One document matched: draft-ietf-mext-rfc3775bis-05.xml
<?xml version="1.0"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
<!-- rfc ipr="pre5378Trust200902" docName='draft-ietf-mext-rfc3775bis-04.txt'-->
<rfc ipr="trust200811" docName='draft-ietf-mext-rfc3775bis-05.txt'
obsoletes="3775">
<?rfc symrefs="no"?>
<front>
<title>Mobility Support in IPv6</title>
<author initials="D." surname="Johnson" fullname="David B. Johnson">
<organization>Rice University</organization>
<address>
<postal>
<street>Dept. of Computer Science, MS 132</street>
<street>6100 Main Street</street>
<city>Houston</city> <code>TX 77005-1892</code>
<country>USA</country>
</postal>
<email>dbj@cs.rice.edu</email>
</address>
</author>
<author initials="C." surname="Perkins (Ed.)" fullname="Charles E. Perkins">
<organization>WiChorus Inc.</organization>
<address>
<postal>
<street>3590 N. 1st Street, Suite 300</street>
<city>San Jose</city> <code>CA 95134</code>
<country>USA</country>
</postal>
<email>charliep@computer.org</email>
</address>
</author>
<author initials="J." surname="Arkko" fullname="Jari Arkko">
<organization>Ericsson</organization>
<address>
<postal>
<city>Jorvas</city> <code>02420</code>
<country>Finland</country>
</postal>
<email>jari.arkko@ericsson.com</email>
</address>
</author>
<date month="October" year="2009" />
<area>Internet</area>
<workgroup>IETF Mobile IP Working Group</workgroup>
<keyword>Mobility</keyword>
<keyword>IPv6</keyword>
<abstract>
<t>This document specifies a protocol which allows nodes to remain
reachable while moving around in the IPv6 Internet.
Each mobile node is always identified by its home address,
regardless of its current point of attachment to the Internet.
While situated away from its home,
a mobile node is also associated with a care-of address,
which provides information about the mobile node's current location.
IPv6 packets addressed to a mobile node's home address
are transparently routed to its care-of address.
The protocol enables IPv6 nodes to cache the binding of a mobile
node's home address with its care-of address,
and to then send any packets destined for the mobile node
directly to it at this care-of address.
To support this operation,
Mobile IPv6 defines a new IPv6 protocol and a new
destination option. All IPv6 nodes, whether mobile or
stationary, can
communicate with mobile nodes.
</t>
</abstract>
</front>
<middle>
<section anchor='intro' title='Introduction'>
<t>This document specifies a protocol which allows nodes to remain
reachable while moving around in the IPv6 Internet.
Without specific support for mobility in <xref target='RFC2460'>IPv6</xref>,
packets destined to a mobile node would not be able to
reach it while the mobile node is away from its home link.
In order to continue communication in spite of its movement,
a mobile node could change its IP address
each time it moves to a new link,
but the mobile node would then not be able to maintain
transport and higher-layer connections when it changes location.
Mobility support in IPv6 is particularly important,
as mobile computers are likely to account for a majority or
at least a substantial fraction of
the population of the Internet during the lifetime of IPv6.
</t>
<t>The protocol defined in this document, known as Mobile IPv6,
allows a mobile node to move from one link to another
without changing the mobile node's "home address".
Packets may be routed to the mobile node using this address regardless of
the mobile node's current point of attachment to the Internet.
The mobile node may also continue to communicate with other nodes
(stationary or mobile) after moving to a new link.
The movement of a mobile node away from its home link
is thus transparent to transport and higher-layer protocols
and applications.
</t>
<t>The Mobile IPv6
protocol is just as suitable for mobility across homogeneous media
as for mobility across heterogeneous media.
For example, Mobile IPv6 facilitates node movement from one Ethernet
segment to another as well as it facilitates node movement from
an Ethernet segment to a wireless LAN cell,
with the mobile node's IP address remaining
unchanged in spite of such movement.
</t>
<t>One can think of the Mobile IPv6 protocol as solving the network-layer
mobility management problem.
Some mobility management applications -- for example,
handover among wireless transceivers,
each of which covers only a very small geographic area -- have
been solved using link-layer techniques.
For example, in many current wireless LAN products,
link-layer mobility mechanisms allow a "handover" of a mobile
node from one cell to another, re-establishing link-layer
connectivity to the node in each new location.
</t>
<t>Mobile IPv6 does not attempt to solve all general problems related
to the use of mobile computers or wireless networks.
In particular, this protocol does not attempt to solve:
</t>
<list style='symbols'>
<t> Handling links with unidirectional connectivity or
partial reachability, such as the hidden terminal
problem where a host is hidden from only
some of the routers on the link.
</t>
<t> Access control on a link being visited by a mobile node.
</t>
<t> Local or hierarchical forms of mobility management (similar
to many current link-layer mobility management solutions).
</t>
<t> Assistance for adaptive applications.
</t>
<t> Mobile routers.
</t>
<t> Service Discovery.
</t>
<t> Distinguishing between packets lost due to bit errors vs.
network congestion.
</t>
</list>
</section>
<section anchor='ipv4' title='Comparison with Mobile IP for IPv4'>
<t>The design of Mobile IP support in IPv6 (Mobile IPv6)
benefits both from
the experiences gained from the development of Mobile IP
support in IPv4 <xref target='RFC3344'>(Mobile IPv4)</xref>
<xref target='RFC2003' /> <xref target='RFC2004' />,
and from the opportunities provided by IPv6.
Mobile IPv6 thus shares many features with Mobile IPv4,
but is integrated into IPv6
and offers many other improvements.
This section summarizes the major differences
between Mobile IPv4 and Mobile IPv6:
</t>
<list style='symbols'>
<t> There is no need to deploy special routers as
"foreign agents", as in Mobile IPv4. Mobile IPv6
operates in any
location without any special support required from the local
router.
</t>
<t> Support for route optimization is a fundamental part
of the protocol, rather than a nonstandard set of extensions.
</t>
<t> Mobile IPv6 route optimization can operate securely even
without pre-arranged security associations. It is
expected that route optimization can be deployed on a global
scale between all mobile nodes and correspondent nodes.
</t>
<t> Support is also integrated into
Mobile IPv6 for allowing route optimization to coexist
efficiently with routers that perform "ingress
filtering" <xref target='RFC2827' />.
</t>
<t> The IPv6 Neighbor Unreachability Detection assures
symmetric reachability between the mobile node and
its default router in the current location.
</t>
<t> Most packets sent to a mobile node while away from home
in Mobile IPv6 are
sent using an IPv6 routing header rather than IP encapsulation,
reducing the amount of resulting overhead compared to Mobile IPv4.
</t>
<t> Mobile IPv6 is decoupled from any particular link layer,
as it uses IPv6 <xref target='RFC4861'>Neighbor Discovery</xref>
instead of ARP.
This also improves the robustness of the protocol.
</t>
<t> The use of IPv6 encapsulation (and the routing header)
removes the need in Mobile IPv6 to manage "tunnel soft state".
</t>
<t> The dynamic home agent address discovery mechanism in Mobile IPv6
returns a single reply to the mobile node. The directed broadcast
approach used in IPv4 returns separate replies from each home
agent.
</t>
</list>
</section>
<section anchor='terms' title='Terminology'>
<t>The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
<xref target='RFC2119'>RFC 2119</xref>.
</t>
<section anchor='terms:general' title='General Terms'>
<list style='hanging'>
<t hangText='IP'>
<vspace blankLines='1' />
Internet Protocol Version 6 (IPv6).
</t>
<t hangText='node'>
<vspace blankLines='1' />
A device that implements IP.
</t>
<t hangText='router'>
<vspace blankLines='1' />
A node that forwards IP packets not explicitly addressed to itself.
</t>
<t hangText='unicast routable address'>
<vspace blankLines='1' />
An identifier for a single interface such that a packet sent to it
from another IPv6 subnet is delivered to the interface identified
by that address. Accordingly, a unicast routable address must
either be global IPv6 address or a unique local IPv6 address.
</t>
<t hangText='host'>
<vspace blankLines='1' />
Any node that is not a router.
</t>
<t hangText='link'>
<vspace blankLines='1' />
A communication facility or medium over which nodes can
communicate at the link layer, such as an Ethernet (simple or bridged).
A link is the layer immediately below IP.
</t>
<t hangText='interface'>
<vspace blankLines='1' />
A node's attachment to a link.
</t>
<t hangText='subnet prefix'>
<vspace blankLines='1' />
A bit string that consists of some number of initial
bits of an IP address.
</t>
<t hangText='interface identifier'>
<vspace blankLines='1' />
A number used to identify a node's interface on a link.
The interface identifier is the
remaining low-order bits in the node's IP address
after the subnet prefix.
</t>
<t hangText='link-layer address'>
<vspace blankLines='1' />
A link-layer identifier for an interface,
such as IEEE 802 addresses on Ethernet links.
</t>
<t hangText='packet'>
<vspace blankLines='1' />
An IP header plus payload.
</t>
<t hangText='security association'>
<vspace blankLines='1' />
An IPsec security association is a cooperative relationship
formed by the sharing of cryptographic keying material and
associated context. Security associations are simplex.
That is, two security associations are needed to protect
bidirectional traffic between two nodes, one for each
direction.
