IETF MONAMI6 Working Group N. Montavont
Internet-Draft GET/ENST-B
Expires: August 30, 2007 R. Wakikawa
Keio University
T. Ernst
INRIA
C. Ng
Panasonic Singapore Labs
K. Kuladinithi
University of Bremen
February 26, 2007
Analysis of Multihoming in Mobile IPv6
draft-ietf-monami6-mipv6-analysis-02.txt
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Abstract
Mobile IPv6 as specified in RFC3775 [1] allows a mobile node to
maintain its IPv6 communications while moving between subnets. This
document investigates configurations where a mobile node running
Mobile IPv6 is multihomed. The use of multiple addresses is foreseen
to provide ubiquitous, permanent and fault-tolerant access to the
Internet, particularly on mobile nodes which are more prone to
failure or sudden lack of connectivity. However, Mobile IPv6
currently lacks support for such multihomed nodes. The purpose of
this document is to detail all the issues arising through the
operation of Mobile IPv6 on multihomed mobile nodes.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Nodes capabilities . . . . . . . . . . . . . . . . . . . . . . 8
4. Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. (H1,C1): 1 HoA, 1 CoA . . . . . . . . . . . . . . . . . . 12
5.2. (Hn,C1): n HoAs, 1 CoA . . . . . . . . . . . . . . . . . . 13
5.3. (H1,Cn): 1 HoA, n CoAs . . . . . . . . . . . . . . . . . . 15
5.4. (Hn,Cn): n HoAs, n CoAs . . . . . . . . . . . . . . . . . 16
5.5. (Hn,C0): n HoAs, no CoAs . . . . . . . . . . . . . . . . . 17
6. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. General IPv6-related Issues . . . . . . . . . . . . . . . 19
6.1.1. Path Selection . . . . . . . . . . . . . . . . . . . . 19
6.1.2. Ingress Filtering . . . . . . . . . . . . . . . . . . 20
6.1.3. Failure detection . . . . . . . . . . . . . . . . . . 21
6.2. MIPv6-specific Issues . . . . . . . . . . . . . . . . . . 21
6.2.1. Binding Multiple CoAs to a given HoA . . . . . . . . . 21
6.2.2. Simultaneous Location in Home and Foreign Networks . . 22
6.3. Considerations for MIPv6 Implementation . . . . . . . . . 22
6.3.1. Using one HoA as a CoA . . . . . . . . . . . . . . . . 22
6.3.2. Binding a new CoA to the Right HoA . . . . . . . . . . 23
6.3.3. Binding HoA to interface . . . . . . . . . . . . . . . 23
6.3.4. Flow redirection . . . . . . . . . . . . . . . . . . . 24
6.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 24
7. TODO List . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 28
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Appendix A. Why a MN may want to redirect flows . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 35
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1. Introduction
The emergence of performant wireless technologies has favored node
mobility within the Internet. Nowadays, and even more tomorrow,
nodes are highly mobile and they can change their point of attachment
to the Internet at any time, even during active network connections.
Mobile IPv6 as specified in RFC3775 [1] allows mobile nodes to
maintain their communication while changing their point of attachment
to the Internet.
Besides, mobile nodes can be connected to multihomed networks or they
may have multiple interfaces to benefit from heterogeneous
technologies. The use of multiple addresses (allocated to either a
single interface or multiple interfaces) is foreseen to provide
ubiquitous, permanent and fault-tolerant access to the Internet,
particularly on mobile nodes which are prone to failure or sudden
lack of connectivity. However, the current specification of Mobile
IPv6 lacks support for mobile nodes with multiple addresses used
simultaneously, i.e. multihomed mobile nodes. These addresses would
be assigned to a single interface or to multiple interfaces, which
poses a number of issues.
This document has two goals. The first goal is to define the
requirements from the point of view of multihomed mobile nodes
operating Mobile IPv6. The second goal is to define the issues
arising when we attempt to fulfill these requirements. The
definition of the potentially needed solutions is out of scope of
this document. These should be defined in a separate document.
In order to reach the goals of this document, we define a taxonomy
which is used to describe the different situations where a mobile
node is multihomed. For each case, we show the configuration a
multihomed node may have (the number of Home Addresses, and the
number of Care-of Addresses). We also illustrate each scenario with
example diagrams.
To understand the foundation of this document, the reader should read
the companion document
draft-ietf-monami6-multihoming-motivation-scenario [2] which outlines
the motivations, the goals and the benefits of multihoming for both
fixed and mobile nodes (i.e. generic IPv6 nodes). Real-life
scenarios as illustrated in that document are the base motivations of
the present study. The reader should also understand the operation
of the Mobile IPv6 protocol (RFC3775 [1]).
The document is organized as follows: in Section 2, we introduce the
terminology related to multihoming and used in this document. In
Section 3, we discuss what is required on the mobile nodes to fully
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benefit from a multihomed configuration. Then we propose in
Section 4 a taxonomy to classify the different cases where mobile
nodes are multihomed. Thereafter the taxonomy is used in Section 5
to describe a number of multihomed configuration specific to Mobile
IPv6. Finally we discuss in Section 6 and Section 6.3 all issues
related to a multihomed mobile node and we identify what is missing
in order to reach the goals outlined in
draft-ietf-monami6-multihoming-motivation-scenario [2]. These issues
are classified into IPv6 issues, Mobile IPv6-specific issues, and
advices to implementers.
