NEMO Working Group C. Ng
Internet-Draft Panasonic Singapore Labs
Expires: January 19, 2006 E. Paik
KT
T. Ernst
WIDE at Keio University
M. Bagnulo
UC3M
July 18, 2005
Analysis of Multihoming in Network Mobility Support
draft-ietf-nemo-multihoming-issues-03
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Copyright (C) The Internet Society (2005).
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
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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). Issues are classified according to the Working Group which
is the best chartered to solve them.
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 . . . . . . . 7
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 . . . . . . . 9
2.6 (n,1,n): Multiple MRs, Single HA, Multiple MNPs . . . . . 10
2.7 (n,n,1): Multiple MRs, Multiple HAs, Single MNP . . . . . 10
2.8 (n,n,n): Multiple MRs, Multiple HAs, Multiple MNPs . . . . 11
3. Deployment Scenarios and Prerequisites . . . . . . . . . . . . 13
3.1 Deployment Scenarios . . . . . . . . . . . . . . . . . . . 13
3.2 Prerequisites . . . . . . . . . . . . . . . . . . . . . . 14
4. Multihoming Issues . . . . . . . . . . . . . . . . . . . . . . 16
4.1 Fault Tolerance . . . . . . . . . . . . . . . . . . . . . 16
4.1.1 Failure Detection . . . . . . . . . . . . . . . . . . 16
4.1.2 Path Exploration . . . . . . . . . . . . . . . . . . . 18
4.1.3 Path Selection . . . . . . . . . . . . . . . . . . . . 19
4.1.4 Re-Homing . . . . . . . . . . . . . . . . . . . . . . 20
4.2 Ingress Filtering . . . . . . . . . . . . . . . . . . . . 21
4.3 Media Detection . . . . . . . . . . . . . . . . . . . . . 22
4.4 HA Synchronization . . . . . . . . . . . . . . . . . . . . 23
4.5 MR Synchronization . . . . . . . . . . . . . . . . . . . . 23
4.6 Prefix Delegation . . . . . . . . . . . . . . . . . . . . 24
4.7 Multiple Bindings/Registrations . . . . . . . . . . . . . 24
4.8 Source Address Selection . . . . . . . . . . . . . . . . . 24
4.9 Loop Prevention in Nested Mobile Networks . . . . . . . . 25
4.10 Prefix Ownership . . . . . . . . . . . . . . . . . . . . . 25
5. Matrix [Issues,(x,y,z) Configuration] . . . . . . . . . . . . 26
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
8. Security Considerations . . . . . . . . . . . . . . . . . . . 27
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9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
10.1 Normative References . . . . . . . . . . . . . . . . . . . 28
10.2 Informative References . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
A. Alternative Classifications Approach . . . . . . . . . . . . . 32
A.1 Ownership-Oriented Approach . . . . . . . . . . . . . . . 32
A.1.1 ISP Model . . . . . . . . . . . . . . . . . . . . . . 32
A.1.2 Subscriber/Provider Model . . . . . . . . . . . . . . 33
A.2 Problem-Oriented Approach . . . . . . . . . . . . . . . . 35
B. Nested Tunneling for Fault Tolerance . . . . . . . . . . . . . 36
B.1 Detecting Presence of Alternate Routes . . . . . . . . . . 36
B.2 Re-Establishment of Bi-Directional Tunnels . . . . . . . . 36
B.2.1 Using Alternate Egress Interface . . . . . . . . . . . 37
B.2.2 Using Alternate Mobile Router . . . . . . . . . . . . 37
B.3 To Avoid Tunneling Loop . . . . . . . . . . . . . . . . . 38
C. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . . 41
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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
basically solves the problem 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 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 doesn't
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 by R.12 in section 5 of [1], 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 objective of this memo
is thus multi-folded:
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o to determine all the potential multihomed configurations for a
NEMO, and then to identify which of those 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 those 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 identify potential solutions to the previously identified
issues.
o to decide which issues are worth to be solved, and to determine
which WG should address each of the issues that are worth solving.