</t>
<t hangText='security policy database'>
<vspace blankLines='1' />
A database that specifies what security services are to be
offered to IP packets and in what fashion.
</t>
<t hangText='destination option'>
<vspace blankLines='1' />
Destination options are carried by the IPv6 Destination Options
extension header.
Destination options
include optional information that need be examined only by the
IPv6 node given as the destination address in the IPv6 header,
not by routers in between.
Mobile IPv6 defines one new destination option,
the Home Address destination option (see <xref target='hao' />).
</t>
<t hangText='routing header'>
<vspace blankLines='1' />
A routing header may be present as an IPv6 header extension,
and indicates that the payload has to be delivered to a
destination IPv6 address in some way that is different
from what would be carried out by standard Internet routing.
In this document, use of the term "routing header"
typically refers to use of a type 2 routing header,
as specified in <xref target='rhtype2' />.
</t>
<t hangText='"|" (concatenation)'>
<vspace blankLines='1' />
Some formulas in this specification use the symbol "|" to
indicate bytewise concatenation, as in A | B.
This concatenation requires that all of the octets of
the datum A appear first in the result, followed
by all of the octets of the datum B.
</t>
<t hangText='First (size, input)'>
<vspace blankLines='1' />
Some formulas in this specification use a functional
form "First (size, input)" to indicate truncation of
the "input" data so that only the first "size" bits
remain to be used.
</t>
</list>
</section>
<section anchor='terms:mobileip' title='Mobile IPv6 Terms'>
<t>
These terms are intended to be compatible with the
definitions given in RFC 3753<xref target='RFC3753'/>.
However, if there is any conflict, the definitions given here
should be considered to supersede those in RFC 3753.
</t>
<list style='hanging'>
<t hangText='home address'>
<vspace blankLines='1' />
A unicast routable address assigned to a mobile node,
used as the permanent address of the mobile node.
This address is within the mobile node's home link.
Standard IP routing mechanisms will deliver packets destined for
a mobile node's home address to its home link.
Mobile nodes can have multiple home addresses, for instance when
there are multiple home prefixes on the home link.
</t>
<t hangText='home subnet prefix'>
<vspace blankLines='1' />
The IP subnet prefix corresponding to a mobile node's home address.
</t>
<t hangText='home link'>
<vspace blankLines='1' />
The link on which a mobile node's home subnet prefix is defined.
</t>
<t hangText='mobile node'>
<vspace blankLines='1' />
A node that can change its point of attachment from one link
to another, while still being reachable via its home address.
</t>
<t hangText='movement'>
<vspace blankLines='1' />
A change in a mobile node's point of attachment to the Internet
such that it is no longer connected to
the same link as it was previously.
If a mobile node is not currently attached to its home link,
the mobile node is said to be "away from home".
</t>
<t hangText='L2 handover'>
<vspace blankLines='1' />
A process by which the mobile node changes from one
link-layer connection to another. For example, a change of wireless
access point is a L2 handover.
</t>
<t hangText='L3 handover'>
<vspace blankLines='1' />
Subsequent to a L2 handover, a mobile node detects
a change in an on-link subnet prefix that would require a
change in the primary care-of address. For example, a change of access
router subsequent to a change of wireless access point typically
results in an L3 handover.
</t>
<t hangText='correspondent node'>
<vspace blankLines='1' />
A peer node with which a mobile node is communicating.
The correspondent node may be either mobile or stationary.
</t>
<t hangText='foreign subnet prefix'>
<vspace blankLines='1' />
Any IP subnet prefix other than the mobile node's home subnet prefix.
</t>
<t hangText='foreign link'>
<vspace blankLines='1' />
Any link other than the mobile node's home link.
</t>
<t hangText='care-of address'>
<vspace blankLines='1' />
A unicast routable address associated with a mobile node
while visiting a foreign link;
the subnet prefix of this IP address is a foreign subnet prefix.
Among the multiple care-of addresses that a mobile node may have at any
given time (e.g., with different subnet prefixes),
the one registered with the mobile node's home agent for a
given home address
is called its "primary" care-of address.
</t>
<t hangText='home agent'>
<vspace blankLines='1' />
A router on a mobile node's home link with which the mobile node
has registered its current care-of address.
While the mobile node is away from home,
the home agent intercepts packets on the home link
destined to the mobile node's home address, encapsulates them,
and tunnels them to the mobile node's registered care-of address.
</t>
<t hangText='binding'>
<vspace blankLines='1' />
The association of the home address of a mobile node
with a care-of address for that mobile node,
along with the remaining lifetime of that association.
</t>
<t hangText='registration'>
<vspace blankLines='1' />
The process during which a mobile node sends a Binding Update
to its home agent or a correspondent node, causing a binding
for the mobile node to be registered.
</t>
<t hangText='mobility message'>
<vspace blankLines='1' />
A message containing a Mobility Header (see
<xref target='mobility-header' />).
</t>
<t hangText='binding authorization'>
<vspace blankLines='1' />
Correspondent registration needs to be authorized to allow the recipient to
believe that the sender has the right to specify a new binding.
</t>
<t hangText='return routability procedure'>
<vspace blankLines='1' />
The return routability procedure authorizes registrations by the
use of a cryptographic token exchange.
</t>
<t hangText='correspondent registration'>
<vspace blankLines='1' />
A return routability procedure followed by a registration,
run between the mobile node and a correspondent node.
</t>
<t hangText='home registration'>
<vspace blankLines='1' />
A registration between the mobile node and its home agent,
authorized by the use of IPsec.
</t>
<t hangText='nonce'>
<vspace blankLines='1' />
Nonces are random numbers used internally by the correspondent
node in the creation of keygen tokens related to the return
routability procedure.
The nonces are not specific to a mobile node, and are
kept secret within the correspondent node.
</t>
<t hangText='nonce index'>
<vspace blankLines='1' />
A nonce index is used to indicate which
nonces have been used when creating keygen token values,
without revealing the nonces themselves.
</t>
<t hangText='cookie'>
<vspace blankLines='1' />
A cookie is a random number used by a mobile node
to prevent spoofing by a bogus correspondent node
in the return routability procedure.
</t>
<t hangText='care-of init cookie'>
<vspace blankLines='1' />
A cookie sent to the correspondent node
in the Care-of Test Init message, to be returned in
the Care-of Test message.
</t>
<t hangText='home init cookie'>
<vspace blankLines='1' />
A cookie sent to the correspondent node
in the Home Test Init message, to be returned in
the Home Test message.
</t>
<t hangText='keygen token'>
<vspace blankLines='1' />
A keygen token is a number supplied by a correspondent
node in the return routability procedure to enable the
mobile node to compute the necessary binding management key
for authorizing a Binding Update.
</t>
<t hangText='care-of keygen token'>
<vspace blankLines='1' />
A keygen token sent by the correspondent node
in the Care-of Test message.
</t>
<t hangText='home keygen token'>
<vspace blankLines='1' />
A keygen token sent by the correspondent node
in the Home Test message.
</t>
<t hangText='binding management key (Kbm)'>
<vspace blankLines='1' />
A binding management key (Kbm) is a key
used for authorizing a binding cache management message
(e.g., Binding Update or Binding Acknowledgement).
Return routability provides a way to create a binding management key.
</t>
</list>
</section>
</section>
<section anchor='overview' title='Overview of Mobile IPv6'>
<section anchor='overview:basic' title='Basic Operation'>
<t>A mobile node is always expected to be addressable at its home
address, whether it is currently attached to its home link or is away
from home. The "home address" is an IP address assigned to the mobile
node within its home subnet prefix on its home link. While a mobile
node is at home, packets addressed to its home address are routed to
the mobile node's home link, using conventional Internet routing
mechanisms.
</t>
<t>While a mobile node is attached to some foreign link away from home,
it is also addressable at one or more care-of addresses. A care-of
address is an IP address associated with a mobile node that has the
subnet prefix of a particular foreign link. The mobile node can
acquire its care-of address through conventional IPv6 mechanisms, such
as stateless or stateful auto-configuration. As long as the mobile node
stays in this location, packets addressed to this care-of address will
be routed to the mobile node. The mobile node may also accept packets
from several care-of addresses, such as when it is moving but still
reachable at the previous link.
</t>
<t>The association between a mobile node's home address and care-of
address is known as a "binding" for the mobile node. While away from
home, a mobile node registers its primary care-of address with a
router on its home link, requesting this router to function as the
"home agent" for the mobile node. The mobile node performs this
binding registration by sending a "Binding Update" message to the home
agent. The home agent replies to the mobile node by returning a
"Binding Acknowledgement" message. The operation of the mobile node
is specified in <xref target='mobile' />, and the operation of the
home agent is specified in <xref target='ha-consider' />.
</t>
<t>Any node communicating with a mobile node is referred to in this
document as a "correspondent node" of the mobile node, and may itself
be either a stationary node or a mobile node. Mobile nodes can
provide information about their current location to correspondent
nodes. This happens through the correspondent registration. As a
part of this procedure, a return routability test is performed in
order to authorize the establishment of the binding. The operation of
the correspondent node is specified in <xref target='corresp' />.
