No Specific Working Group T. Ernst
Internet-Draft WIDE at Keio University
Expires: August 9, 2004 N. Montavont
LSIIT - ULP
R. Wakikawa
Keio University
E. Paik
Seoul National University
C. Ng
Panasonic Singapore Labs
K. Kuladinithi
University of Bremen
T. Noel
LSIIT - ULP
February 9, 2004
Goals and Benefits of Multihoming
draft-multihoming-generic-goals-and-benefits-00
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on August 9, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document attempts to define the goals and benefits of
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multihoming for fixed and mobile hosts and routers. Those goals and
benefits are illustrated with a set of real-life scenarios.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Goals and Benefits of Multihoming . . . . . . . . . . . . . . 4
4. Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Configurations . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
New equipments shipped on the market now often integrate several
wireless technologies.
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) since a permanent Internet connectivity
is now required by some applications. Unfortunately, there is no
network interfaces assuring global scale connectivity. Nodes must
thus use various type of network interfaces to obtain wide area
network connectivity [8].
New equipments also integrate several access technologies 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 should thus be able
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 show real-life scenarios in order
to illustrate 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. Specific
mobility issues pertaining to mobile nodes and mobile networks are
discussed in companion drafts [6], [5] and [7].
This document is organized as follows: we first define the terms used
in the document before emphasizing the goals and benefits of
multihoming. Next follows a differentiation between cases where a
multihomed node has a single interface and the cases where it has
multiple interfaces. Then, we describe some real-life scenarios to
illustrate the goals and benefits of multihoming and we conclude with
the description of possible configurations at the network layer.
2. Terminology
In this section we define terms related to multihoming:
Interface (from [2])
A node's attachment to a link
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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:
multi-prefixed: multiple prefixes are advertised on the link(s)
the node is attached to.
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 of multihoming from the benefits of
being multihomed, but we can identify several situations where it is
either advisable or beneficial to be multihomed:
Ubiquitous Access:
To provide an extended coverage area. Multiple interfaces bound to
distinct technologies can be used to ensure a permanent
connectivity is offered.
Redundancy/Fault-Recovery:
To act upon failure of one point of attachment, i.e. the functions
of a network 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.
Load Sharing:
To spread network traffic load among several routes. This is
achieved when traffic load is distributed simultaneously among
different connections between the node and the Internet [4].
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Load Balancing:
To balance load between multiple point of attachments
(simultaneously active or not), usually chosing the lest loaded
connection.
Bi-casting:
Bi-casting (n-casting) duplicates a particular flow for
simultaneous transmission through different routes. It minimizes
packet loss typically for real-time communication and burst
traffic. It also minimizes delay of packet delivery caused by
congestions and achieves more reliable real-time communication
than single-casting. For mobile computing, bi-casting is useful
not to drop packets when a mobile node changes its interface
during communication [1].
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.
When considering these goals/benefits, one has to consider whether
these goals can be achieved with transparency or without
transparency. Transparency is achieved when switching to a different
point of attachment does not cause on-going sessions to break.
For instance, ubiquity with transparency is achieved when switching
between two different access mechanism does not cause on-going
sessions to be disrupted.
In order to achieve transparency, a necessary (may or may not be
sufficient) condition is for the end-point addresses to remain
unchanged. This is in-view of the large amount of Internet traffic
today are carried by TCP, which unlike SCTP, cannot handle multiple
end-point address pairs.
4. Cases
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. A rough analysis of these two
cases is detailed below.
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Case 1: One Interface
The node has one interface. It is multihomed when several prefixes
are advertised on its interface. The node must therefore configure
several IPv6 addresses. An address selection mechanism is needed.
This multihomed configuration may yield the following benefits for
the node:
* Load sharing can be performed in the network
* Redundancy: In case of failure of one IPv6 prefix, one of the
address of the node will not be valid anymore. The fact that
the node has another address built with other prefixes should
allow it to recover this sort of failure, however transparency
may not achieved since on-going sessions using the invalid
address would have to be terminated, and restarted using the
new address. It is also to be noted that all sessions on the
node will be disrupted if all prefixes fail to be announced
(e.g. all were announced by the same router and this router
broke down).