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2. Terminology
The terms used in the present document are defined in RFC3753 [3],
RFC3775 [1] and draft-ietf-monami6-multihoming-motivation-scenario
[2].
In this draft we are using the following terms and abbreviations:
o MIPv6
The Mobile IPv6 protocol specified in RFC3775 [1]
o MN
a Mobile Node using MIPv6
o HA
a Mobile IPv6 Home Agent
o HoA
a Mobile IPv6 Home Address
o CoA
a Mobile IPv6 Care-of Address
o Multihomed MN
In the companion document [2], a node is said to be multihomed
when it has multiple IPv6 addresses, either because multiple
prefixes are advertised on the link(s) the node is attached to, or
because the node has multiple interfaces (see the entire
definition). For a mobile node operating MIPv6, this may
translate into the following definition:
A MN (as defined above) is said multihomed when it has either i)
multiple addresses which are used as source addresses or ii)
multiple tunnels to transmit packets, or both. A MN may have
multiple HoAs/CoAs in the following cases:
* A MN would have multiple HoAs if multiple prefixes are
available on the home link or if it has multiple interfaces
named on (presumably) distinct home links.
* A MN would have multiple CoAs if multiple prefixes are
available on the foreign link or if it has multiple interfaces
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attached to (presumably) distinct foreign links.
o A valid address
An address that is topologically correct (it is configured from
the prefix available on the link the interface is attached to) and
routable.
o Simultaneously using multiple addresses
A MN is simultaneously using multiple addresses at the same time
when an incoming packet with the destination address set to any of
these addresses reaches the MN, or when any of these addresses can
be used by the MN to set the source address of outcoming packets.
o Simultaneously using multiple interfaces
A MN is simultaneously using multiple interfaces when it can
exchange IP packets over any of these interfaces.
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3. Nodes capabilities
Generic goals and benefits of multihoming are discussed in
draft-ietf-monami6-multihoming-motivation-scenario [2]. These goals
are ubiquitous access, flow redirection, reliability, load sharing,
inteface switching, preference settings, and aggregate bandwidth.
These generic goals overlap, i.e., they are not totally independent.
Reaching one of them may imply to reach another goal as well. For
this reason, the following non-overlapping goals may be extracted
from these generic goals:
1. Reliability
2. Load Sharing
3. Interface switching/Flow Distribution
In this section, after determining a set of capabilities that allows
a MN to achieve these design goals, we explicitly indicate which
capability is needed to reach which goal. We will determine later in
the document (in Section 5) which capabilities are already fulfilled
by Mobile IPv6 and what issues remain.
Basically, Internet connectivity is guaranteed for a MN as long as at
least one path is maintained between the MN and the fixed Internet.
This path can be divided into two portions: the path between the MN
to its HA(s) and the path between the HA(s) and the CN. If RO is in
place between the MN and a CN, an additional path between the MN and
the CN must be guaranteed. In some cases, it may be necessary to
divert packets from a (perhaps failed) path to an alternative
(perhaps newly established) path (e.g. for matters of reliability,
preferences), or to split traffic between multiple paths (e.g. for
load sharing, interface switching). The use of an alternative path
must be transparent at layers above layer 3 if broken sessions and
the establishment of new transport sessions has to be avoided.
In order to meet some of the goals (particularly interface switching
and load sharing), multiple paths must be maintained simultaneously
between the mobile node and its CN.
This translates into the following capabilities:
1. A MN equipped with multiple global addresses must be able to use
them simultaneously
2. A MN equipped with multiple interfaces must be able to attach
distinct interfaces to distinct access networks (distinct foreign
links or distinct home links, or a combination of both).
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3. A MN should be able to share its traffic load among its valid
global addresses
4. A mechanism should be available to quickly activate a backup
interface and redirect traffic when an interface fails (e.g.,
loss of connection).
5. A mechanism should be available to quickly redirect flow from one
address to another when it is needed. Some of the triggers of
flow redirection are given in Appendix A.
6. If multiple HAs are available to manage bindings for a given HoA,
the MN should be able to use them simultaneously.
One has to consider whether these goals can be achieved with
transparency or without transparency. Transparency is achieved when
switching between interfaces does not cause the disruption of on-
going sessions. To be achieved with transparency, a necessary (may
or may not be sufficient) condition is for the end-point addresses at
the transport/application layer to remain unchanged. This is in view
of the large amount of Internet traffic currently carried by TCP,
which unlike SCTP, cannot handle multiple end-point address pairs.
Each of the aforementioned goals can be achieved independently. We
define here what of the above capabilities are needed for each goal:
1. Reliability: 2, 4, 5, 6
2. Load Sharing: 1, 6
3. Interface switching/Flow Distribution: 1, 2, 3, 5
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4. Taxonomy
In order to examine the issues preventing a MIPv6 mobile node to meet
the requirements listed in Section 3 we use in the present document
the following taxonomy (Hx,Cy) where:
o x = number of Home Addresses (HoAs)
o y = number of Care-of Addresses (CoAs)
A value of '1' implies there is a single instance of the parameter,
whereas a value of 'n' indicates that there are multiple instances of
the parameter. A value '*' indicates that the number can be '1' or
'n'.
