakikawa
Keio University
E. Paik
KT
C. Ng
Panasonic Singapore Labs
K. Kuladinithi
University of Bremen
T. Noel
LSIIT - ULP
February 21, 2005
Goals and Benefits of Multihoming
draft-ernst-generic-goals-and-benefits-01
Status of this Memo
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Copyright Notice
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Copyright (C) The Internet Society (2005).
Abstract
This document attempts to define the goals and benefits of
multihoming for fixed and mobile hosts and routers. Those goals and
benefits are illustrated with a set of scenarios.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Goals and Benefits of Multihoming . . . . . . . . . . . . . . 5
3.1 Permanent and Ubiquitous Access . . . . . . . . . . . . . 5
3.2 Redundancy/Fault-Recovery . . . . . . . . . . . . . . . . 5
3.3 Load Sharing . . . . . . . . . . . . . . . . . . . . . . . 5
3.4 Load Balancing . . . . . . . . . . . . . . . . . . . . . . 6
3.5 Bi-casting (n-casting) . . . . . . . . . . . . . . . . . . 6
3.6 Preference Settings . . . . . . . . . . . . . . . . . . . 6
3.7 Increased Bandwidth . . . . . . . . . . . . . . . . . . . 6
4. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 Load Balancing, Increased Bandwidth (no mobility) . . . . 7
4.2 Preference Settings and Transparent Flow Handoffs
(with mobility) . . . . . . . . . . . . . . . . . . . . . 7
4.3 Preference Settings for House Networking (fixed) . . . . . 7
4.4 Load Balancing, Preference Settings, Increased
Bandwidth (no mobility) . . . . . . . . . . . . . . . . . 8
4.5 Ubiquitous Access and Load Sharing (with mobility) . . . . 8
4.6 Redundancy and Bi-Casting (with no mobility) . . . . . . . 8
5. Classification . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1 Case 1: One Interface, Multiple Prefixes . . . . . . . . . 9
5.2 Case 2: Several Interfaces . . . . . . . . . . . . . . . . 11
6. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1 Router Selection . . . . . . . . . . . . . . . . . . . . . 14
6.2 Source Address Selection . . . . . . . . . . . . . . . . . 14
6.3 Flow Redirection and Broken Sessions . . . . . . . . . . . 14
6.4 Recovery Delay . . . . . . . . . . . . . . . . . . . . . . 14
6.5 Mobility . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
A. Change Log From Previous Version . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . 19
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1. Introduction
New equipments shipped on the market now often integrate several
access technologies (both wired and wireless). The main purpose of
this integration is to federate all means of communications in order
to access the Internet ubiquitously (from everywhere and at any time)
as no single technology can be expected to be deployed everywhere.
Flows may thus be redirected from one interface to the other due to
the loss of connectivity or change of the network conditions.
Several access technologies are also integrated in order to increase
bandwidth availability or to select the technology the most
appropriate according to the type of flow or choices of the user.
Basically, each network interface has different cost, performance,
bandwidth, access range, and reliability. Users are also willing to
select the most appropriate set of network interface(s) depending on
the network environment, particularly in wireless networks which are
mutable and less reliable than wired networks. Users should also be
able to select the most appropriate interface per communication type
or to combine a set of interfaces to get sufficient bandwidth.
The purpose of this document is to emphasize the goals and benefits
of multihoming for fixed and mobile hosts and routers in a generic
fashion, i.e. without focusing on issues pertaining to hosts, or
routers, or mobility.
Issues pertaining to site multihoming in fixed networks are discussed
in [3]. Mobility issues pertaining to mobile nodes and mobile
networks are respectively discussed in companion drafts [9] and [8].
Our document is targetted to IPv6, although our analysis may be
applicable to IPv4 as well. The readers may refer to [10] for a
description of the problem specific to Mobile IPv4.
This document is organized as follows: we first define the terms used
in the document before emphasizing the goals and benefits of
multihoming in Section 3. Then, the needs for multihoming are
illustrated through a set of scenarios in Section 4. Next follows an
analysis in Section 5 of two different cases where a multihomed node
has either a single interface or multiple interfaces.
