NEMO Working Group C. Ng
Internet-Draft Panasonic Singapore Labs
Expires: April 16, 2004 J. Charbon
Keio and Louis Pasteur University
E. Paik
Seoul National University
October 17, 2003
Multihoming Issues in Network Mobility Support
draft-ng-nemo-multihoming-issues-02
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes deployment scenario of multihomed mobile
network and attempts to identify issues that arises when supporting
multihoming in Network Mobility Support (NEMO. It is also the
objective of this document to build a full taxonomy covering
multihomed scenarios in NEMO, and to identify different cases of
multihomed scenario that is supported by NEMO Basic Support [1].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Motivations . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Organization . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Terms and Abbreviation . . . . . . . . . . . . . . . . . . . 4
2. Classification . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Configuration-Oriented Approach . . . . . . . . . . . . . . 5
2.1.1 (1,1,1): Single MR, Single HA, Single Prefix . . . . . . . . 6
2.1.2 (1,1,N): Single MR, Single HA, Multiple Prefixes . . . . . . 7
2.1.3 (1,N,1): Single MR, Multiple HAs, Single Prefix . . . . . . 7
2.1.4 (1,N,N): Single MR, Multiple HAs, Multiple Prefixes . . . . 8
2.1.5 (N,1,1): Multiple MRs, Single HA, Single Prefix . . . . . . 9
2.1.6 (N,1,N): Multiple MRs, Single HA, Multiple Prefixes . . . . 9
2.1.7 (N,N,1): Multiple MRs, Multiple HAs, Single Prefix . . . . . 10
2.1.8 (N,N,N): Multiple MRs, Multiple HAs, Multiple Prefixes . . . 11
2.2 Ownership-Oriented Approach . . . . . . . . . . . . . . . . 11
2.3 Problem-Oriented Approach . . . . . . . . . . . . . . . . . 13
3. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . 15
4. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1 Benefits/Issues of Multihoming in NEMO . . . . . . . . . . . 17
4.1.1 Fault Tolerance . . . . . . . . . . . . . . . . . . . . . . 17
4.1.2 Load Sharing . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2 Ownership-Oriented Approach . . . . . . . . . . . . . . . . 20
4.3 Configuration-Oriented Approach . . . . . . . . . . . . . . 21
5. Security Considerations . . . . . . . . . . . . . . . . . . 26
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 26
References . . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 27
A. Nested Tunneling for Fault Tolerance . . . . . . . . . . . . 27
A.1 Detecting Presence of Alternate Routes . . . . . . . . . . . 28
A.2 Re-Establishment of Bi-Directional Tunnels . . . . . . . . . 28
A.2.1 Using Alternate Egress Interface . . . . . . . . . . . . . . 28
A.2.2 Using Alternate Mobile Router . . . . . . . . . . . . . . . 29
A.3 To Avoid Tunneling Loop . . . . . . . . . . . . . . . . . . 29
A.4 Other Considerations . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . 31
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1. Introduction
1.1 Motivations
The problem of Network Mobility Support (NEMO) is identified in
various previous works [2]. In essence, the problem of network in
motion is to provide continuous Internet connectivity to nodes in a
network that moves as a whole. Nodes within the network that moves
may not be aware of the network changing its point of attachment to
the Internet. This differs from the traditional problem of mobility
support as addressed by Mobile IPv6 [3].
In Mobile IP, each mobile node has a permanent home domain. When the
mobile node is attached to its home network, it is assigned a
permanent global address known as a home-address (HoA). When the
mobile node is away, i.e. attached to some other foreign networks, it
is usually assigned a temporary global address known as a
care-of-address (CoA). The idea of mobility support is such that the
mobile node can be reached at the home-address even when it is
attached to other foreign networks. This is done in [3] with the
introduction of an entity at the home network known as a home agent
(HA). Mobile nodes register their care-of-addresses with the home
agents using messages known as Binding Updates. The home agent is
responsible to intercept messages that are addressed to the mobile
node's home-address, and forward the packet to the mobile node's
care-of-address using IP-in-IP Tunneling [4].
Extending the concept of mobility support for individual hosts to
mobility support for a network of nodes, the objective of a network
in motion solution is to provide a mechanism where nodes in a mobile
network can be reached by their permanent addresses, no matter where
on the Internet the mobile network is attached to. There exist a few
prior attempts to provide network mobility support, most of them
based on using bi-directional tunnels between the mobile routers and
the home agents of the mobile routers [1].
In bi-directional tunnels between mobile routers and home agents, the
mobile router controlling a mobile network performs routing of
packets to and from the mobile network when it is in its home domain.
When the mobile router and its mobile network move to a foreign
domain, the mobile router registers its care-of-address with its home
agent. An IP-in-IP tunnel is then set up between the mobile router
and the home agent. Every packet going to the mobile network will be
intercepted by the home agent and forwarded to the mobile router
through the IP-in-IP tunnel. The mobile router then forwards the
packet to a host in its mobile network. When a node in its mobile
network wishes to send a packet out of the network, the mobile router
intercepts the packet and forward the packet to the home agent
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through the IP-in-IP tunnel. The home agent then sends the packet out
to the intended recipient.
It is the interest of this memo to investigate if such a
bi-directional tunneling approach can be extended to a mobile network
that is multihomed. More specifically, we wish to identify issues
that may arise in bi-directional tunneling between mobile router and
home agent when the mobile network is multihomed.