An illustration of this taxonomy is given in Figure 1.
Mobile Node
HoA1 HoA2 ... HoAn --> Permanent Address (Hx)
| | |
+-----+--------+ | |
| | | | |
CoA1 +--CoA2 +---CoA3 ... CoAn --> Temporary Address (Cy)
| | | |
Link1 Link2 Link3 ... Linkn --> IPv6 Link (n/a *)
| | | |
+-----+----+ | |
| | |
IF1 IF2 ... IFn --> Physical layer
CoA1, CoA2, CoA3 are bound to HoA1 on IF1 and IF2
CoA3 is bound to HoA2 on IF2
* because number of IPv6 links is equal to the number of CoAs, y
Figure 1: Illustration of the Taxonomy
As the taxonomy suggests, the fact that a mobile node has several
HoAs is independent from the fact that this mobile node has multiple
interfaces. The fact that a mobile node has multiple interfaces does
not imply that it has multiple HoAs and vice-versa. Similarly, the
number of CoAs is independent from the number of HoAs and the number
of interfaces. While a node would probably have at least one CoA per
interface, multiple prefixes available on a link may lead the node to
configure several CoAs on that link.
The proposed taxonomy has two parameters because each of them has an
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influence on either the mobile node behavior / management, or the
potential benefits the mobile node may obtain from such a
configuration.
The configurations denoted by these parameters will have an impact on
the way multihoming is supported. Depending on the number of HoAs
and CoAs, different policies will be needed, such as "which CoA has
to be mapped to which HoA", "must all the CoAs be mapped with all the
HoAs", etc.
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5. Scenarios
In this section, we detail the reachable multihoming goals under each
type of configuration. For each configuration, we give a basic
explanation and we list which of the goals outlined in Section 3 are
achievable provided that supporting mechanisms are either already
existing or could be added. Other goals are not achievable due to
the inherent configuration of the node. Then, we briefly discuss the
current situation with MIPv6 and we point to issues discussed later
in this document in Section 6.1, Section 6.2 and Section 6.3.
5.1. (H1,C1): 1 HoA, 1 CoA
A mobile node in this configuration with a single network interface
is not multihomed. This scenario is the common case of a MN running
Mobile IPv6 and away from its home link: the node has one HoA and one
CoA which is configured on the foreign link. None of the multihoming
goals are achievable.
When the node in the same configuration has several interfaces, it
may lead to a special situation where a node is connected to both its
home link and a foreign link. The MN is multihomed since it would be
able to use both interfaces. The Home Address might be directly used
on the interface which is connected to the home link, and a Care-of
Address is configured on the other interface which is connected to a
foreign link. There cannot be more than two active interfaces,
otherwise the mobile node would either have (A) multiple interfaces
on the home link, or (B) multiple interfaces on foreign links. For
(A), there would be multiple HoAs. For (B) there would be multiple
CoAs. These two cases would fall in another scenario, either (Hn,C*)
(see Section 5.2, Section 5.4 and Section 5.5) or (H*,Cn) (see
Section 5.3 and Section 5.4).
Achievable goals (when the node has a single interface): Reliability,
Load Sharing.
Achievable goals (when the node has multiple interfaces):
Reliability, Load Sharing, Interface switching
Current situation with MIPv6 (when the node has multiple interfaces):
o Reliability
These goals are achievable, but in a limited manner. The MN can
build a CoA on the interface connected to the foreign network, but
it cannot register the CoA with its HA and receive packets from
the HA via tunnel to the CoA at the same time it receives packet
on the HoA from the Home Link. As a result, without binding
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separately to CNs (i.e. route optimization), the MN is not able to
use both addresses simultaneously. In addition, in case of
failure, the MN cannot redirect flows from one valid address to
another. If the MN looses its connection established using the
address on the foreign link, all flows must be re-initiated with
another address (either the HoA, or a new address obtained on
another foreign link). Fault recovery is thus only possible
without transparency, and the MIPv6 features can only recover the
failure of the Home Address. This issue is detailed in
Section 6.2.2.
It might be possible for MN to register the CoA with selected CNs
(i.e. route optimization). In this case, the MN can enjoy
benefits of increased reliability for communications sessions
opened with these CNs. When the CoA fails, the MN can either bind
a new CoA, or remove the binding and directly get the packets to
its HoA.
Reliability could be achieved through bi-casting since the MN has
two addresses and should be able to request any CN to duplicate
traffic to both of them. However, MIPv6 does not allow the MN to
request bi casting on the CN (see Section 6.2.2).
o Load Sharing, Interface Switching
The MN is able to use both interfaces at the same time, according
to some preference settings on its side to decide which one to use
for which application. Therefore load sharing and interface
switching can be achieved when sessions are initiated by the MN.
When a CN initiates a session with the MN, it would choose the
destination address depending on the available information about
the MN (e.g., DNS request). Presently there is no mechanism
allowing the MN to indicate on which interface (i.e., address) a
CN may reach it. If only one address is known by the distant
node, load sharing and interface switching cannot be achieved.
See in Section 6.1.1 where such path selection issues are
discussed.
5.2. (Hn,C1): n HoAs, 1 CoA
The MN is multihomed, since it has several HoAs. This case may
happen when a node is getting access to the Internet through
different HAs (possibly distinct operators) and each offers a Mobile
IPv6 service to the node. That way, the node will have a HoA per HA.
