NEMO Working Group C. Ng
Internet-Draft Panasonic Singapore Labs
Expires: August 9, 2007 T. Ernst
INRIA
E. Paik
KT
M. Bagnulo
UC3M
February 5, 2007
Analysis of Multihoming in Network Mobility Support
draft-ietf-nemo-multihoming-issues-07
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Abstract
This document is an analysis of multihoming in the context of network
mobility (NEMO) in IPv6. As there are many situations in which
mobile networks may be multihomed, a taxonomy is proposed to classify
the possible configurations. The possible deployment scenarios of
multihomed mobile networks are described together with the associated
issues when network mobility is supported by RFC 3963 (NEMO Basic
Support). Recommendations are offered on how to address these
issues.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Classification . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. (1,1,1): Single MR, Single HA, Single MNP . . . . . . . . 7
2.2. (1,1,n): Single MR, Single HA, Multiple MNPs . . . . . . . 8
2.3. (1,n,1): Single MR, Multiple HAs, Single MNP . . . . . . . 8
2.4. (1,n,n): Single MR, Multiple HAs, Multiple MNPs . . . . . 9
2.5. (n,1,1): Multiple MRs, Single HA, Single MNP . . . . . . . 10
2.6. (n,1,n): Multiple MRs, Single HA, Multiple MNPs . . . . . 10
2.7. (n,n,1): Multiple MRs, Multiple HAs, Single MNP . . . . . 11
2.8. (n,n,n): Multiple MRs, Multiple HAs, Multiple MNPs . . . . 12
3. Deployment Scenarios and Prerequisites . . . . . . . . . . . . 13
3.1. Deployment Scenarios . . . . . . . . . . . . . . . . . . . 13
3.2. Prerequisites . . . . . . . . . . . . . . . . . . . . . . 15
4. Multihoming Issues . . . . . . . . . . . . . . . . . . . . . . 17
4.1. Fault Tolerance . . . . . . . . . . . . . . . . . . . . . 17
4.1.1. Failure Detection . . . . . . . . . . . . . . . . . . 17
4.1.2. Path Exploration . . . . . . . . . . . . . . . . . . . 19
4.1.3. Path Selection . . . . . . . . . . . . . . . . . . . . 20
4.1.4. Re-Homing . . . . . . . . . . . . . . . . . . . . . . 22
4.2. Ingress Filtering . . . . . . . . . . . . . . . . . . . . 22
4.3. HA Synchronization . . . . . . . . . . . . . . . . . . . . 24
4.4. MR Synchronization . . . . . . . . . . . . . . . . . . . . 24
4.5. Prefix Delegation . . . . . . . . . . . . . . . . . . . . 25
4.6. Multiple Bindings/Registrations . . . . . . . . . . . . . 26
4.7. Source Address Selection . . . . . . . . . . . . . . . . . 26
4.8. Loop Prevention in Nested Mobile Networks . . . . . . . . 26
4.9. Prefix Ownership . . . . . . . . . . . . . . . . . . . . . 27
4.10. Preference Settings . . . . . . . . . . . . . . . . . . . 27
5. Recommendations to the Working Group . . . . . . . . . . . . . 29
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6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 32
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
8. Security Considerations . . . . . . . . . . . . . . . . . . . 32
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1. Normative References . . . . . . . . . . . . . . . . . . . 33
10.2. Informative References . . . . . . . . . . . . . . . . . . 33
Appendix A. Alternative Classifications Approach . . . . . . . . 36
A.1. Ownership-Oriented Approach . . . . . . . . . . . . . . . 36
A.1.1. ISP Model . . . . . . . . . . . . . . . . . . . . . . 36
A.1.2. Subscriber/Provider Model . . . . . . . . . . . . . . 37
A.2. Problem-Oriented Approach . . . . . . . . . . . . . . . . 39
Appendix B. Nested Tunneling for Fault Tolerance . . . . . . . . 40
B.1. Detecting Presence of Alternate Routes . . . . . . . . . . 40
B.2. Re-Establishment of Bi-Directional Tunnels . . . . . . . . 41
B.2.1. Using Alternate Egress Interface . . . . . . . . . . . 41
B.2.2. Using Alternate Mobile Router . . . . . . . . . . . . 41
B.3. To Avoid Tunneling Loop . . . . . . . . . . . . . . . . . 42
B.4. Points of Considerations . . . . . . . . . . . . . . . . . 42
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . . . 48
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1. Introduction
The design goals and objectives of Network Mobility Support (NEMO) in
IPv6 are identified in [1] while the terminology is being described
in [2] and [3]. NEMO Basic Support (RFC 3963) [4] is the solution
proposed by the NEMO Working Group to provide continuous Internet
connectivity to nodes located in an IPv6 mobile network, e.g. like in
an in-vehicle embedded IP network. The NEMO Basic Support solution
does so by setting up bi-directional tunnels between the mobile
routers (MRs) connecting the mobile network to the Internet and their
respective home agents (HAs), much like how this is done in Mobile
IPv6 [5], the solution for host mobility support. NEMO Basic Support
is transparent to nodes located behind the mobile router (i.e. the
mobile network nodes, or MNNs) and as such does not require MNNs to
take any action in the mobility management.
However, mobile networks are typically connected by means of wireless
and thus less reliable links; there could also be many nodes behind
the MR. A loss of connectivity or a failure to connect to the
Internet has thus a more significant impact than for a single mobile
node. Scenarios illustrated in [6] demonstrate that providing 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 governed by distinct
access control policies.
As specified in section 5 of the NEMO Basic Support Requirements [1]
(R.12), the NEMO WG must ensure that NEMO Basic Support does not
prevent mobile networks to be multihomed, i.e. when there is more
than one point of attachment between the mobile network and the
Internet (see definitions in [3]). This arises either:
o when a MR has multiple egress interfaces, or
o the mobile network has multiple MRs, or
o the mobile network is associated with multiple HAs, or
o multiple global prefixes are available in the mobile network.-->
Using NEMO Basic Support, this would translate into having multiple
bi-directional tunnels between the MR(s) and the corresponding HA,
and may result into multiple MNPs available to the MNNs. However,
NEMO Basic Support does not specify any particular mechanism to
manage multiple bi-directional tunnels. The objectives of this memo
are thus multifold:
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o to determine all the potential multihomed configurations for a
NEMO, and then to identify which of these may be useful in a real
life scenario;
o to capture issues that may prevent some multihomed configurations
to be supported under the operation of NEMO Basic Support. It
doesn't necessarily mean that the ones not supported will not work
with NEMO Basic Support, as it may be up to the implementors to
make it work (hopefully this memo will be helpful to these
implementors);
o to decide which issues are worth solving and to determine which WG
is the most appropriate to address these;
o to identify potential solutions to the previously identified
issues.
In order to reach these objectives, a taxonomy for classifying the
possible multihomed configurations is described in Section 2.
Deployment scenarios, their benefits, and requirements to meet these
benefits are illustrated in Section 3. Following this, the related
issues are studied in Section 4. The issues are then summarized in a
matrix for each of the deployment scenario, and recommendations are
made on which of the issues should be worked on and where in
Section 5. This memo concludes with an evaluation of NEMO Basic
Support for multihomed configurations. Alternative classifications
are outlined in the Appendix.
The readers should note that this document considers multihoming only
from the point of view of an IPv6 environment. In order to
understand this memo, the reader is expected to be familiar with the
above cited documents, i.e. with the NEMO terminology as defined in
[2] (section 3) and [3], Motivations and Scenarios for Multihoming
[6], Goals and Requirements of Network Mobility Support [1], and the
NEMO Basic Support specification [4]. Goals and benefits of
multihoming as discussed in [6] are applicable to fixed nodes, mobile
nodes, fixed networks and mobile networks.
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2. Classification
As there are several configurations in which mobile networks are
multihomed, there is a need to classify them into a clearly defined
taxonomy. This can be done in various ways. A Configuration-
Oriented taxonomy is described in this section. Two other
taxonomies, namely, the Ownership-Oriented Approach, and the Problem-
Oriented Approach are outlined in Appendix A.1 and Appendix A.2.
Multihomed configurations can be classified depending on how many
mobile routers are present, how many egress interfaces, Care-of
Address (CoA) and Home Addresses (HoA) the mobile routers have, how
many prefixes (MNPs) are available to the mobile network nodes, etc.