In order to reach these objectives, a taxonomy to classify 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 (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], Goals and Benefits of 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 a mobile network has only a single MR, presumably
multihomed.
x=n implies 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
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 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 visited link), or (ii) the MR is equipped with
multiple interfaces. In such a case, the MR would have multiple Home
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Address / Care-of Address 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 mad available through Dynamic Host Configuration
Protocol (DHCP).
2.1 (1,1,1): Single MR, Single HA, Single MNP
The (1,1,1) mobile network 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, or
o Multiple global prefixes are available on the visited link, or
o Multiple global prefixes are available on the home link (Note that
in this case, those prefixes are not all available in the NEMO,
since there is only a single MNP in the NEMO)
A bi-directional tunnel would then be established between each pair
of Home Address / Care-of Address.
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
2.2 (1,1,n): Single MR, Single HA, Multiple MNPs
The (1,1,n) mobile network has only one MR, it is associated with a
single HA and two or more MNPs are available in the NEMO.
The MR may be itself multihomed, as detailed in Section 2.1. In such
a case, a bi-directional tunnel would be established between each
pair of Home Address / Care-of Address.
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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.
_____
_ 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) mobile network has only one MR and a single MNP is
available in the NEMO. The MR, however, is associated with multiple
HAs, possibly one HA per Home Address, or one HA per egress
interface.
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 then be established between each pair of
Home Address / Care-of Address.
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.
AR HA2
_ |
|-|_|-| _
_____ | |-|_|
_ p _ | |-|
|_|-|<-_ |-|_|-| |
_ |-|_|=| |_____|-| _
|_|-| | | _ |-|_|
|-|_|-|
|
MNNs MR AR Internet AR HA1
Figure 3: (1,n,1): 1 MR, multiple HAs, 1 MNP
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2.4 (1,n,n): Single MR, Multiple HAs, Multiple MNPs
The (1,n,n) mobile network has only one MR. However, the MR is
associated with multiple HAs, possibly one per Home Address (or one
HA per egress interface), 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 then be established between each pair of Home Address /
Care-of Address.
Regarding MNNs, they are generally multihomed since they would
configure a global address from each MNP available on the link they
are attached to.
AR HA2
_ | _
_____ |-|_|-|-|_|
_ p1,p2 _ | |-| |
|_|-|<-_ |-|_|-| | _
_ |-|_|=| |_____|-| _ |-|_|
|_|-| | |-|_|-|
| |
MNNs MR AR Internet AR HA1
Figure 4: (1,n,n): 1 MR, multiple HAs, multiple MNPs
2.5 (n,1,1): Multiple MRs, Single HA, Single MNP
The (n,1,1) mobile network 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 then be established between each pair of
Home Address / Care-of Address.
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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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) mobile network 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 then be established between each pair of
Home Address / Care-of Address.
Regarding MNNs, they are generally multihomed since they would
configure a global address from each MNP available on the link they
are attached to.
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) mobile network 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
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addition the conditions detailed in Section 2.1 may also hold for
each MR. A bi-directional tunnel would then be established between
each pair of Home Address / Care-of Address.
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
2.8 (n,n,n): Multiple MRs, Multiple HAs, Multiple MNPs
The (n,n,n) mobile network 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 then be established between
each pair of Home Address / Care-of Address.
Regarding MNNs, they are generally multihomed since they would
configure a global address from each MNP available on the link they
are attached to.
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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. Redundancy/Fault-Recovery
3. Load Sharing
4. Load Balancing
5. Preference Settings
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: an MR with dual/multiple access interfaces (e.g.
802.11 and GPRS capabilities). This is a S/P-(1,1,*) if both
accesses are subscribed to the same ISP. If the two accesses
are offered by independent ISPs, this is a S/mP-(1,n,n) [for
the meaning of this abbreviation, see Appendix A.1].
