IETF MIP6 Working Group N. Montavont
Internet-Draft NIST
Expires: December 10, 2005 R. Wakikawa
Keio University
T. Ernst
WIDE at Keio University
C. Ng
Panasonic Singapore Labs
K. Kuladinithi
University of Bremen
June 8, 2005
Analysis of Multihoming in Mobile IPv6
draft-montavont-mobileip-multihoming-pb-statement-04.txt
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Abstract
The use of multiple interfaces is foreseen to provide ubiquitous,
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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. Individual solutions have been proposed to extend
Mobile IPv6 but all issues have not been addressed in a single
document. The purpose of the present document is thus to fill up
this gap and to raise the discussion in order to make sure that
forthcoming solutions will address all the issues. In this document,
we propose a taxonomy to classify the situations where a mobile node
could be multihomed. This taxonomy is then used to highlight the
issues preventing mobile nodes operating Mobile IPv6 to be
multihomed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1 (1,1,1): 1 iface, 1 HoA, 1 CoA . . . . . . . . . . . . . . 12
5.2 (1,n,1): 1 iface, n HoAs, 1 CoA . . . . . . . . . . . . . 12
5.3 (1,1,n): 1 iface, 1 HoA, n CoAs . . . . . . . . . . . . . 13
5.4 (1,n,n): 1 iface, n HoAs, n CoAs . . . . . . . . . . . . . 14
5.5 (n,1,1): n ifaces, 1 HoA, 1 CoA . . . . . . . . . . . . . 14
5.6 (n,1,n): n ifaces, 1 HoA, n CoAs . . . . . . . . . . . . . 16
5.7 (n,n,1): n ifaces, n HoAs, 1 CoA . . . . . . . . . . . . . 17
5.8 (n,n,n): n ifaces, n HoAs, n CoAs . . . . . . . . . . . . 18
5.9 (n,n,0): n ifaces, n HoAs, no CoAs . . . . . . . . . . . . 18
6. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1 General IPv6-related Issues . . . . . . . . . . . . . . . 20
6.1.1 Path Selection . . . . . . . . . . . . . . . . . . . . 20
6.1.2 Ingress Filtering . . . . . . . . . . . . . . . . . . 21
6.1.3 Media Detection . . . . . . . . . . . . . . . . . . . 22
6.2 MIPv6-specific Issues . . . . . . . . . . . . . . . . . . 22
6.2.1 Binding Multiple CoAs to a given HoA . . . . . . . . . 22
6.2.2 Using one HoA as a CoA . . . . . . . . . . . . . . . . 22
6.2.3 Simultaneous location in home and foreign networks . . 23
7. Considerations for MIPv6 Implementation . . . . . . . . . . . 25
7.1 Binding a new CoA to the right HoA . . . . . . . . . . . . 25
7.2 Binding HoA to interface . . . . . . . . . . . . . . . . . 25
7.3 Flow redirection . . . . . . . . . . . . . . . . . . . . . 26
8. TODO List . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 28
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 31
Intellectual Property and Copyright Statements . . . . . . . . 33
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1. Introduction
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.
Mobile IPv6 [1],[2] is designed to allow a mobile node to maintain
its IPv6 communications while moving between IPv6 subnets. 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.
Individual solutions have been proposed to extend Mobile IPv6 for
multihomed mobile nodes, but all issues have not been addressed in a
single document. The purpose of the present document is thus to fill
up this gap by listing such issues, raising the discussion at the
IETF, and placing some requirements in order to propose comprehensive
solutions in forthcoming standards.
This document has two goals. The first goal of this document is to
define the requirements from the point of view of multihomed mobile
nodes operating Mobile IPv6. The second goal of this document 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 the analysis document. These should be defined in a
separate document once the IETF community agrees on which issues
should be solved.
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 (number of interfaces, number of Home
Addresses or number of Care-of Addresses). We also illustrate each
scenario.
To understand the foundation of this document, the reader must read
our companion document [3] 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
must also understand the operation of the Mobile IPv6 protocol ([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
benefit from a multihomed configuration. Then we propose in
Section 4 a taxonomy to classify the different cases where mobile
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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 7 all issues
related to a multihomed MN and we identify what is missing to reach
the goals outlined in [3]. 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 [4], [1] and
[3].