2. Terminology
This draft is based on the terminology defined in [2]. For the
purpose of clarity, we remind the definition of interface. Terms
related to multihoming are not known to be defined in existing IETF
RFCs.
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Interface
A node's point of attachment to a link (from [2])
Multihomed Node
A node (either a host or a router) is multihomed when it has
several IPv6 addresses to choose between, i.e. in the following
cases when it is either:
* multi-prefixed: multiple prefixes are advertised on the link(s)
the node is attached to, or.
* multi-interfaced: the node has multiple interfaces to choose
between, on the same link or not.
Multihomed Network
From the above definition, it follows that a network is multihomed
when either the network is simultaneously connected to the
Internet via more than one router, or when a router is
multi-prefixed or multi-interfaced.
3. Goals and Benefits of Multihoming
We cannot distinguish the goals from the benefits of multihoming, but
there are several situations where it is either advisable or
beneficial to be multihomed:
3.1 Permanent and Ubiquitous Access
To provide an extended coverage area via distinct access
technologies. Multiple interfaces bound to distinct technologies can
be used to ensure a permanent connectivity is offered.
3.2 Redundancy/Fault-Recovery
To act upon failure of one point of attachment, i.e. the functions
of a system component are assumed by secondary system components when
the primary component becomes unavailable (e.g. failure).
Connectivity is guaranteed as long as at least one connection to the
Internet is maintained.
3.3 Load Sharing
To spread network traffic load among several routes. This is
achieved when traffic load is distributed among different connections
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between the node and the Internet [7].
3.4 Load Balancing
To balance load between multiple points of attachment (simultaneously
active or not), usually chosing the less loaded connection or
according to preferences on the mapping between flows and interfaces.
3.5 Bi-casting (n-casting)
To duplicate a particular flow simultaneously through different
routes. This minimizes packet loss typically for real-time
communication and burst traffic. It also minimizes delay of packet
delivery caused by congestion and achieves more reliable real-time
communication than single-casting. For mobile computing, bi-casting
avoids dropping packets when a mobile node changes its interface
during communication [1].
3.6 Preference Settings
To provide the user or the application or the ISP the ability to
choose the preferred transmission technology or access network. for
matters of cost, efficiency, politics, bandwidth requirement, delay,
etc.
3.7 Increased Bandwidth
To provide the user or the application with more bandwidth than is
available with any one interface. Multiple interfaces connected to
different links can increase the total available bandwith.
4. Scenarios
The following real-life scenarios highlight the benefits of
multihoming. Each scenario usually yields more than one of the
benefits outlined in the above section. All scenarios focus on
wireless technologies though no mobility management may be involved
(one can use wireless access at office).
The first scenario focuses on using two wireless interfaces for the
purpose of increasing bandwidth while the second shows the usage of
preference settings. The third is a combination of the first two.
The fourth and fifth illustrate how multiple connections can provide
ubiquitous Internet access and how load can be balanced according to
some preferences. The last one illustrates redundancy and
bi-casting.
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4.1 Load Balancing, Increased Bandwidth (no mobility)
Alice is at the airport waiting to board the plane. She receives a
call from her husband. This audio communication is received via a
wireless local area network (WLAN) link realized over one of the
available hot-spots. She knows this is going to be a long flight and
wishes to catch up on some work. Alice uses a WLAN connection to
download the necessary data. However, there is not enough time and
Alice decides to accelerate the download. Her notebook is equipped
with an additional WLAN interface. Alice decides to use this
additional WLAN interface to connect to another access point, and
distribute the different download flows between the two wireless
interfaces.
4.2 Preference Settings and Transparent Flow Handoffs (with mobility)
Mr. Smith is on his way to work waiting at a train station. He uses
this opportunity and the presence of a WLAN hot-spot to download the
news from his favorite on-line news channel. His train is announced.
Mr. Smith decides to buy a ticket. However, the ticket reservation
service is only available via a wide area cellular link of a specific
provider. While Mr. Smith is downloading the news and accessing the
train ticket reservation service, he receives a phone call over a
wide area cellular link. Mr. Smith decides he wishes to initiate a
video flow for this communication. The bandwidth and traversal delay
of the wide area cellular link is not adequate for the video
conference, so both flows (video/audio) are transferred to the WLAN
link provided by the hot-spot. This transfer occurs transparently
and without affecting any other active flows.