</t>
<t>There are two possible modes for communications between the mobile
node and a correspondent node. The first mode, bidirectional
tunneling, does not require Mobile IPv6 support from the correspondent
node and is available even if the mobile node has not registered its
current binding with the correspondent node. Packets from the
correspondent node are routed to the home agent and then tunneled to
the mobile node. Packets to the correspondent node are tunneled from
the mobile node to the home agent ("reverse tunneled") and then routed
normally from the home network to the correspondent node. In this
mode, the home agent uses proxy Neighbor Discovery to intercept any
IPv6 packets addressed to the mobile node's home address (or home
addresses) on the home link. Each intercepted packet is tunneled to
the mobile node's primary care-of address. This tunneling is performed
using <xref target='RFC2473'>IPv6 encapsulation</xref>.
</t>
<t>The second mode, "route optimization", requires the mobile node to
register its current binding at the correspondent node. Packets from
the correspondent node can be routed directly to the care-of address
of the mobile node. When sending a packet to any IPv6 destination, the
correspondent node checks its cached bindings for an entry for the
packet's destination address. If a cached binding for this
destination address is found, the node uses a new type of IPv6
<xref target='RFC2460'>routing header</xref> (see <xref target='rhtype2' />) to route the packet
to the mobile node by way of the care-of address indicated in this
binding.
</t>
<t>
Routing packets directly to the mobile node's care-of address allows
the shortest communications path to be used. It also
eliminates congestion at the mobile node's home agent and home
link. In addition, the impact of temporary failures of the home
agent or networks on the path to or from the home agent is reduced.
</t>
<t>When routing packets directly to the mobile node, the correspondent
node sets the Destination Address in the IPv6 header to the care-of
address of the mobile node. A new type of IPv6 routing header (see
<xref target='rhtype2' />) is also added to the packet to carry the
desired home address. Similarly, the mobile node sets the Source
Address in the packet's IPv6 header to its current care-of
addresses. The mobile node adds a new IPv6 "Home Address" destination
option (see <xref target='hao' />) to carry its home address. The
inclusion of home addresses in these packets makes the use of the
care-of address transparent above the network layer (e.g., at the
transport layer).
</t>
<t>Mobile IPv6 also provides support for multiple home agents, and a limited
support for the
reconfiguration of the home network. In these cases, the mobile node
may not know the IP address of its own home agent, and even the home
subnet prefixes may change over time. A mechanism, known as "dynamic
home agent address discovery" allows a mobile node to dynamically
discover the IP address of a home agent on its home link, even when
the mobile node is away from home. Mobile nodes can also learn new
information about home subnet prefixes through the "mobile prefix discovery"
mechanism. These mechanisms are described starting from
<xref target='ha-disc-req' />.
</t>
<t>
This document is written under the assumption that the mobile node
is configured with the home prefix for the mobile node to be able
to discover a home agent and configure a home address.
This might be limiting in deployments where
the home agent and the home address for the mobile node needs to be
assigned dynamically. Additional mechanisms have been specified for
the mobile node to dynamically configure a home agent, a home address
and the home prefix. These mechanisms are described in
<xref target='RFC5026'>"Mobile IPv6 Bootstrapping in Split Scenario"</xref>
and <xref target='I-D.ietf-mip6-bootstrapping-integrated-dhc'>
"MIP6 bootstrapping for the Integrated Scenario" </xref>.
<!-- This is a comment... -->
</t>
</section>
<section anchor='overview:protocols' title='New IPv6 Protocol'>
<t>Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header
(see <xref target='mobility-header' />). This
Header is used to carry the following messages:
</t>
<list style='hanging'>
<t hangText='Home Test Init'>
</t>
<t hangText='Home Test'>
</t>
<t hangText='Care-of Test Init'>
</t>
<t hangText='Care-of Test'>
<vspace blankLines='1' />
These four messages are used to perform the return
routability procedure from the mobile node to a correspondent
node. This ensures authorization of subsequent Binding Updates,
as described in <xref target='rrp' />.
</t>
<t hangText='Binding Update'> <vspace blankLines='1' />
A Binding Update is used by a mobile node to notify a
correspondent node or the mobile node's home agent of its
current binding. The Binding Update sent to the mobile node's
home agent to register its primary care-of address is marked
as a "home registration".
</t>
<t hangText='Binding Acknowledgement'> <vspace blankLines='1' />
A Binding Acknowledgement is used to acknowledge
receipt of a Binding Update, if an acknowledgement was
requested in the Binding Update (e.g., the binding update
was sent to a home agent), or an error occurred.
</t>
<t hangText='Binding Refresh Request'> <vspace blankLines='1' />
A Binding Refresh Request is used by a correspondent node to
request that a mobile node re-establish its binding with the
correspondent node.
This message is typically used
when the cached binding is in active use but the
binding's lifetime is close to expiration. The correspondent
node may use, for instance, recent traffic and open transport
layer connections as an indication of active use.
</t>
<t hangText='Binding Error'>
<vspace blankLines='1' />
The Binding Error is used by the correspondent node to signal an
error related to mobility, such as an inappropriate attempt to use
the Home Address destination option without an existing binding.
The Binding Error message is also used by the Home Agent to signal
an error to the mobile node, if it receives an unrecognized
Mobility Header Message Type from the mobile node.
</t>
</list>
</section>
<section anchor='overview:options' title='New IPv6 Destination Option'>
<t>Mobile IPv6 defines a new IPv6 destination option, the Home Address
destination option. This option is described in detail
in <xref target='hao' />.
</t>
</section>
<section anchor='overview:icmp' title='New IPv6 ICMP Messages'>
<t>Mobile IPv6 also introduces four new ICMP message types,
two for use in the dynamic home agent address discovery mechanism,
and two for renumbering and mobile configuration mechanisms.
As described in <xref target='ha-disc' /> and <xref target='mn-ha-disc' />,
the following two new ICMP message types are used for home agent address
discovery:
</t>
<list style='symbols'>
<t> Home Agent Address Discovery Request,
described in <xref target='ha-disc-req' />.
</t>
<t> Home Agent Address Discovery Reply,
described in <xref target='ha-disc-reply' />.
</t>
</list>
<t>The next two message types are used for network renumbering and
address configuration on the mobile node, as described in
<xref target='ha:renum' />:
</t>
<list style='symbols'>
<t> Mobile Prefix Solicitation, described in <xref target='renum-mps' />.
</t>
<t> Mobile Prefix Advertisement,
described in <xref target='renum-mpa' />.
</t>
</list>
</section>
<section anchor='overview:data' title='Conceptual Data Structure Terminology'>
<t>This document describes the Mobile IPv6 protocol in terms of
the following conceptual data structures:
</t>
<list style='hanging'>
<t hangText='Binding Cache'>
<vspace blankLines='1' />A cache of bindings for other nodes.
This cache is maintained by home agents and correspondent nodes.
The cache contains both "correspondent registration"
entries (see <xref target='cn:data' />) and "home registration" entries
(see <xref target='ha:data' />).
</t>
<t hangText='Binding Update List'>
<vspace blankLines='1' />
<vspace blankLines='1' />This list is maintained by each mobile node. The list has an item for
every binding that the mobile node has or is trying to establish with
a specific other node. Both correspondent and home registrations are
included in this list. Entries from the list are deleted as the
lifetime of the binding expires. See
<xref target='mn:data' />.
</t>
<t hangText='Home Agents List'>
<vspace blankLines='1' />
<vspace blankLines='1' />
Home agents need to know which other home agents are on the same link.
This information is stored in the Home Agents List, as described in
more detail in <xref target='ha:data' />. The list is used for
informing mobile nodes during dynamic home agent address discovery.
</t>
</list>
</section>
<section anchor='addressability' title='Unique-Local Addressability'>
<t>
This specification requires that home and care-of addresses MUST be
unicast routable addresses. Unique-local IPv6 unicast
addresses (ULAs) <xref target='RFC4193'>RFC4193</xref>
may be usable on networks that use such non-globally routable addresses
but this specification does not define when such usage is safe and when it
is not. Mobile nodes may not be able to distinguish between their home
site and the site at which they are currently located. This can make it
hard to prevent accidental attachment to other sites, because the mobile node
might use the ULA at another site, which could not be used to successfully
send packets to the mobile node's HA. This would result
in unreachability between the MN and the HA, when unique-local IPv6
routable addresses are used as care-of addresses.
Similarly, CNs outside the MN's own site will not be reachable when
ULAs are used as home addresses. Therefore, unique-local IPv6 unicast
addresses SHOULD NOT be used as home or care-of addresses when other
address choices are available. If such
addresses are used, however, according to
<xref target='RFC4193'>RFC4193</xref>, they are treated as any global
unicast IPv6 address so, for the remainder of this specification, use
of unique-local IPv6 unicast
addresses is not differentiated from other globally unique IPv6 addresses.
</t>
</section>
</section>
<section anchor='overview:security' title='Overview of Mobile IPv6 Security'>
<t>This specification provides a number of security features. These
include the protection of Binding Updates both to home agents and
correspondent nodes, the protection of mobile prefix discovery, and
the protection of the mechanisms that Mobile IPv6 uses for transporting
data packets.
</t>
<t>Binding Updates are protected by the use of IPsec extension headers,
or by the use of the Binding Authorization Data option. This option
employs a binding management key, Kbm, which can be established
through the return routability procedure. Mobile prefix discovery is
protected through the use of IPsec extension headers. Mechanisms
related to transporting payload packets - such as the Home Address
destination option and type 2 routing header - have been specified in
a manner which restricts their use in attacks.