We use three key parameters to differentiate the multihomed
configurations. Using these parameters, each configuration is
referred 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 that a mobile network has only a single MR,
presumably multihomed.
x=n implies that a mobile network has more than one MR.
o 'y' indicates the number of HAs associated with the entire mobile
network, where:
y=1 implies that a single HA is assigned to the mobile network.
y=n implies that multiple HAs are assigned to the mobile network.
o 'z' indicates the number of MNPs available within the NEMO, where:
z=1 implies that a single MNP is available in the NEMO.
z=N implies that multiple MNPs are available in the NEMO
It can be seen that the above three parameters are fairly orthogonal
with one another. Thus different values of 'x', 'y' and 'z' result
into different combinations of the 3-tuple (x,y,z).
As described in the sub-sections below, a total of 8 possible
configurations can be identified. One thing the reader has to keep
in mind is that in each of the following 8 cases, the MR may be
multihomed if either (i) multiple prefixes are available (on the home
link, or on the foreign link), or (ii) the MR is equipped with
multiple interfaces. In such a case, the MR would have multiple HoA-
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CoA pairs. Issues pertaining to a multihomed MR are also addressed
in [7].
In addition, the readers should also keep in mind that when "MNP(s)
is/are available in the NEMO", the MNP(s) may either be explicitly
announced by the MR via router advertisement, or made available
through Dynamic Host Configuration Protocol (DHCP).
2.1. (1,1,1): Single MR, Single HA, Single MNP
The (1,1,1) configuration has only one MR, it is associated with a
single HA, and a single MNP is available in the NEMO. To fall into a
multihomed configuration, at least one of the following conditions
must hold:
o The MR has multiple interfaces and thus it has multiple CoAs;
o Multiple global prefixes are available on the foreign link, and
thus it has multiple CoAs; or
Note that the case where multiple prefixes are available on the
foreign link does not have any bearing on the MNPs. MNPs are
independent of prefixes available on the link where the MR is
attached to, thus prefixes available on the foreign link are not
announced on the NEMO link. For the case where multiple prefixes are
available on the home link, these are only announced on the NEMO link
if the MR is configured to do so. In this configuration, only one
MNP is announced.
A bi-directional tunnel would then be established between each {HA
address,CoA} pair.
Regarding MNNs, they are (usually) not multihomed since they would
configure a single global address from the single MNP available on
the link they are attached to.
_____
_ p _ | |
|_|-|<-_ |-|_|-| |-| _
_ |-|_|=| |_____| | _ |-|_|
|_|-| | |-|_|-|
|
MNNs MR AR Internet AR HA
Figure 1: (1,1,1): 1 MR, 1 HA, 1 MNP
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2.2. (1,1,n): Single MR, Single HA, Multiple MNPs
The (1,1,n) configuration has only one MR, it is associated with a
single HA and two or more MNPs are available in the NEMO.
The MR may itself be multihomed, as detailed in Section 2.1. A bi-
directional tunnel would be established between each {HA address,CoA}
pair.
Regarding MNNs, they are multihomed because several MNPs are
available on the link they are attached to. The MNNs would then
configure a global address from each MNP available on the link.
_____
_ p1,p2 _ | |
|_|-|<-_ |-|_|-| |-| _
_ |-|_|=| |_____| | _ |-|_|
|_|-| | |-|_|-|
|
MNNs MR AR Internet AR HA
Figure 2: (1,1,n): 1 MR, 1 HA, multiple MNPs
2.3. (1,n,1): Single MR, Multiple HAs, Single MNP
The (1,n,1) configuration has only one MR and a single MNP is
available in the NEMO. The MR, however, is associated with multiple
HAs.
The NEMO is multihomed since it has multiple HAs, but in addition the
conditions detailed in Section 2.1 may also hold for the MR. A bi-
directional tunnel would be established between each {HA address,CoA}
pair.
Regarding MNNs, they are (usually) not multihomed since they would
configure a single global address from the single MNP available on
the link they are attached to.
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AR HA2
_ |
|-|_|-| _
_____ | |-|_|
_ p _ | |-|
|_|-|<-_ |-|_|-| |
_ |-|_|=| |_____|-| _
|_|-| | | _ |-|_|
|-|_|-|
|
MNNs MR AR Internet AR HA1
Figure 3: (1,n,1): 1 MR, multiple HAs, 1 MNP
2.4. (1,n,n): Single MR, Multiple HAs, Multiple MNPs
The (1,n,n) configuration has only one MR. However, the MR is
associated with multiple HAs and more than one MNP is available in
the NEMO.
The MR is multihomed since it has multiple HAs, but in addition the
conditions detailed in Section 2.1 may also hold. A bi-directional
tunnel would be established between each {HA address,CoA} pair.
Regarding MNNs, they are multihomed because several MNPs are
available on the link they are attached to. The MNNs would then
configure a global address with each MNP available on the link.
AR HA2
_ | _
_____ |-|_|-|-|_|
_ p1,p2 _ | |-| |
|_|-|<-_ |-|_|-| | _
_ |-|_|=| |_____|-| _ |-|_|
|_|-| | |-|_|-|
| |
MNNs MR AR Internet AR HA1
Figure 4: (1,n,n): 1 MR, multiple HAs, multiple MNPs
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2.5. (n,1,1): Multiple MRs, Single HA, Single MNP
The (n,1,1) configuration has more than one MR advertising global
routes. However, the MR(s) are associated with as single HA, and
there in a single MNP available in the NEMO.
The NEMO is multihomed since it has multiple MRs, but in addition the
conditions detailed in Section 2.1 may also hold for each MR. A bi-
directional tunnel would be established between each {HA address,CoA}
pair.
Regarding MNNs, they are (usually) not multihomed since they would
configure a single global address from the single MNP available on
the link they are attached to.
MR2
p<-_ |
_ |-|_|-| _____
|_|-| |-| |
_ | | |-| _
|_|-| _ |-|_____| | _ |-|_|
|-|_|-| |-|_|-|
p<- | |
MNNs MR1 Internet AR HA
Figure 5: (n,1,1): Multiple MRs, 1 HA, 1 MNP
2.6. (n,1,n): Multiple MRs, Single HA, Multiple MNPs
The (n,1,n) configuration has more than one MR; multiple global
routes are advertised by the MRs and multiple MNPs are available
within the NEMO.
The NEMO is multihomed since it has multiple MRs, but in addition the
conditions detailed in Section 2.1 may also hold for each MR. A bi-
directional tunnel would be established between each {HA address,CoA}
pair.
Regarding MNNs, they are multihomed because several MNPs are
available on the link they are attached to. The MNNs would then
configure a global address with each MNP available on the link.
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MR2
p2<-_ |
_ |-|_|-| _____
|_|-| |-| |
_ | | |-| _
|_|-| _ |-|_____| | _ |-|_|
|-|_|-| |-|_|-|
p1<- | |
MNNs MR1 Internet AR HA
Figure 6: (n,1,n): Multiple MRs, 1 HA, multiple MNPs
2.7. (n,n,1): Multiple MRs, Multiple HAs, Single MNP
The (n,n,1) configuration has more than one MR advertising multiple
global routes. The mobile network is simultaneously associated with
multiple HAs and a single MNP is available in the NEMO.
The NEMO is multihomed since it has multiple MRs and HAs, but in
addition the conditions detailed in Section 2.1 may also hold for
each MR. A bi-directional tunnel would be established between each
{HA address,CoA} pair.
Regarding MNNs, they are (usually) not multihomed since they would
configure a single global address from the single MNP available on
the link they are attached to.
MR2 AR HA2
p _ |
<-_ | |-|_|-| _
_ |-|_|-| _____ | |-|_|
|_|-| |-| |-|
_ | | |
|_|-| _ |-|_____|-| _
|-|_|-| | _ |-|_|
<- | |-|_|-|
p |
MNNs MR1 Internet AR HA1
Figure 7: (n,n,1): Multiple MRs, Multiple HAs, 1 MNP
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2.8. (n,n,n): Multiple MRs, Multiple HAs, Multiple MNPs
The (n,n,n) configuration has multiple MRs advertising different
global routes. The mobile network is simultaneously associated with
more than one HA and multiple MNPs are available in the NEMO.