1.2 Objectives
This document is written with two main objectives in mind:
o To capture issues of deploying a multihomed mobile network
o To identify which multihoming scenario NEMO basic solution would
support. It does not't mean that those not supported will not
work with NEMO, just that it is up to the implementors to make it
work (hopefully issues discussed in the document will be helpful
to these implementors).
1.3 Organization
In this document, we first look into different classifications of
multihomed mobile network in Section 2. This section outlines 3
different approaches to classifying multihomed mobile network. Next,
we described deployment scenarios of multihomed mobile networks in
Section 3. In Section 4, we go into detailed analysis of multihomed
networks. Benefits and issues of multihoming in network mobility
support is discussed.
1.4 Terms and Abbreviation
It is assumed that readers are familiar with the terminologies and
abbreviations as defined in [5].
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2. Classification
Various discussions on the topic of multihoming issues in NEMO has
been carried out on the Mailing List. As there can be a lot of
different deployments of a multihomed mobile network, there is a need
for us to classify multihomed mobile network into a clearly defined
set of taxonomy.
There are various ways in which multihomed mobile network can be
classified. We identify three main approach. These are, namely, (i)
Configuration-Oriented Approach, (iii) Ownership-Oriented Approach,
and (ii) Problem-Oriented Approach. These are described in greater
detail in the following sub-sections.
2.1 Configuration-Oriented Approach
There are various configurations of a multihomed mobile network,
depending on how many mobile routers are present, how many egress
interfaces and home addresses the mobile routers have, how many
subnet prefixes are advertised to the mobile network nodes, etc. In
order to facilitate discussions on multihomed mobile network, it is
necessary to identify what kind of configuration the mobile network
is in. Here, we identify three key parameters differentiating
different multihomed configurations. With these parameters, we can
refer to each configuration by the 3-tuple (w,x,y), where 'w', 'x',
'y' are defined as follows:
o 'w' differentiates the case of single mobile router (with multiple
egress interfaces or multiple home addresses) versus the case of
multiple mobile routers, where
w=1 implies a mobile network has only a single mobile router. In
this case, the mobile router either has multiple egress
interfaces or multiple home addresses bound to a single egress
interface.
w=N implies a mobile network has more than one mobile router
advertising an egress route.
o 'x' differentiates the case of a single home agent for the mobile
network versus the case of multiple home agents for the mobile
network, where
x=1 implies that a single home agent is assigned to manage binding
updates of the mobile network.
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x=N implies that more than one home agents (possibly in different
administrative domains) manage the binding updates of the
mobile network.
o 'y' differentiates the case of single mobile network prefix versus
multiple mobile network prefixes that is/are advertised to the
mobile network node, where
y=1 implies that a single subnet prefix is advertised to the
mobile network nodes.
y=N implies that more than one subnet prefixes are advertised to
the mobile network nodes.
It can be seen that the above three parameters are fairly orthogonal
to one another. Thus different values of 'w', 'x' and 'y' give rise
to different combinations of the 3-tuple (w,x,y). A total of 8
possible configurations can be identified. These are described
further in the following sub-sections.
2.1.1 (1,1,1): Single MR, Single HA, Single Prefix
The (1,1,1) mobile network has only one mobile router advertising a
single subnet prefix. In addition, the mobile router associates with
only one home agent at any one time. This makes the mobile network
very similar to a non-multihomed mobile network, except for the fact
that the mobile router has multiple simultaneously active connections
at the same time, e.g either (i) has multiple active egress links, or
(ii) multiple active egress addresses.
Since only one subnet prefix is advertised, the mobile network nodes
are (usually) not multihomed.
_____
_ p _ | |
|_|-|<-_ |-|_|-| |-| _
_ |-|_|=| |_____| | _ |-|_|
|_|-| | |-|_|-|
|
MNNs MR AR Internet AR HA
Figure 2.1 - (1,1,1) Multihomed Mobile Network
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2.1.2 (1,1,N): Single MR, Single HA, Multiple Prefixes
The (1,1,N) mobile network has only one mobile router, which
associates to only one home agent at any one time. However, two or
more subnet prefixes are advertised to the mobile network nodes. No
associations is assumed between the subnet prefixes and the home
addresses of the mobile router.
Since a plurality of subnet prefixes are advertised, mobile network
nodes can generally be multihomed themselves, where each mobile
network node is allocated one address in each subnet prefix.
_____
_ p1,p2 _ | |
|_|-|<-_ |-|_|-| |-| _
_ |-|_|=| |_____| | _ |-|_|
|_|-| | |-|_|-|
|
MNNs MR AR Internet AR HA
Figure 2.2 - (1,1,N) Multihomed Mobile Network
2.1.3 (1,N,1): Single MR, Multiple HAs, Single Prefix
The (1,N,1) mobile network has only one mobile router advertising a
single subnet prefix. The mobile router, however, associates to
multiple home agents, possibly one home agent per home addresses. No
assumption is made on whether or not the home agents belongs to the
same administrative domain.
Since only one subnet prefix is advertised, the mobile network nodes
are (usually) not multihomed.
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AR HA2
_ |
|-|_|-| _
_____ | |-|_|
_ p _ | |-|
|_|-|<-_ |-|_|-| |
_ |-|_|=| |_____|-| _
|_|-| | | _ |-|_|
|-|_|-|
|
MNNs MR AR Internet AR HA1
Figure 2.3 - (1,N,1) Multihomed Mobile Network
2.1.4 (1,N,N): Single MR, Multiple HAs, Multiple Prefixes
The (1,n,n) mobile network has only one mobile router. However, the
mobile router advertises more than one subnet prefix, and also
associates to multiple home agents at the same time, possibly one
home agent per home address. No assumptions is made on whether or
not the home agents belongs to the same administrative domain.
