Network Working Group H. Soliman
Request for Comments: 5380 Elevate Technologies
Obsoletes: 4140 C. Castelluccia
Category: Standards Track INRIA
October 2008 Hierarchical Mobile IPv6 (HMIPv6) Mobility Management
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
This document introduces extensions to Mobile IPv6 and IPv6 Neighbour
Discovery to allow for local mobility handling. Hierarchical
mobility management for Mobile IPv6 is designed to reduce the amount
of signalling between the mobile node, its correspondent nodes, and
its home agent. The Mobility Anchor Point (MAP) described in this
document can also be used to improve the performance of Mobile IPv6
in terms of handover speed.
This specification introduces the concept of a hierarchical Mobile
IPv6 network, utilising a new node called the Mobility Anchor Point
Mobile IPv6 [RFC3775] allows nodes to move within the Internet
topology while maintaining reachability and ongoing connections
between mobile and correspondent nodes. To do this, a mobile node
sends binding updates (BUs) to its home agent (HA) every time it
The mobile node may send data packets via its home agent immediately
after sending the binding update, but the home agent will not be able
to route traffic back to the mobile node before it receives the
binding update. This incurs at least half a round-trip delay before
packets are again forwarded to the right place. There is an
additional delay for sending data packets if the mobile node chooses
to wait for a binding acknowledgement (BA). The round-trip times can
be relatively long if the mobile node and its home agent are in
different parts of the world.
Additional delay is also incurred if the mobile node employs route
optimisation. Authenticating binding updates requires approximately
1.5 round-trip times between the mobile node and each correspondent
node (for the entire return routability procedure in a best-case
scenario, i.e., no packet loss). This can be done in parallel with
sending binding updates to the home agent, and there are further
optimisations that reduce the required 1.5 round-trips [RFC4449]
Nevertheless, the signalling exchanges required to update your
location will always cause some disruption to active connections.
Some packets will be lost. Together with link layer and IP layer
connection setup delays, there may be effects to upper-layer
protocols. Reducing these delays during the time-critical handover
period will improve the performance of Mobile IPv6.
Moreover, in the case of wireless links, such a solution reduces the
number of messages sent over the air interface to all correspondent
nodes and the home agent. A local anchor point will also allow
Mobile IPv6 to benefit from reduced mobility signalling with external
For these reasons, a new Mobile IPv6 node, called the Mobility Anchor
Point, is used and can be located at any level in a hierarchical
network of routers, including the Access Router (AR). The MAP will
limit the amount of Mobile IPv6 signalling outside the local domain.
The introduction of the MAP provides a solution to the issues
outlined earlier, in the following way:
o The mobile node sends binding updates to the local MAP rather than
the home agent (HA) (which is typically further away) and
correspondent nodes (CNs).
o Only one binding update message needs to be transmitted by the
mobile node (MN) before traffic from the HA and all CNs is
re-routed to its new location. This is independent of the number
of CNs with which the MN is communicating.
A MAP is essentially a local home agent. The aim of introducing the
hierarchical mobility management model in Mobile IPv6 is to enhance
the performance of Mobile IPv6 while minimising the impact on Mobile
IPv6 or other IPv6 protocols. Furthermore, HMIPv6 allows mobile
nodes to hide their location from correspondent nodes and home
agents, while using Mobile IPv6 route optimisation. The security
differences between the MAP and the home agent are described in
It is pertinent to note that the use of the MAP does not rely on, or
assume the presence of, a permanent home agent. In other words, a
mobile node need not have a permanent home address or home agent in
order to be HMIPv6-aware or use the features in this specification.
A MAP may be used by a mobile node in a nomadic manner to achieve
mobility management within a local domain. Section 6.5 describes
such a scenario.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
In addition, the following terms are introduced:
Access Router (AR)
The AR is the mobile node's default router. The AR aggregates the
outbound traffic of mobile nodes.
Mobility Anchor Point (MAP)
A Mobility Anchor Point is a router located in a network visited
by the mobile node. The MAP is used by the MN as a local HA. One
or more MAPs can exist within a visited network.
Regional Care-of Address (RCoA)
An RCoA is an address allocated by the MAP to the mobile node.
HMIPv6-Aware Mobile Node
An HMIPv6-aware mobile node is a mobile node that can receive and
process the MAP option received from its default router. An
HMIPv6-aware mobile node must also be able to send local binding
updates (binding update with the M flag set).
On-Link Care-of Address
The LCoA is the on-link CoA configured on a mobile node's
interface based on the prefix advertised by its default router.
In [RFC3775], this is simply referred to as the care-of address.