The NEMO is multihomed since it has multiple MRs and HAs, but in
addition the conditions detailed in Section 2.1 may also hold for
each MR. A bi-directional tunnel would be established between each
{HA address,CoA} pair.
Regarding MNNs, they are multihomed because several MNPs are
available on the link they are attached to. The MNNs would then
configure a global address with each MNP available on the link.
MR2 AR HA2
p2 _ |
<-_ | |-|_|-| _
_ |-|_|-| _____ | |-|_|
|_|-| |-| |-|
_ | | |
|_|-| _ |-|_____|-| _
|-|_|-| | _ |-|_|
<- | |-|_|-|
p1 |
MNNs MR1 Internet AR HA1
Figure 8: (n,n,n): Multiple MRs, HAs, and MNPs
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3. Deployment Scenarios and Prerequisites
The following generic goals and benefits of multihoming are discussed
in [6]:
1. Permanent and Ubiquitous Access
2. Reliability
3. Load Sharing
4. Load Balancing/Flow Distribution
5. Preference Settings
6. Aggregate Bandwidth
These benefits are now illustrated from a NEMO perspective with a
typical instance scenario for each case in the taxonomy. We then
discuss the prerequisites to fulfill these.
3.1. Deployment Scenarios
x=1: Multihomed mobile networks with a single MR
o Example 1:
MR with dual/multiple access interfaces (e.g. 802.11 and GPRS
capabilities). This is a (1,1,*) if both accesses are
performed with the same ISP. If the two accesses are offered
by independent ISPs, this is a (1,n,n) configuration.
Benefits: Ubiquitous Access, Reliability, Load Sharing,
Preference Settings, Aggregate Bandwidth.
x=N: Multihomed mobile networks with multiple MRs
o Example 1:
Train with one MR in each car, all served by the same HA, thus
a (n,1,*) configuration. Alternatively, the train company
might be forced to use different ISPs when the train crosses
different countries, thus a (n,n,n) configuration.
Benefits: Ubiquitous Access, Reliability, Load Sharing,
Aggregate Bandwidth.
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o Example 2:
W-PAN with a GPRS-enabled phone and a WiFi-enabled PDA. This
is a (n,n,n) configuration if the two access technologies are
subscribed separately.
Benefits: Ubiquitous Access, Reliability, Preference Settings,
Aggregate Bandwidth.
y=1: Multihomed mobile networks with a single HA
o Example:
Most single ISP cases in above examples.
y=N: Multihomed mobile networks with multiple HAs
o Example 1:
Most multiple ISP cases in above examples.
o Example 2:
Transatlantic flight with a HA in each continent. This is a
(1,n,1) configuration if there is only one MR.
Benefits: Ubiquitous Access, Reliability, Preference Settings
(reduced delay, shortest path).
z=1: Multihomed mobile networks with a single MNP
o Example:
Most single ISP cases in above examples.
z=N: Multihomed mobile networks with multiple MNPs
o Example 1:
Most multiple ISP cases in above examples.
o Example 2:
Car with a prefix taken from home (personal traffic is
transmitted using this prefix and is paid by the owner) and one
that belongs to the car manufacturer (maintenance traffic is
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paid by the car manufacturer). This will typically be a
(1,1,n) or a (1,n,n,) configuration.
Benefits: Preference Settings
3.2. Prerequisites
In this section, requirements are stated in order to comply with the
expected benefits of multihoming as detailed in [6].
At least one bi-directional tunnel must be available at any point in
time between the mobile network and the fixed network to meet all
expectations. But for most goals to be effective, multiple tunnels
must be maintained simultaneously:
o Permanent and Ubiquitous Access:
At least one bi-directional tunnel must be available at any point
in time.
o Reliability:
Both the inbound and outbound traffic must be transmitted or
diverted over another bi-directional tunnel once a bi-directional
tunnel is broken or disrupted. It should be noted that the
provision of fault tolerance capabilities does not necessarily
require the existence of multiple bi-directional tunnels
simultaneously.
o Load Sharing and Load Balancing:
Multiple tunnels must be maintained simultaneously.
o Preference Settings:
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Implicitly, multiple tunnels must be maintained simultaneously if
preferences are set for deciding which of the available bi-
directional tunnels should be used. To allow user/application to
set the preference, a mechanism should be provided to the user/
application for the notification of the availability of multiple
bi-directional tunnels, and perhaps also to set preferences.
Similar mechanism should also be provided to network
administrators to manage preferences.
o Aggregate Bandwidth:
Multiple tunnels must be maintained simultaneously in order to
increase the total aggregated bandwidth available to the mobile
network.
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4. Multihoming Issues
As discussed in the previous section, multiple bi-directional tunnels
need to be maintained either sequentially (e.g. for fault tolerance)
or simultaneously (e.g. for load sharing).
In some cases, it may be necessary to divert packets from a (perhaps
failed) bi-directional tunnel to an alternative (perhaps newly
established) bi-directional tunnel (i.e. for matters of fault
recovery, preferences), or to split traffic between multiple tunnels
(load sharing, load balancing).
So, depending on the configuration under consideration, the issues
discussed below may need to be addressed sometimes dynamically. For
each issue, potential ways to solve the problem are investigated.
4.1. Fault Tolerance
One of the goals of multihoming is the provision of fault tolerance
capabilities. In order to provide such features, a set of tasks need
to be performed, including: failure detection, alternative available
path exploration, path selection, re-homing of established
communications. These are also discussed in [8] and [8] by the Shim6
WG. In the following sub-sections, we look at these issues in the
specific context of NEMO, rather than the general Shim6 perspective
in [8]. In addition, in some scenarios, it may also be required to
provide the mechanisms for coordination between different HAs (see
Section 4.3) and also the coordination between different MRs (see
Section 4.4).
4.1.1. Failure Detection
It is expected for faults to occur more readily at the edge of the
network (i.e. the mobile nodes), due to the use of wireless
connections. Efficient fault detection mechanisms are necessary to
recover in timely fashion.
Depending on the NEMO configuration considered, the failure
protection domain greatly varies. In some configurations, the
protection domain provided by NEMO multihoming is limited to the
links between the MR(s) and the HA(s). In other configurations, the
protection domain allows to recover from failures in other parts of
the path, so an end to end failure detection mechanism is required.
Below are detailed which failure detection capabilities are required
for each configuration:
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o For the (1,1,*) cases, multiple paths are available between a
single MR and a single HA. All the traffic from and to the NEMO
flows through these MR and HA. Failure detection mechanisms need
only to be executed between these two devices. This is a NEMO/
MIPv6 specific issue.
o For the (n,1,*) cases, there is a single HA, so all the traffic
from and to the NEMO will flow through it. The failure detection
mechanisms need to be able to detect failure in the path between
the used MR and the only HA. Hence, the failure detection
mechanism needs only to involve the HA and the MRs. This is a
NEMO/MIPv6 specific issue.
o For the (n,n,*) cases, there are multiple paths between the
different HAs and the different MRs. Moreover, the HAs may be
located in different networks, and have different Internet access
links. This implies that changing the HA used may not only allow
recovering from failures in the link between the HA and the MR,
but also from other failure modes, affecting other parts of the
path. In this case, an end-to-end failure detection mechanism
would provide additional protection. However, a higher number of
failures is likely to occur in the link between the HA and the MR,
so it may be reasonable to provide optimized failure detection
mechanisms for this failure mode. The (n,n,n) case is hybrid,
since selecting a different prefix results in a change of path.
For this case the Shim6 protocols (such as those discussed in [8])
may be useful.
Most of the above cases involve the detection of tunnel failures
between HA(s) and MR(s). This is no different from the case of
failure detection between a mobile host and its HA(s). As such, a
solution for MIPv6 should apply to NEMO as well. For case (n,*,*), a
MR synchronization solution (see Section 4.4) should be able to
complement a MIPv6 failure detection solution to achieve the desired
functionality for NEMO.
In order for fault recovery to work, the MRs and HAs must first
possess a means to detect failures:
o On the MR's side, the MR can rely on router advertisements from
access routers, or other layer-2 trigger mechanisms to detect
faults, e.g. [9] and [10].
o On the HA's side, it is more difficult to detect tunnel failures.