</t>
<section anchor='securing-regs-with-home-agents' title='Binding Updates to Home Agents'>
<t>The mobile node and the home agent MUST use an IPsec security
association to protect the integrity and authenticity of the Binding
Updates and Acknowledgements. Both the mobile nodes and the home
agents MUST support and SHOULD use the Encapsulating Security Payload <xref target='RFC2406'>(ESP)</xref>
header in transport mode and MUST use a non-NULL payload
authentication algorithm to provide data origin authentication,
connectionless integrity and optional anti-replay protection. Note
that Authentication Header <xref target='RFC2402'>(AH)</xref> is also possible but
for brevity not discussed in this specification.
</t>
<t>In order to protect messages exchanged between the mobile node and the
home agent with IPsec, appropriate security policy database entries
must be created. A mobile node must be prevented from using its
security association to send a Binding Update on behalf of another
mobile node using the same home agent. This MUST be achieved by
having the home agent check that the given home address has been used
with the right security association. Such a check is provided in the
IPsec processing, by having the security policy database entries
unequivocally identify a single security association for protecting Binding
Updates between any given
home address and home agent. In order to make this possible, it is
necessary that the home address of the mobile node is visible in the
Binding Updates and Acknowledgements. The home address is used in
these packets as a source or destination, or in the Home Address
Destination option or the type 2 routing header.
</t>
<t>As with all IPsec security associations in this specification, manual
configuration of security associations MUST be supported. The used
shared secrets MUST be random and unique for different mobile nodes,
and MUST be distributed off-line to the mobile nodes.
</t>
<t>
Automatic key management with <xref target='RFC2409'>IKE</xref>
or <xref target='RFC4306'>IKEv2</xref>
MAY be supported. When IKE is used, either the security policy
database entries or the Mobile IPv6 processing MUST unequivocally
identify the IKE phase 1 credentials which can be used to
authorize the creation of security associations for protecting
Binding Updates for a particular home address. How these mappings
are maintained is outside the scope of this specification, but
they may be maintained, for instance, as a locally administered
table in the home agent. If the phase 1 identity is a Fully
Qualified Domain Name (FQDN), secure forms of DNS may also be used.
</t>
<t><xref target='outbound-ipsec' /> discusses how IKE connections to
the home agent need a careful treatment of the addresses used for
transporting IKE. This is necessary to ensure that a Binding Update is
not needed before the IKE exchange which is needed for securing the
Binding Update.
</t>
<t>When IKE version 1 is used with preshared secret authentication
between the mobile node and the home agent, aggressive mode MUST be
used.</t>
<t>The ID_IPV6_ADDR Identity Payload MUST NOT be
used in IKEv1 phase 1.
</t>
<t>
<xref target='RFC3776'>References</xref>
<xref target='RFC4877'>and</xref>
contain more detailed descriptions and examples on using IPsec
to protect the communications between the mobile node and the
home agent.
</t>
</section>
<section anchor='securing-regs-with-cns' title='Binding Updates to Correspondent Nodes'>
<t>The protection of Binding Updates sent to correspondent nodes does not
require the configuration of security associations or the existence of
an authentication infrastructure between the mobile nodes and
correspondent nodes. Instead, a method called the return routability
procedure is used to assure that the right mobile node is sending the
message. This method does not protect against attackers who are on the
path between the home network and the correspondent node. However,
attackers in such a location are capable of performing the same attacks
even without Mobile IPv6. The main advantage of the return routability
procedure is that it limits the potential attackers to those having an
access to one specific path in the Internet, and avoids forged Binding
Updates from anywhere else in the Internet. For a more in depth
explanation of the security properties of the return routability
procedure, see <xref target='security'/>.
Also, consult <xref target='RFC4225'/>
</t>
<t>The integrity and authenticity of the Binding Updates messages to
correspondent nodes is protected by using a keyed-hash algorithm. The
binding management key, Kbm, is used to key the hash algorithm for
this purpose. Kbm is established using data exchanged during the
return routability procedure. The data exchange is accomplished by use
of node keys, nonces, cookies, tokens, and certain cryptographic
functions. <xref target='rrp' /> outlines the basic return routability
procedure. <xref target='rr-cnbind' /> shows how the results of this
procedure are used to authorize a Binding Update to a correspondent
node.
</t>
<section anchor='nodekeys' title='Node Keys'>
<t>Each correspondent node has a secret key, Kcn, called the "node key",
which it uses to produce the keygen tokens sent to the mobile nodes.
The node key MUST be a random number, 20 octets in length.
The node key allows the correspondent
node to verify that the keygen tokens used by the
mobile node in authorizing a Binding Update are indeed its own.
This key MUST NOT be shared with any other entity.
</t>
<t>A correspondent node MAY generate a fresh node key at any time;
this avoids the need for secure persistent key storage.
Procedures for optionally updating
the node key are discussed later in <xref target='tokenupdate' />.
</t>
</section>
<section anchor='nonces' title='Nonces'>
<t>Each correspondent node also generates nonces at regular intervals.
The nonces should be generated by using a random number generator that
is known to have good <xref target='RFC4086'>randomness properties</xref>.
A correspondent node may use the same Kcn and nonce
with all the mobiles it is in communication with.
</t>
<t>Each nonce is identified by a nonce index.
When a new nonce is generated, it must be associated with
a new nonce index; this may be done, for example, by incrementing
the value of the previous nonce index, if the nonce index
is used as an array pointer into a linear array of nonces.
However, there is no requirement that nonces be stored that
way, or that the values of subsequent nonce indices have any
particular relationship to each other.
The index value is communicated in the protocol, so that if a
nonce is replaced by new nonce during the run of a protocol, the
correspondent node can distinguish messages that should be checked
against the old nonce from messages that should be checked against the
new nonce. Strictly speaking, indices are not necessary in the
authentication, but allow the correspondent node to efficiently find
the nonce value that it used in creating a keygen token.
</t>
<t>Correspondent nodes keep both the current nonce and a small
set of valid previous nonces whose lifetime has not yet expired.
Expired values MUST be discarded, and messages using
stale or unknown indices will be rejected.
</t>
<t>The specific nonce index values cannot be used by mobile nodes to
determine the validity of the nonce. Expected validity times for the
nonces values and the procedures for updating them are discussed later
in <xref target='tokenupdate' />.
</t>
<t>A nonce is an octet string of any length. The recommended length is 64 bits.
</t>
</section>
<section title='Cookies and Tokens'>
<t>The return routability address test procedure uses cookies and
keygen tokens as opaque values within the test init and
test messages, respectively.
</t>
<list style='symbols'>
<t> The "home init cookie" and "care-of init cookie" are 64 bit
values sent to the correspondent node from the mobile
node, and later returned to the mobile node.
The home init cookie is sent in the Home Test
Init message, and returned in the Home Test message.
The care-of init cookie is sent in the Care-of Test
Init message, and returned in the Care-of Test message.
</t>
<t> The "home keygen token" and "care-of keygen token" are 64-bit
values sent by the correspondent node to the mobile node via
the home agent (via the Home Test message) and the care-of
address (by the Care-of Test message), respectively.
</t>
</list>
<t>The mobile node should set the home init or care-of init cookie to
a newly generated random number in every Home or Care-of Test Init
message it sends.
The cookies are used to verify that
the Home Test or Care-of Test message matches the Home Test
Init or Care-of Test Init message, respectively.
These cookies also serve to ensure that parties
who have not seen the request cannot spoof responses.
</t>
<t>Home and care-of keygen tokens are produced by the correspondent
node based on its currently active secret key (Kcn) and
nonces, as well as the home or care-of address (respectively).
A keygen token is valid as long as both the secret key
(Kcn) and the nonce used to create it are valid.
</t>
</section>
<section title='Cryptographic Functions'>
<t>In this specification, the
function used to compute hash values is <xref target='FIPS.180-1.1995'>SHA1</xref>.
Message Authentication Codes (MACs) are computed using
<xref target='RFC2104'>HMAC_SHA1</xref> <xref target='FIPS.180-1.1995' />.
HMAC_SHA1(K,m) denotes such a MAC computed on message m
with key K.
</t>
</section>
<section anchor='rrp' title='Return Routability Procedure'>
<t>The Return Routability Procedure enables the correspondent node to
obtain some reasonable assurance that the mobile node is in fact
addressable at its claimed care-of address as well as at
its home address. Only with this assurance is the correspondent
node able to accept Binding Updates from the mobile node which
would then instruct the correspondent node to direct that mobile
node's data traffic to its claimed care-of address.
</t>
<t>This is done by testing whether packets addressed to the
two claimed addresses are routed to the mobile node.
The mobile node can pass the test only if it is able to
supply proof that it received certain data (the "keygen tokens")
which the correspondent node sends to those addresses.
These data are combined by the
mobile node into a binding management key, denoted Kbm.
</t>
<t>The figure below shows the message flow for
the return routability procedure.
</t>
<figure><artwork>
Mobile node Home agent Correspondent node
| |
| Home Test Init (HoTI) | |
|------------------------->|------------------------->|
| | |
| Care-of Test Init (CoTI) |
|---------------------------------------------------->|
| |
| | Home Test (HoT) |
|<-------------------------|<-------------------------|
| | |
| Care-of Test (CoT) |
|<----------------------------------------------------|
| |
</artwork></figure>
<t>The Home and Care-of Test Init messages are sent at the same time.
