NEMO Working Group C. Ng
Internet-Draft Panasonic Singapore Labs
Expires: August 16, 2004 J. Charbon
Keio and Louis Pasteur University
E. Paik
Seoul National University
T. Ernst
WIDE at Keio University
February 16, 2004
Analysis of Multihoming in Network Mobility Support
draft-ng-nemo-multihoming-issues-03
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document is an analysis of multihoming in the context of network
mobility (NEMO). As there are many situations in which mobile
networks may be multihomed, we outline possible approaches to
classify the multihomed mobile networks. We also describe possible
deployment scenarios and we attempt to identify issues that arise
when mobile networks are multihomed while mobility supports is taken
care by NEMO Basic Support.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Classification . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 (1,1,1): Single MR, Single HA, Single Prefix . . . . . . . . 6
2.2 (1,1,N): Single MR, Single HA, Multiple Prefixes . . . . . . 6
2.3 (1,N,1): Single MR, Multiple HAs, Single Prefix . . . . . . 7
2.4 (1,N,N): Single MR, Multiple HAs, Multiple Prefixes . . . . 7
2.5 (N,1,1): Multiple MRs, Single HA, Single Prefix . . . . . . 8
2.6 (N,1,N): Multiple MRs, Single HA, Multiple Prefixes . . . . 8
2.7 (N,N,1): Multiple MRs, Multiple HAs, Single Prefix . . . . . 9
2.8 (N,N,N): Multiple MRs, Multiple HAs, Multiple Prefixes . . . 9
3. Benefits/Issues of Multihoming in NEMO . . . . . . . . . . . 11
3.1 Deployment Scenarios . . . . . . . . . . . . . . . . . . . . 11
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . 13
4.1 Connection Availability . . . . . . . . . . . . . . . . . . 13
4.2 Connection Selection . . . . . . . . . . . . . . . . . . . . 13
4.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . . 13
4.4 Route Optimization Considerations . . . . . . . . . . . . . 13
4.5 Ingress Filtering . . . . . . . . . . . . . . . . . . . . . 14
4.6 Failure Detection . . . . . . . . . . . . . . . . . . . . . 15
5. Evaluation of Basic NEMO Solution . . . . . . . . . . . . . 16
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 18
References . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19
A. Alternative Classifications Approach . . . . . . . . . . . . 21
A.1 Ownership-Oriented Approach . . . . . . . . . . . . . . . . 21
A.1.1 ISP Model . . . . . . . . . . . . . . . . . . . . . . . . . 21
A.1.2 Subscriber/Provider Model . . . . . . . . . . . . . . . . . 22
A.2 Problem-Oriented Approach . . . . . . . . . . . . . . . . . 24
B. Nested Tunneling for Fault Tolerance . . . . . . . . . . . . 25
B.1 Detecting Presence of Alternate Routes . . . . . . . . . . . 25
B.2 Re-Establishment of Bi-Directional Tunnels . . . . . . . . . 25
B.2.1 Using Alternate Egress Interface . . . . . . . . . . . . . . 26
B.2.2 Using Alternate Mobile Router . . . . . . . . . . . . . . . 26
B.3 To Avoid Tunneling Loop . . . . . . . . . . . . . . . . . . 27
B.4 Other Considerations . . . . . . . . . . . . . . . . . . . . 27
C. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 28
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Intellectual Property and Copyright Statements . . . . . . . 29
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1. Introduction
The goals and objectives of Network Mobility Support (NEMO) are
identified in [2] while the terminology is being described in [5]. A
solution to provide continuous Internet connectivity to nodes in a
mobile network, that is, a network which changes its point of
attachment to the Internet, is currently designed by the NEMO Working
Group [1]. This solutions basically solves the problem by setting up
bi-directional tunnels betweens the mobile routers (MRs) and their
Home Agent (HAs), much how this is done in Mobile IPv6 [3], the
solution for host mobility.
The purpose of this memo to investigate issues related to such a
bi-directional tunneling mechanism when mobile networks are
multihomed, i.e. when there is more than one point of attachment
between the mobile network and the Internet (see definitions in draft
[1]). Goals and objectives of multihoming are discussed in a separate
document [6] with fits to both fixed nodes, mobile nodes, fixed
networks and mobile networks. Our objectives are three-folds:
o To capture issues for deploying a multihomed mobile network
o To identify which multihoming configurations are useful
o To identify issues in NEMO Basic Support that prevent to support
the useful configurations. It doesn't mean that those not
supported will not work with NEMO Basic Support, just that it is
up to the implementors to make it work (hopefully issues discussed
in the document will be helpful to these implementors).