However, in this memo LCoA is used to distinguish it from the
Local Binding Update
The MN sends a local binding update to the MAP in order to
establish a binding between the RCoA and LCoA.
3. Overview of HMIPv6
This hierarchical Mobile IPv6 scheme introduces a new function, the
MAP, and minor extensions to the mobile node operation. The
correspondent node and home agent operations will not be affected.
Just like Mobile IPv6, this solution is independent of the underlying
access technology, allowing mobility within or between different
types of access networks.
A mobile node entering a MAP domain will receive Router
Advertisements containing information about one or more local MAPs.
The MN can bind its current location (on-link CoA) with an address on
the MAP's subnet (RCoA). Acting as a local HA, the MAP will receive
all packets on behalf of the mobile node it is serving and will
encapsulate and forward them directly to the mobile node's current
address. If the mobile node changes its current address within a
local MAP domain (LCoA), it only needs to register the new address
with the MAP. Hence, only the Regional CoA (RCoA) needs to be
registered with correspondent nodes and the HA. The RCoA does not
change as long as the MN moves within a MAP domain (see below for
definition). This makes the mobile node's mobility transparent to
correspondent nodes it communicates with.
A MAP domain's boundaries are defined by the Access Routers (ARs)
advertising the MAP information to the attached mobile nodes. The
detailed extensions to Mobile IPv6 and operations of the different
nodes will be explained later in this document.
It should be noted that the HMIPv6 concept is simply an extension to
the Mobile IPv6 protocol. An HMIPv6-aware mobile node with an
implementation of Mobile IPv6 SHOULD choose to use the MAP when
discovering such capability in a visited network. However, in some
cases the mobile node may prefer to simply use the standard Mobile
IPv6 implementation. For instance, the mobile node may be located in
a visited network within its home site. In this case, the HA is
located near the visited network and could be used instead of a MAP.
In this scenario, the mobile node would only update the HA whenever
it moves. The method to determine whether the HA is in the vicinity
of the MN (e.g., same site) is outside the scope of this document.
3.1. HMIPv6 Operations
The network architecture shown in Figure 1 illustrates an example of
the use of the MAP in a visited network.
In Figure 1, the MAP can help in providing seamless mobility for the
mobile node as it moves from Access Router 1 (AR1) to Access Router 2
(AR2), while communicating with the correspondent node. A
multi-level hierarchy is not required for a higher handover
performance. Hence, it is sufficient to locate one or more MAPs
(possibly covering the same domain) at any position in the operator's
| HA |
| | CN |
| MAP |
| AR1 | | AR2 |
| MN |
Figure 1: Hierarchical Mobile IPv6 domain
Upon arrival in a visited network, the mobile node will discover the
global address of the MAP. This address is stored in the Access
Routers and communicated to the mobile node via Router Advertisements
(RAs). A new option for RAs is defined later in this specification.
This is needed to inform mobile nodes about the presence of the MAP
(MAP Discovery). The discovery phase will also inform the mobile
node of the distance of the MAP from the mobile node. For example,
the MAP function could be implemented as shown in Figure 1, and, at
the same time, also be implemented in AR1 and AR2. In this case, the
mobile node can choose the first hop MAP or one further up in the
hierarchy of routers. The details on how to choose a MAP are
provided in Section 10.
The process of MAP Discovery continues as the mobile node moves from
one subnet to the next. Every time the mobile node detects movement,
it will also detect whether it is still in the same MAP domain. The
Router Advertisement used to detect movement will also inform the
mobile node, through Neighbour Discovery [RFC4861] and the MAP
option, whether it is still in the same MAP domain. As the mobile
node roams within a MAP domain, it will continue to receive the same
MAP option included in Router Advertisements from its AR. If a
change in the advertised MAP's address is received, the mobile node
MUST act on the change by sending binding updates to its HA and
If the mobile node is not HMIPv6-aware, then no MAP Discovery will be
performed, resulting in the mobile node using the Mobile IPv6
[RFC3775] protocol for its mobility management. On the other hand,
if the mobile node is HMIPv6-aware it SHOULD choose to use its HMIPv6
implementation. If so, the mobile node will first need to register
with a MAP by sending it a BU containing its home address and on-link
address (LCoA). The home address used in the BU is the RCoA, which
the mobile node acquires via RFC 4877 [RFC4877] Section 9 mechanisms
when it first contacts a given MAP. The MAP MUST store this
information in its binding cache to be able to forward packets to
their final destination when received from the different
correspondent nodes or HAs.
The mobile node will always need to know the original sender of any
received packets to determine if route optimisation is required.
This information will be available to the mobile node because the MAP
does not modify the contents of the original packet. Normal
processing of the received packets (as described in [RFC3775]) will
give the mobile node the necessary information.