4.3 Preference Settings for House Networking (fixed)
Mr. Verne works at home for a publishing company. He has an
in-house network and get access to the Internet via ADSL, a public
802.11b WLAN from the street and satellite. He has subscribed to the
lowest ADSL service with limited upward bandwidth. The satellite
link he has access to is only downward but is extremely cheap for TV
broadcasting. He has noticed the 802.11b is unreliable at some point
in time during the day, so he chooses to send requests and periodic
refreshments for joining the TV broadcasting via ADSL rather the
802.11b although 802.11b in the street is free. On the other hand,
he has configured his network to use the 802.11b link at night to
publish web content comprising video. Once a week, he communicate
with overseas peer staff by videoconferencing. Voice being the most
important, he has configured his VoIP session over ADSL. Video is
sent at maximum rate when 802.11b is working fine, otherwise the
video is sent at lower rate.
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4.4 Load Balancing, Preference Settings, Increased Bandwidth (no
mobility)
An ambulance is called at the scene of a car accident. A paramedic
initiates a communication to a hospital via a wide area cellular link
for the relay of low bit-rate live video from the site of the crash
to assess the severity of the accident. It is identified that one of
the passengers has suffered a severe head injury. The paramedic
decides to consult a specialist via video conferencing. This session
is initiated from the specialist via the same wide area cellular
link. Meanwhile, the paramedic requests for the download of the
patient medical records from the hospital servers. The paramedic
decides in mid-session that the wide area cellular link is too slow
for this download and transfers the download to the ambulance
satellite link. Even though this link provides a significantly
faster bit rate it has a longer traversal delay and only down-link is
available. For this, only the down-stream of the download is
transferred while up-stream proceeds over the wide area cellular
link. Connectivity with the ambulance is managed over a WLAN link
between the paramedic and the ambulance. Even though the paramedic
has performed a partial hand-off for the transfer of the download
down-stream to the satellite link, the upstream and the video
conferencing session remains on the wide area cellular link. This
serves best the time constraint requirements of the real time
communication.
4.5 Ubiquitous Access and Load Sharing (with mobility)
Jules drives his car and constantly keeps some sort of Internet
connectivity through one of the available access technologies. His
car navigator downloads road information from the Internet and his
car-audio plays on-line audio streaming. When his car passes an area
where both high-data-rate WLAN and low-data-rate cellular network are
available, it distributes load to the WLAN access and the cellular
network access. When his car passes an area where only a wide
coverage-range cellular network is available, it maintains its
connection via the cellular network.
4.6 Redundancy and Bi-Casting (with no mobility)
Dr. Catherine performs an operation via long-distance medical
system. She watches a patient in a battle field over the screen
which delivers real-time images of the patient. Sensors on her arms
deliver her operational action and a robot performs her operation in
the battle field. Since the operation is critical, the delivery of
patient images and Catherine's action is done by bi-casting from/to
multiple interfaces bound to a distinct technology or distinct radio
range. So in case packets are delayed or one of the interface fails
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to maintain connectivity to the network, her distant can be
continued.
5. Classification
From the definition of a multihomed node it follows that a multihomed
node has several IPv6 addresses to choose between. In order to
expose the goals and benefits to manage multihomed nodes, we propose
to distinguish two main cases: either the node has only one
interface, or the node has several interfaces.
5.1 Case 1: One Interface, Multiple Prefixes
The single-interfaced node is multihomed when several prefixes are
advertised on its interface. The node must therefore configure
several IPv6 addresses. The node has to choose which address to use
when an IPv6 communication is established (e.g. open a TCP
connection). This choice can be influenced by many parameters: user
preference, different price on prefixes, preference flag in Router
Advertisement, destination prefix, etc. An address selection
mechanism is needed.
A typical example is a node with an IEEE 802.11 interface, connected
to an access point. The access point is connected trough an Ethernet
link to two access routers. Each access router is configured to send
Router Advertisements on the link and can be used as default router.
Several reasons may lead to configure two access routers are on the
same link: for instance, the access points may be shared between
different ISPs, or two access routers may be used for redundancy or
load sharing purposes. The node will then build two global IPv6
addresses on its interface.