In [3], 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 MN having multiple HoAs and/or
multiple CoAs:
o A MN would have multiple HoAs if multiple prefixes are advertised
on the home link or if it has multiple interfaces named on
(presumably) distinct home links.
o A MN would have multiple CoAs if multiple prefixes are advertised
on the foreign link or if it has multiple interfaces attached to
(presumably) distinct foreign links.
In this draft we are using the following terms and abbreviations:
o MIPv6
The Mobile IPv6 protocol specified in [1]
o MN
a Mobile Node operating MIPv6
o HA
a Home Agent
o HoA
Home Address
o CoA
Care-of Address
o A valid address
An address that is topologically correct (it is named after the
prefix advertised on the link the interface is attached to) and
routable.
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o Simultaneously using multiple addresses
This indicates a scenario where the MN has the ability to use any
of the said multiple addresses at the same time. This implies
that the MN is able to receive packets with destination address
field equals to any of the said multiple addresses, and the MN is
able to choose any of the said multiple addresses as the source
address of the packets it is sending.
o Simultaneously using multiple interfaces
This indicates a scenario when there is at least one valid address
named for each of the said multiple interfaces, and that the MN is
able to simultaneously use these addresses.
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3. Requirements
The following generic goals and benefits of multihoming are discussed
in the companion document [3]:
1. Permanent and Ubiquitous Access
2. Redundancy/Fault-Recovery
3. Load Sharing
4. Load Balancing
5. Preference Settings
In this section, we are determining what is required for a mobile
node to achieve these design goals. We will determine later in the
document (from Section 5) which requirements are already fulfilled by
MIPv6 and what issues remain in order to meet the requirements not
currently fulfilled by MIPv6.
Basically, Internet connectivity is guaranteed for a MN as long as at
least one path is maintained between the MN and the fixed Internet.
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 fault recovery, preferences), or to split traffic
between multiple paths (e.g. for load sharing, load balancing). 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 load balancing and
load sharing), multiple paths must be maintained simultaneously
between the mobile node and the home network(s).
Basically, this can translate into the following enumeration of
requirements:
o The MN must have either multiple interfaces with at least a single
valid IP address on each interface, or a single interface with
more than one valid address.
o A MN equipped with multiple interfaces must be able to use
multiple interfaces simultaneously.
o A MN equipped with multiple interfaces must be able to attach
distinct interfaces to different access networks (distinct foreign
links or distinct home links, or a combination of both).
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o If several interfaces are activated and configured with valid
addresses, the MN should be able to share its traffic load among
these interfaces.
o If an interface is used as backup and the primary interface failed
(loss of connection), a mechanism should be available to quickly
activate the backup interface and redirect traffic.
o The MN must be able to use multiple HAs simultaneously for a given
home address.
o The MN must be able to use a distinct HA for each home address
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 to
remain unchanged. This is in-view of the large amount of Internet
traffic today are carried by TCP, which unlike SCTP, cannot handle
multiple end-point address pairs.
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4. Taxonomy
In order to aid the discussion of the benefits of multihoming as
listed in [3] from the perspective of a mobile node, we will use in
the present document the taxonomy (x,y,z) where:
o x = number of active interfaces
o y = number of Home Addresses (HoAs)
o z = 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 (y)
| | |
+-----+--------+ | |
| | | | |
CoA1 +--CoA2 +---CoA3 ... CoAn --> Temporary Address (z)
| | | |
Link1 Link2 Link3 ... Linkn --> IPv6 Link (n/a *)
| | | |
+-----+----+ | |
| | |
IF1 IF2 ... IFn --> Physical layer (x)
HoA1 ::= {CoA1, 2, 3} [IF1 and IF2]
HoA2 ::= {CoA3} [IF2]
Mobile Node(x = 2, y = 2, z = 3)
* because number of IPv6 links is equal to the number of CoAs, z
Figure 1: Illustration of the Taxonomy
As the taxonomy suggests, the fact that the mobile node has several
HoAs is independent from the fact that the mobile node has multiple
interfaces. The fact that the mobile node has multiple interfaces
does not imply that it has multiple HoAs and vice-versa. Similarly,
the number of CoAs the node has 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 advertised on a
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link may lead to the node building several CoAs on that link.