Benefits: Ubiquity, Redundancy/Fault-Recovery, Load Sharing,
Preference Settings
x=N: Multihomed mobile networks with multiple MRs
o Example 1: a train with one MR in each car, all served by the
same HA, thus a (n,1,*). Alternatively, the train company
might be forced to use different ISPs when the train go to
different locations, thus it is a (n,n,n).
Benefits: Load Sharing, Redundancy/Fault-Recovery, Ubiquity
o Example 2: W-PAN with a GPRS-enabled phone and a WiFi-enabled
PDA. This is a S/mP-(n,n,n) if the two access technologies are
subscribed separately.
Benefits: Ubiquity, Redundancy/Fault-Recovery, Preference
Settings
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y=1: Multihomed mobile networks with a single HA
o Most single ISP cases in above examples.
y=N: Multihomed mobile networks with multiple HAs
o Most multiple ISP cases in above examples.
o Example: a transatlantic flight with a HA in each continent.
This is a (1,n,1) network if there is only one MR.
Benefits: Ubiquity, Preferences (reduced delay, shortest path)
z=1: Multihomed mobile networks with a single MNP
o Most single ISP cases in above examples.
z=N: Multihomed mobile networks with multiple MNPs
o Most multiple ISP cases in above examples.
o Example: a car with a prefix taken from home (personal traffic
transit on this prefix and is paid by the owner) and one that
belongs to the car manufacturer (maintenance traffic is paid by
the car-manufacturer). This will typically be a (1,1,n) or a
(1,n,n,).
Benefits: Preference Settings
3.2 Prerequisites
In this section, requirements are started 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.
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o Redundancy and Fault-Recovery:
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:
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 for the administration of the preferences.
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4. Multihoming Issues
As discussed in the previous section, multiple bi-directional tunnels
need to be maintained either simultaneously (e.g. for load sharing)
or sequentially (e.g. for fault tolerance)
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 and
an a recommendation is made on which IETF WG(s) should look into
these issues.
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] [9]. In the
following sub-sections, we look at these issues in the specific
context of NEMO, rather than the general Multi6/Shim6 perspective in
[8] [9]. In addition, in some scenarios, it may also be required to
provide the mechanisms for coordination between different HAs (see
Section 4.4) and also the coordination between different MRs (see
Section 4.5).
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
specific issue (MIPv6?).
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 specific issue (MIPv6?).
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
to recover 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 in the link between the HA and the MR, so it
may be reasonable to provide optimized failure detection
mechanisms for this failure mode even in this scenario, as it is
considered next. 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 for those cases that selecting a
different prefix results in a change of path, the SHIM/multi6
protocols may be useful. The other cases, are NEMO specific.
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. [10], [11], [12] or [13]. (For a related issue, see
Section 4.3.)
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 S/P model (see
Appendix A.2), a lack of standardized "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
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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 need 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 in alternative paths (not in 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 need to be explored to identify an available one. The
mechanism involves only the HA and the MR. This is a NEMO
specific issue (MIPv6?).
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 needs only to involve
the HA and the MRs. This is a NEMO specific issue (MIPv6?).
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 each other.
In this case, an end to end path exploration mechanism would be
able to discover if any of the end to end paths is available.
However, a higher number of failures is likely in the link between
the HA and the MR, so it may be reasonable to provide optimized
path exploration mechanism for this failure mode even in this
scenario. The (n,n,1) case, however, seems to be pretty NEMO
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specific, because of the absence of multiple prefixes. The
(n,n,n) case is hybrid, since for those cases that selecting a
different prefix result in a change of path, the SHIM/multi6
protocols may be useful. The other cases, are NEMO specific.
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 may affect the full end to end path.
Related to this, depending on the configuration, path selection may
be performed by the HA(s), the MR(s) or the hosts themselves through
address selection, as it will be presented next.
In the (*,*,1) cases, the multiple available paths mostly 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, as it is presented next.
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.
In the (*,*,n) cases, 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
address implies selecting a given prefix, which is associated with a
given ISP serving one of the home networks.