For doing so, Section 2 first outlined several taxonomies to classify
multihomed mobile networks. This section outlines 3 different
approaches to classifying multihomed mobile network. Benefits and
issues of multihoming peculiar to network mobility support are
discussed in Section 3. Next, we described deployment scenarios of
multihomed mobile networks in Section 4. Following this, we study the
general issues, and we conclude with an evaluation of NEMO Basic
Support for multihomed configurations.
In order to understand this memo, the reader is expected to be
familiar with the aboved cited documents, i.e. with the NEMO
terminology [5], Goals and Objectives of Multihoming [6], Goals and
Requirements of Network Mobility Support [2], and the NEMO Basic
Support specification [1].
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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 are several
configurations in which mobile networks are multihomed, there is a
need to classify multihomed mobile network into a clearly defined
taxonomy. This can be done in various ways. Three main approaches
have been proposed on the NEMO mailing list. These are, namely, (i)
Configuration-Oriented Approach, (iii) Ownership-Oriented Approach,
and (ii) Problem-Oriented Approach. As the WG consensus seems to
have converged to the Configuration-Oriented dApproach, we described
only this approach here. The other two appraoches can be found in
Appendix A.1 and Appendix A.2.
Configuration-Oriented Approach
Multihomed configurations can be classified depending on how many
mobile routers are present, how many egress interfaces and home
addresses the mobile routers have, how many prefixes (NEMO-prefixes)
are advertised to the mobile network nodes, etc. For doing so, we use
three key parameters differentiating different multihomed
configurations. With these parameters, we can refer to each
configuration by the 3-tuple (x,y,z), where 'x', 'y', 'z' are defined
as follows:
o 'x' indicates the number of MRs where:
x=1 implies a mobile network has only a single mobile router.
presumably with multiple egress interfaces or multiple home
addresses.
x=N implies a mobile network has more than one mobile router
advertising an egress route.
o 'y' indicates the number of HAs associated with the mobile
network, where:
y=1 implies that a single home agent is assigned to the mobile
network.
y=N implies that more than one home agents (possibly in different
administrative domains) are assigned the mobile network.
o 'z' indicates the number of NEMO-prefix announced to MNNs, where:
z=1 implies that a single NEMO-prefix is advertised to the mobile
network nodes.
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z=N implies that more than one NEMO-prefix 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 'x', 'y' and 'z' give rise
to different combinations of the 3-tuple (x,y,z). As described
below, a total of 8 possible configurations can be identified.
2.1 (1,1,1): Single MR, Single HA, Single Prefix
The (1,1,1) mobile network has only one mobile router advertising a
single NEMO-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 may either (i) use more than one egress links
at the same time, or (ii) use more than one home address at the same
time.
Since only one NEMO-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
2.2 (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 NEMO-prefixes are advertised to the mobile network nodes. No
associations is assumed between the NEMO-prefixes and the home
addresses of the mobile router.
Since a plurality of NEMO-prefixes are advertised, mobile network
nodes can generally be multihomed themselves, where each mobile
network node is allocated one address in each NEMO-prefix.
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_____
_ p1,p2 _ | |
|_|-|<-_ |-|_|-| |-| _
_ |-|_|=| |_____| | _ |-|_|
|_|-| | |-|_|-|
|
MNNs MR AR Internet AR HA
Figure 2.2 - (1,1,N) Multihomed Mobile Network
2.3 (1,N,1): Single MR, Multiple HAs, Single Prefix
The (1,N,1) mobile network has only one mobile router advertising a
single NEMO-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 NEMO-prefix is advertised, the mobile network nodes
are (usually) not multihomed.
AR HA2
_ |
|-|_|-| _
_____ | |-|_|
_ p _ | |-|
|_|-|<-_ |-|_|-| |
_ |-|_|=| |_____|-| _
|_|-| | | _ |-|_|
|-|_|-|
|
MNNs MR AR Internet AR HA1
Figure 2.3 - (1,N,1) Multihomed Mobile Network
2.4 (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 NEMO-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 NEMO-prefixes are advertised, mobile network
nodes can generally be multihomed themselves, where each mobile
network node is allocated one address in each NEMO-prefix.
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AR HA2
_ | _
_____ |-|_|-|-|_|
_ p1,p2 _ | |-| |
|_|-|<-_ |-|_|-| | _
_ |-|_|=| |_____|-| _ |-|_|
|_|-| | |-|_|-|
| |
MNNs MR AR Internet AR HA1
Figure 2.4 - (1,N,N) Multihomed Mobile Network
2.5 (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 NEMO-prefix and associate to the same home agent. Since
only one NEMO-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
2.6 (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 NEMO-prefixes.
However, these mobile routers associate to the same home agents.
Since a plurality of NEMO-prefixes are advertised, mobile network
nodes can generally be multihomed themselves, where each mobile
network node is allocated one address in each NEMO-prefix.