Alternatively, a single home network may be multihomed to the
Internet, leading to having multiple prefixes on the home link. Thus
the MN would have multiple HoAs for a single home link. In either
case, the node would need to configure a single CoA on the visited
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IPv6 subnet, and bind that single CoA to all the HoAs it owns. If
the MN has multiple interfaces, only one interface is connected to a
foreign network. The other interfaces are connected to their home
links, or are inactive.
Achievable goals: Reliability, Load Sharing, Interface switching (if
the MN has more than one interface)
Current situation with MIPv6:
o Reliability
In case a HoA fails, (a failure could happen in the home network
of the MN, e.g. when one HA of the MN is disconnected from the
network), the session using the HoA must be restarted since MIPv6
does not provide any mechanism to hand-over transparently from a
fail HoA to another one. Currently, fault tolerance cannot be
achieved in this case, since established communications cannot be
preserved. See the corresponding discussion in Section 6.3.4.
The CoA may change when the MN has multiple interfaces and is
disconnected from its home link (e.g. failure of the interface, or
movement). In such a situation, MIPv6 allows to transparently
redirect flows using the old CoA as a temporarily address (i.e.
the HoA was used as the main address) to another CoA. For
sessions directly opened via the CoA, the loss of the address
implies a re-initiation of the session.
In conclusion, fault recovery can only be done in some cases, when
flows were initiated via a HoA.
Achieving reliability through bi-casting could be achieved in this
scenario by registering two addresses with a single HoA. However
MIPv6 does not provide any mechanism to associate more than one
CoA with one HoA. Moreover, in this particular case, one HoA
should be taken as a CoA regarding the other HoA. (see discussions
in Section 6.2.1 and Section 6.3.1).
o Load Sharing
In Bidirectional Tunnel (BT) mode, load sharing only affects the
path between the CN and the HA(s), and not the path between the MN
and the HA(s), as long as the CoA does not change. In RO mode,
the path between the MN and the CN does not change if the CoA does
not change. As an entry in the binding cache is identified by a
HoA, the MN can register the same CoA with all HoAs, on any
distant node. A mechanism would then be needed for the MN to
select which HoA should be used when a new communication flow is
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initiated. A similar mechanism is needed on the CN side if it
knows that multiple HoAs are assigned to the same MN. With such
mechanisms, it would be possible to use multiple HoAs at the same
time, and load sharing could be performed. However, it can be
noted that load sharing on the path between the CN and the MN's HA
might not be the most bandwidth contraint part of the overall path
from the CN to the MN. Thus this load sharing might not show
important enhancements. See in Section 6.1.1 where such path
selection issues are discussed. It is also possible that the MN
register one HoA as a CoA for another HoA (see discussions in
Section 6.3.1).
o Interface Switching
Interface switching is achievable when the MN has several
interfaces. In this case, the MN is attached to one foreign link
via one of its interfaces, and it is attached to home link(s) with
its other interface(s). In this case, the MN can spread flows
over its interfaces. Note that if a CN initiates a communication,
the interface that it will use on the MN would depend on the
information it has about this MN.
5.3. (H1,Cn): 1 HoA, n CoAs
The MN is multihomed since it has several CoAs. This case may for
instance occur when the interface of the node is connected to a link
where multiple IPv6 prefixes are available. One possible reason for
this is that the visited network is multihomed and because of that,
multiple prefixes are available in it, one per provider. Another
possible configuration is a MN with several interfaces connected to
different links. Note that one of the interfaces of the MN may be
connected to its home link.
Achievable goals: Reliability, Load Sharing, Interface switching (if
the MN is equipped with several interfaces)
Current situation with MIPv6:
o Reliability
Fault recovery will be limited to the case where one of the CoAs
become unreachable for a particular peer. For instance, a CoA may
become unreachable because the ISP which provides the IPv6 prefix
fails. Fault recovery in MIPv6 is achieved by associating an
alternate CoA to replace the failed one. However, efficient
mechanisms are needed to determine that a CoA has failed (see
Section 6.1.3) and to determine which CoA should be used instead
(see below).
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o Load Sharing and Interface Switching
This configuration allows to share the load and set preferences
among different paths between the HA and the MN when BT mode is
used, and between the CN and the MN when RO mode is used. In
order to achieve load sharing and interface switching under this
scenario, the MN would need to register several CoAs with its
unique HoA. However, the present specification of MIPv6 only
allows the MN to register a single CoA per HoA. This is discussed
in Section 6.2.1. When a single HoA is bounded to several CoAs at
the same time, the MN or HA/CN must be able to select the
appropriate CoA. This selection could be done based on user/
application preferences (see Section 6.1.1).
5.4. (Hn,Cn): n HoAs, n CoAs
The MN is multihomed since it has multiple addresses. This case can
be viewed as a combination of the two cases described above: the MN
has several HoAs (e.g. given by different operators) and several CoAs
(e.g. because the node is receiving multiple IPv6 prefixes). As an
example, we can consider a node with three interfaces, two of them
connected to their home link (two different HoAs) and the last one
connected to a visited link where two IPv6 prefixes are available.