* 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 [3], or by ISP.
Case 2: Several Interfaces
The node has more than one interface. The node must therefore
configure several IPv6 addresses (one on each interface). An
address selection mechanism is needed.
This multihomed configuration may yield the following benefits for
the node:
* Load balancing: between the interfaces
* Redundancy: another interface can be used if one fails
* Ubiquity: it is more likely to have access to another
technology if a technology cannot be used
* 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
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supplementary parameter in the address selection algorithm.
5. Scenarios
The following scenarios highlight the usage of several interfaces and
the benefit of such configuration. Each scenario usually yields more
than one benefit. The first scenario focuses on using two wireless
interfaces for the purpose of balancing the load while 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.
Scenario 1: Load Balancing
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 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 wireless LAN 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 wireless local area network interface. Alice
decides to distribute the different download flows between the
wireless interfaces to accelerate the download.
Scenario 2: Preference Settings and Transparent Flow Handoffs
Mr. Smith is on his way to work waiting at a train station. He
uses this opportunity and the presence of a wireless LAN 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 a wireless local area link
via a hot-spot. This transfer occurs transparently and without
affecting any other active flows.
Scenario 3: Preference Settings for House Networking
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Mr. Vernes 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. The satellite link he
has access to is only downward but is extremely cheap for TV
broadcasting. He chooses to send requests for joining the TV
broadcasting via ADSL rather the 802.11b although 802.11b in the
street is free. He has noticed the 802.11b is unreliable at some
point in time during the day. 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 one
picture every 5 sec over ADSL.
Scenario 4: Ubiquitous Access, Load Balancing, Preference Setting
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 wireless local area 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.
Scenario 5: Ubiquitous Access and Load Sharing
Jules drives his car to a new place and constantly keeps some sort
of Internet access through one the access technology. His car
navigator downloads road information from the internet and his
car-audio serves on-line audio streaming. When his car passes an
area where both high-data-rate WLAN and low-data-rate cellular
network available, it distributes load to WLAN and cellular
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connection. When his car passes an area where only
wide-coverage-range cellular network is available, it maintains
its connection via the cellular network. When Bob passes an area
where even cellular networks can not be reached, he can switch to
the expensive satellite network with multiple interfaces that his
car support.
Scenario 6: Redundancy and Bi-Casting
Catherine performs an operation via long-distance medical system.
She watches a patient in a battle field over the screen which
delivers real-time image 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 image and Catherine's action is done by bi-casting
from/to multiple interface. So in case of one of the interface
failed, Catherine can continue her long-distance operation.
6. Configurations
This section details the possible network configurations that are
considered important. Possible configurations may involve either a
fixed host, a mobile host, a fixed router or a mobile router.
Case 1: One Interface
A typical case of this scenario 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 Advertisement on the
link and can be used as default router. Several reasons may lead
to the fact that 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.
In that case, the node will build two global IPv6 addresses on its
interface. When the node establishes an IPv6 communication (e.g.
open a TCP connection), it has to choose which address to use.
This choice can be influenced by many parameters: user preference,
different price on prefixes, preference flag in Router
Advertisement, destination prefix...
The critical points that can be highlighted here are the
following:
* Choice of the router: each access router the node is be
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connected to might have different capacity. As an example, if
the node is inside a mobile network and is connected to two
mobile routers, each mobile router may implement different
technology on their egress interface. This would have a strong
impact on the node traffic.
* Load Sharing: This benefit is mainly for the network side: if
different access routers or routes can be used to forward the
node traffic, it will share the traffic load on the network.
* Lost of a prefix: If the node looses one of its prefix, it can
not use the corresponding address anymore. So the node needs a
recovery mechanism allowing to remove 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.
* 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 communication,
it will need to redirect the flows to its new location,
whatever the old address used for the flow. The scalability of
the redirection can also be considered here.