Since a plurality of subnet prefixes are advertised, mobile network
nodes can generally be multihomed themselves, where each mobile
network node is allocated one address in each subnet prefix.
AR HA2
_ |
|-|_|-| _
_____ | |-|_|
_ p1,p2 _ | |-|
|_|-|<-_ |-|_|-| |
_ |-|_|=| |_____|-| _
|_|-| | | _ |-|_|
|-|_|-|
|
MNNs MR AR Internet AR HA1
Figure 2.4 - (1,N,N) Multihomed Mobile Network
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2.1.5 (N,1,1): Multiple MRs, Single HA, Single Prefix
The (N,1,1) mobile network has more than one mobile router
advertising global routes. These mobile routers, however, advertise
the same subnet prefix and associate to the same home agent. Since
only one subnet prefix is advertised, the mobile network nodes are
(usually) not multihomed.
MR2
p
<-_ |
_ |-|_|-| _____
|_|-| |-| |
_ | | |-| _
|_|-| _ |-|_____| | _ |-|_|
|-|_|-| |-|_|-|
<- | |
p
MNNs MR1 Internet AR HA
Figure 2.5 - (N,1,1) Multihomed Mobile Network
2.1.6 (N,1,N): Multiple MRs, Single HA, Multiple Prefixes
The (N,1,N) mobile network has more than one mobile router
advertising different global routes and different subnet prefixes.
However, these mobile routers associate to the same home agents.
Since a plurality of subnet prefixes are advertised, mobile network
nodes can generally be multihomed themselves, where each mobile
network node is allocated one address in each subnet prefix.
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MR2
p2
<-_ |
_ |-|_|-| _____
|_|-| |-| |
_ | | |-| _
|_|-| _ |-|_____| | _ |-|_|
|-|_|-| |-|_|-|
<- | |
p1
MNNs MR1 Internet AR HA
Figure 2.6 - (N,1,N) Multihomed Mobile Network
2.1.7 (N,N,1): Multiple MRs, Multiple HAs, Single Prefix
The (N,N,1) mobile network has more than one mobile router
advertising different global routes. The mobile routers are also
associated to more than one home agents at any one time. No
assumptions is made on whether or not the home agents belongs to the
same administrative domain. However, the mobile routers advertises
the same subnet prefix. Since only one subnet prefix is advertised,
the mobile network nodes are (usually) not multihomed.
MR2 AR HA2
p _ |
<-_ | |-|_|-| _
_ |-|_|-| _____ | |-|_|
|_|-| |-| |-|
_ | | |
|_|-| _ |-|_____|-| _
|-|_|-| | _ |-|_|
<- | |-|_|-|
p |
MNNs MR1 Internet AR HA1
Figure 2.7 - (N,N,1) Multihomed Mobile Network
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2.1.8 (N,N,N): Multiple MRs, Multiple HAs, Multiple Prefixes
The (N,N,N) mobile network has more than one mobile router
advertising different global routes and different subnet prefixes.
The mobile routers are also associated to more than one home agent at
any one time. No assumptions is made on whether or not the home
agents belongs to the same administrative domain.
Since a plurality of subnet prefixes are advertised, mobile network
nodes can generally be multihomed themselves, where each mobile
network node is allocated one address in each subnet prefix.
MR2 AR HA2
p2 _ |
<-_ | |-|_|-| _
_ |-|_|-| _____ | |-|_|
|_|-| |-| |-|
_ | | |
|_|-| _ |-|_____|-| _
|-|_|-| | _ |-|_|
<- | |-|_|-|
p1 |
MNNs MR1 Internet AR HA1
Figure 2.8 - (N,N,N) Multihomed Mobile Network
2.2 Ownership-Oriented Approach
A second approach to classifying multihomed mobile networks is
proposed by Eric Nordmark (Sun Microsystems) by breaking the
classifications of multihomed network based on ownership. This is
more of a tree-like top-down classifications. Starting from the
control and ownership of the HA(s) and MR(s), there are two different
possibilities: either (i) the HA(s) and MR(s) are controlled by a
single entity, or (ii) the HA(s) and MR(s) are controlled by separate
entities.
The case of the HA(s) and MR(s) are controlled by the same entity can
be best illustrated as an Internet Service Provider (ISP) installing
mobile routers on trains, ships or planes. It is up to the ISP to
deploy a certain configuration of mobile network; all 8
configurations as described in the Configuration-Oriented Approach
are possible. In the remaining portion of this document, when
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specifically referring to a mobile network configuration that is
controlled by a single entity, we will add an 'ISP' prefix: for
example: ISP-(1,1,1) or ISP-(1,N,N).
The case of the HA(s) and MR(s) are controlled by the separate
entities can be best illustrated with a subscriber/provider model,
where the mobile routers belongs to a single subscriber and
subscribes to one or more ISPs for home agent services. There is two
sub-categories in this case: when the subscriber subscribes to a
single ISP, and when the subscriber subscribes to multiple ISPs. In
the remaining portion of this document, when specifically referring
to a mobile network configuration that is in the subscriber/provider
model where the subscriber subscribes to only one ISP, we will add an
'S/P' prefix: for example: S/P-(1,1,1) or S/P-(1,N,N). When
specifically referring to a mobile network configuration that is in
the subscriber/provider model where the subscriber subscribes to
multiple ISPs, we will add an 'S/mP' prefix: for example: S/
mP-(1,1,1) or S/mP-(1,N,N).