We now analyse which of the benefits detailed in Section 3 can be
attained for this configuration.
o Ubiquitous Access
Ubiquitous access cannot be guaranteed when the node looses
Internet connectivity through its sole interface (e.g. the node
is going outside the coverage area of its access point).
o Redundancy
In case of failure of one IPv6 prefix, one of the address of the
node will not be valid anymore. Another available address built
from other prefixes should allow the node to recover this sort of
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failure. However transparency may not be achieved since on-going
sessions using the invalid address would have to be terminated,
and restarted using the new address. To avoid this, the node
needs a recovery mechanism allowing to redirect all current
communication to one of its other IPv6 address. The time needed
for the detection of the prefix failure and the time to redirect
communications to one of its other addresses is considered as
critical.
o Load sharing
Load Sharing can be performed in the network, according to the
address used by the node. The choice of the address used by the
node and the router selection can be influenced by the load
sharing rules. This mostly benefits the network side: if
different access routers or routes can be used to forward the
node's traffic, it will share the traffic load on the network.
o Load balancing
Load balancing cannot be performed as the node has only one
interface.
o Bi-casting
Bi-casting can be performed to ensure the delivery of packets on
the node. To do so, more than one IPv6 address must be used
simultaneously for one flow. Although packets can not be
distributed to different interfaces on the node, bi-casting would
allow the node to seamlessly change the address used on the node
if such a protocol is used to change address of on-going flow.
Time synchronization can be an issue in this case. If we use
different access technologies or routes for each casting, the
round trip time (RTT) can differ from casting to casting. Thus
the receiver will receive the same contents at different times.
o Preference
The source address can be chosen according to preferences set up
by the user, or according to preferences set up in the network
(such as with the default router preferences option introduced in
Router Advertisement [6], or by the ISP.
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o Increased Bandwidth
With only one interface connected to a link, the node generally
will not be able to enjoy increased bandwidth with multiple
prefixes. However, this benefit might be gained indirectly. For
instance, by alternating between different addresses, the total
throughput may be higher (eg. due to load sharing). Also, some
web and file transfer servers limit transfer bandwidths based on
the client's address. By using different addresses to connect to
the same server, the node may also see an increase in file
transfer rate.
5.2 Case 2: Several Interfaces
In this case, the node may use its multiple interfaces either
alternatively or simultaneously. If used alternatively, the node is
either multihomed if multiple prefixes are advertised on its current
link (case 1, one interface), or not multihomed if only one prefix is
advertised on its current link. We will thus assume that multiple
interfaces are used simultaneously. At least one IPv6 address will
be configured per interface (or several addresses per interface if
several prefixes are announced on the link(s) it is connected to).
Also, multiple interfaces can be connected to the same link as well
as to different links. These configurations will imply different
issues.
An address selection mechanism is also needed, but this time the
interface on which the address is bound to will be a supplementary
parameter in the address selection. The different characteristics of
each interface may help to decide first which interface to use.
A typical example is a node with two interfaces, each one on a
different technology (e.g. a WLAN IEEE 802.11b interface and a 3GPP
GPRS interface), in order to benefit from a better coverage area
(ubiquitous access) and the characteristics of each technology.
This multihomed configuration may yield different benefits to the
node. We now analyse how each of the benefits listed in Section 3
could be applied:
o Ubiquitous Access
It is easier to guarantee ubiquitous access when the node has
multiple interfaces since distinct technologies may be available
at a given time according to the location and administrative
policies. However, the node must be able to use several
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technologies at the same time and to maintain Internet
connectivity while a technology can not be used. It is obvious
that the node must have the choice to use any of the available
technologies, and that this choice must not prevent the node to
redirect a communication to another interface/address.
o Redundancy
Two levels of redundancy can be seen in this case: either one
address of one interface is not valid anymore (e.g. because the
corresponding prefix is not advertised on the link), or the node
loses its internet connectivity through one interface. In the
former case, another IPv6 address available on the interface would
allow the node to switch addresses for on-going flows. In the
latter case, another connection to the internet through another
interface would allow it to redirect on-going flow from the
previous interface to the new one. In either cases the node needs
to change the IPv6 address for on-going sessions from the no
longer valid address to one of the address available on the target
interface. The redirection will trigger a decision process to
choose the best target interface to redirect the flow. In both
cases, transparency of the addressses switching is an important
issue.