Case 2: Multiple Interfaces
The typical case of this scenario is a node that has two
interfaces, each on a different technology, in order to benefit a
better coverage area (ubiquitous access) and the capacity and
specification of each technology. Such an example is to have a
WLAN interface (e.g. IEEE 802.11b) and a 3GPP interface (e.g.
GPRS). In this case, the node may use its two interfaces either
alternatively or simultaneously. If it alternatively uses its two
interfaces, the node falls into case one (multiple prefixes and
one interface) and is multihomed or the node is not multihomed. In
the following, we thus consider that the node simultaneously uses
its two interfaces.
In this case, the node will have one or several addresses per
interface according to the number of prefixes announced on the
link(s) it is connected to. Also, the two interfaces can be
connected to the same link as well as to different link. When
considering such a multihoming management, it might imply
different issues. Once more this node will have to make a choice
between its different addresses, but the interface on which the
address is bound to will be a supplementary parameter in the
address selection. The different characteristics of each interface
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may help to decide first which interface to use.
The critical points that can be highlighted here are the
following:
* Choice of the router: each access router the node is be
connected to might have different capacity. As an example, if
the node is inside a mobile network and is connected to two
mobile routers, each mobile router may implement different
technology on their egress interface. This would have a strong
impact on the node traffic.
* Load Balancing: As the node has several available interfaces at
the same time, it is interesting to simultaneously use them for
different flows. To do so, the node must be able to choose a
different interface for each new communication (through the
address selection).
* Load Sharing: This benefit is mainly for the network side: if
different access routers or routes can be used to forward the
node traffic, it will share the traffic load on the network.
* Lost of a prefix: If the node looses one of its prefix, it can
not use the corresponding address anymore. So the node needs a
recovery mechanism allowing to remove 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.
* Interface failure: If one of the used interface breaks down
(lost of network connection of access router is not reachable
anymore), the node must be able to redirect all its flows from
that interface to one of the alive interface. 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 communication,
it will need to redirect the flows to its new localization,
whatever the old address used for the flow. The scalability of
the redirection can also be considered here.
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7. Acknowledgments
References
[1] Elmalki, K. and H. Soliman, "Simultaneous Bindings for Mobile
IPv6 Fast Handovers", draft-elmalki-mobileip-bicasting-v6-05.txt
(work in progress), October 2003.
[2] Manner, J. and M. Kojo, "Mobility Related Terminology",
draft-ietf-seamoby-terminology-04 (work in progress), April
2003.
[3] Draves, R. and D. Thaler, "Default Router Preferences,
More-Specific Routes, and Load Sharing",
draft-ietf-ipv6-router-selection-03 (work in progress), December
2003.
[4] Hinden, R. and D. Thaler, "IPv6 Host to Router Load Sharing",
draft-ietf-ipv6-host-load-sharing-01 (work in progress), January
2004.
[5] Ng, C., "Multihoming Issues in Network Mobility Support",
draft-xxx-nemo-multihoming-00 (work in progress), Feb 2004.
[6] Montavont, N., "Analysis of Multihoming in Mobile IPv6",
draft-xxx-nemo-multihoming-00 (work in progress), Feb 2004.
[7] Fikouras, N., "Mobile IPv4 Flow Mobility",
draft-nomad-mip4-flow-mobility-problem-statement-00.txt (work in
progress), Feb 2004.
[8] Stemm, M. and R. Katz, "Vertical Handoffs in Wireless Overlay
Networks", Journal Mobile Networks and Applications, vol. 3,
number 4, pages 335-350, 1998.
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Authors' Addresses
Ernst Thierry
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/
Wakikawa Ryuji
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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Paik, Eun Kyoung
Seoul National University
Multimedia Communications Lab., Seoul National Univ.
Shillim-dong, Kwanak-gu
Seoul 151-744
Korea
Phone: +82-2-880-1832
Fax: +82-2-872-2045
EMail: eun@mmlab.snu.ac.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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