To use the network bandwidth in a more efficient manner, a mobile
node may decide to register with more than one MAP simultaneously and
to use each MAP address for a specific group of correspondent nodes.
For example, in Figure 1, if the correspondent node happens to exist
on the same link as the mobile node, it would be more efficient to
use the first hop MAP (in this case assume it is AR1) for
communication between them. This will avoid sending all packets via
the "highest" MAP in the hierarchy and thus will result in more
efficient usage of network bandwidth. The mobile node can also use
its current on-link address (LCoA) as a CoA, as specified in
[RFC3775]. Note that the mobile node MUST NOT present an RCoA from a
MAP's subnet as an LCoA in a binding update sent to another MAP. The
LCoA included in the binding update MUST be the mobile node's
address, derived from the prefix advertised on its link.
4. Mobile IPv6 Extension - Local Binding Update
This section outlines the extensions proposed to the binding update
specified in [RFC3775].
A new flag is added: the M flag, which indicates MAP registration.
When a mobile node registers with the MAP, the M and A flags MUST be
set to distinguish this registration from a BU being sent to the HA
or a correspondent node.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Sequence # |
|A|H|L|K|M| Reserved | Lifetime |
Figure 2: Local Binding Update
If set to 1, it indicates a MAP registration.
5. Neighbour Discovery Extension: The MAP Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Type | Length | Dist | Pref |R| Reserved |
| Valid Lifetime |
+ Global IP Address for MAP +
Figure 3: The MAP option
IPv6 Neighbour Discovery option. Its value is 23.
8-bit unsigned integer. The length of the option and MUST be set
A 4-bit unsigned integer identifying the distance between MAP and
the receiver of the advertisement, measure in the number of hops
and starting from 1 if the MAP is on the same link as the mobile
node. A distance value of zero MUST NOT be used.
The preference field, used as an indicator of operator preference.
A 4-bit unsigned integer. A decimal value of 15 indicates the
highest preference. When comparing two potential MAPs, the mobile
node SHOULD inspect this field as a tie-breaker for MAPs that have
equal Dist values.
When set to 1, it indicates that the mobile node is allocated the
RCoA by the MAP based on Section 9 of [RFC4877].
The minimum value (in seconds) of both the valid lifetime of the
prefix used to form the MAP's address and that used to form the
RCoA. This value indicates the validity of the MAP's address and
One of the MAP's global addresses.
6. Protocol Operation
This section describes the HMIPv6 protocol. In HMIPv6, the mobile
node has two addresses, an RCoA on the MAP's link and an on-link CoA
(LCoA). This RCoA is formed in a stateless manner by combining the
mobile node's interface identifier and the subnet prefix received in
the MAP option.
As illustrated in this section, this protocol requires updating the
mobile nodes' implementation only. The HA and correspondent node are
unchanged. The MAP performs the function of a "local" HA that binds
the mobile node's RCoA to an LCoA.
6.1. Mobile Node Operation
When a mobile node moves into a new MAP domain (i.e., its MAP
changes), it needs to configure two CoAs: an RCoA on the MAP's link
and an on-link CoA (LCoA). After employing [RFC4877] to acquire an
RCoA, the mobile node sends a local BU to the MAP with the A and M
flags set. The local BU is a BU defined in [RFC3775] and includes
the mobile node's RCoA in the Home Address option. No alternate-CoA
option is needed in this message. The LCoA is used as the source
address of the BU. This BU will bind the mobile node's RCoA (similar
to a home address) to its LCoA. The MAP (acting as an HA) will then
return a binding acknowledgement to the MN. This acknowledgement
either identifies the binding as successful or contains the
appropriate fault code. No new error codes need to be supported by
the mobile node for this operation. The mobile node MUST silently
ignore binding acknowledgements that do not contain a routing header
type 2, which includes the mobile node's RCoA.
Following a successful registration with the MAP, a bi-directional
tunnel between the mobile node and the MAP is established. All
packets sent by the mobile node are tunnelled to the MAP. The outer
header contains the mobile node's LCoA in the source address field,
and the MAP's address in the destination address field. The inner
header contains the mobile node's RCoA in the source address field,
and the peer's address in the destination address field. Similarly,
all packets addressed to the mobile node's RCoA are intercepted by
the MAP and tunnelled to the mobile node's LCoA.
This specification allows a mobile node to use more than one RCoA if
it received more than one MAP option. In this case, the mobile node
MAY perform the binding update procedure for each RCoA. In addition,
the mobile node MUST NOT use one RCoA (e.g., RCoA1) derived from a
MAP's prefix (e.g., MAP1) as a care-of address in its binding update
to another MAP (e.g., MAP2). This would force packets to be
encapsulated several times (twice in this example) on their path to
the mobile node. This form of multi-level hierarchy will reduce the
protocol's efficiency and performance.
