Network Working Group C. Ng
Request for Comments: 4980 Panasonic Singapore Labs
Category: Informational T. Ernst
INRIA
E. Paik
KT
M. Bagnulo
UC3M
October 2007
Analysis of Multihoming in Network Mobility Support
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Classification . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. (1,1,1): Single MR, Single HA, Single MNP . . . . . . . . 6
2.2. (1,1,n): Single MR, Single HA, Multiple MNPs . . . . . . . 6
2.3. (1,n,1): Single MR, Multiple HAs, Single MNP . . . . . . . 7
2.4. (1,n,n): Single MR, Multiple HAs, Multiple MNPs . . . . . 8
2.5. (n,1,1): Multiple MRs, Single HA, Single MNP . . . . . . . 8
2.6. (n,1,n): Multiple MRs, Single HA, Multiple MNPs . . . . . 9
2.7. (n,n,1): Multiple MRs, Multiple HAs, Single MNP . . . . . 9
2.8. (n,n,n): Multiple MRs, Multiple HAs, Multiple MNPs . . . . 10
Ng, et al. Informational [Page 1]RFC 4980 Analysis of Multihoming in NEMO October 2007
3. Deployment Scenarios and Prerequisites . . . . . . . . . . . . 11
3.1. Deployment Scenarios . . . . . . . . . . . . . . . . . . . 11
3.2. Prerequisites . . . . . . . . . . . . . . . . . . . . . . 13
4. Multihoming Issues . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Fault Tolerance . . . . . . . . . . . . . . . . . . . . . 14
4.1.1. Failure Detection . . . . . . . . . . . . . . . . . . 15
4.1.2. Path Exploration . . . . . . . . . . . . . . . . . . . 16
4.1.3. Path Selection . . . . . . . . . . . . . . . . . . . . 17
4.1.4. Re-Homing . . . . . . . . . . . . . . . . . . . . . . 19
4.2. Ingress Filtering . . . . . . . . . . . . . . . . . . . . 19
4.3. HA Synchronization . . . . . . . . . . . . . . . . . . . . 21
4.4. MR Synchronization . . . . . . . . . . . . . . . . . . . . 22
4.5. Prefix Delegation . . . . . . . . . . . . . . . . . . . . 23
4.6. Multiple Bindings/Registrations . . . . . . . . . . . . . 23
4.7. Source Address Selection . . . . . . . . . . . . . . . . . 23
4.8. Loop Prevention in Nested Mobile Networks . . . . . . . . 24
4.9. Prefix Ownership . . . . . . . . . . . . . . . . . . . . . 24
4.10. Preference Settings . . . . . . . . . . . . . . . . . . . 25
5. Recommendations to the Working Group . . . . . . . . . . . . . 26
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7. Security Considerations . . . . . . . . . . . . . . . . . . . 28
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.1. Normative References . . . . . . . . . . . . . . . . . . . 29
9.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix 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 . . . . . . . . . . . . . . . . 34
Appendix B. Nested Tunneling for Fault Tolerance . . . . . . . . 35
B.1. Detecting Presence of Alternate Routes . . . . . . . . . . 35
B.2. Re-Establishment of Bi-Directional Tunnels . . . . . . . . 36
B.2.1. Using Alternate Egress Interface . . . . . . . . . . . 36
B.2.2. Using Alternate Mobile Router . . . . . . . . . . . . 36
B.3. To Avoid Tunneling Loop . . . . . . . . . . . . . . . . . 37
B.4. Points of Considerations . . . . . . . . . . . . . . . . . 37
Ng, et al. Informational [Page 2]RFC 4980 Analysis of Multihoming in NEMO October 2007
1. Introduction
The design goals and objectives of Network Mobility (NEMO) support in
IPv6 are identified in [1], while the terminology is 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 (NEMO) 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 MR (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 typically require the use of
several interfaces and technologies. For example, this is
particularly useful for Intelligent Transport Systems (ITS)
applications since vehicles are moving across distant geographical
locations. Access would be provided through different access
technologies (e.g., Wimax, Wifi, 3G) and through different access
operators.