The procedure requires very little processing at the correspondent
node, and the Home and Care-of Test messages can be returned quickly,
perhaps nearly simultaneously.
These four messages form the return routability procedure.
</t>
<list style='hanging'>
<t hangText='Home Test Init'>
<vspace blankLines='1' />
<vspace blankLines='1' /> A mobile node sends a Home Test Init message to the
correspondent node (via the home agent) to acquire the home keygen token.
The contents of the message can be summarized as follows:
<list style='symbols'>
<t hangText=''>
Source Address = home address
</t>
<t hangText=''>
Destination Address = correspondent
</t>
<t hangText=''>
Parameters:
<list style='symbols'>
<t hangText='-'>
home init cookie
</t>
</list>
</t>
</list>
<vspace blankLines='1' />The Home Test Init message conveys the mobile node's home address to the
correspondent node.
The mobile node also sends along a home init cookie that the
correspondent node must return later.
The Home Test Init message is reverse tunneled through the home agent.
(The headers and addresses related to reverse tunneling have
been omitted from the above discussion of the message contents.)
The mobile node remembers these cookie values to obtain some
assurance that its protocol messages are being processed by the
desired correspondent node.
</t>
<t hangText='Care-of Test Init'>
<vspace blankLines='1' />
<vspace blankLines='1' /> The mobile node sends a Care-of Test Init message to the
correspondent node (directly, not via the home agent) to acquire the care-of keygen token.
The contents of this message can be summarized as follows:
<list style='symbols'>
<t hangText=''>
Source Address = care-of address
</t>
<t hangText=''>
Destination Address = correspondent
</t>
<t hangText=''>
Parameters:
<list style='symbols'>
<t hangText='-'>
care-of init cookie
</t>
</list>
</t>
</list>
<vspace blankLines='1' />The Care-of Test Init message conveys
the mobile node's care-of address to the correspondent node.
The mobile node also sends along a care-of init cookie that the
correspondent node must return later.
The Care-of Test Init message is sent directly to the correspondent node.
</t>
<t hangText='Home Test'>
<vspace blankLines='1' />
<vspace blankLines='1' />The Home Test message is sent in response to a Home Test Init message.
It is sent via the home agent.
The contents of the message are:
<list style='symbols'>
<t hangText=''>
Source Address = correspondent
</t>
<t hangText=''>
Destination Address = home address
</t>
<t hangText=''>
Parameters:
<list style='symbols'>
<t hangText='-'>
home init cookie
</t>
<t hangText='-'>
home keygen token
</t>
<t hangText='-'>
home nonce index
</t>
</list>
</t>
</list>
<vspace blankLines='1' />When the correspondent node receives the Home Test Init message, it
generates a home keygen token as follows:
<figure><artwork>
home keygen token :=
First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0)))
</artwork></figure>
<vspace blankLines='1' />where | denotes concatenation.
The final "0" inside the HMAC_SHA1 function is a single
zero octet, used to
distinguish home and care-of cookies from each other.
<vspace blankLines='1' />
The home keygen token is formed from the first 64 bits of the MAC.
The home keygen token tests that the mobile node can receive
messages sent to its home address.
Kcn is used in the production of home keygen token in order to allow
the correspondent node to verify that it generated the home and care-of
nonces, without forcing the correspondent node to remember a list
of all tokens it has handed out.
<vspace blankLines='1' />The Home Test message is sent to the mobile node via the home network, where
it is presumed that the home agent will tunnel the message to the mobile node.
This means that the mobile node needs to already have sent a
Binding Update to the home agent, so that the home agent will
have received and authorized the new care-of address for the
mobile node before the return routability procedure.
For improved security, the data passed between the
home agent and the mobile node is made immune to inspection and passive attacks.
Such protection is gained by encrypting the home keygen token
as it is tunneled from the home agent to the mobile node as
specified in <xref target='ha:rrprot' />.
The security properties of this additional security are discussed
in <xref target='security-cn-updates-overview' />.
<vspace blankLines='1' />The home init cookie from the mobile node is returned in the Home Test
message, to ensure that the message comes from a node on the route between
the home agent and the correspondent node.
<vspace blankLines='1' />The home nonce index is delivered to the mobile node to later allow
the correspondent node to efficiently find the nonce value that it
used in creating the home keygen token.
</t>
<t hangText='Care-of Test'>
<vspace blankLines='1' />
<vspace blankLines='1' />This message is sent in response to a Care-of Test Init message.
This message is not sent via the home agent, it is sent directly to the
mobile node.
The contents of the message are:
<list style='symbols'>
<t hangText=''>
Source Address = correspondent
</t>
<t hangText=''>
Destination Address = care-of address
</t>
<t hangText=''>
Parameters:
<list style='symbols'>
<t hangText='-'>
care-of init cookie
</t>
<t hangText='-'>
care-of keygen token
</t>
<t hangText='-'>
care-of nonce index
</t>
</list>
</t>
</list>
<vspace blankLines='1' />When the correspondent node receives the Care-of Test Init message, it
generates a care-of keygen token as follows:
<figure><artwork>
care-of keygen token :=
First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1)))
</artwork></figure>
<vspace blankLines='1' />Here, the final "1" inside the HMAC_SHA1 function is a single
octet containing the hex value 0x01, and is used to
distinguish home and care-of cookies from each other.
The keygen token is formed from the first 64 bits of the MAC,
and sent directly to the mobile node at its care-of address.
The care-of init cookie from the Care-of Test Init message is
returned to ensure that the message comes from a node on the route
to the correspondent node.
<vspace blankLines='1' />The care-of nonce index is provided to identify the nonce
used for the care-of keygen token.
The home and care-of nonce indices MAY be the same,
or different, in the Home and Care-of Test messages.
</t>
</list>
<t>When the mobile node has received both the Home and Care-of Test messages,
the return routability procedure is complete. As a result of the
procedure, the mobile node has the data it needs to send a Binding Update
to the correspondent node.
The mobile node hashes the tokens together to form a 20 octet binding key Kbm:
</t>
<figure><artwork>
Kbm = SHA1 (home keygen token | care-of keygen token)
</artwork></figure>
<t>A Binding Update may also be used to delete a previously
established binding (<xref target='bind-upd' />). In this case, the
care-of keygen token is not used. Instead, the binding management key
is generated as follows:
</t>
<figure><artwork>
Kbm = SHA1(home keygen token)
</artwork></figure>
<t>Note that the correspondent node does not create any state specific to the
mobile node, until it receives the Binding Update from that mobile node.
The correspondent node does not maintain the value for the binding
management key Kbm; it creates Kbm when given the nonce indices and
the mobile node's addresses.
</t>
</section>
<section anchor='rr-cnbind' title='Authorizing Binding Management Messages'>
<t>After the mobile node has created the binding management key (Kbm),
it can supply a verifiable Binding Update to the correspondent node.
This section provides an overview of this registration.
The below figure shows the message flow.
</t>
<figure><artwork>
Mobile node Correspondent node
| |
| Binding Update (BU) |
|---------------------------------------------->|
| (MAC, seq#, nonce indices, care-of address) |
| |
| |
| Binding Acknowledgement (BA) (if sent) |
|<----------------------------------------------|
| (MAC, seq#, status) |
</artwork></figure><list style='hanging'>
<t hangText='Binding Update'>
<vspace blankLines='1' />
<vspace blankLines='1' />To authorize a Binding Update, the mobile node creates a binding management
key Kbm from the keygen tokens as described in the previous section.
The contents of the Binding Update include the following:
<list style='symbols'>
<t hangText=''>
Source Address = care-of address
</t>
<t hangText=''>
Destination Address = correspondent
</t>
<t hangText=''>
Parameters:
<list style='symbols'>
<t hangText='-'>
home address (within the Home Address
destination option if different from the Source Address)
</t>
<t hangText='-'>
sequence number (within the Binding
Update message header)
</t>
<t hangText='-'>
home nonce index (within the Nonce Indices option)
</t>
<t hangText='-'>
care-of nonce index (within the Nonce Indices option)
</t>
<t hangText='-'>
First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | BU)))
</t>
</list>
</t>
</list>
<vspace blankLines='1' />The Binding Update contains a Nonce Indices option, indicating to
the correspondent node which home and care-of nonces to use to
recompute Kbm, the binding management key.
The MAC is computed as described in <xref target='auth-option' />, using the
correspondent node's address as the destination address and
the Binding Update message itself ("BU" above) as the MH Data.
<vspace blankLines='1' />Once the correspondent node has verified the MAC, it can create
a Binding Cache entry for the mobile.
</t>
<t hangText='Binding Acknowledgement'>
<vspace blankLines='1' />
<vspace blankLines='1' />The Binding Update is in some cases acknowledged by the
correspondent node.
The contents of the message are as follows:
<list style='symbols'>
<t hangText=''>
Source Address = correspondent
</t>
<t hangText=''>
Destination Address = care-of address
</t>
<t hangText=''>
Parameters:
<list style='symbols'>
<t hangText='-'>
sequence number (within the Binding
Update message header)
</t>
<t hangText='-'>
First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | BA)))
</t>
</list>
</t>
</list>
<vspace blankLines='1' />The Binding Acknowledgement contains the same sequence number as
the Binding Update.