For an ISP deployment model, the HAs and MRs can use proprietary
methods (such as constant transmission of heartbeat signals) to
detect failures and check tunnel liveness. In the subscriber
model (see Appendix A.2: S/P model), a lack of standardized
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"tunnel liveness" protocol means that it is harder to detect
failures.
A possible method is for the MRs to send binding updates more
regularly with shorter Lifetime values. Similarly, HAs can return
binding acknowledgment messages with smaller Lifetime values, thus
forcing the MRs 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 HAs must be protected (and possibly
encrypted) with security associations.
4.1.2. Path Exploration
Once a failure in the currently used path is detected, alternative
paths have to be explored in order to identify an available one.
This process is closely related to failure detection in the sense
that paths being explored need to be alternative paths to the one
that has failed. There are, however, subtle but significant
differences between path exploration and failure detection. Failure
detection occurs on the currently used path while path exploration
occurs on the alternative paths (not on the one currently being used
for exchanging packets). Although both path exploration and failure
detection are likely to rely on a reachability or keepalive test
exchange, failure detection also relies on other information, such as
upper layer information (e.g. positive or negative feedback form
TCP), lower layer information (e.g. an interface is down), and
network layer information (e.g. as an address being deprecated or
ICMP error message).
Basically, the same cases as in the analysis of the failure detection
(Section 4.1.1) issue are identified:
o For the (1,1,*) cases, multiple paths are available between a
single MR and a single HA. The existing paths between the HA and
the MR have to be explored to identify an available one. The
mechanism involves only the HA and the MR. This is a NEMO/MIPv6
specific issue.
o For the (n,1,*) cases, there is a single HA, so all the traffic
from and to the NEMO will flow through it. The available
alternative paths are the different ones between the different MRs
and the HA. The path exploration mechanism only involves the HA
and the MRs. This is a NEMO/MIPv6 specific issue.
o For the (n,n,*) cases, there are multiple paths between the
different HAs and the different MRs. In this case, alternative
paths may be routed completely independently one from one another.
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An end-to-end path exploration mechanism would be able to discover
if any of the end-to-end paths is available. The (n,n,1) case,
however, seems to be pretty NEMO specific, because of the absence
of multiple prefixes. The (n,n,n) case is hybrid, since selecting
a different prefix results in a change of path. For this case the
Shim6 protocols (such as those discussed in [8]) may be useful.
Most of the above cases involve the path exploration of tunnels
between HA(s) and MR(s). This is no different from the case of path
exploration between a mobile host and its HA(s). As such, a solution
for MIPv6 should apply to NEMO as well. For case (n,*,*), a MR
synchronization solution (see Section 4.4) should be able to
compliment a MIPv6 path exploration solution to achieve the desired
functionality for NEMO.
In order to perform path exploration, it is sometimes also necessary
for the mobile router to detect the availability of network media.
This may be achieved using layer 2 triggers [9], or other mechanism
developed/recommended by the Detecting Network Attachment (DNA)
Working Group [10]. This is related to Section 4.1.1, since the
ability to detect media availability would often implies the ability
to detect media un-availability.
4.1.3. Path Selection
A path selection mechanism is required to select among the multiple
available paths. Depending on the NEMO multihoming configuration
involved, the differences between the paths may affect only the part
between the HA and the MR, or they may affect the full end-to-end
path. In addition, depending on the configuration, path selection
may be performed by the HA(s), the MR(s) or the hosts themselves
through address selection, as will be described in details next.
The multiple available paths may differ on the tunnel between the MR
and the HA used to carry traffic to/from the NEMO. In this case,
path selection is performed by the MR and the intra-NEMO routing
system for traffic flowing from the NEMO, and path selection is
performed by the HA and intra-Home Network routing system for traffic
flowing to the NEMO.
Alternatively, the multiple paths available may differ in more than
just the tunnel between the MR and the HA, since the usage of
different prefixes may result in using different providers, hence in
completely different paths between the involved endpoints. In this
case, besides the mechanisms presented in the previous case,
additional mechanisms for the end-to-end path selection may be
needed. This mechanism may be closely related to source address
selection mechanisms within the hosts, since selecting a given
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address implies selecting a given prefix, which is associated with a
given ISP serving one of the home networks.
A dynamic path selection mechanism is thus needed so that this path
could be selected by:
o The HA: it should be able to select the path based on some
information recorded in the binding cache.
o The MR: it should be able to select the path based on router
advertisements received on both its egress interfaces or on its
ingress interfaces for the (n,*,*) case.
o The MNN: it should be able to select the path based on "Default
Router Selection" (see [Section 6.3.6. Default Router Selection]
[11]) in the (n,*,*) case or based on "Source Address Selection"
in the (*,*,n) cases (see Section 4.7 of the present memo).
o The user or the application: e.g. in case where a user wants to
select a particular access technology among the available
technologies for reasons e.g. of cost or data rate.
o A combination of any of the above: a hybrid mechanism should be
also available, e.g. one in which the HA, the MR, and/or the MNNs
are coordinated to select the path.
When multiple bi-directional tunnels are available and possibly used
simultaneously, the mode of operation may be either primary-secondary
(one tunnel is precedent over the others and used as the default
tunnel, while the other serves as a back-up) or peer-to-peer (no
tunnel has precedence over one another, they are used with the same
priority). This questions which of the bi-directional tunnels would
be selected, and based on which of the parameters (e.g. type of flow
that goes into/out of the mobile network).
The mechanisms for the selection among the different tunnels between
the MR(s) and the HA(s) seems to be quite NEMO/MIPv6 specific.
For (1,*,*) cases, they are no different from the case of path
selection between a mobile host and its HA(s). As such, a solution
for MIPv6 should apply to NEMO as well. For the (n,*,*) cases, a MR
synchronization solution (see Section 4.4) should be able to
compliment a MIPv6 path selection solution to achieve the desired
functionality for NEMO.
The mechanisms for selecting among different end-to-end paths based
on address selection are similar to the ones used in other
multihoming scenarios, as those considered by Shim6 (e.g. [12]).
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4.1.4. Re-Homing
After an outage has been detected and an available alternative path
has been identified, a re-homing event takes place, diverting the
existing communications from one path to the other. Similar to the
previous items involved in this process, the re-homing procedure
heavily varies depending on the NEMO multihoming configuration.
o For the (*,*,1) configurations, the re-homing procedure involves
only the MR(s) and the HA(s). The re-homing procedure may involve
the exchange of additional BU messages. These mechanisms are
shared between NEMO Basic Support and MIPv6.
o For the (*,*,n) cases, in addition to the previous mechanisms, end
to end mechanisms may be required. Such mechanisms may involve
some form of end to end signaling or may simply rely on using
different addresses for the communication. The involved
mechanisms may be similar to those required for re-homing Shim6
communications (e.g. [12]).
4.2. Ingress Filtering
Ingress filtering mechanisms [13][14] may drop the outgoing packets
when multiple bi-directional tunnels end up at different HAs. This
could particularly occur if different MNPs are handled by different
HAs. If a packet with a source address configured from a specific
MNP is tunneled to a home agent that does not handle that specific
MNP the packet may be discarded either by the home agent or by a
border router in the home network.
The ingress filtering compatibility issue is heavily dependent on the
particular NEMO multihoming configuration:
o For the (*,*,1) cases, there is not such an issue, since there is
a single MNP.
o For the (1,1,*) and (n,1,1) cases, there is not such a problem,
since there is a single HA, accepting all the MNPs.
o For the (n,1,n) case, though ingress filtering would not occur at
the HA, it may occur at the MRs, when each MR is handling
different MNPs.
o (*,n,n) are the cases where the ingress filtering presents some
difficulties. In the (1,n,n) case, the problem is simplified
because all the traffic from and to the NEMO is routed through a
single MR. Such configuration allows the MR to properly route
packets respecting the constraints imposed by ingress filtering.
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In this case, the single MR may face ingress filtering problems
that a multihomed mobile node may face, as documented in [7]. The
more complex case is the (n,n,n) case. A simplified case occurs
when all the prefixes are accepted by all the HAs, so that no
problems occur with the ingress filtering. However, this cannot
be always assumed, resulting in the problem described below.