Not all 8 configurations are likely to be deployed for the S/P and S/
mP models. For instance, it is unlikely to foresee a S/mP-(*,1,1)
mobile network where there is only a single HA. For the S/P model,
the following configurations are likely to be deployed:
o S/P-(1,1,1): Single Provider, Single MR, Single HA, Single Prefix
o S/P-(1,1,N): Single Provider, Single MR, Single HA, Multiple
Prefixes
o S/P-(1,N,1): Single Provider, Single MR, Multiple HAs, Single
Prefix
o S/P-(1,N,N): Single Provider, Single MR, Multiple HAs, Multiple
Prefixes
o S/P-(N,N,1): Single Provider, Multiple MRs, Single HA, Single
Prefix
o S/P-(N,1,N): Single Provider, Multiple MRs, Single HA, Multiple
Prefixes
o S/P-(N,N,1): Single Provider, Multiple MRs, Multiple HAs, Single
Prefix
o S/P-(N,N,N): Single Provider, Multiple MRs, Multiple HAs, Multiple
Prefixes
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For the S/mP model, the following configurations are likely to be
deployed:
o S/mP-(1,N,1): Multiple Providers, Single MR, Multiple HAs, Single
Prefix
o S/mP-(1,N,N): Multiple Providers, Single MR, Multiple HAs,
Multiple Prefixes
o S/mP-(N,N,N): Multiple Providers, Multiple MRs, Multiple HAs,
Multiple Prefixes
2.3 Problem-Oriented Approach
A third approach to classifying multihomed mobile networks is
proposed by Pascal Thubert (Cisco System). This focused on the
problems of multihomed mobile networks rather than the configuration
or ownership. With this approach, there is a set of 4 categories
based on two orthogonal parameters: the number of home agents, and
the number of subnet prefixes advertised. Since the two parameters
are orthogonal, the categories are not mutually exclusive. The four
categories are:
o Tarzan: Single HA for Different Care-ofs of Same Prefix
This is the case where one mobile router registers different
care-of-addresses to the same home agent for the same subnet
prefix. This is equivalent to the case of x=1, i.e. the (1,1,N)
mobile network.
o JetSet: Multiple HA for Different Care-ofs of Same Prefix
This is the case where the mobile router registers different
care-of-addresses to different home agents for the same subnet
prefix. This is equivalent to the case of x=N, i.e. the (1,N,*)
mobile network.
o Shinkansen: Single Prefix Advertised by Mobile Router(s)
This is the case where one subnet prefix is announced by different
mobile routers. This is equivalent to the case of y=N, i.e. the
(1,*,N) mobile network.
o DoubleBed: Multiple Prefixes Advertised by Mobile Router(s)
This is the case where more than one subnet prefixes are announced
by the different mobile routers. This is equivalent to the case
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of y=N, i.e. the (N,*,N) mobile network.
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3. Deployment Scenarios
One example of the S/P-(1,1,*) mobile network is that a single ISP
offers two different wireless public access methods such as IEEE
802.11 and GPRS. A mobile router with both access interfaces (i.e.
802.11 and GPRS capabilities) may subscribe to the same ISP and is
allowed to use both access methods. The ISP will choose to provide a
single home agent for the same mobile router for ease of management.
This configuration is useful for maintaining connectivity between
several interfaces. An example will be to use 802.11 in town and
GPRS in the country side. In addition, it can also provide some
multihoming benefits (such as Fault Tolerance / Load Sharing) to MNNs
without having to involve the MNNs.
Extending the above example to a S/mP-(1,N,*) mobile network, the
mobile router may subscribe to different ISPs for different access
technologies. For instance, it may subscribe to 802.11 public access
services using one ISP, and subscribe to GPRS services from another
ISP. In this case, the two different ISPs will provide two different
home agents for the same mobile router. Since the two ISPs are
independent, under normal situation, each ISP will delegates
different subnet prefixes to the mobile network, thus forming a S/
mP-(1,N,N) mobile network.
An example of the (N,*,*) mobile network is when a mobile network
contains more than one device with independent routes to the global
Internet. An excellent illustration is the Wireless Personal Area
Network (W-PAN) where a mobile phone on the W-PAN connects to the
Internet via GPRS services, and a Personal Digital Assistant (PDA) on
the same W-PAN connects to the Internet via 802.11 public access. If
the ISPs provide both access technologies, then the subscriber can
subscribes to a all-in-one package where the ISP provides a single
home agent to manage the mobile network, and delegates a single
subnet prefix to the mobile network. This forms a S/P-(N,1,1) mobile
network. Alternatively, the subscriber can subscribes to two ISPs for
each access mechanism, thus giving a S/mP-(N,N,N) mobile network.
The (N,*,1) configuration provides easily a router redundancy and/or
HA redundancy for big mobile networks, such as within a train or a
plane, or critical mobile networks, such as those deployed in
ambulances, fire engines, or military vehicles.
Figure 3.1 below illustrate a (1,N,1) mobile network deployed on a
plane. In this example, the MR sends the same PBU to both HAs in
different cities, and communicates with both simultaneously. Thus a
Correspondent Node near Paris can choose the Paris's HA to send its
packets, and the MR inside the plane should send its packet to the
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New York's HA (which is nearer).