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 an 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(s),
and may result in multiple Mobile Network Prefixes (MNPs) available
Ng, et al. Informational [Page 3]
RFC 4980 Analysis of Multihoming in NEMO October 2007
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:
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
does not 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.
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
Ng, et al. Informational [Page 4]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 MRs
are present, how many egress interfaces, Care-of Address (CoA), and
Home Addresses (HoA) the MRs 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
in different combinations of the 3-tuple (x,y,z).
As will be 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,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)
[8].
Ng, et al. Informational [Page 5]RFC 4980 Analysis of Multihoming in NEMO October 2007
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. The MR and the
AR are connected to the Internet via a single Access Router (AR). 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
o Multiple global prefixes are available on the home link, and thus
the MR has more than one path to reach the HA.
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 the present (1,1,1)
configuration, only one MNP is announced.
A bi-directional tunnel would then be established between each
(HoA,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
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.
Ng, et al. Informational [Page 6]RFC 4980 Analysis of Multihoming in NEMO October 2007
The MR may itself be multihomed, as detailed in Section 2.1. In such
a case, a bi-directional tunnel would be established between each
(HoA,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 then be established between each
(HoA,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.
AR HA2
_ |
|-|_|-| _
_____ | |-|_|
_ p _ | |-|
|_|-|<-_ |-|_|-| |
_ |-|_|=| |_____|-| _
|_|-| | | _ |-|_|
|-|_|-|
|
MNNs MR AR Internet AR HA1
Figure 3: (1,n,1): 1 MR, multiple HAs, 1 MNP
Ng, et al. Informational [Page 7]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 then be established between each (HoA,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
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 a single HA, and
there is 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 (HoA,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.
Ng, et al. Informational [Page 8]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 then be established between each
(HoA,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
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.
Ng, et al. Informational [Page 9]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 (HoA,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
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 then be established between
each (HoA,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.
Ng, et al. Informational [Page 10]RFC 4980 Analysis of Multihoming in NEMO October 2007
MR2 AR HA2
p2 _ |
<-_ | |-|_|-| _
_ |-|_|-| _____ | |-|_|
|_|-| |-| |-|
_ | | |
|_|-| _ |-|_____|-| _
|-|_|-| | _ |-|_|
<- | |-|_|-|
p1 |
MNNs MR1 Internet AR HA1
Figure 8: (n,n,n): Multiple MRs, HAs, and MNPs
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 a single HA is used for
both. If two independent HAs are used, this is a (1,n,n)
configuration.
Benefits: Ubiquitous Access, Reliability, Load Sharing,
Preference Settings, Aggregate Bandwidth.
Ng, et al. Informational [Page 11]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 use different HAs, in different countries, thus a (n,n,n)
configuration.
Benefits: Ubiquitous Access, Reliability, Load Sharing,
Aggregate Bandwidth.
o Example 2:
Wireless personal area network with a GPRS-enabled phone and a
WiFi-enabled PDA. This is a (n,n,n) configuration if different
HAs are also used.
Benefits: Ubiquitous Access, Reliability, Preference Settings,
Aggregate Bandwidth.
y=1: Multihomed mobile networks with a single HA
o Example:
Most single HA cases in above examples.
y=n: Multihomed mobile networks with multiple HAs
o Example 1:
Most multiple HAs 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 HA cases in above examples.
Ng, et al. Informational [Page 12]RFC 4980 Analysis of Multihoming in NEMO October 2007
z=n: Multihomed mobile networks with multiple MNPs
o Example 1:
Most multiple HAs 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
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.
Ng, et al. Informational [Page 13]RFC 4980 Analysis of Multihoming in NEMO October 2007
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. A
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.
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, and re-homing of established
communications. These are also discussed in [9] 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 [9].
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).
Ng, et al. Informational [Page 14]RFC 4980 Analysis of Multihoming in NEMO October 2007
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.
The failure detection capabilities required for each configuration
are detailed below:
o For the (1,1,*) cases, multiple paths are available between a
single MR and a single HA. All the traffic to and from the NEMO
flows through the 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 to
and from 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
[9]) 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
Ng, et al. Informational [Page 15]RFC 4980 Analysis of Multihoming in NEMO October 2007
solution for MIPv6 should apply to NEMO as well. For case (n,*,*),
an 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., [10] and [11].