In this case we need to consider additional mechanisms for path
selection based on:
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o The MNN: it should be able to select the path based on "Default
Router Selection" (see [Section 6.3.6. Default Router Selection]
[14]) in the (n,*,*) case or based on "Source Address Selection"
in the (*,*,n) cases (see Section 4.8 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 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 bi-directional tunnel would be
selected, and based on which parameter (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 specific. 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/multi6 (e.g. [15]).
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
existent 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-
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homing Shim6/multi6 communications (e.g. [15]).
4.2 Ingress Filtering
Ingress filtering mechanisms [16][17] 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
home agents. If a packet with a source address configured from a
specific MNP is tunnelled 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 gateway 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,n) cases, there is not such a problem since there is
a single HA, accepting all the 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.
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. 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). 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 [14] practice that an IPv6 node is free
to choose any router as its default router regardless of the
prefix it chooses to use.
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o If the tunnel to HA1 is broken, packets would be sent through the
tunnel to HA1 are diverted through the tunnel to HA2. If HA2 (or
some border gateway in the domain of HA2) performs ingress
filtering, packets with source address configured from MNP P1 may
be discarded.
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
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
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
The ingress filtering incompatibilities problems that appear in some
NEMO multihoming configurations are similar to those considered in
Shim6/multi6.
4.3 Media Detection
In order to achieve benefits such as ubiquity or fault recovery, it
is necessary for the mobile router to detect the availability of
network media. This may be achieved using layer 2 triggers [10], or
other mechanism developed/recommended by the Detecting Network
Attachment (DNA) Working Group [11][18].
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.
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4.4 HA Synchronization
In the (*,n,*) mobile networks, 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 [19] and
[20]. Such synchronization might also be necessary in a (*,n,*)
mobile network, 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 MNP is delegated to the MR (see Section 4.6), some
level of co-ordination between the HAs may also be neceesary.
This issue is a general mobility issue that will also have to be
dealt with Mobile IPv6. This issue should be dealt within a
potentially forthcoming Monami6 WG.
4.5 MR Synchronization
In the (n,*,*) mobile network, different MRs may need to be
synchronized in order to take common decisions. The mode of
synchronization may be either primary-secondary or peer-to-peer.
This may include:
o In the (n,*,1) case, advertising the same MNP (see also "prefix
delegation" in Section 4.6).
o In the (n,*,n) case, a MR relaying the advertisement of the MNP
from another failed MR.
o In the (n,*,*) cases, relaying between MRs everything that needs
to be relayed, such as data packets, creating a tunnel from the
ingress interface, etc.
This problem is general in the sense that a multi-router site in the
fixed network would require the same level of router synchronization.
It is recommended that the issue be looked at in the IPv6 or Shim6
WG, and the NEMO WG to contribute any NEMO specific requirement.
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4.6 Prefix Delegation
In the (*,*,1) mobile network, the same MNP must be advertised to the
MNNs through different paths. This questions how to perform prefix
delegation:
o For the (*,n,1) mobile network, how multiple HAs would 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 4.4).
o For the (n,*,n) mobile network, how multiple mobile routers would
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 4.5).
This could be configured manually, or dynamically. Alternatively,
prefix delegation mechanisms [21] [22] could be used to ensure all
routers advertise the same MNP.
Prefix delegation is currently being explored in the NEMO WG. Should
the WG decides to standardize a prefix delegation mechanism, the WG
should also consider its application to a multihomed mobile network.
4.7 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 [23] can solve this for both Mobile IPv6
and NEMO Basic Support. This issue should be dealt within a
potentially forthcoming Monami6 WG.
4.8 Source Address Selection
In the (*,*,n) mobile networks, MNNs would be configured with
multiple addresses. Source address selection mechanisms are needed
to decide which address to choose from.
In addition, 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).
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 these specified in [24] to be
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used. Further exploration on setting such "preference" information
in Router Advertisement based on performance of the bi-directional
tunnel might prove to be useful.