[The Jet]
_ |\_______ _
-|_|- <--------------> |____~___\ <--> -|_|-
HA1 MN inside the HA2
plane.
Paris New York
Figure 3.1 - Deployment example for (1,N,1)
Example for a (*,*,N) mobile network is a car network, where there
may be different logical subnets:
o the User Network which provides Internet connectivity to
passengers;
o the Control Network which exchanges car information (e.g.
position, movement, intern constants) with the others cars, or
with the society who use this car; and
o the Safeguard Network which shares state information of the car
with the emergency/repairing companies, or the emergency agencies
in case of accidents.
Because of these differences it can be useful to attribute a
different network prefix for each network to clearly separate each
entity and each network prefix should be send to a different subnet
link.
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4. Analysis
4.1 Benefits/Issues of Multihoming in NEMO
Multihoming brings along its benefits and issues to mobile networks.
These are (1) Fault Tolerance and (2) Load Sharing. They are
discussed in greater details below.
4.1.1 Fault Tolerance
A multihomed mobile network, by definition, has multiple paths to the
global network (i.e. Internet). This brings along the obvious
benefit of having an alternate path to the global network when one of
path is down. Used in the context of mobile networks, it means that
either of the following:
o There is a single bi-directional tunnel established between the
mobile network and a home agent. When this bi-directional tunnel
is broken due to one egress link of the mobile network is down,
the mobile network set up a new bi-directional tunnel.
This form of multihoming is more appropriate for a dual-mode
mobile router which has multiple egress interfaces (i.e. the
(1,*,*) mobile networks). This is because in this way the mobile
router is at full control of which bi-directional tunnel to set
up. For a mobile network with multiple mobile routers, to use
only one bi-directional tunnel would require some form of
co-ordinations between the mobile routers.
o There is multiple bi-directional tunnels established between the
mobile network and one or more home agents. When any of these
bi-directional tunnels is broken, packets to and from the mobile
network can traverse along the other bi-directional tunnels. This
form of multihoming can be deployed by any configuration of
multihomed mobile network.
In both cases, when a bi-directional tunnel fails and packets are
diverted to an alternative (perhaps newly established) bi-directional
tunnel, care has to be taken to prevent ingress filtering from
dropping the outgoing packets when the two tunnels end at different
home agents. Ingress filtering occurs when different mobile network
prefixes are handled by different home agents. For example, consider
the case when a mobile network has two tunnel connections to home
agents HA1 and HA2. The mobile network prefix P1 is registered to
HA1, and mobile network prefix P2 is registered to HA2. Mobile
network nodes are free to auto-configure their addresses based on any
of P1 or P2. When the tunnel to HA1 is broken, packets sent through
the tunnel to HA1 are diverted to send through the tunnel to HA2. If
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HA2 (or some border gateway in the domain of HA2) performs ingress
filtering, packets with a source address prefix of P1 may be
discarded.
To avoid ingress filtering for such cases, the mobile router(s) can
stop advertising the network prefix P1. This will stop mobile
network node from using source address auto-configured from prefix
P1. However, such a method suffers from the following two
limitations:
o Switching of source address is a long process since nodes have to
wait for source address to get deprecated [6].
o In addition, switching of source address will force transport
sessions without multihoming capabilities (such as TCP) to be
terminated, and re-established using the new source address.
Transport sessions with multihoming capabilities (such as SCTP)
may be able to continue without disruption.
It is possible to overcome these limitations by using nested tunnels.
Appendix A describes one such approach.
In order for fault tolerance to work, the mobile routers and home
agents must first possess a means to detect failures. It is expected
for faults to occur more readily at the attachment point to the
Internet of the mobile network, due to the use of wireless
connections. The mobile router can then rely on router
advertisements from access routers, or other layer two trigger
mechanisms to detect faults. In comparison, it is more difficult for
home agents to detect tunnel failures. For an ISP deployment model,
the home agents and mobile routers can use proprietary methods (such
as constant transmission of heartbeat signals) to detect failures and
check tunnel liveness. In the S/P model, a lack of standardized
"tunnel liveness" protocol means that it is harder to detect
failures.
A possible method is for the mobile routers to send binding updates
more regularly with shorter Lifetime value. Similarly the home
agents can return binding acknowledgment messages with smaller
Lifetime values as well, thus forcing the mobile routers to send
binding updates more frequently. These binding updates can be used
to emulate "tunnel heartbeats". This however may lead to more
traffic and processing overhead, since binding updates sent to home
agents must be encrypted.
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4.1.2 Load Sharing
With multiple tunnels simultaneously established to connect the
mobile network to the global Internet, packets going to and from the
mobile network can take different paths. This allows home agents and
mobile routers to distribute traffic loads among the bi-directional
tunnels, thus allowing more efficient use of network resources and
alleviate traffic congestion. Such is known as load sharing.
Load sharing can be achieved in various forms and using different
techniques. Considerations into each individual load sharing methods
is out of scope of this document. Instead, we discuss load sharing
in multihomed mobile networks based on only two broad
classifications: static load sharing, and dynamic load sharing.
o Static Load Sharing
This means that the distribution of traffic among the
bi-directional tunnels are configured statically. This may be
based on destination address, source address, or even simple round
robin.
o Dynamic Load Sharing
This means that the distribution of traffic among the
bi-directional tunnels are configured dynamically. This usually
involves a form of policy that is loaded to the mobile routers
and/or home agents at the time when the bi-directional tunnels are
established. Such policy can specify rules for distribution of
traffic based on a richer set of parameters, such as
quality-of-service, as well as destination and source addresses.