The MAC is computed as described in <xref target='auth-option' />, using the
correspondent node's address as the destination address and
the message itself ("BA" above) as the MH Data.
</t>
</list>
<t>Bindings established with correspondent nodes using keys
created by way of the return routability procedure MUST NOT
exceed MAX_RR_BINDING_LIFETIME
seconds (see <xref target='constants' />).
</t>
<t>The value in the Source Address field in the IPv6 header carrying the
Binding Update is normally also the care-of address which is
used in the binding. However, a different care-of address MAY be
specified by including an Alternate Care-of Address mobility option in
the Binding Update (see <xref target='alt-option' />). When such a
message is sent to the correspondent node and the return routability
procedure is used as the authorization method, the Care-of Test Init
and Care-of Test messages MUST have been performed for the address in
the Alternate Care-of Address option (not the Source Address). The
nonce indices and MAC value MUST be based on information gained in this
test.
</t>
<t>Binding Updates may also be sent to delete a previously established
binding. In this case, generation of the binding management key depends
exclusively on the home keygen token and the care-of nonce index is
ignored.
</t>
</section>
<section anchor='tokenupdate' title='Updating Node Keys and Nonces'>
<t>Correspondent nodes generate nonces at regular intervals. It is
recommended to keep each nonce (identified by a nonce index) acceptable
for at least MAX_TOKEN_LIFETIME seconds (see <xref target='constants' />)
after it has been first used in constructing a return routability message
response. However, the correspondent node MUST NOT accept nonces beyond
MAX_NONCE_LIFETIME seconds (see <xref target='constants' />) after the first use.
As the difference between these two constants is 30 seconds, a convenient
way to enforce the above lifetimes is to generate a new nonce
every 30 seconds. The node can then continue to accept tokens
that have been based on the last 8 (MAX_NONCE_LIFETIME / 30)
nonces. This results in tokens being acceptable MAX_TOKEN_LIFETIME
to MAX_NONCE_LIFETIME seconds after they have been sent to the mobile
node, depending on whether the token was sent at the beginning or
end of the first 30 second period. Note that the correspondent
node may also attempt to generate new nonces on demand, or only if
the old nonces have been used. This is possible, as long as the
correspondent node keeps track of how long a time ago the nonces
were used for the first time, and does
not generate new nonces on every return routability request.
</t>
<t>Due to resource limitations, rapid deletion of bindings, or
reboots the correspondent node may not in all cases recognize the
nonces that the tokens were based on. If a nonce index is
unrecognized, the correspondent node replies with an error code
in the Binding Acknowledgement (either 136, 137, or 138 as discussed in
<xref target='bind-ack' />). The mobile node can then retry the
return routability procedure.
</t>
<t>An update of Kcn SHOULD be done at the same time as an update of
a nonce, so that nonce indices can identify both the nonce and the
key. Old Kcn values have to be therefore remembered as long as old
nonce values.
</t>
<t>Given that the tokens are normally expected to be usable for
MAX_TOKEN_LIFETIME seconds, the mobile node MAY use them beyond a
single run of the return routability procedure until
MAX_TOKEN_LIFETIME expires.
After this the mobile node SHOULD NOT use the tokens. A fast moving
mobile node MAY reuse a recent home keygen token from a correspondent
node when moving to a new location, and just acquire a new care-of
keygen token to show routability in the new location.
</t>
<t>While this does not save the number of round-trips due to the
simultaneous processing of home and care-of return routability tests,
there are fewer messages being exchanged, and a potentially long
round-trip through the home agent is avoided. Consequently, this
optimization is often useful. A mobile node that has multiple home
addresses, MAY also use the same care-of keygen token for Binding
Updates concerning all of these addresses.
</t>
</section>
<section anchor='rrreplay' title='Preventing Replay Attacks'>
<t>The return routability procedure also protects the participants against
replayed Binding Updates through the use of the sequence number and a MAC.
Care must be taken
when removing bindings at the correspondent node, however.
Correspondent nodes must retain bindings and the associated
sequence number information at least as long as the nonces
used in the authorization of the binding are still valid.
Alternatively, if memory is very constrained, the correspondent
node MAY invalidate the nonces that were used for the binding
being deleted (or some larger group of nonces that they belong to).
This may, however, impact the ability to accept Binding Updates
from mobile nodes that have recently received keygen tokens.
This alternative is therefore recommended only as a last measure.
</t>
</section>
<section anchor='simul' title='Handling Interruptions to Return Routability'>
<t>
In some scenarios, such as simultaneous mobility, where both
correspondent host and mobile host move at the same time, or in the case
where the correspondent node reboots and loses data, route optimization
may not complete, or relevant data in the binding cache might be lost.
</t>
To provide graceful recovery from those cases, the following steps
should be taken:
<list style='symbols'>
<t>
Return Routability signalling MUST be sent to the correspondent node's
home address if it has one (i.e. not to the correspondent nodes care-of
address if the correspondent node is also mobile).
</t>
<t>
If Return Routability signalling timed out after MAX_RO_FAILURE
attempts, the mobile node MUST revert to sending packets to the
correspondent node's home address through its home agent.
</t>
</list>
<t>
The mobile node may run the bidirectional tunnelling in parallel with the
return routability procedure until it is successful. Exponential backoff
SHOULD be used for retransmission of return routability messages.
</t>
<t>
The return routability procedure may be triggered by movement of the mobile
node or by sustained loss of end-to-end communication with a correspondent
node (e.g. based on indications from upper-layers) that has been using a route
optimised connection to the mobile node.
If such indications are received, the mobile
node MAY revert to bi-directional tunnelling while re-starting the return
routability procedure.
</t>
</section>
</section>
<section title='Dynamic Home Agent Address Discovery'>
<t>
Dynamic home agent address discovery has been designed for use
in deployments where security is not needed. For this reason,
no security solution is provided in this document for
dynamic home agent address discovery.
</t>
</section>
<section anchor='security-prefix' title='Mobile Prefix Discovery'>
<t>The mobile node and the home agent SHOULD use an IPsec security
association to protect the integrity and authenticity of the Mobile
Prefix Solicitations and Advertisements. Both the mobile nodes and the
home agents MUST support and SHOULD use the Encapsulating Security Payload (ESP) header
in transport mode with a non-NULL payload authentication algorithm to
provide data origin authentication, connectionless integrity and
optional anti-replay protection.
</t>
</section>
<section anchor='overview:security-tunnel' title='Payload Packets'>
<t>Payload packets exchanged with mobile nodes can be protected in the
usual manner, in the same way as stationary hosts can protect them.
However, Mobile IPv6 introduces the Home Address destination option, a
routing header, and tunneling headers in the payload packets. In the
following we define the security measures taken to protect these, and
to prevent their use in attacks against other parties.
</t>
<t>This specification limits the use of the Home Address destination
option to the situation where the correspondent node already has a
Binding Cache entry for the given home address. This avoids the use of
the Home Address option in attacks described in
<xref target='seccons:threats' />.
</t>
<t>
Mobile IPv6 uses a type of routing header specific to Mobile IPv6.
This type provides the necessary functionality but does not open
vulnerabilities discussed in <xref target='seccons:threats' /> and
<xref target='RFC5095'> RFC 5095 </xref>.
</t>
<t>
Tunnels between the mobile node and the home agent are protected by
ensuring proper use of source addresses, and optional cryptographic
protection. The mobile node verifies that the outer IP address
corresponds to its home agent. The home agent verifies that the outer
IP address corresponds to the current location of the mobile node
(Binding Updates sent to the home agents are secure). The home agent
identifies the mobile node through the source address of the inner
packet. (Typically, this is the home address of the mobile node, but it
can also be a link-local address, as discussed in
<xref target='ha-deliver' />. To recognize the latter type of addresses,
the home agent requires that the Link-Local Address Compatibility (L)
was set in the Binding Update.) These measures protect the tunnels
against vulnerabilities discussed in <xref target='seccons:threats' />.
</t>
<t>
For traffic tunneled via the home agent, additional IPsec ESP
encapsulation MAY be supported and used. If multicast group membership
control protocols or stateful address autoconfiguration protocols are
supported, payload data protection MUST be supported.
</t>
</section>
</section>
<section anchor='new-dos-and-messages' title='New IPv6 Protocol, Message Types, and Destination Option'>
<section anchor='mobility-header' title='Mobility Header'>
<t>The Mobility Header is an extension header used by mobile nodes,
correspondent nodes, and home agents in all messaging related to
the creation and management of bindings.
The subsections within this section describe the
message types that may be sent using the Mobility Header.
</t>
<t>Mobility Header messages MUST NOT be sent with a type 2 routing
header, except as described in <xref target='send-ba' /> for Binding
Acknowledgement. Mobility Header messages also MUST NOT be used with a
Home Address destination option, except as described in
<xref target='update-home' /> and <xref target='update-corresp' /> for Binding
Update. Binding Update List or Binding Cache information (when present) for the destination
MUST NOT be used in sending Mobility Header messages. That is,
Mobility Header messages bypass both the Binding Cache check described in
<xref target='corresp-deliver' /> and the Binding Update List check described
in <xref target='mobile-sending' /> which are normally performed for all
packets. This applies even to messages sent to or from a correspondent
node which is itself a mobile node.