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MR2
p2<-_ |
_ |-|_|-| _____
|_|-| |-| |
_ | | |-| _
|_|-| _ |-|_____| | _ |-|_|
|-|_|-| |-|_|-|
p1<- | |
MNNs MR1 Internet AR HA
Figure 2.6 - (N,1,N) Multihomed Mobile Network
2.7 (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 NEMO-prefix. Since only one NEMO-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
2.8 (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 NEMO-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 NEMO-prefixes are advertised, mobile network
nodes can generally be multihomed themselves, where each mobile
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network node is allocated one address in each NEMO-prefix.
MR2 AR HA2
p2 _ |
<-_ | |-|_|-| _
_ |-|_|-| _____ | |-|_|
|_|-| |-| |-|
_ | | |
|_|-| _ |-|_____|-| _
|-|_|-| | _ |-|_|
<- | |-|_|-|
p1 |
MNNs MR1 Internet AR HA1
Figure 2.8 - (N,N,N) Multihomed Mobile Network
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3. Benefits/Issues of Multihoming in NEMO
The following generic goals and benefits of multihoming are discussed
in a companion document [6]:
1. Ubiquitous Acccess
2. Redundancy/Fault-Recovery
3. Load Sharing
4. Load Balancing
5. Ubiquity
6. Preference Settings
This section discusses these from a NEMO perspective and we give
typical instances for each case of our taxonomy.
Mobile networks are typically connected by means of wireless and thus
less reliable links. In addition, there could be many nodes behind
the MR, so a failure to connect to the Internet has a more important
impact than once only one node is concerned by a lack a failure or
loss of connectivity. Real-life scenarios highlighted in [6] have
illustrated that offering a permanent access to mobile networks such
as vehicles typically require the use of several interfaces and
technologies since the mobile network may be moving in distant
geographical locations where different access technologies are
provided and gouverned by distinct access control policies.
3.1 Deployment Scenarios
Here, we list some example scenarios for each configurations
x=1: Multihomed mobile network with one mobile router
o A mobile router with dual/multiple access interfaces (i.e.
802.11 and GPRS capabilities). When it subscribed to same ISP
for both accesses, this is a S/P-(1,1,*). If it different ISPs
are offering the two accesses independently, this is a S/
mP-(1,N,N).
Benefits: Ubiquity, Redundancy/Fault-Recovery
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x=N: Mutlihomed mobile networks with multiple mobile routers
o A train with one MR in each car. This is usually served by
same home agent, thus usually a (N,1,*). Alternatively, the
train company might be forced to use different ISP when the
train go to different locations, thus it is a (N,N,N).
Benefits: Load Sharing
o A W-PAN with a GPRS_enabled phone and a WiFi-enabled PDA. The
two access technology are usually separately subsribed, thus it
is likely to be S/mP-(N,N,N).
Benefits: Ubiquity, Redundancy/Fault-Recovery
y=1: Multihomed mobile networks with one home agent
o Most single ISP cases in above examples.
y=N: Multihomed mobile networks with multiple home agents
o Most multiple ISP cases in above examples.
o A transalantic flight that cahnge its home agent when its in
different continents. This is a (1,N,1) network if there is
only one mobile router.
Benefits: Ubiquity (reduce delays)
z=1: Multihomed mobile networks with one prefix
o Most single ISP cases in above examples.
z=N: Multihomed mobile networks with multiple prefixes
o Most multiple ISP cases in above examples.
o A car with a prefix taken from my home (I pay the traffic on
thisprefix) and one that belong to the car-manufacturer (for
maintenance, traffic is paid by the car-manufacturer [indeed me
when I buy the car:-)]). This will typically be a (1,1,N).
Benefits: preference settings
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4. Problem Statement
4.1 Connection Availability
Multiple connections of MRs can be used simultaneously or one at a
time.
o When multiple connections are used simultaneously, the mode of
operation can be either primary-secondary or peer-to-peer. These
configurations can be useful especially for large mobile networks,
but there are many implementation issues which need to be
addressed, e.g. which connection will be selected for each traffic
folw that goes into/out of the mobile network ?
o When only one connection can be used at a time, e.g. in the case
where a single connection has to substitute for all of the other
failed connections, a connection selection mechanism is needed.
The connection selection can depend on which connection is
available at that time.
4.2 Connection Selection
The connection can be selected by the home agent (HA), the MR, and/or
the MNN.
o The HA can select a connection based on the binding update
information in the binding cache.
o The MR can select a connection since the MR is one of the main
bodies of the connection.
o The MNN should be able to select a connection, e.g. in case where
a user wants to select a particular access technology among the
available technologies for reasons of cost or data rate.
o A hybrid mechanism should be also available, e.g. one in which the
HA, the MR, and/or the MNN coordinate to select a connection.