As an example of how this could happen, consider the deployment
scenario illustrated in Figure 9: the mobile network has two mobile
routers MR1 and MR2, with home agents HA1 and HA2 respectively. Two
bi-directional tunnels are established between the two pairs. Each
mobile router advertises a different MNP (P1 and P2 respectively).
MNP P1 is registered to HA1, and MNP P2 is registered to HA2. Thus,
MNNs should be free to auto-configure their addresses on any of P1 or
P2. Ingress filtering could thus happen in two cases:
o If the two tunnels are available, MNN cannot forward packet with
source address equals P1.MNN to MR2. This would cause ingress
filtering at HA2 to occur (or even at MR2). This is contrary to
normal Neighbor Discovery [11] practice that an IPv6 node is free
to choose any router as its default router regardless of the
prefix it chooses to use.
o If the tunnel to HA1 is broken, packets that would normally be
sent through the tunnel to HA1 should be diverted through the
tunnel to HA2. If HA2 (or some border router in HA2's domain)
performs ingress filtering, packets with source address configured
from MNP P1 may be discarded.
Prefix: P1 +-----+ +----+ +----------+ +-----+
+--| MR1 |--| AR |--| |---| HA1 |
| +-----+ +----+ | | +-----+
IP: +-----+ | | | Prefix: P1
P1.MNN or | MNN |--+ | Internet |
P2.MNN +-----+ | | | Prefix: P2
| +-----+ +----+ | | +-----+
+--| MR2 |--| AR |--| |---| HA2 |
Prefix: P2 +-----+ +----+ +----------+ +-----+
Figure 9: An (n,n,n) mobile network
Possible solutions to the ingress filtering incompatibility problem
may be based on the following approaches:
o Some form of source address dependent routing, whether host-based
and/or router-based where the prefix contained in the source
address of the packet is considered when deciding which exit
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router to use when forwarding the packet.
o The usage of nested tunnels for (*,n,n) cases. Appendix B
describes one such approach.
o Deprecating those prefixes associated to non-available exit
routers.
The ingress filtering incompatibilities problems that appear in some
NEMO multihoming configurations are similar to those considered in
Shim6 (e.g. see [15]).
4.3. HA Synchronization
In the (*,n,*) configuration, a single MNP would be registered at
different HAs. This gives rise to the following cases:
o Only one HA may actively advertise a route to the MNP,
o Multiple HAs at different domains may advertise a route to the
same MNP.
This may pose a problem in the routing infrastructure as a whole if
the HAs are located in different administrative domains. The
implications of this aspect needs further exploration. Certain level
of HA co-ordination may be required. A possible approach is to adopt
a HA synchronization mechanism such as that described in [16] and
[17]. Such synchronization might also be necessary in a (*,n,*)
configuration, when a MR sends binding update messages to only one HA
(instead of all HAs). In such cases, the binding update information
might have to be synchronized between HAs. The mode of
synchronization may be either primary-secondary or peer-to-peer. In
addition, when a MNP is delegated to the MR (see Section 4.5), some
level of co-ordination between the HAs may also be necessary.
This issue is a general mobility issue that will also have to be
dealt with by Mobile IPv6 as well as NEMO Basic Support.
4.4. MR Synchronization
In the (n,*,*) configurations, there are common decisions which may
require synchronization among different MRs [18], such as:
o advertising the same MNP in the (n,*,1) configurations (see also
"prefix delegation" in Section 4.5);
o one MR relaying the advertisement of the MNP from another failed
MR in the (n,*,n) configuration; and
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o relaying between MRs everything that needs to be relayed, such as
data packets, creating a tunnel from the ingress interface, etc,
in the (n,*,*) configuration.
However, there is no known standardized protocols for this kind of
router-to-router synchronization. Without such synchronization, it
may not be possible for a (n,*,*) configuration to achieve various
multihoming goals, such as fault tolerance.
Such a synchronization mechanism can be primary-secondary (i.e. a
master MR, with the other MRs as backup) or peer-to-peer (i.e. there
is no clear administrative hierarchy between the MRs). The need for
such mechanism is general in the sense that a multi-router site in
the fixed network would require the same level of router
synchronization.
Thus, this issue is not specific to NEMO Basic Support, though there
is a more pressing need to develop a MR to MR synchronization scheme
for proving fault tolerances and failure recovery in NEMO
configurations due to the higher possibility of links failure.
In conclusion it is recommended to investigate a generic solution to
this issue although the solution would first have to be developed for
NEMO deployments.
4.5. Prefix Delegation
In the (*,*,1) configurations, the same MNP must be advertised to the
MNNs through different paths. There is, however, no synchronization
mechanism available to achieve this. Without a synchronization
mechanism, MR may end up announcing incompatible MNPs. Particularly,
o for the (*,n,1) cases, how can multiple HAs delegate the same MNP
to the mobile network? For doing so, the HAs may be somehow
configured to advertise the same MNP (see also "HA
Synchronization" in Section 4.3).
o for the (n,*,1) cases, how can multiple MRs be synchronized to
advertise the same MNP down the NEMO-link? For doing so, the MRs
may be somehow configured to advertise the same MNP (see also "MR
Synchronization" in Section 4.4).
Prefix delegation mechanisms [19][20][21] could be used to ensure all
routers advertise the same MNP. Their applicability to a multihomed
mobile network should be considered.
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4.6. Multiple Bindings/Registrations
When a MR is configured with multiple Care-of Addresses, it is often
necessary for it to bind these Care-of Addresses to the same MNP.
This is a generic mobility issue, since Mobile IPv6 nodes face a
similar problem. This issue is discussed in [7]. It is sufficient
to note that solutions like [22] can solve this for both Mobile IPv6
and NEMO Basic Support. This issue is being dealt with in the
Monami6 WG.
4.7. Source Address Selection
In the (*,*,n) configurations, MNNs would be configured with multiple
addresses. Source address selection mechanisms are needed to decide
which address to choose from.
However, currently available source address selection mechanisms do
not allow MNNs to acquire sufficient information to select their
source addresses intelligently (such as based on the traffic
condition associated with the home network of each MNP). It may be
desirable for MNNs to be able to acquire "preference" information on
each MNP from the MRs. This would allow default address selection
mechanisms such as those specified in [23] 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. Note that source address selection may be
closely related to path selection procedures (see Section 4.1.3) and
re-homing techniques (see Section 4.1.4).
This is a general issue faced by any node when offered multiple
prefixes.
4.8. Loop Prevention in Nested Mobile Networks
When a multihomed mobile network is nested within another mobile
network, it can result in very complex topologies. For instance, a
nested mobile network may be attached to two different root-MRs, thus
the aggregated network no longer forms a simple tree structure. In
such a situation, infinite loop within the mobile network may occur.
This problem is specific to NEMO Basic Support. However, at the time
of writing, more research is recommended to assess the probability of
loops occurring in a multihomed mobile network. For related work,
see [24] for a mechanism to avoid loops in nested NEMO.
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4.9. Prefix Ownership
When a (n,*,1) network splits, (i.e. the two MRs split themselves
up), MRs on distinct links may try to register the only available
MNP. This cannot be allowed, as the HA has no way to know which node
with an address configured from that MNP is attached to which MR.
Some mechanism must be present for the MNP to either be forcibly
removed from one (or all) MRs, or the implementors must not allow a
(n,*,1) network to split.
A possible approach to solving this problem is described in [25].
This problem is specific to NEMO Basic Support. However, it is
unclear whether there is sufficient deployment scenario for this
problem to occur.
It is recommended that the NEMO WG standardizes a solution to solve
this problem if there is sufficient vendor/operator interest, or
specifies that the split of a (n,*,1) network cannot be allowed
without a router renumbering.
4.10. Preference Settings
When a mobile network is multihomed, the MNNs may be able to benefit
from this configuration, such as to choose among the available paths
based on cost, transmission delays, bandwidth, etc. However, in some
cases, such a choice is not made available to the MNNs.
Particularly:
o In the (*,*,n) configuration, the MNNs can influence the path by
source address selection (see Section 4.1.3, Section 4.7).
o In the (n,*,*) configuration, the MNNs can influence the path by
default router selection (see Section 4.1.3).
o In the (1,*,1) configuration, the MNNs cannot influence the path
selection.