Achievable goals: reliability, load sharing and interface switching
Current situation with MIPv6:
o Reliability
If one CoA becomes unreachable (similar to scenario (H1,Cn) in
Section 5.3), the MN can redirect flows to another CoA by
associating any other available CoAs to the corresponding HoA. If
the MN can not use one of its HoA anymore (similar to scenario
(Hn,C1) in Section 5.2), the MN will have to re-initiate all flows
which were using the corresponding HoA. Redirection between the
addresses available for the MN will be different depending on this
HoA / CoA binding policies.
o Load Sharing, Interface Switching
Currently, the MN can register only one CoA per HoA (see
Section 6.2.1), but it can register the same or different CoAs
with multiple HoAs. If the MN chooses to bind the same CoA to all
its HoAs, we fall in the (Hn,C1) case. In this case, load sharing
can only be performed if route optimization is not used, on the
CN-HA path, as a different HoA may be used per CN. If the MN
chooses to bind a different CoA for each HoA, load sharing will be
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done along the whole path across the MN and its CNs. Preference
settings may define which CoA (eventually several if bi-casting is
used) should be bound to which HoA (see Section 6.1.1).
In a very specific situation, one of the routable address of the
MN (i.e., which can be directly used without tunneling to reach
the MN) can be one of its HoA. In this case, this HoA can be
viewed as a CoA to bind with another HoA. Thus the MN may be able
to register one HoA as CoA regarding another HoA (see
Section 6.3.1). MIPv6 does not prevent this behavior, which allow
to set a certain preference on addresses usage.
5.5. (Hn,C0): n HoAs, no CoAs
This case happens when all the interfaces are connected to their
respective home links. This situation is quite similar to a non-
mobile node which is multihomed. The node would no longer be in the
(Hn,C0) configuration when one or more of the interfaces are attached
to foreign links.
The mobile node may wish to use one or more HoAs to serve as the CoA
of another HoA (see Section 6.3.1). In such situations, this
scenario is reduced to a (H1,C1) or (H1,Cn) configuration as
described in Section 5.1 or Section 5.3.
Achievable goals: Reliability, Load Sharing, interface switching
Current situation with MIPv6
o Reliability
If the MN is disconnected from one of its interfaces, the MN
should be able to register another valid HoA to its failed HoA
(see issue Section 6.3.1).
o Load Sharing, Interface Switching
This can be achieved when the MN is initiating the communication
flow, as it can choose which HoA should be used. Depending on how
CN can discover HoAs of the MN, these goals might also be achieved
when the CN is initiating the communication flow. See previous
scenario and discussions in Section 6.1.1 about the path
selection. If the flows binding on interfaces preferences change
over time, the MN should be able to redirect one flow from one
interface to another. However, MIPv6 only allows to redirect all
flows from one interface to another, assuming one HoA is
registered as CoA (see issue Section 6.3.1). If the MN policies
indicate to redirect only one flow, a supplementary mechanism
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would be needed.
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6. Issues
Existing protocols may not be able to handle the requirements
expressed in Section 3. For doing so, the issues discussed in this
section must be addressed, and solved preferably by dynamic
mechanisms. Note that some issues are pertaining to MIPv6 solely,
whereas other issues are not at all related to MIPv6. However, such
non MIPv6 issues must be solved in order to meet the requirements
outlined in Section 3.
In this section, we describe some of these issues in two main
headings: general IPv6-related issues, and issues that are specific
to MIPv6. Other concerns related to implementations of MIPv6 are
described in Section 6.3. Issues and their area of application are
summarized in Section 6.4
6.1. General IPv6-related Issues
6.1.1. Path Selection
When there exists multiple paths from and to the MN, the MN ends up
choosing a source address, and possibly the interface that should be
used. A CN that wants to establish a communication with such a MN
may end up by choosing a destination address for this MN.
o Interface selection
When the node has multiple available interfaces, the simultaneous
or selective use of several interfaces would allow a mobile node
to spread flows between its different interfaces.
Each interface could be used differently according to some user
and applications policies and preferences that would define which
flow would be mapped to which interface and/or which flow should
not be used over a given interface. How such preferences would be
set on the MN is out of scope of MIPv6 and might be implementation
specific. On the other hand, if the MN wishes to influence how
preferences are set on distant nodes (Correspondent Node or Home
Agent), mechanisms such as those proposed in
draft-soliman-flow-binding [4] could be used.
o HoA Selection
When multiple HoAs are available, the MN and its communicating
peers (HA and CNs) must be able to select the appropriate HoA to
be used for a particular packet or flow.
This choice would be made at the time of a new communication flow
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set up. Usual IPv6 mechanisms for source and destination address
selection, such as "Default Address Selection for IPv6" (RFC3484)
[5] or DNS SRV Protocol (RFC2782) [6] could be used.
However, in RFC3484 it is said that "If the eight rules fail to
choose a single address, some unspecified tie-breaker should be
used". Therefore more specific rules in addition to those
described in RFC3484 may be defined for HoA selection.
o CoA Selection
When multiple CoAs are available, the MN and its communicating
peers (HA and CNs) must be able to select the appropriate CoA to
be used for a particular packet or flow. The MN may use its
internal policies to (i) distribute its flow, and (ii) distribute
policies on distant nodes to allow them to select the preferred
CoA. Solutions like draft-soliman-flow-binding [4] may be used.