Loss of a prefix: If the node loses one of its prefix, it can no
longer use the corresponding address anymore. So the node needs a
recovery mechanism that allows it to transfer all current
communications to one of its other IPv6 address(es). The time
needed for the detection of the prefix failure and the time to
redirect communications to one of its other addresses may be
critical.
Interface failure: If one of the used interface breaks down (loss
of network connection or access router is not reachable anymore),
the node must be able to redirect all its flows from that
interface to one of the alive interfaces. The time needed to
discover the failure and to redirect each flow has to be
considered. The scalability of such a solution is also an issue.
Mobility of the node: If the node moves to a new point of
attachment in another subnet, it will need to change its IPv6
addresses. In order to maintain all its previous communications,
it will need to redirect the flows to its new point of attachment,
whatever the old address used for the flow. The scalability of
the redirection can also be considered here.
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o Load Sharing
This benefit is mainly for the network side: if different access
routers or routes can be used to forward traffic going into and
out of the node, they can share the traffic load on the network.
If the node uses several addresses at the same time for its
on-going sessions, load sharing can be performed in the network.
This goal can be a parameter that helps the source address
selection.
o Load balancing
Load balancing can be achieved on the node if several interfaces
are used simultaneously. Several interfaces can be used to spread
the traffic load on the node. This implies the choice of the IPv6
address to use for each flow and the ability to choose a different
address for each flow.
o Bi-casting
Bi-casting might be used to ensure the packets delivery on the
node. It also allow seamless redirection between two addresses /
interfaces with zero packets loss. Bi-casting can be performed if
several IPv6 addresses can be simultaneously used for one flow.
One entity between the CN (included) and the node (excluded) must
duplicate the traffic to the destination node.
o Preferences
Interface and address selection is required. The problem can be
seen exactly as in the first case (the node has only one
interface) if we consider that the interface preference is a
parameter for the address selection. Therefore in this case, the
interface selection/preference is a supplementary parameter in the
address selection algorithm.
o Increased Bandwidth
With multiple interface connected to a link, the node generally
will be able to enjoy increased bandwidth.
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6. Issues
In this section, we attempt to list a number of generic issues
[[Comment.1: Tentative section under construction --Note]]
6.1 Router Selection
Each access router the node is connected to might have different
capacity. For example, if the network the node is located in (either
a fixed network or a mobile network) is connected to the Internet via
2 routers, each path may have very different bandwidth and delay.
This would have a strong impact on the node behind those routers.
Therefore, it is desirable for the node to obtain enough information
so that it can choose its best default router.
6.2 Source Address Selection
The node has to choose which address to use when an IPv6
communication is established (e.g. open a TCP connection). This
choice can be influenced by many parameters: user preference,
different price on prefixes, preference flag in Router Advertisement,
destination prefix, etc. An address selection mechanism is needed.
6.3 Flow Redirection and Broken Sessions
Sessions may break as a result of diverting from one interface or
prefix to another. With no support mechanism, an address change
would cause on-going sessions using the invalid former address to
terminate, and to be restarted using the new address. To avoid this,
the node needs a recovery mechanism allowing to redirect all current
communication to one of its other IPv6 address.
6.4 Recovery Delay
The time needed for the detection an address has become invalid and
the time to redirect communications to one of its other addresses is
considered as critical.
6.5 Mobility
A node which moves to a new point of attachment in another subnet
must obtain a new IPv6 address on the new link. In order to maintain
sessions, all flows must be redirected to the new location and a
mobility management solution may be required, such as Mobile IPv6 [4]
or NEMO Basic Support [5]. More mechanisms may be needed if the node
was using several addresses on its old link, such as which flow to
redirect, which address must be associated with the new address(es).
The scalability of the operations involved in the redirection of
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flows may also be an issue, if we consider that the node had several
addresses on the old link and several flows and/or correspondents.
Issues pertaining to Mobile IPv6 are explained in companion drafts
[9] and [8].