After registering with the MAP, the mobile node MUST register its new
RCoA with its HA by sending a BU that specifies the binding (RCoA,
home address), as in Mobile IPv6. The mobile node's home address is
used in the Home Address option and the RCoA is used as the care-of
address in the source address field. The mobile node may also send a
similar BU (i.e., that specifies the binding between the home address
and the RCoA) to its current correspondent nodes.
The mobile node SHOULD wait for the binding acknowledgement from the
MAP before registering the RCoA with other nodes (e.g., CN or HA, if
available). It should be noted that when binding the RCoA with the
HA and correspondent nodes, the binding lifetime MUST NOT be larger
than the mobile node's binding lifetime with the MAP, which is
received in the binding acknowledgement.
In order to speed up the handover between MAPs and reduce packet
loss, a mobile node SHOULD send a local BU to its previous MAP,
specifying its new LCoA. Packets in transit that reach the previous
MAP are then forwarded to the new LCoA.
The MAP will receive packets addressed to the mobile node's RCoA
(from the HA or correspondent nodes). Packets will be tunnelled from
the MAP to the mobile node's LCoA. The mobile node will de-capsulate
the packets and process them in the normal manner.
When the mobile node moves within the same MAP domain, it should only
register its new LCoA with its MAP. In this case, the RCoA remains
Note that a mobile node may send a BU containing its LCoA (instead of
its RCoA) to correspondent nodes. If these nodes are connected to
the same link, packets will then be routed directly, without going
through the MAP.
6.1.1. Sending Packets to Correspondent Nodes
The mobile node can communicate with a correspondent node through the
HA, or in a route-optimised manner, as described in [RFC3775]. When
communicating through the HA, the message formats in [RFC3775] are
If the mobile node communicates directly with the correspondent node
(i.e., the CN has a binding cache entry for the mobile node), the
mobile node MUST use the same care-of address used to create a
binding cache entry in the correspondent node (RCoA) as a source
address. According to [RFC3775], the mobile node MUST also include a
Home Address option in outgoing packets. The Home Address option
MUST contain the mobile node's home address.
6.2. MAP Operations
The MAP acts like an HA; it intercepts all packets addressed to
registered mobile nodes and tunnels them to the corresponding LCoA,
which is stored in its binding cache.
A MAP has no knowledge of the mobile node's home address. The mobile
node will send a local BU to the MAP with the M and A flags set. The
aim of this BU is to bind the RCoA (contained in the BU as a home
address) to the mobile node's LCoA. If successful, the MAP MUST
return a binding acknowledgement to the mobile node, indicating a
successful registration. This is identical to the HA operation in
[RFC3775]. No new error codes are introduced for HMIPv6. The
binding acknowledgement MUST include a routing header type 2 that
contains the mobile node's RCoA.
The MAP MUST be able to accept packets tunnelled from the mobile
node, with the mobile node being the tunnel entry point and the MAP
being the tunnel exit point.
The MAP employs [RFC4877] Section 9 procedures for the allocation of
RCoA, and subsequently acts as an HA for the RCoA. Packets addressed
to the RCoA are intercepted by the MAP, using proxy Neighbour
Advertisement, and then encapsulated and routed to the mobile node's
LCoA. This operation is identical to that of the HA described in
A MAP MAY be configured with the list of valid on-link prefixes that
mobile nodes can use to derive LCoAs. This is useful for network
operators that need to stop mobile nodes from continuing to use the
MAP after moving to a different administrative domain. If a mobile
node sent a binding update containing an LCoA that is not in the
MAP's "valid on-link prefixes" list, the MAP could reject the binding
update using existing error code 129 (administratively prohibited).
6.3. Home Agent Operations
The support of HMIPv6 is completely transparent to the HA's
operation. Packets addressed to a mobile node's home address will be
forwarded by the HA to its RCoA, as described in [RFC3775].
6.4. Correspondent Node Operations
HMIPv6 is completely transparent to correspondent nodes.
6.5. Local Mobility Management Optimisation within a MAP Domain
In [RFC3775], it is stated that for short-term communication,
particularly communication that may easily be retried upon failure,
the mobile node MAY choose to directly use one of its care-of
addresses as the source of the packet, thus not requiring the use of
a Home Address option in the packet. Such use of the CoA will reduce
the overhead of sending each packet due to the absence of additional
options. In addition, it will provide an optimal route between the
mobile node and correspondent node.