A mechanism that allows the MNN to indicate its preference for a
given traffic might be helpful. In addition, there may also be a
need to exchange some information between the MRs and the MNNs. This
problem is general in the sense that any IPv6 nodes may wish to
influence the routing decision done by the upstream routers. Such
mechanism is currently being explored by various WGs, such as the
NSIS and IPFIX WGs. It is also possible that a Shim6 layer in the
MNNs may possess such capability.
It is recommended that vendors or operators to investigate into the
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solutions developed by these WGs when providing multihoming
capabilities to mobile networks.
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5. Recommendations to the Working Group
Several issues that might impact the deployment of NEMO with
multihoming capabilities were identified in Section 4. These are
shown in the matrix below, for each of the eight multihoming
configurations, together with indications of from which WG(s) a
solution to each issue is most likely to be found.
+=================================================================+
| # of MRs: | 1 | 1 | 1 | 1 | n | n | n | n |
| # of HAs: | 1 | 1 | n | n | 1 | 1 | n | n |
| # of Prefixes: | 1 | n | 1 | n | 1 | n | 1 | n |
+=================================================================+
| Fault Tolerance | * | * | * | * | * | * | * | * |
+---------------------------------+---+---+---+---+---+---+---+---+
| Failure Detection |N/M|N/M|N/M|N/M|N/M|N/M| N | S |
+---------------------------------+---+---+---+---+---+---+---+---+
| Path Exploration |N/M|N/M|N/M|N/M|N/M|N/M| N | S |
+---------------------------------+---+---+---+---+---+---+---+---+
| Path Selection | N |S/M| M |S/M| N |S/N| N |S/N|
+---------------------------------+---+---+---+---+---+---+---+---+
| Re-Homing |N/M| S |N/M| S |N/M| S |N/M| S |
+---------------------------------+---+---+---+---+---+---+---+---+
| Ingress Filtering | . | . | . | t | . | . | . | N |
+---------------------------------+---+---+---+---+---+---+---+---+
| HA Synchronization | . | . |N/M|N/M| . | . |N/M|N/M|
+---------------------------------+---+---+---+---+---+---+---+---+
| MR Synchronization | . | . | . | . | G | G | G | G |
+---------------------------------+---+---+---+---+---+---+---+---+
| Prefix Delegation | . | . | N | N | N | N | N | N |
+---------------------------------+---+---+---+---+---+---+---+---+
| Multiple Binding/Registrations | M | M | M | M | M | M | M | M |
+---------------------------------+---+---+---+---+---+---+---+---+
| Source Address Selection | . | G | . | G | . | G | . | G |
+---------------------------------+---+---+---+---+---+---+---+---+
| Loop Prevention in Nested NEMO | N | N | N | N | N | N | N | N |
+---------------------------------+---+---+---+---+---+---+---+---+
| Prefix Ownership | . | . | . | . | N | . | N | . |
+---------------------------------+---+---+---+---+---+---+---+---+
| Preference Settings | G | G | G | G | G | G | G | G |
+=================================================================+
N - NEMO Specific M - MIPv6 Specific G - Generic IPv6
S - SHIM6 WG D - DNA WG
. - Not an Issue t - trivial
* - Fault Tolerance is a combination of Failure Detection, Path
Exploration, Path Selection, and Re-Homing
Figure 10: Matrix of NEMO Multihoming Issues
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The above matrix serves to identify which issues are NEMO-specific,
and which are not. The readers are reminded that this matrix is a
simplification of Section 4 as subtle details are not represented in
Figure 10.
As can be seen from Figure 10, the following have some concerns which
are specific to NEMO: Failure Detection, Path Exploration, Path
Selection, Re-Homing, Ingress Filtering, HA Synchronization, Prefix
Delegation, Loop Prevention in Nested NEMO, and Prefix Ownership.
Based on the authors' best knowledge of the possible deployments of
NEMO, it is recommended that:
o A solution for Failure Detection, Path Exploration, Path
Selection, and Re-Homing be solicited from other WGs.
Although Path Selection is reflected in Figure 10 as NEMO-
Specific, the technical consideration of the problem is believed
to be quite similar to the selection of multiple paths in mobile
nodes. As such, we would recommend vendors to solicit a solution
for these issues from other WGs in the IETF, for instance the
Monami6 or Shim6 WG.
o Ingress Filtering on the (n,n,n) configuration be solved by the
NEMO WG.
This problem is clearly defined, and can be solved by the WG.
Deployment of the (n,n,n) configuration can be envisioned on
vehicles for mass transportation (such as buses, trains) where
different service providers may install their own mobile routers
on the vehicle/vessel.
It should be noted that the Shim6 WG may be developing a mechanism
for overcoming ingress filtering in a more general sense. We thus
recommend the NEMO WG to concentrate only on the (n,n,n)
configuration should the WG decide to work on this issue.
o A solution for Home Agent Synchronization be looked at in a
mobility specific WG and taking into consideration both mobile
hosts operating Mobile IPv6 and mobile routers operating NEMO
Basic Support.
o A solution for Multiple Bindings/Registrations be presently looked
at by the Monami6 WG.
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o Prefix Delegation be reviewed and checked by the NEMO WG.
The proposed solutions [21] and [20] providing prefix delegation
functionality to NEMO Basic Support should be reviewed in order to
make sure concerns as discussed in Section 4.5 are properly
handled.
o Loop Prevention in Nested NEMO be investigated.
Further research is recommended to assess the risk of having a
loop in the nesting of multihomed mobile networks.
o Prefix Ownership should be considered by the vendors and
operators.
The problem of Prefix Ownership only occurs when a mobile network
with multiple MRs and a single MNP can arbitrarily join and split.
Vendors and operators of mobile networks are encouraged to input
their views on the applicability of deploying such kind of mobile
networks.
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6. Conclusion
This memo presented an analysis of multihoming in the context of
network mobility under the operation of NEMO Basic Support (RFC
3963). The purpose was to investigate issues related to such a bi-
directional tunneling mechanism where mobile networks are multihomed
and multiple bi-directional tunnels established between home agent
and mobile router pairs. For doing so, mobile networks were
classified into a taxonomy comprising eight possible multihomed
configurations. Issues were explained one by one and then summarized
into a table showing the multihomed configurations where they apply
and suggesting the most relevant IETF working group where they could
be solved. This analysis will be helpful to extend the existing
standards to support multihoming and to implementors of NEMO Basic
Support and multihoming-related mechanisms.
7. IANA Considerations
This is an informational document and as such does not require any
IANA action.
8. Security Considerations
This is an informational document where the multihoming
configurations under the operation of NEMO Basic Support are
analyzed. Security considerations of these multihoming
configurations, should they be different from those that concern NEMO
Basic Support, must be considered by forthcoming solutions.
9. 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 - 61st IETF Meetings.
The initial evaluation of NEMO Basic Support on multihoming
configurations is a contribution from Julien Charbon.
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10. References
10.1. Normative References
[1] Ernst, T., "Network Mobility Support Goals and Requirements",
draft-ietf-nemo-requirements-06 (work in progress),
November 2006.
[2] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[3] Ernst, T. and H. Lach, "Network Mobility Support Terminology",
draft-ietf-nemo-terminology-06 (work in progress),
November 2006.
[4] Devarapalli, V., Wakikawa, R., Petrescu, A., and P. Thubert,
"Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
January 2005.
[5] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
10.2. Informative References
[6] Ernst, T., Montavont, N., Wakikawa, R., Ng, C., and K.
Kuladinithi, "Motivations and Scenarios for Using Multiple
Interfaces and Global Addresses",
draft-ietf-monami6-multihoming-motivation-scenario-01 (work in
progress), October 2006.
[7] Montavont, N., Wakikawa, R., Ernst, T., Ng, C., and K.
Kuladinithi, "Analysis of Multihoming in Mobile IPv6",
draft-ietf-monami6-mipv6-analysis-00 (work in progress),
February 2006.
[8] Arkko, J. and I. Beijnum, "Failure Detection and Locator Pair
Exploration Protocol for IPv6 Multihoming",
draft-ietf-shim6-failure-detection-07 (work in progress),
December 2006.
[9] Krishnan, S., Montavont, N., Yegin, A., Veerepalli, S., and A.
Yegin, "Link-layer Event Notifications for Detecting Network
Attachments", draft-ietf-dna-link-information-05 (work in
progress), November 2006.