The policy may be communicated to the mobile routers using a
dynamic routing protocol that is run over the bi-direction
tunnels. This form of load sharing is more likely to be deployed
by a ISP multihomed model rather than the S/P model, due to the
need for the mobile routers and home agents to constantly exchange
parameters.
As is the case of fault tolerance, when distributing packets among
different bi-directional tunnels, there is the issue of address
filtering to consider. Sending packets with source address belonging
to a network prefix P1 to a home agent that does not accept P1 as a
valid ingress prefix will cause the packet to be discarded. In view
of this, we expect load sharing to be used for ISP model and the S/P
model, but less likely in a S/mP model.
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4.2 Ownership-Oriented Approach
o ISP Model
When the HA(s) and MR(s) are controlled by a single entity (such
as an ISP), the ISP can decide whether it wants to assign one or
multiple network prefixes to the mobile network just like it can
make the same decision for any other link in its network (wired or
otherwise). In any case, the ISP will make the routing between
the mobile networks and its core routers (such as the home agents)
work. This include not introducing any aggregation between the
home agents which will filter out routing announcements for the
mobile prefix(es).
To such ends, the ISP has various means and mechanisms. For one,
the ISP can run its Interior Gateway Protocol (IGP) over
bi-directional tunnels between the MR(s) and HA(s).
Alternatively, static routes may be used with the tunnels. When
static routes are used, a mechanism to test "tunnel liveness"
might be necessary to avoid maintaining stale routes. Such
"tunnel liveness" may be tested by sending heartbeats signals from
MR(s) to the HA(s). A possibility is to simulate heartbeats using
Binding Updates messages by controlling the "Lifetime" field of
the Binding Acknowledgment message to force the MR to send Binding
Update messages at regular interval. However, a more appropriate
tool might be the Binding Refresh Request message, though
conformance to the Binding Refresh Request message may be less
strictly enforced in implementations since it serves a somewhat
secondary role when compared to Binding Update messages.
o Subscriber/Provider Model
When the HA(s) and MR(s) are controlled by different entities, it
is more likely the scenario where the MR is controlled by one
entity (i.e. the subscriber), and the MR is establishing multiple
bi-directional tunnels to one or more HA(s) provided by one or
more ISP(s). In such case, it is unlikely for the ISP to run IGP
over the bi-directional tunnel, since ISP would most certainly
wish to retain full control of its routing domain.
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4.3 Configuration-Oriented Approach
This section, we attempt to analyze what are the problems faced in
each of the 8 categories. It shouldn't matter if some of the
categories share the same problem(s).
[To think about how to merge in Julien's evaluation stuff -- cwng]
o (1,1,1) Mobile Network
The (1,1,1) mobile network has only one mobile router registering
more than one care-of-addresses to the same home agent, and
advertising only one prefix. The mobile router can either have
more than one care-of-addresses bound to the same home-address, or
it can have various care-of-address and home-address pairs.
Either way, this is a MIPv6 problem. Multiple pairs of different
care-of-address and home-address is perfectly alright with MIPv6.
The fact that they specify the same subnet prefix in binding
updates shouldn't cause a problem either. Having a home-address
bound to multiple care-of-address simultaneously may be a problem
for MIPv6. This will require a solution like [7].
o (1,1,N) Mobile Network
The (1,1,N) mobile network is similar to the (1,1,1) mobile
network, and thus face the same problem when there is only one
home-address bound to multiple care-of-addresses. In addition, it
is possible for the mobile router to include multiple mobile
network prefix options in a single binding update, thus having
multiple network prefixes should not create additional issues.
o (1,N,1) Mobile Network
The (1,N,1) mobile network has one mobile router registering to
multiple home agents. There is the question of whether a mobile
router can register the same home-address to different home agents
simultaneously with the 'H' bit set. If not, the mobile router
can only register different home-address and care-of-address pairs
to different home agents. In any case, this is a MIPv6 issue.
The NEMO-specific problem is the fact that a subnet prefix has a
care-of in different home agents. It might be possible that only
one home-agent will actively advertise a route to the subnet
prefix. The case of multiple home agents at different domains
advertising a route to the same subnet prefix may pose a problem
in the routing infrastructure as a whole. The implications of
this aspect needs further exploration.
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o (1,N,N) Mobile Network
The (1,N,N) mobile network has one mobile router registering to
multiple home agents multiple subnet prefixes. The same question
of whether the same home-address can be simultaneously registered
to multiple home agents.
This (1,N,N) network can avoid the problem of registering care-ofs
for the same prefix to different home agents by registering
care-of for one prefix at one home-agent.
o (N,1,1) Mobile Network
The (N,1,1) mobile network has two or more active egress mobile
routers, registering to same home agents, and advertising the same
prefix. May not have any problem at all if the mobile routers are
manually configured to announce the same prefix. It is also
possible that prefix delegation is used to ensure all routers
advertise the same subnet prefix since all routers are handled by
the same home agent. The home-agent will see multiple HoA-CoA
pairs taking care of the same subnet prefix.
o (N,1,N) Mobile Network
The (N,1,N) mobile network has multiple active egress mobile
routers registering to the same home-agent, and advertising
multiple prefixes. If a mobile router is advertising more than one
prefix, we have the same situation as (1,1,N), except with an
additional mobile router. It is possible for the mobile router to
include multiple mobile network prefix options in a single binding
update, thus having multiple network prefixes should not create
additional issues.