HMIPv6-aware mobile nodes can use their RCoA as the source address
without using a Home Address option. In other words, the RCoA can be
used as a source address for upper layers. Using this feature, the
mobile node will be seen by the correspondent node as a fixed node
while moving within a MAP domain.
This usage of the RCoA does not have the cost of Mobile IPv6 (i.e.,
no bindings or Home Address options are sent over the Internet), but
still provides local mobility management to the mobile nodes with
near-optimal routing. Although such use of RCoA does not provide
global mobility (i.e., communication is broken when a mobile node
changes its RCoA), it would be useful for several applications (e.g.,
web browsing). The validity of the RCoA as a source address used by
applications will depend on the size of a MAP domain and the speed of
the mobile node. Furthermore, because the support for BU processing
in correspondent nodes is not mandated in [RFC3775], this mechanism
can provide a way of obtaining route optimisation without sending BUs
to the correspondent nodes.
Enabling this mechanism can be done by presenting the RCoA as a
temporary home address for the mobile node. This may require an
implementation to augment its source address selection algorithm with
the knowledge of the RCoA in order to use it for the appropriate
6.6. Location Privacy
In HMIPv6, a mobile node hides its LCoA from its correspondent nodes
and its home agent by using its RCoA in the source field of the
packets that it sends. As a result, address-based location tracking
of a mobile node by its correspondent nodes or its home agent is more
difficult because they only know its RCoA and not its LCoA.
7. MAP Discovery
This section describes how a mobile node obtains the MAP address and
subnet prefix, and how ARs in a domain discover MAPs.
This specification requires network administrators to manually
configure the MAP option information in ARs; future mechanisms may be
defined to allow MAPs to be discovered dynamically.
7.1. Mobile Node Operation
When an HMIPv6-aware mobile node receives a Router Advertisement, it
should search for the MAP option. One or more options may be found
for different MAP IP addresses. A mobile node SHOULD register with
the MAP having the highest preference value. A MAP with a preference
value of zero SHOULD NOT be used for new local BUs (i.e., the mobile
node can refresh existing bindings but cannot create new ones).
However, a mobile node MAY choose to register with one MAP over
another, depending on the value received in the distance field,
provided that the preference value is above zero.
A MAP option containing a valid lifetime value of zero means that
this MAP MUST NOT be selected by the MN. A valid lifetime of zero
indicates a MAP failure. When this option is received, a mobile node
MUST choose another MAP and create new bindings. Any existing
bindings with this MAP can be assumed to be lost. If no other MAP is
available, the mobile node MUST NOT attempt to use HMIPv6.
If a multi-homed mobile node has access to several ARs simultaneously
(on different interfaces), it SHOULD use an LCoA on the link defined
by the AR that advertises its current MAP.
A mobile node MUST store the received option(s) in order to choose at
least one MAP to register with. Storing the options is essential, as
they will be compared to other options received later for the purpose
of the movement detection algorithm.
If the R flag is set, the mobile node MUST place its RCoA in place of
the home address in the binding update message. This causes the RCoA
to be bound to the LCoA in the MAP's binding cache.
A mobile node MAY choose to register with more than one MAP
simultaneously, or use both the RCoA and its LCoA as care-of
addresses simultaneously with different correspondent nodes.
8. Updating Previous MAPs
When a mobile node moves into a new MAP domain, the mobile node may
send a BU to the previous MAP requesting it to forward packets
addressed to the mobile node's new CoA. An administrator MAY
restrict the MAP from forwarding packets to LCoAs outside the MAP's
domain. However, it is RECOMMENDED that MAPs be allowed to forward
packets to LCoAs associated with some of the ARs in neighbouring MAP
domains, provided that they are located within the same
For instance, a MAP could be configured to forward packets to LCoAs
associated with ARs that are geographically adjacent to ARs on the
boundary of its domain. This will allow for a smooth inter-MAP
handover as it allows the mobile node to continue to receive packets
while updating the new MAP, its HA and, potentially, correspondent
9. Note on MAP Selection by the Mobile Node
HMIPv6 provides a flexible mechanism for local mobility management
within a visited network. As explained earlier, a MAP can exist
anywhere in the operator's network (including the AR). Several MAPs
can be located within the same domain independently of each other.
In addition, overlapping MAP domains are also allowed and
recommended. Both static and dynamic hierarchies are supported.
When the mobile node receives a Router Advertisement including a MAP
option, it should perform actions according to the following movement
detection mechanisms. In a hierarchical Mobile IP network, such as
the one described in this document, the mobile node should be:
o "Eager" to perform new bindings.
o "Lazy" in releasing existing bindings.