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
"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 from
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).
Ng, et al. Informational [Page 16]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 to
and from 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 independent from one another. 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 [9]) 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,*,*), an MR
synchronization solution (see Section 4.4) should be able to
complement an MIPv6 path-exploration solution to achieve the desired
functionality for NEMO.
In order to perform path exploration, it is sometimes also necessary
for the MR 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]. This is related to Section 4.1.1, since the ability to detect
media availability would often imply the ability to detect media
unavailability.
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
Ng, et al. Informational [Page 17]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 detail 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
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]
[12]) 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
Ng, et al. Informational [Page 18]RFC 4980 Analysis of Multihoming in NEMO October 2007
tunnel, while the other serves as a backup) 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) seem 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, an MR
synchronization solution (see Section 4.4) should be able to
complement an 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., [13]).
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., [13]).
4.2. Ingress Filtering
Ingress filtering mechanisms [14][15] 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
Ng, et al. Informational [Page 19]RFC 4980 Analysis of Multihoming in NEMO October 2007
MNP is tunneled to a HA that does not handle that specific MNP, the
packet may be discarded either by the HA 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 to and from the NEMO is routed through a
single MR. Such configuration allows the MR to properly route
packets respecting the constraints imposed by ingress filtering.
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
MR 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 [12] practice that an IPv6 node is free
to choose any router as its default router regardless of the
prefix it chooses to use.
Ng, et al. Informational [Page 20]RFC 4980 Analysis of Multihoming in NEMO October 2007
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
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 [16]).
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.
Ng, et al. Informational [Page 21]RFC 4980 Analysis of Multihoming in NEMO October 2007
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. A certain
level of HA coordination may be required. A possible approach is to
adopt an HA synchronization mechanism such as that described in [17]
and [18]. Such synchronization might also be necessary in a (*,n,*)
configuration, when an 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 coordination 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 (see Section 6.2.3 in [7]) as well as NEMO
Basic Support.
4.4. MR Synchronization
In the (n,*,*) configurations, there are common decisions that may
require synchronization among different MRs [19], 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
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 protocol 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 an MR-to-MR synchronization scheme
for proving fault tolerances and failure recovery in NEMO
configurations due to the higher possibility of links failure.
Ng, et al. Informational [Page 22]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 [20][21][22] could be used to ensure all
routers advertise the same MNP. Their applicability to a multihomed
mobile network should be considered.
4.6. Multiple Bindings/Registrations
When an MR is configured with multiple CoAs, it is often necessary
for it to bind these CoAs 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 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
Ng, et al. Informational [Page 23]RFC 4980 Analysis of Multihoming in NEMO October 2007
mechanisms, such as those specified in [24], 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 [25] for a mechanism to avoid loops in nested NEMO.
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 [26].
This problem is specific to NEMO Basic Support. However, it is
unclear whether there is a sufficient deployment scenario for this
problem to occur.
It is recommended that the NEMO WG standardize a solution to solve
this problem if there is sufficient vendor/operator interest, or
specify that the split of a (n,*,1) network cannot be allowed without
router renumbering.
Ng, et al. Informational [Page 24]
RFC 4980 Analysis of Multihoming in NEMO October 2007
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 and 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,n,1) configuration, the MNNs cannot influence the path
selection.
One aspect of preference setting is that the preference of the MNN
(e.g., application or transport layer configuration) may not be the
same as the preference used by MR. Thus, forwarding choices made by
the MR may not be the best for a particular flow, and may even be
detrimental to some transport control loops (i.e., the flow control
algorithm for TCP may be messed up when MR unexpectedly performs load
balancing on a TCP flow). A mechanism that allows the MNN to
indicate its preference for a given traffic might be helpful here.
Another aspect of preference setting is that the MNN may not even be
aware of the existence of multiple forwarding paths, e.g., the
(1,n,1) configuration. A mechanism for the MR to advertise the
availability of multiple tunneling paths would allow the MNN to take
advantage of this, coupled with the previously mentioned mechanism
that allows the MNN to indicate its preference for a given traffic.