Another related aspect of path selection is the concern of ingress
filtering. This is covered below in Section 6.1.2.
6.1.2. Ingress Filtering
In the (H*,Cn) case, a MN may be connected to multiple access
networks or multiple home networks each practicing ingress filtering
(such as those specified in RFC3704 [7] and RFC2827 [8]). Thus, to
avoid ingress filtering, the selection of path would be limited by
the choice of address used. This is related to Section 6.1.1. The
problem of ingress filtering however, is two-fold. It can occur at
the access network, as well as the home network.
For instance, consider Figure 2 below. In the access network, when
the MN sends a packet through AR-A, it must use CoA=PA.MN; and when
MN sends a packet through AR-B, it must use CoA=PB.MN. In the home
network, when the MN tunnels the packet to home agent HA-1, it must
use HoA=P1.MN; and when MN tunnels the packet to home agent HA-2, it
must use HoA=P2.MN. This greatly limits the way MN can benefit from
its multihoming configuration.
As an illustration, suppose MN is choosing the interface (which would
determine the CoA used) and the home network to use (which would
determine the HoA used), it might be possible that the chosen CoA is
not registered with the chosen HoA.
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Prefix: PA +------+ +----------+ +------+
HoA: P1.MN /-----| AR-A |----| |----| HA-1 |
CoA: PA.MN / +------+ | | +------+
+----+ | | Prefix: P1
| MN | | Internet |
+----+ | | Prefix: P2
HoA: P2.MN \ +------+ | | +------+
CoA: PB.MN \-----| AR-B |----| |----| HA-2 |
Prefix: PB +------+ +----------+ +------+
Figure 2: MN connected to Multiple Access/Home Networks
It must be noted that should the MN have a way of binding both CoAs
PA.MN and PB.MN simultaneously to each of HoAs P1.MN and P2.MN (see
Section 6.2.1), then it can choose the CoA to use based on the access
network it wishes to use for outgoing packets. This solves the first
part of the problem: ingress filtering at the access network. It is,
nonetheless, still limited to using only a specific home agent for
the home-address used (i.e. the second problem of ingress filtering
at the home network remains unsolved). For this, mechanism such as
those provided by Shim6 (see RFC3582 [9] and draft-ietf-shim6-proto
[10]) may be used.
6.1.3. Failure detection
Currently, IPv6 has not clearly defined mechanism for failure
detection. A failure in the path between two nodes can be located at
many different places: the media of one of the node is broken (i.e.,
loss of connectivity), the path between the MN and the HA is broken,
the home link is disconnected from the Internet, etc. By now, MIPv6
only relies on the ability or the inability to receive Router
Advertisements within a stipulated period to detect the availability
or loss of media (local failure). Current effort [11] in the DNA
Working Group aims to address this, such as through the use of layer
2 triggers [12]. Movement detection might be extended to include
other triggers such as the loss of connectivity on one interface.
Further mechanisms would be needed to detect a failure in the path
between a MN and its CN(s), as well as between the MN and its HA(s),
between the MN and CN(s), or between the HA and CN(s).
6.2. MIPv6-specific Issues
6.2.1. Binding Multiple CoAs to a given HoA
In the (H1,Cn) cases, multiple CoAs would be available to the MN. In
order to use them simultaneously, the MN must be able to bind and
register multiple CoAs for a single HoA with both the HA and the CNs.
The MIPv6 specification is currently lacking such ability.
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Although in the (Hn,Cn) cases, MIPv6 allows MN to have multiple HoA
and CoA pairs, the upper layer's choice of using a particular HoA
would mean that the MN is forced to use the corresponding CoA. This
constrains the MN from choosing the best (in terms of cost, bandwidth
etc) access link for a particular flow, since CoA is normally bound
to a particular interface. If the MN can register all available CoAs
with each HoA, this would completely decouple the HoA from the
interface, and allow the MN to fully exploit its multihoming
capabilities.
To counter this issue, solutions like draft-ietf-mcoa [13] may be
used.
6.2.2. Simultaneous Location in Home and Foreign Networks
Rule 4 of RFC3484 specifies that a HoA will always be preferred to a
CoA. While this rule allows the host to choose which address to use,
it does not allow the MN to benefit from being multihomed in some
cases. When a MN is multihomed, it may have one of its interfaces
directly connected to a home link. This may have an impact on the
way multihoming is managed, since addresses from other interfaces
cannot be registered as CoAs for the HoA associated to the home link
the mobile node is connected on.
In the special case of (H1,C*) where one of the interface is
connected to the home link, none of the other addresses can be used
to achieve multihoming goals with the HA.
6.3. Considerations for MIPv6 Implementation
In addition to the issues described in Section 6.1 and Section 6.2,
there are other concerns implementers should take into consideration
so that their MIPv6 implementations are more "friendly" to the use of
multiple interfaces. These implementation-related considerations are
described in the sub-sections below.
6.3.1. Using one HoA as a CoA
In (Hn,C*) cases, the MN has multiple HoAs. A HoA may be seen as a
CoA from the perspective of another home link of the same MN.
As an example, a MN has two HoAs (HoA1 and HoA2) on two distinct home
links. MN is connected to these two home links via two interfaces.
If the MN looses its connectivity on its first interface, HoA1 is not
reachable. It may then want to register HoA2 as a CoA for HoA1 in
order to keep receiving packets intended to HoA1, via the second
interface.