</t>
<section anchor='mobility-header-format' title='Format'>
<t> The Mobility Header is identified by a Next Header value of
135 in the immediately preceding
header, and has the following format:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Proto | Header Len | MH Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. Message Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Payload Proto'>
<vspace blankLines='1' /> 8-bit selector. Identifies the type of header
immediately following the Mobility Header.
Uses the same values as
the IPv6 <xref target='RFC2460'>Next Header field</xref>.
<vspace blankLines='1' /> This field is intended to be used by a future
extension (see <xref target='future:piggybacking' />).
<vspace blankLines='1' /> Implementations conforming to this
specification SHOULD set the payload protocol
type to IPPROTO_NONE (59 decimal).
</t>
<t hangText='Header Len'>
<vspace blankLines='1' /> 8-bit unsigned integer, representing the length of the
Mobility Header in units of 8 octets, excluding the first 8 octets.
<vspace blankLines='1' /> The length of the Mobility Header MUST be a multiple of 8
octets.
</t>
<t hangText='MH Type'>
<vspace blankLines='1' /> 8-bit selector. Identifies the particular
mobility message in question. Current values
are specified in <xref target='bind-req' /> and onward.
An unrecognized MH Type field causes an error indication to be sent.
</t>
<t hangText='Reserved'>
<vspace blankLines='1' /> 8-bit field reserved for future use. The value
MUST be initialized to zero by the sender,
and MUST be ignored by the receiver.
</t>
<t hangText='Checksum'>
<vspace blankLines='1' /> 16-bit unsigned integer. This field contains the checksum of
the Mobility Header. The checksum is calculated from the octet
string consisting of a "pseudo-header" followed by the entire
Mobility Header starting with the Payload Proto field.
The checksum is the 16-bit one's complement of the one's
complement sum of this string.
<vspace blankLines='1' /> The pseudo-header contains IPv6 header fields, as specified in
Section 8.1 of <xref target='RFC2460'>RFC 2460</xref>.
The Next Header value used in the pseudo-header is 135. The addresses
used in the pseudo-header are the addresses that appear in the
Source and Destination Address fields in the IPv6 packet carrying
the Mobility Header.
<vspace blankLines='1' /> Note that the procedures of calculating upper layer checksums while away
from home described in <xref target='mobile-sending' />
apply even for the Mobility Header.
If a mobility message has a Home Address destination option, then
the checksum calculation uses the home address in this option as
the value of the IPv6 Source Address field.
The type 2 routing header is treated as explained in <xref target='RFC2460' />.
<vspace blankLines='1' /> The Mobility Header is considered as the upper layer protocol
for the purposes of calculating the pseudo-header. The
Upper-Layer Packet Length field in the pseudo-header MUST be
set to the total length of the Mobility Header.
<vspace blankLines='1' /> For computing the
checksum, the checksum field is set to zero.
</t>
<t hangText='Message Data'>
<vspace blankLines='1' /> A variable length field containing the data
specific to the indicated Mobility Header type.
</t>
</list>
<t>Mobile IPv6 also defines a number of "mobility options" for use within
these messages; if included, any options MUST appear after the fixed
portion of the message data specified in this document. The presence of
such options will be indicated by the Header Len field within the
message. When the Header Len value is greater than the length required
for
the message specified here, the remaining octets are interpreted as
mobility options. These options include padding options that can be
used to ensure that other options are aligned properly, and that the
total length of the message is divisible by 8.
The encoding and format of defined options are
described in <xref target='params' />.
</t>
<t>Alignment requirements for the Mobility Header are the same as for any
IPv6 protocol Header. That is, they MUST be aligned on an 8-octet
boundary.
</t>
</section>
<section anchor='bind-req' title='Binding Refresh Request Message'>
<t>The Binding Refresh Request (BRR) message requests a mobile node to
update its mobility binding. This message is sent by correspondent
nodes according to the rules in <xref target='corresp-send-brr' />. When a
mobile node receives a packet containing a Binding Refresh Request
message it processes the message according to the rules in
<xref target='receive-req' />.
</t>
<t>The Binding Refresh Request message uses the MH Type value 0. When this
value is indicated in the MH Type field, the format of the
Message Data field in the Mobility Header is as follows:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Reserved'>
<vspace blankLines='1' /> 16-bit field reserved for future use. The value
MUST be initialized to zero by the sender,
and MUST be ignored by the receiver.
</t>
<t hangText='Mobility Options'>
<vspace blankLines='1' /> Variable-length field of such length that the
complete Mobility Header is an integer multiple
of 8 octets long. This field contains zero or more TLV-encoded
mobility options. The
encoding and format of defined options are
described in <xref target='params' />.
The receiver MUST ignore and skip any options which it does
not understand.
<vspace blankLines='1' />There MAY be additional information, associated with this Binding
Refresh Request message that need not be present in all Binding
Refresh Request messages sent.
Mobility options allow future extensions to the format of
the Binding Refresh Request message to be defined.
This specification does not define any options valid for the Binding
Refresh Request message.
</t>
</list>
<t>If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 0.
</t>
</section>
<section anchor='hoti' title='Home Test Init Message'>
<t>A mobile node uses the Home Test Init (HoTI) message to initiate
the return routability procedure and request a home keygen token
from a correspondent node (see <xref target='update-corresp-rr' />).
The Home Test Init message uses the MH Type value 1. When this value is
indicated in the MH Type field, the format of the Message Data
field in the Mobility Header is as follows:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Reserved'>
<vspace blankLines='1' /> 16-bit field reserved for future use. This value
MUST be initialized to zero by the sender,
and MUST be ignored by the receiver.
</t>
<t hangText='Home Init Cookie'>
<vspace blankLines='1' /> 64-bit field which contains a random value, the home init cookie.
</t>
<t hangText='Mobility Options'>
<vspace blankLines='1' /> Variable-length field of such length that the complete
Mobility Header is an integer multiple of 8 octets
long. This field contains zero or more TLV-encoded mobility
options. The receiver MUST ignore and skip any options
which it does not understand.
This specification does not define
any options valid for the Home Test Init message.
</t>
</list>
<t>If no actual options are present in this message, no padding
is necessary and the Header Len field will be set to 1.
</t>
<t>This message is
tunneled through the home agent when the mobile node is away from home.
Such tunneling SHOULD employ IPsec ESP in tunnel mode between the home
agent and the mobile node.
This protection is indicated by the IPsec security policy database.
The protection of Home Test Init messages is unrelated to the requirement
to protect regular payload traffic, which MAY use such tunnels as well.
</t>
</section>
<section anchor='coti' title='Care-of Test Init Message'>
<t>A mobile node uses the Care-of Test Init (CoTI) message to initiate
the return routability procedure and request a care-of keygen token
from a correspondent node (see <xref target='update-corresp-rr' />).
The Care-of Test Init message uses the MH Type value 2. When this
value is indicated in the MH Type field, the format of the
Message Data field in the Mobility Header is as follows:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Reserved'>
<vspace blankLines='1' /> 16-bit field reserved for future use. The value
MUST be initialized to zero by the sender,
and MUST be ignored by the receiver.
</t>
<t hangText='Care-of Init Cookie'>
<vspace blankLines='1' /> 64-bit field which contains a random value, the care-of init cookie.
</t>
<t hangText='Mobility Options'>
<vspace blankLines='1' /> Variable-length field of such length that the
complete Mobility Header is an integer multiple
of 8 octets long. This field contains zero or more TLV-encoded
mobility options. The receiver MUST ignore and
skip any options which it does not understand.
This specification does not define
any options valid for the Care-of Test Init message.
</t>
</list>
<t>If no actual options are present in this message, no padding
is necessary and the Header Len field will be set to 1.
</t>
</section>
<section anchor='hot' title='Home Test Message'>
<t>The Home Test (HoT) message is a response to the Home Test Init message, and
is sent from the correspondent node to the mobile node (see <xref target='rrp' />).
The Home Test message uses the MH Type value 3. When this value is
indicated in the MH Type field, the format of the Message Data
field in the Mobility Header is as follows:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Keygen Token +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Home Nonce Index'>
<vspace blankLines='1' /> This field will be echoed back by the mobile node to the
correspondent node in a subsequent Binding Update.
</t>
<t hangText='Home Init Cookie'>
<vspace blankLines='1' /> 64-bit field which contains the home init cookie.
</t>
<t hangText='Home Keygen Token'>
<vspace blankLines='1' /> This field contains the 64 bit home keygen token used in the
return routability procedure.
</t>
<t hangText='Mobility Options'>
<vspace blankLines='1' /> Variable-length field of such length that the
complete Mobility Header is an integer multiple
of 8 octets long. This field contains zero or more TLV-encoded
mobility options. The receiver MUST ignore and skip any options
which it does not understand.
This specification does not define
any options valid for the Home Test message.
</t>
</list>
<t>If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 2.
</t>
</section>
<section anchor='cot' title='Care-of Test Message'>
<t>The Care-of Test (CoT) message is a response to the Care-of Test Init
message, and is sent from the correspondent node to the mobile node
(see <xref target='mobile-recv-rr' />).