This is a general issue faced by any node when offered multiple
prefixes. It is recommended that the issue be examined by other WG
(such as IPv6).
4.9 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 two different root-MRs, thus
the aggregated network no longer forms a simple tree structure. As
such, a solution to prevent an infinite loop of multiple mobile
routers must be provided.
This problem is specific to NEMO Basic Support. It is recommended
that the NEMO WG standardizes a solution to solve this problem (such
as the use of a tree-spanning algorithm, or [25]).
4.10 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 [26].
This problem is specific to NEMO Basic Support. It is recommended
that the NEMO WG standardizes a solution to solve this problem, or
specifies that the split of a (n,*,1) network cannot be allowed
without a router renumbering.
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5. Matrix [Issues,(x,y,z) Configuration]
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 the WG(s) in which each
issue is the most likely to be solved.
+=================================================================+
| # 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/N| N |S/N| N |S/N| N |S/N|
+---------------------------------+---+---+---+---+---+---+---+---+
| Re-Homing |N/M| S |N/M| S |N/M| S |N/M| S |
+---------------------------------+---+---+---+---+---+---+---+---+
| Ingress Filtering | . | . | . | t | . | . | . | N |
+---------------------------------+---+---+---+---+---+---+---+---+
| Media Detection | D | D | D | D | D | D | D | D |
+---------------------------------+---+---+---+---+---+---+---+---+
| 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 | . |
+=================================================================+
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
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6. Conclusion
This memo is an analysis of multihoming in the context of network
mobility under the operation of NEMO Basic Support. The purpose of
this memo is to investigate issues related to such a bi-directional
tunneling mechanism when mobile networks are multihomed. For doing
so, a taxonomy was proposed to classify the mobile networks in eight
possible multihomed configurations. The issue were explained and
were then summarized in a table showing under which multihoming
configuration it applies, and which IETF working group is the best
chartered to solve it. 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 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 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-04 (work in progress),
February 2005.
[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-03 (work in progress),
February 2005.
[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., "Goals and Benefits of Multihoming",
draft-multihoming-generic-goals-and-benefits-00 (work in
progress), February 2004.
[7] Montavont, N., Wakikawa, R., Ernst, T., Ng, C., and K.
Kuladinithi, "Analysis of Multihoming in Mobile IPv6",
draft-montavont-mobileip-multihoming-pb-statement-04 (work in
progress), June 2005.
[8] Arkko, J., "Failure Detection and Locator Selection Design
Considerations", draft-ietf-shim6-failure-detection-00 (work in
progress), July 2005.
[9] Beijnum, I., "Shim6 Reachability Detection",
draft-ietf-shim6-reach-detect-00 (work in progress), July 2005.
[10] Yegin, A., "Link-layer Event Notifications for Detecting
Network Attachments", draft-ietf-dna-link-information-01 (work
in progress), February 2005.
[11] Narayanan, S., "Detecting Network Attachment in IPv6 - Best
Current Practices for hosts.", draft-ietf-dna-hosts-00 (work in
progress), April 2005.
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[12] Yegin, A., "Link-layer Hints for Detecting Network
Attachments", draft-yegin-dna-l2-hints-01 (work in progress),
February 2004.
[13] Yegin, A., "Supporting Optimized Handover for IP Mobility
-Requirements for Underlying Systems",
draft-manyfolks-l2-mobilereq-02 (work in progress), July 2002.
[14] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[15] Bagnulo, M. and J. Arkko, "Functional decomposition of the
multihoming protocol", draft-ietf-shim6-functional-dec-00 (work
in progress), July 2005.
[16] 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.
[17] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[18] Narayanan, S., "Detecting Network Attachment in IPv6 - Best
Current Practices for Routers", draft-ietf-dna-routers-00 (work
in progress), July 2005.
[19] Wakikawa, R., Devarapalli, V., and P. Thubert, "Inter Home
Agents Protocol (HAHA)", draft-wakikawa-mip6-nemo-haha-01 (work
in progress), February 2004.