The above means that the mobile node should register with any "new"
MAP advertised by the AR (Eager). The method by which the mobile
node determines whether the MAP is a "new" MAP is described in
Section 9.1. The mobile node should not release existing bindings
until it no longer receives the MAP option (or receives it with a
lifetime of zero) or the lifetime of its existing binding expires
(Lazy). This Eager-Lazy approach, described above, will assist in
providing a fallback mechanism in case of the failure of one of the
MAP routers, as it will reduce the time it takes for a mobile node to
inform its correspondent nodes and HA about its new care-of address.
9.1. MAP Selection in Distributed MAP Environment
The mobile node needs to consider several factors to optimally select
one or more MAPs, where several MAPs are available in the same
There are no benefits foreseen in selecting more than one MAP and
forcing packets to be sent from the higher MAP down through a
hierarchy of MAPs. This approach may add forwarding delays and
eliminate the robustness of IP routing between the highest MAP and
the mobile node; therefore, it is prohibited by this specification.
Allowing more than one MAP ("above" the AR) within a network should
not imply that the mobile node forces packets to be routed down the
hierarchy of MAPs. However, placing more than one MAP "above" the AR
can be used for redundancy and as an optimisation for the different
mobility scenarios experienced by mobile nodes. The MAPs are used
independently of each other by the MN (e.g., each MAP is used for
communication to a certain set of CNs).
In terms of the distance-based selection in a network with several
MAPs, a mobile node may choose to register with the furthest MAP to
avoid frequent re-registrations. This is particularly important for
fast mobile nodes that will perform frequent handoffs. In this
scenario, the choice of a more distant MAP would reduce the
probability of having to change a MAP and informing all correspondent
nodes and the HA.
In a scenario where several MAPs are discovered by the mobile node in
one domain, the mobile node may need sophisticated algorithms to be
able to select the appropriate MAP. These algorithms would have the
mobile node speed as an input (for distance-based selection) combined
with the preference field in the MAP option. However, this
specification proposes that the mobile node use the following
algorithm as a default, where other optimised algorithms are not
available. The following algorithm is simply based on selecting the
MAP that is most distant, provided that its preference value did not
reach a value of zero. The mobile node operation is shown below:
1. Receive and parse all MAP options.
2. Arrange MAPs in a descending order, starting with the furthest
MAP (i.e., MAP option having largest Dist field).
3. Select first MAP in list.
4. If either the preference value or the valid lifetime fields are
set to zero, select the following MAP in the list.
5. Repeat step (4) while new MAP options still exist, until a MAP is
found with a non-zero preference value and a non-zero valid
Implementing the steps above would result in mobile nodes selecting,
by default, the most distant or furthest available MAP. This will
continue until the preference value reduces to zero. Following this,
mobile nodes will start selecting another MAP.
9.2. MAP Selection in a Flat Mobility Architecture
Network operators may choose a flat architecture in some cases where
a Mobile IPv6 handover may be considered a rare event. In these
scenarios, operators may choose to include the MAP function in ARs
only. The inclusion of the MAP function in ARs can still be useful
to reduce the time required to update all correspondent nodes and the
HA. In this scenario, a mobile node may choose a MAP (in the AR) as
an anchor point when performing a handoff. This kind of dynamic
hierarchy (or anchoring) is only recommended for cases where inter-AR
movement is not frequent.
10. Detection and Recovery from MAP Failures
This specification introduces a MAP that can be seen as a local home
agent in a visited network. A MAP, like a home agent, is a single
point of failure. If a MAP fails, its binding cache content will be
lost, resulting in loss of communication between mobile and
correspondent nodes. This situation may be avoided by using more
than one MAP on the same link and by utilising a form of context
transfer protocol between them. However, MAP redundancy is outside
the scope of this document.
In cases where such protocols are not supported, the mobile node
would need to detect MAP failures. The mobile node can detect this
situation when it receives a Router Advertisement containing a MAP
option with a lifetime of zero. The mobile node should then start
the MAP Discovery process and attempt to register with another MAP.
After it has selected and registered with another MAP, it will also
need to inform correspondent nodes and the home agent if its RCoA has
changed. Note that in the presence of a protocol that transfers
binding cache entries between MAPs for redundancy purposes, a new MAP
may be able to provide the same RCoA to the mobile node (e.g., if
both MAPs advertise the same prefix in the MAP option). This would
save the mobile node from updating correspondent nodes and the home
Access Routers can be triggered to advertise a MAP option with a
lifetime of zero (indicating MAP failure) in different ways:
o By manual intervention.
o In a dynamic manner.
One way of performing dynamic detection of MAP failure can be done by
probing the MAP regularly (e.g., every 10 seconds). If no response
is received, an AR MAY try to aggressively probe the MAP for a short
period of time (e.g., once every 5 seconds for 15 seconds); if no
reply is received, a MAP option may be sent with a valid lifetime
value of zero. The exact mechanisms for probing MAPs is outside the
scope of this document. The above text simply shows one example of
This specification does not mandate a particular recovery mechanism.