7. Acknowledgments
We would like to thank all the people who have provided comments on
this draft, and also co-authors of earlier documents in which authors
of this present document have been engaged. As such, we would like
to thank Niko A. Figouras, Hesham Soliman, Ken Nagami, and many
others.
8. References
[1] Malki, K. and H. Soliman, "Simultaneous Bindings for Mobile
IPv6 Fast Handovers",
Internet-Draft draft-elmalki-mobileip-bicasting-v6-05, November
2003.
[2] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[3] Abley, J., Black, B. and V. Gill, "Goals for IPv6
Site-Multihoming Architectures", RFC 3582, August 2003.
[4] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[5] Devarapalli, V., Wakikawa, R., Petrescu, A. and P. Thubert,
"Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
January 2005.
[6] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes",
Internet-Draft draft-ietf-ipv6-router-selection-06, October
2004.
[7] Hinden, R., "IPv6 Host to Router Load Sharing",
Internet-Draft draft-ietf-ipv6-host-load-sharing-03, October
2004.
[8] Ng, C., Paik, E. and T. Ernst, "Analysis of Multihoming in
Network Mobility Support",
Internet-Draft draft-ietf-nemo-multihoming-issues-02, February
2005.
[9] Montavont, N., Wakikawa, R. and T. Ernst, "Analysis of
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Multihoming in Mobile IPv6",
Internet-Draft draft-montavont-mobileip-multihoming-pb-statement-03
, January 2005.
[10] Fikouras, N., "Mobile IPv4 Flow Mobility Problem Statement",
Internet-Draft draft-nomad-mip4-flow-mobility-pb-00.txt, Feb
2004.
Authors' Addresses
Thierry Ernst
WIDE at Keio University
Jun Murai Lab., Keio University.
K-square Town Campus, 1488-8 Ogura, Saiwa-Ku
Kawasaki, Kanagawa 212-0054
Japan
Phone: +81-44-580-1600
Fax: +81-44-580-1437
Email: ernst@sfc.wide.ad.jp
URI: http://www.sfc.wide.ad.jp/~ernst/
Nicolas Montavont
LSIIT - Univerity Louis Pasteur
Pole API, bureau C444
Boulevard Sebastien Brant
Illkirch 67400
FRANCE
Phone: (33) 3 90 24 45 87
Email: montavont@dpt-info.u-strasbg.fr
URI: http://www-r2.u-strasbg.fr/~montavont/
Ryuji Wakikawa
Keio University
Jun Murai Lab., Keio University.
5322 Endo
Fujisawa, Kanagawa 252-8520
Japan
Phone: +81-466-49-1100
Fax: +81-466-49-1395
Email: ryuji@sfc.wide.ad.jp
URI: http://www.mobileip.jp/
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Eun Kyoung Paik
KT
Portable Internet Team, Convergence Lab., KT
17 Woomyeon-dong, Seocho-gu
Seoul 137-792
Korea
Phone: +82-2-526-5233
Fax: +82-2-526-5200
Email: euna@kt.co.kr
URI: http://mmlab.snu.ac.kr/~eun/
Chan-Wah Ng
Panasonic Singapore Laboratories Pte Ltd
Blk 1022 Tai Seng Ave #06-3530
Tai Seng Industrial Estate
Singapore 534415
SG
Phone: +65 65505420
Email: cwng@psl.com.sg
Koojana Kuladinithi
University of Bremen
ComNets-ikom,University of Bremen.
Otto-Hahn-Allee NW 1
Bremen, Bremen 28359
Germany
Phone: +49-421-218-8264
Fax: +49-421-218-3601
Email: koo@comnets.uni-bremen.de
URI: http://www.comnets.uni-bremen.de/~koo/
Thomas Noel
LSIIT - Univerity Louis Pasteur
Pole API, bureau C444
Boulevard Sebastien Brant
Illkirch 67400
FRANCE
Phone: (33) 3 90 24 45 92
Email: noel@dpt-info.u-strasbg.fr
URI: http://www-r2.u-strasbg.fr/~noel/
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Appendix A. Change Log From Previous Version
o Added tentative section "Issues"
o Typos, rephrasing, added sub-sections into TOC, updated
references, added ACK section
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