We propose a taxonomy with three parameters because each of them has
an influence on either the mobile node behavior / management, or the
potential benefits the mobile node may obtain from such
configuration.
The configurations denoted by these parameters will have an impact on
the way multihoming is supported. According to the number of HoAs
and CoAs, different policies will be needed, such as which CoA has to
be mapped with which HoA, do all the CoAs need to be mapped with all
the HoAs, etc.
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5. Scenarios
In this section, we are detailing the possible benefits for 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 defined. 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 and Section 7.
5.1 (1,1,1): 1 iface, 1 HoA, 1 CoA
The MN is not multihomed. The node has only one interface, with one
HoA and is currently away from its home link. A CoA is configured on
the foreign link. This is the usual scenario considered by Mobile
IPv6 which would bind that CoA to the HoA.
Achievable goals: none.
5.2 (1,n,1): 1 iface, 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, though less commonly practiced, a single operator may
have a multiple prefixed home link assigned to the mobile node, thus
the mobile node would have multiple HoAs for a single home link.
Either case, the node would need to configure a single CoA on the
visited IPv6 subnet, and bind that single CoA to all the HoAs it
owns.
Achievable goals: fault recovery, load sharing, preference settings.
Current situation with MIPv6:
o Fault Recovery
If there is a failure in one of the home network of the MN (e.g.,
one HA of the MN is disconnected from the network) MIPv6 does not
provide any mechanism to hand-over the communication to another
HoA. If a HoA, which is used at the transport layer by an
application, needs to be changed, the application must be
restarted. MIPv6 does not allow changing of the HoA transparently
for a given transport session. Currently, fault recovery can
therefore only be performed without transparency. See the
corresponding discussion in Section 7.3
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o Preference Settings and Load Sharing
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 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. See in Section 6.1.1 where such
path selection issues are discussed.
5.3 (1,1,n): 1 iface, 1 HoA, n CoAs
The MN is multihomed since it has several CoAs. This case may occur
when the interface of the node is connected to a link where multiple
IPv6 prefixes are advertised.
Achievable goals: fault recovery, load sharing, bicasting, preference
settings.
Current situation with MIPv6:
o Fault Recovery
Fault recovery will be limited to the case where a prefix in the
visited network is not advertised anymore, i.e. when the MN looses
a CoA. This is possible with MIPv6 as it can associate an
alternate CoA to replace the failed CoA. 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).
o Preference Setting, Load Sharing and Bicasting
In order to achieve load sharing 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 with several CoAs at the same time, the MN
or HA/CN would need to select which CoA should be used. This
selection could be done based on user/application preferences (see
Section 6.1.1).
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5.4 (1,n,n): 1 iface, 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).
Achievable goals: fault recovery, load sharing, bicasting,
preferences settings.
Current situation with MIPv6
o Fault Revovery
If one CoA fails (similar to scenario (1,1,n) 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 (1,n,1) 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 Preference Settings, Load Sharing and Bicasting
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 (1,n,1) 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
done along the whole path across the MN and its CNs. Preference
settings may define which CoA (eventually several if bicasting 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 6.2.2). MIPv6 does not prevent this behavior, which allow
to set a certain preference on addresses usage.
5.5 (n,1,1): n ifaces, 1 HoA, 1 CoA
This is a very special case of a node with multiple interfaces
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connected to different IPv6 subnets. The MN is multihomed if one of
the interface is attached to a foreign link, the other one is
attached to the home link. Thus, the MN only configures one CoA on
the foreign subnet, and one HoA on the home link.
There cannot be more than two 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.
Achievable goals: ubiquitous access, fault recovery, load sharing,
load balancing, preference settings, bicasting
Current situation with MIPv6:
o Fault Recovery and Ubiquitous Access
These goals are achievable, but in a limited manner. The MN can
build a temporary IPv6 address on its other interface but it
cannot register the temporary address with its HA because the node
is using its HoA. If the node needs to update its location
following a movement, it will not be able to use its HoA on the
interface connected to its home link. This issue is detailed in
Section 6.2.3.
As MIPv6 cannot be used on these addresses, the MN will not be
able to redirect any flow to another address. For instance, if
the MN looses its connection to its home link, all flows must be
initiated again with the CoA address valid on the other interface
of the MN. Fault recovery is then only possible without
transparency and none of the MIPv6 features can help in case of a
failure.
o Preference Settings, Load Sharing and Load Balancing
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 load balancing
can be achieved when communication are initiated by the MN. When
a CN initiates a communication 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 distant node,
load sharing and load balancing will not be achieved. See in
Section 6.1.1 where such path selection issues are discussed.