However, any mechanism between the MAP and an AR SHOULD be secure to
allow for message authentication, integrity protection, and
protection against replay attacks.
Note that the above suggestion for detecting MAP failure may not
detect MAP failures that might take place between probes, i.e.,if a
MAP reboots between probes.
11. Tunelling Impacts on MTU
This specification requires the mobile node to tunnel outgoing
traffic to the MAP. Similarly, the MAP tunnels inbound packets to
the mobile node. If the mobile node has a home agent elsewhere on
the Internet, this will result in double encapsulations of inbound
and outbound packets. This may have impacts on the mobile node's
path MTU. Hence, mobile nodes MUST consider the encapsulation of
traffic between the node and the MAP when calculating the available
MTU for upper layers.
12. Security Considerations
This specification introduces a new concept to Mobile IPv6, namely, a
Mobility Anchor Point that acts as a local home agent. It is crucial
that the security relationship between the mobile node and the MAP is
strong; it MUST involve mutual authentication, integrity protection,
and protection against replay attacks. Confidentiality may be needed
for payload traffic, such as when the mobile node is unwilling to
reveal any traffic to the access network beyond what is needed for
the mobile node to attach to the network and communicate with a MAP.
Confidentiality is not required for binding updates to the MAP. The
absence of any of these protections may lead to malicious mobile
nodes impersonating other legitimate ones or impersonating a MAP.
Any of these attacks will undoubtedly cause undesirable impacts to
the mobile node's communication with all correspondent nodes having
knowledge of the mobile node's RCoA.
Three different relationships (related to securing binding updates)
need to be considered:
1. The mobile node - MAP
2. The mobile node - correspondent node
3. The mobile node - home agent
12.1. Mobile Node - MAP Security
In order to allow a mobile node to use the MAP's forwarding service,
initial authorisation (specifically for the service, not for the
RCoA) MAY be needed. Authorising a mobile node to use the MAP
service can be done based on the identity of the mobile node
exchanged during the security association (SA) negotiation process.
The authorisation may be granted based on the mobile node's identity
or based on the identity of a Certificate Authority (CA) that the MAP
trusts. For instance, if the mobile node presents a certificate
signed by a trusted entity (e.g., a CA that belongs to the same
administrative domain, or another trusted roaming partner), it would
be sufficient for the MAP to authorise the use of its service. Note
that this level of authorisation is independent of authorising the
use of a particular RCoA. Similarly, the mobile node trusts the MAP
if it presents a certificate signed by the same CA or by another CA
that the mobile node is configured to trust (e.g., a roaming
partner). It is likely that some deployments would be satisfied with
the use of self-signed certificates for either the mobile node or the
MAP or both. This guarantees that the mobile node and the MAP are
authenticated for address allocation and future binding updates
without the need for identity authentication. Hence, the use of
trusted third-party certificates is not required by this
It is important to note that in this specification, authentication
and authorisation are effectively the same thing. All the MAP needs
in order to allocate the mobile node an RCoA is to authenticate the
mobile node and verify that it belongs to a trusted group (based on
IKEv2 MUST be supported by the mobile node and the MAP. IKEv2 allows
the use of Extensible Authentication Protocol (EAP) as a mechanism to
bootstrap the security association between the communicating peers.
Hence, EAP can be used with IKEv2 to leverage the Authentication,
Authorization, and Accounting (AAA) infrastructure to bootstrap the
SA between the mobile node and the MAP. Such a mechanism is useful
in scenarios where an administrator wishes to avoid the configuration
and management of certificates on mobile nodes. A MAP MAY support
the use of EAP over IKEv2.
If EAP is used with IKEv2, the EAP method runs between the mobile
node and a AAA server. Following a successful authentication, the
resulting keying material can be used to bootstrap IKEv2 between the
MAP and the mobile node. The specification of which EAP methods
should be used or how keys are transported between the MAP and the
AAA server is outside the scope of this document.
HMIPv6 uses an additional registration between the mobile node and
its current MAP. As explained in this document, when a mobile node
moves into a new domain (i.e., served by a new MAP), it obtains an
RCoA and an LCoA and registers the binding between these two
addresses with the new MAP. The MAP then verifies the BU and creates
a binding cache entry with the RCoA and LCoA. Whenever the mobile
node gets a new LCoA, it needs to send a new BU that specifies the
binding between its RCoA and its new LCoA. This BU needs to be
authenticated; otherwise, any host could send a BU for the mobile
node's RCoA and hijack the mobile node's packets.