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According to the definition of a CoA, the current MIPv6 specification
does not prohibit a HoA to be a CoA from the perspective of another
HoA.
In RFC3775 section 6.1.7 it is written: " Similarly, the Binding
Update MUST be silently discarded if the care-of address appears as a
home address in an existing Binding Cache entry, with its current
location creating a circular reference back to the home address
specified in the Binding Update (possibly through additional
entries)."
In order to benefit from any multihoming configuration, a MN must be
able to register whatever address it owns with any of its HoA, as
long as the above statement is observed.
6.3.2. Binding a new CoA to the Right HoA
In the (Hn,C*) cases, the MN has multiple HoAs. When the MN moves
and configures a new CoA, the newly obtained CoA must be bound to a
specific HoA. The current MIPv6 specification doesn't provide a
decision mechanism to determine to which HoA this newly acquired CoA
should be bound to.
The result might be to bind the CoA to the same HoA the previous CoA
was bound to or to another one, depending on the implementation. It
would indeed be better to specify the behavior so that all
implementations are compliant.
6.3.3. Binding HoA to interface
In (Hn,C*) cases, MIPv6 does not provide any information on how HoAs
should be bound on a device, and particularly there is no mechanism
to bind HoAs to interfaces.
This may be troublesome, for example, when we consider a MN
configured with two HoAs and equipped with three interfaces. When
the MN is connected to a home link via one interface, it will need to
bind the corresponding HoA to this interface, even if the HoA was
initially assigned to another one.
HoA1 HoA2
CoA1 CoA2 CoA3
Iface1 Iface2 Iface3
Figure 3: Illustration of the case (H2,C3)
HoA must always be assigned to an activated interface and if the MN
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is connected to its home link, the corresponding HoA must be used on
this interface. In some cases, the HoA then would have to be re-
assigned to another interface in case of connection loss or
attachment to the home link.
6.3.4. Flow redirection
Internet connectivity is guaranteed for a MN as long as at least one
path is maintained between the MN and its CN. When the current path
fails, an alternative path must be found to substitute the failed
one. The redirection from the old path to the new path may result in
broken sessions. In this case, new transport sessions would have to
be established over the alternate path if no mechanism is provided to
redirect flow transparently at layers above layer 3. The need for
flow redirection is given in Appendix A.
The different mechanisms that can be used to provide flow redirection
can be split into two categories, depending on the part of the path
that needs to be changed. The first category is when a CoA changes
(i.e., the used CoA become invalid or new preferences indicate that
the used CoA is no more the CoA to use): if one of the MN's CoA needs
to be changed, it influences the path between the MN and its HA, and
the path between the MN and its CN in RO mode. If the MN has
multiple interfaces and one fails, established sessions on the failed
interface would break if no support mechanism is used to redirect
flows from the failed interface to another.
The second category is when the HoA changes (i.e. the currently used
HoA is not valid anymore, e.g., because IPv6 renumbering on the home
link): if one of the MN's HoA needs to be changed, it influences the
path between the CN and the HoA. In (Hn,C*) cases, the MN has
multiple HoAs. If one fails, established sessions on the failed HoA
would break if no support mechanism is used to redirect flows from a
failed HoA to another, unless the transport session has multihoming
capabilities, such as SCTP, to allow dynamic changing of addresses
used.
6.4. Summary
THIS TABLE IS A WORK IN PROGRESS (so all boxes may not have been
filled appropriately). For now, please comment on the need for the
table rather than the content)
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+=====================================================+
| # of HoAs: | 1 | 1 | n | n | n |
| # of CoAs: | 1 | n | 0 | 1 | n |
+=====================================================+
| General IPv6 Issues |
+---------------------------------+---+---+---+---+---+
| Path Selection | | o | ? | o | o |
+---------------------------------+---+---+---+---+---+
| Ingress Filtering | | | ? | o | o |
+---------------------------------+---+---+---+---+---+
| Failure detection |o | o | ? | o | o |
+---------------------------------+---+---+---+---+---+
| MIPv6-Specific Issues |
+---------------------------------+---+---+---+---+---+
| Binding Multiple CoAs to a | | o | ? | o | o |
| given HoA | | | | | |
+---------------------------------+---+---+---+---+---+
| Simultaneous location in home | | o | ? | o | o |
| and foreign networks | | | ? | | |
+---------------------------------+---+---+---+---+---+
| Implementation-Related Concerns |
+---------------------------------+---+---+---+---+---+
| Using one HoA as a CoA | | | ? | o | o |
+---------------------------------+---+---+---+---+---+
| Binding a new CoA to the | | | ? | o | o |
| right HoA | | | | | |
+---------------------------------+---+---+---+---+---+
| Binding HoA to interface(s) | o | o | ? | o | o |
+---------------------------------+---+---+---+---+---+
| Flow redirection | o | o | ? | o | o |
+=====================================================+
Figure 4: Summary of Issues and Categorization
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7. TODO List
Study security concerns
Possibly discuss the possibility to use HoA on both home link and
foreign link as in case (H1,C1):
Mention about relation with Shim6.
Reword all the text about the "returning home case" throughout the
entire draft
Consider the multiple HA addresses throughout the entire draft.