4.3 Scalability
Should a new solution meets the all the eight configurations and the
scenarios mentioned in section 3 ?
4.4 Route Optimization Considerations
RO problems in multihomed mobile networks are dependant on how the
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connections are available and selected.
o In case of multiple HAs and HoAs, new route optimization may be
possible by routing between CN and multiple HAs with different
HoAs.
o When multiple connections are available simultaneously, how the CN
knows about the availability and optimizes route ?
4.5 Ingress Filtering
To enjoy the benefits of multihoming as descriebd earlier, it is
often necessary to divert packets from the same session between two
different bi-directional tunnels. This is especially treu when we
consider the fault recovery feature of multihoming when packets froma
failed bi-directional tunnel is sent via an alternative (perhaps
newly established) bi-directional tunnel.
> When doing so, 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 HA2 (or some border gateway in the domain of HA2)
performs ingress filtering, packets will 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 [7].
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.
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It is possible to overcome these limitations by using nested tunnels.
Appendix B describes one such approach.
4.6 Failure Detection
In order for fault recovery 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 end 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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5. Evaluation of Basic NEMO Solution
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).
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 NEMO-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 [8].
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 MR 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 NEMO-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 NEMO-prefix.
The case of multiple home agents at different domains advertising
a route to the same NEMO-prefix may pose a problem in the routing
infrastructure as a whole. The implications of this aspect needs
further exploration.
o (1,N,N) Mobile Network
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The (1,N,N) mobile network has one mobile router registering to
multiple home agents multiple NEMO-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 NEMO-prefix since all routers are handled by
the same home agent. The home-agent will see two HoA-CoA pairs
taking care of the same NEMO-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 problem as (1,1,N) as in how to register
more than one NEMO-prefix to the same home-agent.
On the other hand, if each mobile router take cares of a separate
(and only one) NEMO-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 NEMO-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.
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.
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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.
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 - 58th IETF Meetings.
References
[1] Devarapalli, V., "Nemo Basic Support Protocol",
draft-ietf-nemo-basic-support-02 (work in progress), December
2003.
[2] Ernst, T., "Network Mobility Support Goals and Requirements",
draft-ietf-nemo-requirements-02 (work in progress), Feb 2004.
[3] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", draft-ietf-mobileip-ipv6-24 (work in progress), July
2003.
[4] Simpson, W., "IP in IP Tunneling", IETF RFC 1853, October 1995.
[5] Ernst, T. and H. Lach, "Network Mobility Support Terminology",
draft-ietf-nemo-terminology-01 (work in progress), Feb 2004.
[6] Ernst, T., "Goals and Benefits of Multihoming",
draft-ernst-generic-multihoming-00 (work in progress), February
2004.
[7] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", IETF RFC 3484, February 2003.
[8] Wakikawa, R., "Multiple Care-of Addresses Registration",
draft-wakikawa-mobileip-multiplecoa-02 (work in progress),
September 2003.
[9] 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
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/
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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/
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Appendix A. Alternative Classifications Approach
A.1 Ownership-Oriented Approach
An alternative approach to classifying multihomed mobile network is
proposed by Eric Nordmark (Sun Microsystems) by breaking the
classification of multihomed network based on ownership. This is
more of a tree-like top-down classification. 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. We called the first posibility the 'ISP Model', and the
second the 'Subscriber/Provider Model'.
A.1.1 ISP Model
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
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).
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
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strictly enforced in implementations since it serves a somewhat
secondary role when compared to Binding Update messages.
A.1.2 Subscriber/Provider Model
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
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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A.2 Problem-Oriented Approach
A third approach 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 y=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 y=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 z=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
of z=N, i.e. the (N,*,N) mobile network.
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Appendix B. 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.
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 [9].
B.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.
B.2 Re-Establishment of Bi-Directional Tunnels
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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.
B.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
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.
B.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
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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.
B.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.
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).
B.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 [7] 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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Appendix C. Change Log
o Changes from version -02 to version -03
* Merged with draft-eun-nemo-multihoming-problem-statement (see
"Problem Statement" (Section 4))
* Included conclusions from
draft-charbon-nemo-multihoming-evaluation
* Re-organize some part of "Benefits/Issues of Multhoming in
NEMO" to "Problem Statement" (Section 4)
* Remove lot of text to be in sync with [6].
* Title change from "Multihoming Issues in NEMO Basic Support" to
"Analysis of Multihoming in NEMO"
* Changed (w,x,y) to (x,y,z) in taxonomy.
* Moved alterntaive approaches of classification to Appendix
* Creation of this Change-Log itself ;-)
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