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o Bicasting
Bicasting should be possible since the MN has two addresses. The
MN should be able to request any CN to duplicate traffic to both
addresses. However, MIPv6 does not allow the MN to request
bicasting on the CN (see Section 6.2.3).
5.6 (n,1,n): n ifaces, 1 HoA, n CoAs
The MN is multihomed: the node has several addresses to choose from.
This case regroups two different scenarios: either the MN is
connected to its home link via one interface, or the MN is only
connected to visited subnets via all its interfaces. For example,
consider a node with several interfaces, each connected to an IPv6
network (the same or not). In this example, at least one global IPv6
address is configured on each interface. The node has only one home
link, and only one HA.
Achievable goals: ubiquitous access, fault recovery, load sharing,
load balancing, bicasting, preference settings.
Current situation with MIPv6:
o Fault Recovery
Fault recovery is possible with MIPv6 whenever the MN looses one
interface (and the CoA configured on this interface) as it can
associate the CoA of an alternate interface to replace the failed
one. However, efficient mechanisms are needed to determine that
an interface has failed (see Section 6.1.3) and to determine which
interface should be used instead (see Section 6.1.1)
o Preference Settings, Load Sharing and Bicasting
In order to achieve these goals 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. Having
multiple interfaces and their associated CoAs available
simultaneously, efficient mechanisms are needed to select the
appropriate path (see Section 6.1.1).
o Flow redirection (Fault Recovery, Load Sharing, preference
settings)
If the MN has a connection to its home link and then uses its HoA
on this interface, it will not be able to use MIPv6 mechanisms.
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The same conclusions can be drawn as in the previous case: the MN
can choose from all its valid addresses to initiate a flow, but
will not be able to redirect any of them (e.g., in case of
preferences/address changing). The interface used to receive a
new flow from a CN will depend on the knowledge of the CN on the
MN's addresses.
If the MN is not connected to its home link, it can register
(only) one CoA with its HoA. In this case, if the registered CoA
fails or changes, the MN will be able to redirect flows on another
CoA. For any flow directly started via one of the CoA, the MN
would have to initiate it again in order to use an alternative
address. Thus fault recovery and preferences changes can only be
done without transparency for other addresses than the registered
CoA. Load sharing and load balancing are subject to the same
consideration as above.
5.7 (n,n,1): n ifaces, n HoAs, 1 CoA
The MN is multihomed. This case extends the case (n,1,1) when the
node has several HoAs, for example from multiple operators. The CoA
can be associated with each HoA.
Achievable goals: ubiquitous access, fault recovery, load sharing,
load balancing, bicasting, preference settings.
Current situation with MIPv6
o Fault Recovery and Ubiquitous Access
If the MN is disconnected from its home link, MIPv6 allows the MN
to redirect flows transparently to another CoA. If the CoA
changes (e.g., failure of the interface, or movement), MIPv6
allows to transparently redirect flows which were using this
address as temporarily address (i.e., communication is using a HoA
as main address) to another address. If sockets were directly
opened via the CoA, loss of the address will imply to initiate
again the communication. In conclusion, fault recovery can only
be done in some cases, when flows were initiated via a HoA.
o Preference Settings, Load Sharing, Load Balancing
That case may be interpreted in different ways. Either the MN is
attached to one or several home links and one foreign link. The
CoA can be registered to HoAs of the MN, but the one used on the
interface(s) connected to the home link. The MN can distribute
its flows among its interfaces by selecting the appropriated
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address. CN can initiate flows intended to the MN via different
addresses depending on the information it has on the MN. It can
be noted, that in some scenarios one HoA will be registered as CoA
for another HoA (see discussions in Section 6.2.2).
o Bicasting
Bicasting 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 6.2.2).
5.8 (n,n,n): n ifaces, n HoAs, n CoAs
The MN is multihomed. Many scenarios may lead to this case. For
example, 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 advertised.
Achievable goals: ubiquitous access, fault recovery, load sharing,
load balancing, bicasting, preference settings.