On the other hand, if each mobile router take cares of a separate
(and only one) subnet prefix, then there should not be any
NEMO-specific problem.
o (N,N,1) Mobile Network
The (N,N,1) mobile network has multiple mobile routers registering
to different home agents, but advertising the same prefix. There
is the same issues as in (1,N,1) of a subnet prefix having a
care-of in different home agents. In addition, there is a
question how to perform prefix delegation such that two home
agents will delegate the same prefix to different mobile routers.
Certain level of home-agent co-ordination may be required here.
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o (N,N,N) Mobile Network
The (N,N,N) mobile network has multiple mobile routers,
registering to multiple home-agents and advertising prefixes.
This may be a case of multiple non-multihomed network superimposed
together, i.e. each mobile router take cares of one prefix, and
register to separate home agents.
On the other hand, if one mobile router takes cares of more than
one prefix, we have similar problems as (1,1,N) and (N,1,N). In
addition, if more than one mobile router takes care of the same
prefix, we have similar issues as (N,N,1). In any case, we see
that the problems within this configurations can be decomposed
into problems from other configurations.
From the above analysis, we can identify the following problems
relating to multihomed mobile network:
o Multiple care-of-addresses to one home-address:
* How to register two care-of-address binding to one
home-address?
* In single or multiple binding message(s)?
* How to selectively update a care-of-address?
* MIPv6 specific?
* Wakikawa's draft [7] specifically addresses this issue.
o Multiple prefixes taken care of by a single home-address:
* How to register multiple prefix scope under the same
home-address?
* In single or multiple binding message(s)?
* How to selectively update the care-of of a subnet prefix?
* Similar to the 'Tarzan' problem illustrated by Thubert.
o A single home-address registered to multiple home agents:
* Is this allowed?
* MIPv6-specific?
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o A single subnet prefix registered to multiple home agents:
* Is this allowed?
* Is this allowed if the prefix is bound to the same
home-address?
* Any routing issue?
* If prefix delegation is used, possibility of requiring home
agents co-ordination.
* Similar to the 'JetSet' problem illustrated by Thubert.
o A single prefix advertised by multiple mobile routers from
multiple home agents:
* If prefix delegation is used, possibility of requiring home
agents co-ordination.
* Similar to the 'Shinkansen' problem illustrated by Thubert.
o [TBD: anymore]
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[Editor's Notes]
It is originally intended that the analysis work done by Julien
Charbon et. al. and Paik Eun Kyoung et. al. to be inserted into this
portion. From these analysis work, we can determine which are the
multihoming scenarios will be supported by the NEMO Basic Support,
and which are not. This part of the work has yet to be merged in as
of the publication date of this draft.
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5. Security Considerations
This document is an on-going work to classify the taxonomy in
multihoming of mobile networks. There should be a separate draft
produced by the working group to analyze security threats for network
in motion. As such, no special security considerations is listed
here. However, since this memo also looks into the analysis of
problems in a multihomed mobile network, we will add problems related
to security threat here as and when they are encountered. We also
encourage interested readers to contribute to this part.
6. Acknowledgments
The authors would like to thank people who have given valuable
comments on various multihoming issues on the mailing list, and also
those who have suggested directions in the 56th and 57th IETF
Meetings.
References
[1] Devarapali, V., et al, "Nemo Basic Support Protocol", Internet
Draft: draft-ietf-nemo-basic-support-01.txt, Work In Progress,
September 2003.
[2] Ernst, T., et al, "Network Mobility Support Goals and
Requirements", Internet Draft:
draft-ietf-nemo-requirements-01.txt, Work In Progress, May 2003.
[3] Johnson, D.B., Perkins, C.E. and Arkko, J., "Mobility Support in
IPv6", Internet Draft: draft-ietf-mobileip-ipv6-21.txt, Work In
Progress, February 2003.
[4] Simpson, W., "IP in IP Tunneling", IETF RFC 1853, October 1995.
[5] Ernst, T. and Lach, H. Y., "Network Mobility Support
Terminology", Internet Draft:
draft-ietf-nemo-terminology-00.txt, Work In Progress, May 2003.
[6] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", IETF RFC 3484, February 2003.
[7] Wakikawa, R., Uehara, K. and Ernst, T., "Multiple
Care-of-Address Registration on Mobile IPv6", Internet Draft:
draft-wakikawa-mobileip-multiplecoa-01.txt, Work In Progress,
June 2003.
[8] Narten, T., Nordmark, E. and Simpson, W., "Neighbour Discovery
for IPv6", IETF RFC 2461, December 1998.
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Authors' Addresses
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
Julien Charbon
Keio University, Louis Pasteur University
Keio University.
5322 Endo
Fujisawa-shi, Kanagawa 252-8520
JP
Phone: +81-466-49-1100
Fax: +81-466-49-1395
EMail: julien@sfc.wide.ad.jp
URI: http://www.sfc.wide.ad.jp/~julien/
Paik, Eun-Kyoung
Seoul National University
Seoul National University
KR
EMail: eun@mmlab.snu.ac.kr
URI: http://mmlab.snu.ac.kr/~eun/
Appendix A. Nested Tunneling for Fault Tolerance
In order to utilize the additional robustness provided by multi-
homing, mobile routers that employ bi-directional tunneling with
their home agents should dynamically change their tunnel exit points
depending on the link status. For instance, if a mobile router
detects that one of its egress interface is down, it should detect if
any other alternate route to the global Internet exists. This
alternate route may be provided by any other mobile routers connected
to one of its ingress interfaces that has an independent route to the
global Internet, or by another active egress interface the mobile
router it self possess. If such an alternate route exists, the
mobile router should re-establish the bi-directional tunnel using
this alternate route.