This problem is general in the sense that IPv6 nodes may wish to
influence the routing decision done by the upstream routers. Such a
mechanism is currently being explored by various WGs, such as the
NSIS and IPFIX WGs. It is also possible that the Shim6 layer in the
MNNs may possess such a capability. It is recommended for vendors or
operators to investigate into the solutions developed by these WGs
when providing multihoming capabilities to mobile networks.
In addition, the Monami6 WG is currently developing a flow filtering
solution for mobile nodes to indicate how flows should be forwarded
by a filtering agent [27] (such as HA and mobile anchor points). It
is recommended that the Monami6 WG consider the issues described here
so that flow filtering can be performed by the MNN to indicate how
flows should be forwarded by the MR.
Ng, et al. Informational [Page 25]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 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
Ng, et al. Informational [Page 26]RFC 4980 Analysis of Multihoming in NEMO October 2007
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 are some concerns that
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 can 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 MRs 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 that the NEMO WG concentrate only on the (n,n,n)
configuration should the WG decide to work on this issue.
o A solution for HA Synchronization can be looked at in a mobility-
specific WG, taking into consideration both mobile hosts operating
Mobile IPv6 and MRs operating NEMO Basic Support.
o A solution for Multiple Bindings/Registrations is presently being
developed by the Monami6 WG.
o Prefix Delegation should be reviewed and checked by the NEMO WG.
The proposed solutions [22] and [21] providing prefix delegation
functionality to NEMO Basic Support should be reviewed in order to
Ng, et al. Informational [Page 27]RFC 4980 Analysis of Multihoming in NEMO October 2007
make sure concerns, as discussed in Section 4.5, are properly
handled.
o Loop Prevention in Nested NEMO should 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.
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 are 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,
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. 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. For
instance, an attacker could try to use the multihomed device as a
means to access another network that would not be normally reachable
through the Internet. Even when forwarding to another network is
turned off by configuration, an attacker could compromise a system to
enable it.
Ng, et al. Informational [Page 28]RFC 4980 Analysis of Multihoming in NEMO October 2007
8. 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.
9. References
9.1. Normative References
[1] Ernst, T., "Network Mobility Support Goals and Requirements",
RFC 4886, July 2007.
[2] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[3] Ernst, T. and H-Y. Lach, "Network Mobility Support
Terminology", RFC 4885, July 2007.
[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.
9.2. Informative References
[6] Ernst, T., Montavont, N., Wakikawa, R., Ng, C., and K.
Kuladinithi, "Motivations and Scenarios for Using Multiple
Interfaces and Global Addresses", Work in Progress,
October 2006.
[7] Montavont, N., Wakikawa, R., Ernst, T., Ng, C., and K.
Kuladinithi, "Analysis of Multihoming in Mobile IPv6", Work
in Progress, February 2006.
[8] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[9] Arkko, J. and I. Beijnum, "Failure Detection and Locator Pair
Exploration Protocol for IPv6 Multihoming", Work in Progress,
December 2006.
Ng, et al. Informational [Page 29]RFC 4980 Analysis of Multihoming in NEMO October 2007
[10] Krishnan, S., Montavont, N., Yegin, A., Veerepalli, S., and A.
Yegin, "Link-layer Event Notifications for Detecting Network
Attachments", Work in Progress, November 2006.
[11] Narayanan, S., "Detecting Network Attachment in IPv6 Networks
(DNAv6)", Work in Progress, October 2006.
[12] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[13] Nordmark, E. and M. Bagnulo, "Level 3 multihoming shim
protocol", Work in Progress, November 2006.
[14] 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.
[15] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[16] Huitema, C. and M. Marcelo, "Ingress filtering compatibility
for IPv6 multihomed sites", Work in Progress, October 2006.
[17] Wakikawa, R., Devarapalli, V., and P. Thubert, "Inter Home
Agents Protocol (HAHA)", Work in Progress, February 2004.
[18] Koh, B., Ng, C., and J. Hirano, "Dynamic Inter Home Agent
Protocol", Work in Progress, July 2004.