Current situation with MIPv6
o Fault Recovery and Ubiquitous Access
Flow redirection between interfaces is only possible if the
corresponding socket was opened with a HoA at the application
layer.
o Preference Settings, Load sharing, Load Balancing and Bicasting
This can be achieved when the MN is initiating communication.
Depending on the information about the MN on the CN side, multiple
addresses may be advertised for the same MN, making possible for
the MN to simultaneously use several interfaces. However, only
one CoA can presently be bound to a HoA (see discussion in
Section 6.2.1). This would limit the selection of addresses per
flow to a selection of which CoA must be bound to which HoA.
5.9 (n,n,0): n ifaces, n HoAs, no CoAs
This case happens when the interfaces are connected to their
respective home links. This node can be considered as a fixed node
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from a multihoming point of view. The node would no longer be in the
(n,n,0) configuration when one or more of the interfaces are attached
to foreign links.
Achievable goals: ubiquitous access, fault recovery, load sharing,
load balancing, bicasting, preference settings.
Current situation with MIPv6
o Fault Recovery and Ubiquitous Access
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 6.2.2).
o Preference Settings, Load Sharing, Load Balancing
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 6.2.2). If the MN policies
indicate to redirect only one flow, a supplementary mechanism
would be needed.
o Bicasting
MN should be able to request bicasting from any CN, to duplicate
traffic to several HoAs. To do so, multiple addresses have to be
registered with one HoA (see Section 6.2.1), and HoA(s) should be
used as CoAs for other HoAs (see Section 6.2.2).
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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 described some of these issues in two main
headings: general IPv6-related issues, and issues that are specific
to MIPv6. Other concerns related to implementation of MIPv6 are
described in Section 7.
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 and destination address, and possibly the interface
that should be used.
o Interface selection
In the (n,*,*) case, 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 [5], [6] and [7]
could be used.
o HoA selection
In the (*,n,*) cases, multiple HoAs would be available on the MN.
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 [8] would be used.
o CoA selection
In the (*,*,n) cases, multiple CoAs would be available on the MN.
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 must use its internal policies to distribute its
flow, but also to distribute policies on distant nodes to allow
them to select the right CoA. Solutions like nomadv6 [7] or HA
filtering [6] may be used.
Another related aspect of path selection is the concern of ingress
filtering. This is detailed in Section 6.1.2.
6.1.2 Ingress Filtering
In the (n,*,n) case, a MN may be connected to multiple access
networks or multiple home networks each practicing ingress filtering
[9], [10]. 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 mobile node 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 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.
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
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It must be noted that should the mobile node 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).
6.1.3 Media Detection
Currently, IPv6 has no clearly defined mechanism for detecting the
availability or loss of media except through the ability or inability
to receive router advertisements within a stipulated period. 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. Moreover, the chosen
mechanism must work regardless the previous bindings the MN has
registered. The redirection between interfaces can be performed
transparently to the MNs if mechanisms such as the one specified in
[13] are brought to the MN.
6.2 MIPv6-specific Issues
6.2.1 Binding Multiple CoAs to a given HoA
In the (*,1,n) 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.
Although in the (*,n,n) 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 [14] may be used.
6.2.2 Using one HoA as a CoA
In (*,n,*) cases, the MN has multiple HoAs. A HoA may be seen as a
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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.
According to the definition of a CoA, the current MIPv6 specification
prohibits registering another HoA as a CoA.
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 other words, the BU must be discarded if the binding cache of the
receiver is:
Binding Cache:
New BU: HoA = addr1 w/ CoA = addr2
existing entries: HoA = addr2 w/ CoA = addr3
HoA = addr3 w/ CoA = addr1
But, in the following situation:
Binding Cache:
New BU: HoA = addr1 w/ CoA = addr2
existing entries: HoA = addr2 w/ CoA = addr3
and no other entries in the binding cache concerning addr1/2/3. In
this case, the BU should be accepted.
In order to counter this, a MN must be able to register whatever
address it owns with any of its HoA. A mechanism is needed to
determine how to decide which HoA will be chosen and the definition
of a HoA and CoA might be extended.
6.2.3 Simultaneous location in home and foreign networks
In the (n,*,*), the mobile node 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.
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In the special case of (n,1,*) 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
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7. Considerations for MIPv6 Implementation
In addition to the issues described in Section 6, 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.