[20] Koh, B., Ng, C., and J. Hirano, "Dynamic Inter Home Agent
Protocol", draft-koh-mip6-nemo-dhap-00 (work in progress),
July 2004.
[21] Miyakawa, S. and R. Droms, "Requirements for IPv6 Prefix
Delegation", RFC 3769, June 2004.
[22] Droms, R. and P. Thubert, "DHCPv6 Prefix Delegation for NEMO",
draft-droms-nemo-dhcpv6-pd-02 (work in progress), April 2005.
[23] Wakikawa, R., Uehara, K., Ernst, T., and K. Nagami, "Multiple
Care-of Addresses Registration",
draft-wakikawa-mobileip-multiplecoa-04 (work in progress),
June 2005.
[24] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
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[25] Thubert, P., "Nested Nemo Tree Discovery",
draft-thubert-tree-discovery-02 (work in progress), July 2005.
[26] Kumazawa, M., "Token based Duplicate Network Detection for
split mobile network (Token based DND)",
draft-kumazawa-nemo-tbdnd-02 (work in progress), July 2005.
[27] Choi, S., "Threat for Multi-homed Mobile Networks",
draft-cho-nemo-threat-multihoming-00 (work in progress),
February 2004.
[28] Montavont, N., Noel, T., and M. Kassi-Lahlou, "MIPv6 for
Multiple Interfaces", draft-montavont-mobileip-mmi-00 (work in
progress), July 2002.
[29] Draves, R. and D. Thaler, "Default Router Preferences and More-
Specific Routes", draft-ietf-ipv6-router-selection-07 (work in
progress), January 2005.
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
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/
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Thierry Ernst
WIDE at Keio University
Jun Murai Lab., Keio University.
K-square Town Campus, 1488-8 Ogura, Saiwa-Ku
Kawasaki, Kanagawa 212-0054
Japan
Phone: +81-44-580-1600
Fax: +81-44-580-1437
Email: ernst@sfc.wide.ad.jp
URI: http://www.sfc.wide.ad.jp/~ernst/
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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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 Care-of Addresses of Same MNP
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,*)
mobile network.
o JetSet: Multiple HAs for Different Care-of Addresses of Same MNP
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 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 section, we will attempt to investigate
methods of performing such re-establishment of bi-directional
tunnels. This method of tunnel re-estblishment is particularly
useful for the (*,n,n) NEMO configuration. 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
[5] and Neighbor Discovery in IPv6 [14].
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.
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-
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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 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 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 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 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 Care-of Address from the alternate MR (i.e. attaches
itself to the alternate MR). Next, it sends binding update to its HA
using the Care-of Address 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 Care-of Address from the alternate MR and use
this as the new (back-up) Care-of Address 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).
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Appendix C. Change Log
o This draft is an update of draft-ng-nemo-multihoming-issues-03.txt
which is itself a merge of 3 previous drafts
draft-ng-nemo-multihoming-issues-02.txt,
draft-eun-nemo-multihoming-problem-statement-00.txt, and
draft-charbon-nemo-multihoming-evaluation-00.txt
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 Section 2.3: Removed text on assumption
* Addressed Issue #6 in Section 3.1: Added benefits
* Addressed Issue #7 in Section 3.2: Modified text
* Addressed Issue #9 in Section 4.2: Modified text
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* Addressed Issue #10 in Section 4.1.1: Added paragraph on other
failure modes
* Addressed Issue #10: New Section 4.3 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 "Problem Statement" (Section 4)
* Merged "Evaluation" Section into "Problem Statement"
(Section 4)
* Cleaned up description in "Classification" (Section 2), and
clearly indicate in each classification, what are the
multihomed entities
* Re-organized "Deployment Scenarios and Prerequisites"
(Section 3), 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
"Problem Statement" (Section 4))
* Included conclusions from
draft-charbon-nemo-multihoming-evaluation-00
* Re-organized some part of "Benefits/Issues of Multihoming in
NEMO" to "Problem Statement" (Section 4)
* 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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