[10] Narayanan, S., "Detecting Network Attachment in IPv6 Networks
(DNAv6)", draft-ietf-dna-protocol-03.txt (work in progress),
October 2006.
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[11] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[12] Nordmark, E. and M. Bagnulo, "Level 3 multihoming shim
protocol", draft-ietf-shim6-proto-07 (work in progress),
November 2006.
[13] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[14] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[15] Huitema, C. and M. Marcelo, "Ingress filtering compatibility
for IPv6 multihomed sites",
draft-huitema-shim6-ingress-filtering-00 (work in progress),
October 2006.
[16] Wakikawa, R., Devarapalli, V., and P. Thubert, "Inter Home
Agents Protocol (HAHA)", draft-wakikawa-mip6-nemo-haha-01 (work
in progress), February 2004.
[17] Koh, B., Ng, C., and J. Hirano, "Dynamic Inter Home Agent
Protocol", draft-koh-mip6-nemo-dhap-00 (work in progress),
July 2004.
[18] Tsukada, M., "Analysis of Multiple Mobile Routers Cooperation",
draft-tsukada-nemo-mr-cooperation-analysis-00 (work in
progress), October 2005.
[19] Miyakawa, S. and R. Droms, "Requirements for IPv6 Prefix
Delegation", RFC 3769, June 2004.
[20] Droms, R. and P. Thubert, "DHCPv6 Prefix Delegation for NEMO",
draft-ietf-nemo-dhcpv6-pd-02 (work in progress),
September 2006.
[21] Thubert, P. and TJ. Kniveton, "Mobile Network Prefix
Delegation", draft-ietf-nemo-prefix-delegation-01 (work in
progress), November 2006.
[22] Wakikawa, R., "Multiple Care-of Addresses Registration",
draft-ietf-monami6-multiplecoa-00 (work in progress),
June 2006.
[23] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
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[24] Thubert, P., Bontous, C., and N. Nicolas, "Nested Nemo Tree
Discovery", draft-thubert-tree-discovery-04 (work in progress),
November 2006.
[25] Kumazawa, M., "Token based Duplicate Network Detection for
split mobile network (Token based DND)",
draft-kumazawa-nemo-tbdnd-02 (work in progress), July 2005.
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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 Erik 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 possibility 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 MNPs 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 HAs) work. This include not
introducing any aggregation between the HAs which will filter out
routing announcements for the MNP(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
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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 MRs belongs to a single subscriber and subscribes to one or
more ISPs for HA 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 MNP
o S/P-(1,1,n): Single Provider, Single MR, Single HA, Multiple MNPs
o S/P-(1,n,1): Single Provider, Single MR, Multiple HAs, Single MNP
o S/P-(1,n,n): Single Provider, Single MR, Multiple HAs, Multiple
MNPs
o S/P-(n,n,1): Single Provider, Multiple MRs, Single HA, Single MNP
o S/P-(n,1,n): Single Provider, Multiple MRs, Single HA, Multiple
MNPs
o S/P-(n,n,1): Single Provider, Multiple MRs, Multiple HAs, Single
MNP
o S/P-(n,n,n): Single Provider, Multiple MRs, Multiple HAs, Multiple
MNPs
For the S/mP model, the following configurations are likely to be
deployed:
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o S/mP-(1,n,1): Multiple Providers, Single MR, Multiple HAs, Single
MNP
o S/mP-(1,n,n): Multiple Providers, Single MR, Multiple HAs,
Multiple MNPs
o S/mP-(n,n,n): Multiple Providers, Multiple MRs, Multiple HAs,
Multiple MNPs
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 HAs, and
the number of MNPs advertised. Since the two parameters are
orthogonal, the categories are not mutually exclusive. The four
categories are:
o Tarzan: Single HA for Different CoAs of Same MNP
This is the case where one MR registers different Care-of
Addresses to the same HA for the same subnet prefix. This is
equivalent to the case of y=1, i.e. the (1,1,*) mobile network.
o JetSet: Multiple HAs for Different CoAs of Same MNP
This is the case where the MR registers different Care-of
Addresses to different HAs for the same subnet prefix. This is
equivalent to the case of y=n, i.e. the (1,n,*) mobile network.
o Shinkansen: Single MNP Advertised by MR(s)
This is the case where one MNP is announced by different MRs.
This is equivalent to the case of x=n and z=1, i.e. the (n,*,1)
mobile network.
o DoubleBed: Multiple MNPs Advertised by MR(s)
This is the case where more than one MNPs are announced by the
different MRs. This is equivalent to the case of x=n and 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
multihoming, MRs that employ bi-directional tunneling with their HAs
should dynamically change their tunnel exit points depending on the
link status. For instance, if a MR 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 MRs connected to one of its ingress interfaces that has an
independent route to the global Internet, or by another active egress
interface the MR itself possess. If such an alternate route exists,
the MR should re-establish the bi-directional tunnel using this
alternate route.
In the remaining part of this Appendix, we will attempt to
investigate methods of performing such re-establishment of bi-
directional tunnels. This method of tunnel re-establishment is
particularly useful for the (*,n,n) NEMO configuration. The method
described is by no means complete and merely serves as a suggestion
on how to approach the problem. It is also not the objective 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 [5] and Neighbor
Discovery in IPv6 [11].
B.1. Detecting Presence of Alternate Routes
To actively utilize the robustness provided by multihoming, a MR must
first be capable of detecting alternate routes. This can be manually
configured into the MR by the administrators if the configuration of
the mobile network is relatively static. It is however highly
desirable for MRs to be able to discover alternate routes
automatically for greater flexibility.
The case where a MR possesses multiple egress interface (bound to the
same HA or otherwise) should be trivial, since the MR should be able
to "realize" it has multiple routes to the global Internet.
In the case where multiple MRs are on the mobile network, each MR has
to detect the presence of other MR. A MR can do so by listening for
Router Advertisement message on its *ingress* interfaces. When a MR
receives a Router Advertisement message with a non-zero Router
Lifetime field from one of its ingress interfaces, it knows that
another MR which can provide an alternate route to the global
Internet is present in the mobile network.
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B.2. Re-Establishment of Bi-Directional Tunnels
When a MR detects that the link by which its current bi-directional
tunnel with its HA 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 MR; secondly, the
alternate route is provided by another MR 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 MR.
B.2.1. Using Alternate Egress Interface
When an egress interface of a MR loses the connection to the global
Internet, the MR can make use of its alternate egress interface
should it possess multiple egress interfaces. The most direct way to
do so is for the MR to send a binding update to the HA of the failed
interface using the CoA assigned to the alternate interface in order
to re-establish the bi-directional tunneling using the CoA on the
alternate egress interface. After a successful binding update, the
MR encapsulates outgoing packets through the bi-directional tunnel
using the alternate egress interface.
The idea is to use the global address (most likely a CoA) assigned to
an alternate egress interface as the new (back-up) CoA of the MR to
re-establish the bi-directional tunneling with its HA.
B.2.2. Using Alternate Mobile Router
When the MR loses a connection to the global Internet, the MR can
utilize a route provided by an alternate MR (if one exists) to re-
establish the bi-directional tunnel with its HA. First, the MR has
to obtain a CoA from the alternate MR (i.e. attaches itself to the
alternate MR). Next, it sends binding update to its HA using the CoA
obtained from the alternate MR. From then on, the MR can
encapsulates outgoing packets through the bi-directional tunnel using
via the alternate MR.
The idea is to obtain a CoA from the alternate MR and use this as the
new (back-up) CoA of the MR to re-establish the bi-directional
tunneling with its HA.
Note that every packet sent between MNNs and their correspondent
nodes will experience two levels of encapsulation. First level of
tunneling occurs between a MR which the MNN uses as its default
router and the MR's HA. The second level of tunneling occurs between
the alternate MR and its HA.
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B.3. To Avoid Tunneling Loop
The method of re-establishing the bi-directional tunnel as described
in Appendix B.2 may lead to infinite loops of tunneling. This
happens when two MRs on a mobile network lose connection to the
global Internet at the same time and each MR tries to re-establish
bi-directional tunnel using the other MR. We refer to this
phenomenon as tunneling loop.
One approach to avoid tunneling loop is for a MR 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 MRs 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 MNNs that are attached to the
MR to stop using the MR as their default router too (in which case,
things are back to square one).