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In the remaining part of this section, we will attempt to investigate
methods of performing such re-establishment of bi-directional
tunnels. It is not the objective of this memo to specify a new
protocol specifically tailored to provide this form of re-
establishments. Instead, we will limit ourselves to currently
available mechanisms specified in Mobile IPv6 and Neighbor Discovery
in IPv6 [8].
A.1 Detecting Presence of Alternate Routes
To actively utilize the robustness provided by multihoming, a mobile
router must first be capable of detecting alternate routes. This can
be manually configured into the mobile router by the administrators
if the configuration of the mobile network is relatively static. It
is however highly desirable for mobile routers to be able to discover
alternate routes automatically for greater flexibility.
The case where a mobile router possesses multiple egress interface
(bound to the same home agent or otherwise) should be trivial, since
the mobile router should be able to "realize" it has multiple routes
to the global Internet.
In the case where multiple mobile routers are on the mobile network,
each mobile router has to detect the presence of other mobile router.
A mobile router can do so by listening for Router Advertisement
message on its *ingress* interfaces. When a mobile router receives a
Router Advertisement message with a non-zero Router Lifetime field
from one of its ingress interfaces, it knows that another mobile
router which can provide an alternate route to the global Internet is
present in the mobile network.
A.2 Re-Establishment of Bi-Directional Tunnels
When a mobile router detects that the link be which its current
bi-directional tunnel with its home agent is using is down, it needs
to re-establish the bi-directional tunnel using an alternate route
detected. We consider two separate cases here: firstly, the
alternate route is provided by another egress interface that belongs
to the mobile router; secondly, the alternate route is provided by
another mobile router connected to the mobile network. We refer to
the former case as an alternate route provided by an alternate egress
interface, and the latter case as an alternate route provided by an
alternate mobile router.
A.2.1 Using Alternate Egress Interface
When an egress interface of a mobile router loses the connection to
the global Internet, the mobile router can make use of its alternate
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egress interface should it possess multiple egress interfaces. The
most direct way to do so is for the mobile router to send a binding
update to the home agent of the failed interface using the
care-of-address assigned to the alternate interface in order to
re-establish the bi-directional tunneling using the care-of-address
on the alternate egress interface. After a successful binding
update, the mobile router encapsulates outgoing packets through the
bi-directional tunnel using the alternate egress interface.
The idea is to use the global address (most likely a care-of-address)
assigned to an alternate egress interface as the new (back-up)
care-of-address of the mobile router to re-establish the
bi-directional tunneling with its home agent.
A.2.2 Using Alternate Mobile Router
When the mobile router loses a connection to the global Internet, the
mobile router can utilize a route provided by an alternate mobile
router (if one exists) to re-establish the bi-directional tunnel with
its home agent. First, the mobile router has to obtain a care-of-
address from the alternate mobile router (i.e. attaches itself to the
alternate mobile router). Next, it sends binding update to its home
agent using the care-of-address obtained from the alternate mobile
router From then on, the mobile router can encapsulates outgoing
packets through the bi-directional tunnel using via the alternate
mobile router.
The idea is to obtain a care-of-address from the alternate mobile
router and use this as the new (back-up) care-of-address of the
mobile router to re-establish the bi-directional tunneling with its
home agent.
Note that every packet sent from/to mobile network nodes to/from
their correspondent nodes will experience two levels of
encapsulation. First level of tunneling is done between a mobile
router which the mobile network node uses as its default router and
the mobile router's home agent. The second level of tunneling is
done between the alternate mobile router and its home agent.
A.3 To Avoid Tunneling Loop
The method of re-establishing the bi-directional tunnel as described
in Section 3.2 may lead to infinite loops of tunneling. This happens
when two mobile routers on a mobile network lose connection to the
global Internet at the same time and each mobile router tries to
re-establish bi-directional tunnel using the other mobile router. We
refer to this phenomenon as tunneling loop.
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One approach to avoid tunneling loop is for a mobile router that has
lost connection to the global Internet to insert an option into the
Router Advertisement message it broadcasts periodically. This option
serves to notify other mobile routers on the link that the sender no
longer provides global connection. Note that setting a zero Router
Lifetime field will not work well since it will cause mobile network
nodes that are attached to the mobile router to stop using the mobile
router as an access router too (in which case, things are back to
square one).
A.4 Other Considerations
When a mobile network is multihomed, mobile network nodes may receive
Router Advertisements that advertise different network prefixes.
This is usually the case when the multihomed mobile network has two
or more mobile routers advertising different routes to the global
Internet. It may also occur when the mobile network has only one
mobile router with multiple egress interfaces bound to different home
agents. In these situations, mobile network nodes typically only
select one to form its global (possibly care-of) address.
In view of this, it may be desirable for mobile network node to be
able to acquire "preference" information on each mobile network
prefix from the mobile routers. This allows default address
selection mechanism such as that specified in [6] to be used.
Further exploration on setting such "preference" information in
Router Advertisement based on performance of the bi-directional
tunnel might prove to be useful.
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