7.1 Binding a new CoA to the right HoA
In the (*,n,*) 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.
With no such mechanism, the MN may be confused and may bind this CoA
to a possibly wrong HoA. 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.
7.2 Binding HoA to interface
In (n,n,*) 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 (3,2,3)
HoA must always be assigned to an activated interface and if the MN
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.
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7.3 Flow redirection
Internet connectivity is guaranteed for a MN as long as at least one
path is maintained between the mobile node and its corresponding
node. When an alternative path must be found to substitute for a
failed one, the loss of one path to the Internet 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. This could
happen in the following situations:
o In the (n,*,*) cases, the MN has multiple interfaces. If one
interface fails, established sessions on the failed interface
would break if no support mechanism is used to redirect flows from
the failed interface to another.
Although MIPv6 does not specify handovers between two interfaces,
implementers should design a mechanism to allow a HoA to be re-
bound to an alternative interface. Such a implementation should
take into consideration the use of connectivity triggers from
multiple interfaces when performing movement detection.
Mechanisms such as those specified in [13] can be used.
o In the (*,n,*) 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.
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8. TODO List
Add a table to summarize issues
Study security concerns
Possibly discuss the possibility to use HoA on both home link and
foreign link as in case (n,1,1):
Mention about relation with Multi6 / Shim6.
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9. 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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10. Contributors
The following people have contributed ideas, text and comments to
this draft: Eun Kyoung Paik from Seoul National University, South
Korea and Thomas Noel from Universite Louis Pasteur, Strasbourg,
France, and Julien Charbon from Keio University, Japan.
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11. Acknowledgments
The authors would like to thank all the people who have sent comments
so far, particularly Tobias Kufner for raising new issues.
12. References
[1] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[2] Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
Agents", RFC 3776, June 2004.
[3] Ernst, T., "Goals and Benefits of Multihoming",
draft-multihoming-generic-goals-and-benefits-00 (work in
progress), February 2004.
[4] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[5] Soliman, H., Malki, K., and C. Castelluccia, "Per-flow movement
in MIPv6", draft-soliman-mobileip-flow-move-02 (work in
progress), July 2002.
[6] Montavont, N. and T. Noel, "Home Agent Filtering for Mobile
IPv6", draft-montavont-mobileip-ha-filtering-v6-00 (work in
progress), January 2004.
[7] Kuladinithi, K., "Filters for Mobile IPv6 Bindings (NOMADv6)",
draft-nomadv6-mobileip-filters-02 (work in progress),
June 2004.
[8] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[9] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[10] 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.
[11] Choi, J., "Goals of Detecting Network Attachment in IPv6",
draft-ietf-dna-goals-04 (work in progress), December 2004.
[12] Yegin, A., "Link-layer Event Notifications for Detecting
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Network Attachments", draft-ietf-dna-link-information-01 (work
in progress), February 2005.
[13] Montavont, N., Noel, T., and M. Kassi-Lahlou, "MIPv6 for
Multiple Interfaces", draft-montavont-mobileip-mmi-00 (work in
progress), July 2002.
[14] Wakikawa, R., "Multiple Care-of Addresses Registration",
draft-wakikawa-mobileip-multiplecoa-03 (work in progress),
July 2004.
[15] Ernst, T. and H. Lach, "Network Mobility Support Terminology",
draft-ietf-nemo-terminology-03 (work in progress),
February 2005.
[16] Stemm, M. and R. Katz, "Vertical Handoffs in Wireless Overlay
Networks", Journal Mobile Networks and Applications, vol. 3,
number 4, pages 335-350, 1998.
Authors' Addresses
Nicolas Montavont
National Institute of Standards and Technology
100 Bureau Drive, Stop 1070
Gaithersburg 20899-1070
USA
Phone: 301 975 2923
Email: nicolas.montavont@nist.gov
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.net/
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Thierry Ernst
WIDE at Keio University
Jun Murai Lab., Keio University.
K-square Town Campus, 1488-8 Ogura, Saiwa-Ku
Kawasaki, Kanagawa 212-0054
Japan
Phone: +81-44-580-1600
Fax: +81-44-580-1437
Email: ernst@sfc.wide.ad.jp
URI: http://www.sfc.wide.ad.jp/~ernst/
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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