B.4. Points of Considerations
This method of using tunnel re-establishments is by no means a
complete solution. There are still points to consider to develop it
into a fully functional solution. For instance, in Appendix B.1, it
was suggested that MR detects the presence of other MRs using Router
Advertisements. However, Router Advertisements are link scoped, so
when there is more than one link, some information may be lost. For
instance, suppose a case where there is three MRs and three different
prefixes and each MR is in a different link with regular routers in
between. Suppose now that only a single MR is working, how do the
other MRs identify which prefix they have to use to configure the new
CoA? In this case, there are three prefixes being announced and a MR
whose link has failed, knows that his prefix is not to be used, but
it has not enough information to decide which one of the other two
prefixes to use to configure the new CoA. In such cases, a mechanism
is needed to allow a MR to withdraw its own prefix when it discovers
that its link is no longer working.
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Appendix C. Change Log
o Changes from draft-ietf-nemo-multihoming-issues-06 to -07:
* Removed in 2.1 the bullet "Multiple global prefixes are
available on the home link, and thus the MR has more than one
path to reach the home agent."
* In all 2.x sub-sections in the sentence similar to "A bi-
directional tunnel would then be established between each HoA-
CoA pair", replaced the part "HoA-CoA" pair with "{HA address,
CoA} pair."
* Removed in 2.3 ", possibly one HA per HoA, or one HA per egress
interface." and in 2.4 ", possibly one per Home Address (or one
HA per egress interface),"
* In 2.4 and 2.6 and 2.8, replaced "Regarding MNNs, they are
generally multihomed since they would configure a global
address from each MNP available on the link they are attached
to." with the better text in 2.2, i.e. "Regarding MNNs, they
are multihomed because several MNPs are available on the link
they are attached to. The MNNs would then configure a global
address with each MNP available on the link."
* In 4.1.1 and 4.1.2 3rd bullet, rephrased the complex sentence
"The (n,n,n) case is hybrid, since for those cases when[4.1.1]/
that[4.1.2] selecting a different prefix result in a change of
path, the Shim6 protocols (such as [9]) may be useful." into
"The (n,n,n) case is hybrid, since selecting a different prefix
results in a change of path. For this case the Shim6 protocols
(such as those discussed in [8]) may be useful."
* Probably due to a typo in the table in section 5 line "Path
Selection", changed "N |S/N| N |S/N| N |S/N| N |S/N|" to "M
|S/M| M |S/M| N |S/N| N |S/N|"
* Removed references to draft-yegin-dna-l2-hints-01 and
draft-manyfolks-l2-mobilereq-02. Should now be covered in
draft-ietf-dna-link-information-05.txt.
* Both draft-droms-nemo-dhcpv6-pd-02 and
draft-ietf-nemo-dhcpv6-pd-00 were cited. Removed the former.
* Replaced references to draft-ietf-dna-hosts-02 and
draft-ietf-dna-routers-01 with draft-ietf-dna-protocol-03.txt
where everything was merged.
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* Replaced draft-ietf-shim6-reach-detect-01 with
draft-ietf-shim6-failure-detection
* Replaced draft-ietf-shim6-functional-dec with
draft-ietf-shim6-proto
* Rephrased paragraph about "Prefix Delegation" in section 5.
* Rephrased the conclusion.
* Replaced "visited link" with "foreign link" and "border
gateway" with "border router" in several places.
* Reordered author list.
* And, minor editorial corrections and reference update.
o Changes from draft-ietf-nemo-multihoming-issues-05 to -06:
* Minor editorial corrections and reference update
o Changes from draft-ietf-nemo-multihoming-issues-04 to -05:
* Addressed Issue #23: modified text in Sect 4.2: "Ingress
Filtering"
* Re-phrase statements in Sect 4 and 5 where we "... recommend
issue XXX to be worked on by Monami6/Shim6/IPv6/DNA WG" to
instead suggest that solution to the issue be solicited
elsewhere within the IETF.
o Changes from draft-ietf-nemo-multihoming-issues-03 to -04:
* Updated Section 3: "Deployment Scenarios and Prerequisites"
* Modifications to Section 4:
+ Removed "Media Detection" and moved the relevant concerns to
"Path Exploration"
+ Added new "Preference Settings" issue
+ Various text clean-ups in most of the sub-sections
* Modifications to Section 5:
+ Changed Section 5: "Matrix" to "Recommendations to the
Working Group"
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+ Identifies which are the issues that are important, and made
recommendations as to how to resolve these multihoming
issues
* Addressed Issue #12: Added Appendix B.4: "Points of
Considerations" to document the concerns raised for this tunnel
re-establishment mechanism.
o Changes from draft-ietf-nemo-multihoming-issues-02 to -03:
* Enlisted Marcelo Bagnulo as co-author
* Restructuring of Section 4:
+ Re-named 'Path Survival' to 'Fault Tolerance'
+ Moved 'Path Failure Detection' and 'Path Selection' as sub-
issues of 'Fault Tolerance'
+ Added 'Path Exploration' and 'Re-homing' as sub-issues of
'Fault Tolerance'
+ Removed 'Impact on Routing Infrastructure'
* Breaking references into Normative and Informative
o Changes from draft-ietf-nemo-multihoming-issues-01 to -02:
* Added recommendations/suggestion of which WG each issue should
be addressed as pointed out in 61st IETF.
* Minor updates on references.
o Changes from draft-ietf-nemo-multihoming-issues-00 to -01:
* Replaced NEMO-Prefix with MNP as decided by the WG at 60th IETF
* Addressed Issue #1 in Section 1: Added a note to remind readers
that IPv6 is implicitly assumed
* Addressed Issue #3 in Sect 2.3: Removed text on assumption
* Addressed Issue #6 in Sect 3.1 "Deployment Scenarios": Added
benefits
* Addressed Issue #7 in Sect 3.2 "Prerequisites": Modified text
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* Addressed Issue #9 in Sect 4.3 "Ingress Filtering": Modified
text
* Addressed Issue #10 in Sect 4.4: Added paragraph on other
failure modes
* Addressed Issue #10: New Sect 4.5 on media detection
* Addressed Issue #11 in Section 4.11: modified text
o Changes from draft-ng-multihoming-issues-03 to
draft-ietf-nemo-multihoming-issues-00:
* Expanded Section 4: "Problem Statement"
* Merged "Evaluation" Section into Section 4: "Problem Statement"
* Cleaned up description in Section 2: "Classification", and
clearly indicate in each classification, what are the
multihomed entities
* Re-organized Section 2: "Deployment Scenarios and
Prerequisites", and created the "Prerequisites" sub-section.
o Changes from draft-ng-multihoming-issues-02 to
draft-ng-multihoming-issues-03:
* Merged with draft-eun-nemo-multihoming-problem-statement (see
Section 4: "Problem Statement")
* Included conclusions from
draft-charbon-nemo-multihoming-evaluation-00
* Re-organized some part of "Benefits/Issues of Multihoming in
NEMO" to Section 4: "Problem Statement"
* Removed lots of text to be in sync with [6].
* Title changed from "Multihoming Issues in NEMO Basic Support"
to "Analysis of Multihoming in NEMO"
* Changed (w,x,y) to (x,y,z) in taxonomy.
* Moved alternative approaches of classification to Appendix
* Creation of this Change-Log itself ;-)
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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: chanwah.ng@sg.panasonic.com
Thierry Ernst
INRIA
INRIA Rocquencourt
Domaine de Voluceau B.P. 105
Le Chesnay, 78153
France
Phone: +33-1-39-63-59-30
Fax: +33-1-39-63-54-91
Email: thierry.ernst@inria.fr
URI: http://www.nautilus6.org/~thierry
Eun Kyoung Paik
KT
Portable Internet Team, Convergence Lab., KT
17 Woomyeon-dong, Seocho-gu
Seoul 137-792
Korea
Phone: +82-2-526-5233
Fax: +82-2-526-5200
Email: euna@kt.co.kr
URI: http://mmlab.snu.ac.kr/~eun/
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad, 30
Leganes, Madrid 28911
Spain
Phone: +34 91624 8837
Email: marcelo@it.uc3m.es
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Internet-Draft Analysis of Multihoming in NEMO February 2007
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Ng, et al. Expires August 9, 2007 [Page 48]