[19] Tsukada, M., "Analysis of Multiple Mobile Routers Cooperation",
Work in Progress, October 2005.
[20] Miyakawa, S. and R. Droms, "Requirements for IPv6 Prefix
Delegation", RFC 3769, June 2004.
[21] Droms, R. and P. Thubert, "DHCPv6 Prefix Delegation for NEMO",
Work in Progress, September 2006.
[22] Thubert, P. and TJ. Kniveton, "Mobile Network Prefix
Delegation", Work in Progress, November 2006.
[23] Wakikawa, R., "Multiple Care-of Addresses Registration", Work
in Progress, June 2006.
[24] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
Ng, et al. Informational [Page 30]RFC 4980 Analysis of Multihoming in NEMO October 2007
[25] Thubert, P., Bontous, C., and N. Nicolas, "Nested Nemo Tree
Discovery", Work in Progress, November 2006.
[26] Kumazawa, M., "Token based Duplicate Network Detection for
split mobile network (Token based DND)", Work in Progress,
July 2005.
[27] Soliman, H., "Flow Bindings in Mobile IPv6 and NEMO Basic
Support", Work in Progress, March 2007.
Ng, et al. Informational [Page 31]RFC 4980 Analysis of Multihoming in NEMO October 2007
Appendix A. Alternative Classifications Approach
A.1. Ownership-Oriented Approach
An alternative approach to classifying a multihomed mobile network
was 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
MRs 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 includes not
introducing any aggregation between the HAs, which will filter out
routing announcements for the MNP(s).
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 intervals.
However, a more appropriate tool might be the Binding Refresh Request
Ng, et al. Informational [Page 32]
RFC 4980 Analysis of Multihoming in NEMO October 2007
message, though conformance to the Binding Refresh Request message
may be less strictly enforced in implementations since it serves a
somewhat secondary role when compared to Binding Update messages.
A.1.2. Subscriber/Provider Model
The case of the HA(s) and MR(s) 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
Ng, et al. Informational [Page 33]RFC 4980 Analysis of Multihoming in NEMO October 2007
For the S/mP model, the following configurations are likely to be
deployed:
o S/mP-(1,n,1): Multiple Providers, Single MR, Multiple HAs, Single
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 that 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
cases, it is unlikely that the ISP will run IGP over the bi-
directional tunnel, since the ISP will most certainly wish to retain
full control of its routing domain.
A.2. Problem-Oriented Approach
A third approach was proposed by Pascal Thubert (Cisco Systems).
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 CoAs 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 CoAs 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.
Ng, et al. Informational [Page 34]RFC 4980 Analysis of Multihoming in NEMO October 2007
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.
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 an MR detects that one of its egress
interface is down, it should detect if alternate routes 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 possesses. 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 [12].
B.1. Detecting Presence of Alternate Routes
To actively utilize the robustness provided by multihoming, an 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 an 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. An MR can do so by listening for
Router Advertisement message on its *ingress* interfaces. When an MR
receives a Router Advertisement message with a non-zero Router
Ng, et al. Informational [Page 35]RFC 4980 Analysis of Multihoming in NEMO October 2007
Lifetime field from one of its ingress interfaces, it knows that
another MR that can provide an alternate route to the global Internet
is present in the mobile network.
B.2. Re-Establishment of Bi-Directional Tunnels
When an 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 an 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., attach 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 encapsulate
outgoing packets through the bi-directional tunnel 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.
Ng, et al. Informational [Page 36]
RFC 4980 Analysis of Multihoming in NEMO October 2007
Note that every packet sent between MNNs and their correspondent
nodes will experience two levels of encapsulation. The first level
of tunneling occurs between an MR that 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.
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 an 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 in order 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 are 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 an MR whose link has failed knows that its
prefix is not to be used, but it does not have 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 an MR to
withdraw its own prefix when it discovers that its link is no longer
working.
Ng, et al. Informational [Page 37]
RFC 4980 Analysis of Multihoming in NEMO October 2007
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
KT Research Center
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
Ng, et al. Informational [Page 38]
RFC 4980 Analysis of Multihoming in NEMO October 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Ng, et al. Informational [Page 39]