The MAP does not need to have prior knowledge of the identity of the
mobile node or its home address. As a result, the SA between the
mobile node and the MAP can be established using any key
establishment protocols such as IKEv2. A return routability test is
The MAP needs to set the SA for the RCoA (not the LCoA). This can be
performed with IKEv2 [RFC4306]. The mobile node uses its LCoA as the
source address, but specifies that the RCoA should be used in the SA.
This is achieved by using the RCoA as the identity in the IKE
CHILD_SA negotiation. This step is identical to the use of the home
address in IKE CHILD_SA when negotiating with the home agent.
The IPsec Peer Authorization Database (PAD) entries and configuration
payloads described in [RFC4877] for allocating dynamic home addresses
SHOULD be used by the MAP to allocate the RCoA for mobile nodes.
Binding updates between the MAP and the mobile node MUST be protected
with either Authentication Header (AH) or Encapsulating Security
Payload (ESP) in transport mode. When ESP is used, a non-null
authentication algorithm MUST be used.
The Security Policy Database (SPD) entries in both the home agent and
the mobile node are identical to those set up for the home agent and
mobile node, respectively, as outlined in [RFC4877].
12.2. Mobile Node - Correspondent Node Security
Mobile IPv6 [RFC3775] defines a return routability procedure that
allows mobile and correspondent nodes to authenticate binding updates
and acknowledgements. This specification does not impact the return
routability test defined in [RFC3775]. However, it is important to
note that mobile node implementers need to be careful when selecting
the source address of the HoTI and CoTI messages, defined in
[RFC3775]. The source address used in HoTI messages SHOULD be the
mobile node's home address unless the mobile node wishes to use the
RCoA for route optimisation. The packet containing the HoTI message
is encapsulated twice. The inner encapsulating header contains the
RCoA in the source address field and the home agent's address in the
destination address field. The outer encapsulating header contains
the mobile node's LCoA in the source address field and the MAP's
address in the destination field.
12.3. Mobile Node - Home Agent Security
The security relationship between the mobile node and its home agent,
as discussed in [RFC3775], is not impacted by this specification.
The relationship between the MAP and the mobile node is not impacted
by the presence of a home agent.
13. IANA Considerations
Both the MAP option and M flag were allocated for RFC 4140 and will
continue to be used by this specification.
The authors would like to thank Conny Larsson (Ericsson) and Mattias
Pettersson (Ericsson) for their valuable input to this document. The
authors would also like to thank the members of the French RNRT
MobiSecV6 project (BULL, France Telecom, and INRIA) for testing the
first implementation and for their valuable feedback. The INRIA
HMIPv6 project is partially funded by the French government.
In addition, the authors would like to thank the following members of
the working group, in alphabetical order: Samita Chakrabarti (Sun),
Gregory Daley, Gopal Dommety (Cisco), Francis Dupont (GET/Enst
Bretagne), Eva Gustaffson (Ericsson), Dave Johnson (Rice University),
Annika Jonsson (Ericsson), James Kempf (Docomo labs), Martti
Kuparinen (Ericsson), Fergal Ladley, Gabriel Montenegro (Microsoft),
Nick "Sharkey" Moore, Vidya Narayanan (Qualcomm), Erik Nordmark
(Sun), Basavaraj Patil (Nokia), Brett Pentland (NEC), Thomas Schmidt,
and Alper Yegin (Samsung) for their comments on the document.
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, December 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
[RFC4877] Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with
IKEv2 and the Revised IPsec Architecture", RFC 4877, April
15.2. Informative References
[RFC4449] Perkins, C., "Securing Mobile IPv6 Route Optimization
Using a Static Shared Key", RFC 4449, June 2006.
[RFC4651] Vogt, C. and J. Arkko, "A Taxonomy and Analysis of
Enhancements to Mobile IPv6 Route Optimization", RFC 4651,
[RFC4866] Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route
Optimization for Mobile IPv6", RFC 4866, May 2007.
Appendix A. Changes from RFC 4140
o Dynamic MAP Discovery was removed.
o Updated the security section to use IKEv2 instead of IKEv1.
o The document clarified that HMIPv6 can be used without the need
for a home agent.
o Several editorials throughout the document.
o IKEv2 only is now used to allocate the RCoA.
RFC 4140 was implemented and interop tested by at least two different
organisations. A test suite including test cases for RFC 4140 was
also developed by Ericsson and run against both implementations. No
major issues were found. The scalability of Dynamic MAP Discovery,
defined in RFC 4140, was seen as inappropriate for large-scale
deployments and prone to loops. It was removed from this
At this time, there is no publicly known deployment of this
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