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8. Conclusion
In this document, we have raised issues related to multihoming. We
have seen that mechanisms are needed to redirect flow from a failed
path to a new path, and mechanisms to decide which path should better
be taken when multiple paths are available. This raises a number of
issues.
Even if MIPv6 can be used as a mechanism to manage multihomed MN,
triggers of flows redirection between interfaces/addresses are not
adapted to the multihoming status of the node. Also, we have shown
that in some scenarios MIPv6 is ambiguous in the definitions of CoA/
HoA and in the mappings between HoAs, CoAs and network interfaces.
Finally, we have also raised issues not directly related to MIPv6,
but solutions for these issues are needed for mobile nodes to fully
enjoy the benefits of being multihomed.
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9. Contributors
The following people have contributed ideas, text and comments to
earlier versions of this document: Eun Kyoung Paik from Seoul
National University, South Korea and Thomas Noel from Universite
Louis Pasteur, Strasbourg, France.
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10. Acknowledgments
The authors would like to thank all the people who have sent comments
so far, particularly Tobias Kufner, Marcelo Bagnulo, Romain Kuntz and
Henrik Levkowetz for their in-depth comments and raising new issues.
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11. References
[1] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[2] Ernst, T., Montavont, N., Wakikawa, R., Ng, C., and K.
Kuladinithi, "Motivations and Scenarios for Using Multiple
Interfaces and Global Addresses",
draft-ietf-monami6-multihoming-motivation-scenario-01 (work in
progress), October 2006.
[3] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[4] Soliman, H., "Flow Bindings in Mobile IPv6",
draft-soliman-monami6-flow-binding-00 (work in progress),
March 2006.
[5] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[6] Gukbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[7] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[8] 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.
[9] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
Multihoming Architectures", RFC 3582, August 2003.
[10] Nordmark, E. and M. Bagnulo, "Level 3 multihoming shim
protocol", draft-ietf-shim6-proto-07 (work in progress),
November 2006.
[11] Choi, J., "Goals of Detecting Network Attachment in IPv6",
draft-ietf-dna-goals-04 (work in progress), December 2004.
[12] Krishnan, S., Montavont, N., Yegin, A., Veerepalli, S., and A.
Yegin, "Link-layer Event Notifications for Detecting Network
Attachments", draft-ietf-dna-link-information-05 (work in
progress), November 2006.
[13] Wakikawa, R., "Multiple Care-of Addresses Registration",
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draft-ietf-monami6-multiplecoa-00 (work in progress),
June 2006.
[14] Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
Agents", RFC 3776, June 2004.
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Appendix A. Why a MN may want to redirect flows
When a MN is multihomed, an addresses selection mechanism is needed
to distribute flows over interfaces. As policies may change over
time, as well as the available addresses/interfaces, flow redirection
mechanisms are needed. While the selection policy is out of scope of
this document, the following reasons may trigger the MN to redirect
flow from one address to another:
o Failure detection: the path between the MN and its CN(s) is
broken. The failure can occur at different places onto this path;
The failure can be local on the MN, where the interface used on
the MN is disconnected from the network (e.g., a wireless
interface which comes out of range from its point of attachment).
Alternatively, the failure can be on the path between the MN and
one of its HA. Yet another alternative is that the failure can be
on the path between the HA and the CN. If route optimization is
used, it can also be a failure between the MN and its CN(s).
o New address: a new address on the MN comes available. This can be
the case when the MN connects to the network with a new interface.
The MN may decide that this new interface is most suitable for its
current flows that are using another interface.
o Uninterrupted horizontal handover in mobility: If the MN is
mobile, it may have to change its point of attachment. When a MN
performs a horizontal handover, the handover latency (the time
during which the MN can not send nor receive packets) can be long
and the flows exchanged on the interface can be interrupted. If
the MN wants to minimize such perturbation, it can redirect some
or all the flows on another available interface. This redirection
can be done prior to the handover if L2 triggering is considered
[12] .
o Change in the network capabilities: the MN can observe a
degradation of service on one of its interface, or conversely an
improvement of capacity on an interface. The MN may then decide
to redirect some or all flows on another interface that it
considers most suitable for the target flows.
o Initiation of a new flow: a new flow is initiated between the MN
and a CN. According to internal policies, the MN may want to
redirect this flow on a most suitable interface.
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Authors' Addresses
Nicolas Montavont
Ecole Nationale Superieure des telecommunications de Bretagne
2, rue de la chataigneraie
Cesson Sevigne 35576
France
Phone: (+33) 2 99 12 70 23
Email: nicolas.montavont@enst-bretagne.fr
URI: http://www-r2.u-strasbg.fr/~montavont/
Ryuji Wakikawa
Keio University
Department of Environmental Information, Keio University.
5322 Endo
Fujisawa, Kanagawa 252-8520
Japan
Phone: +81-466-49-1100
Fax: +81-466-49-1395
Email: ryuji@sfc.wide.ad.jp
URI: http://www.wakikawa.org/
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
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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
Koojana Kuladinithi
University of Bremen
ComNets-ikom,University of Bremen.
Otto-Hahn-Allee NW 1
Bremen, Bremen 28359
Germany
Phone: +49-421-218-8264
Fax: +49-421-218-3601
Email: koo@comnets.uni-bremen.de
URI: http://www.comnets.uni-bremen.de/~koo/
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