The Care-of Test message uses the MH Type value 4. When this
value is indicated in the MH Type field, the format of the
Message Data field in the Mobility Header is as follows:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Keygen Token +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Care-of Nonce Index'>
<vspace blankLines='1' /> This value will be echoed back by the mobile node to the
correspondent node in a subsequent Binding Update.
</t>
<t hangText='Care-of Init Cookie'>
<vspace blankLines='1' /> 64-bit field which contains the care-of init cookie.
</t>
<t hangText='Care-of Keygen Token'>
<vspace blankLines='1' /> This field contains the 64 bit care-of keygen token used in the
return routability procedure.
</t>
<t hangText='Mobility Options'>
<vspace blankLines='1' /> Variable-length field of such length that the complete
Mobility Header is an integer multiple of 8
octets long. This field contains zero or more TLV-encoded
mobility options. The receiver MUST ignore and
skip any options which it does not understand.
This specification does not define
any options valid for the Care-of Test message.
</t>
</list>
<t>If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 2.
</t>
</section>
<section anchor='bind-upd' title='Binding Update Message'>
<t>The Binding Update (BU) message is used by a mobile node to notify other
nodes of a new care-of address for itself. Binding Updates are sent as
described in <xref target='update-home' /> and <xref target='update-corresp' />.
</t>
<t>The Binding Update uses the MH Type value 5. When this
value is indicated in the MH Type field, the format of the Message
Data field in the Mobility Header is as follows:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|L|K| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Acknowledge (A)'>
<vspace blankLines='1' /> The Acknowledge (A) bit is set by the sending
mobile node to request a Binding
Acknowledgement (<xref target='bind-ack' />) be returned
upon receipt of the Binding Update.
</t>
<t hangText='Home Registration (H)'>
<vspace blankLines='1' /> The Home Registration (H) bit is set by the
sending mobile node to request that the receiving
node should act as this node's home agent. The
destination of the packet carrying this message
MUST be that of a router sharing the same
subnet prefix as the home address of the
mobile node in the binding.
</t>
<t hangText='Link-Local Address Compatibility (L)'>
<vspace blankLines='1' /> The Link-Local Address Compatibility (L) bit is set when
the home address reported by the mobile node has the same
interface identifier as the mobile node's link-local
address.
</t>
<t hangText='Key Management Mobility Capability (K)'>
<vspace blankLines='1' /> If this bit is cleared, the protocol used for establishing the
IPsec security associations between the mobile node and the
home agent does not survive movements. It may then have to be
rerun. (Note that the IPsec security associations themselves
are expected to survive movements.) If manual IPsec
configuration is used, the bit MUST be cleared.
<vspace blankLines='1' />
This bit is valid only in Binding Updates sent to the home
agent, and MUST be cleared in other Binding Updates.
Correspondent nodes MUST ignore this bit.
</t>
<t hangText='Reserved'> <vspace blankLines='1' />
These fields are unused. They MUST be initialized to
zero by the sender and MUST be ignored by the
receiver.
</t>
<t hangText='Sequence #'> <vspace blankLines='1' />
A 16-bit unsigned integer used by the receiving node to
sequence Binding Updates and by the sending
node to match a returned Binding
Acknowledgement with this Binding Update.
</t>
<t hangText='Lifetime'>
<vspace blankLines='1' /> 16-bit unsigned integer. The number of
time units remaining before the binding MUST be
considered expired. A value of
zero indicates that the Binding Cache entry
for the mobile node MUST be deleted. (In this
case the specified care-of address MUST also
be set equal to the home address.)
One time unit is 4 seconds.
</t>
<t hangText='Mobility Options'> <vspace blankLines='1' />
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. This field contains
zero or more TLV-encoded mobility options. The encoding and format of
defined options are described in <xref target='params' />. The
receiver MUST ignore and skip any options which it does not
understand.
<vspace blankLines='1' /> The following options are valid in a Binding Update:
<list style='symbols'>
<t> Binding Authorization Data option (this option is mandatory
in Binding Updates sent to a correspondent node)
</t>
<t> Nonce Indices option.
</t>
<t> Alternate Care-of Address option
</t>
</list>
</t>
</list>
<t>If no options are present in this message, 4 octets of padding are
necessary and the Header Len field will be set to 1.
</t>
<t>The care-of address is specified either by the Source Address field
in the IPv6 header or by the Alternate Care-of Address option, if
present. The care-of address MUST be a unicast routable address.
IPv6 Source Address MUST be a topologically correct source address.
Binding Updates for a care-of address which is not
a unicast routable address MUST be silently discarded.
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).
</t>
<t>The deletion of a binding can be indicated by setting the Lifetime
field to 0 and by setting the care-of address equal to the home
address. In deletion, the generation of the binding management key
depends exclusively on the home keygen token, as explained in
<xref target='rrp' />. (Note that while the senders are required to set
both the Lifetime field to 0 and the care-of address equal to the home
address, <xref target='recv-updates' /> rules for receivers are more
liberal, and interpret either condition as a deletion.)
</t>
<t>Correspondent nodes SHOULD NOT delete the Binding Cache entry before
the lifetime expires, if any application hosted by the correspondent node
is still likely to require communication with the mobile node.
A Binding Cache entry that is de-allocated prematurely
might cause subsequent packets to be dropped from the mobile
node, if they contain the Home Address destination option.
This situation is recoverable, since a Binding Error message
is sent to the mobile node (see <xref target='binding-err' />); however,
it causes unnecessary delay in the communications.
</t>
</section>
<section anchor='bind-ack' title='Binding Acknowledgement Message'>
<t>The Binding Acknowledgement is used to acknowledge receipt of
a Binding Update (<xref target='bind-upd' />). This packet is sent
as described in <xref target='send-ba' /> and <xref target='home-reg' />.
</t>
<t>The Binding Acknowledgement has the MH Type value 6. When this
value is indicated in the MH Type field, the format of the Message
Data field in the Mobility Header is as follows:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |K| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Status'>
<vspace blankLines='1' />
8-bit unsigned integer indicating the disposition of the
Binding Update. Values of the Status field less than 128
indicate that the Binding Update was accepted by the receiving
node.
Values greater than or equal to 128
indicate that the Binding Update was rejected by the receiving
node. The following Status values are currently defined:
<list style='empty'>
<list style='hanging'>
<t hangText=' 0'>
Binding Update accepted
</t>
<t hangText=' 1'>
Accepted but prefix discovery necessary
</t>
<t hangText='128'>
Reason unspecified
</t>
<t hangText='129'>
Administratively prohibited
</t>
<t hangText='130'>
Insufficient resources
</t>
<t hangText='131'>
Home registration not supported
</t>
<t hangText='132'>
Not home subnet
</t>
<t hangText='133'>
Not home agent for this mobile node
</t>
<t hangText='134'>
Duplicate Address Detection failed
</t>
<t hangText='135'>
Sequence number out of window
</t>
<t hangText='136'>
Expired home nonce index
</t>
<t hangText='137'>
Expired care-of nonce index
</t>
<t hangText='138'>
Expired nonces
</t>
<t hangText='139'>
Registration type change disallowed
</t>
</list>
</list>
<vspace blankLines='1' /> Up-to-date values of the Status field are to be specified in
the IANA registry of <xref target='RFC3232'>assigned numbers</xref>.
</t>
<t hangText='Key Management Mobility Capability (K)'> <vspace blankLines='1' />
If this bit is cleared, the protocol used by the home agent for
establishing the IPsec security associations between the
mobile node and the home agent does not survive movements. It
may then have to be rerun. (Note that the IPsec security
associations themselves are expected to survive movements.)
<vspace blankLines='1' />
Correspondent nodes MUST set the K bit to 0.
</t>
<t hangText='Reserved'> <vspace blankLines='1' />
This field is unused. It MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
</t>
<t hangText='Sequence #'> <vspace blankLines='1' />
The Sequence Number in the Binding Acknowledgement is copied
from the Sequence Number field in the Binding Update. It
is used by the mobile node in matching this Binding
Acknowledgement with an outstanding Binding Update.
</t>
<t hangText='Lifetime'>
<vspace blankLines='1' />
The granted lifetime, in time units of 4 seconds, for which this
node SHOULD retain the entry for this mobile node in its Binding
Cache.
<vspace blankLines='1' />
The value of this field is
undefined if the Status field indicates that the Binding Update
was rejected.
</t>
<t hangText='Mobility Options'>
<vspace blankLines='1' />
Variable-length field of such length that the complete
Mobility Header is an integer multiple of 8 octets long.
This field contains zero or more TLV-encoded mobility options.
The encoding and format of defined options are described in
<xref target='params' />. The receiver MUST ignore and skip any
options which it does not understand.
<vspace blankLines='1' />
There MAY be additional information, associated with this
Binding Acknowledgement that need not be present in
all Binding Acknowledgements sent.
Mobility options allow future extensions to the format of the
Binding Acknowledgement to be defined.
The following options are valid for the Binding Acknowledgement:
<list style='symbols'>
<t> Binding Authorization Data option (this option is mandatory
in Binding Acknowledgements sent by a correspondent node,
except where otherwise noted in <xref target='send-ba' />)
</t>
<t> Binding Refresh Advice option
</t>
</list>
</t>
</list>
<t> If no options are present in this message, 4 octets of padding are
necessary and the Header Len field will be set to 1.
</t>
</section>
<section anchor='binding-err' title='Binding Error Message'>
<t>The Binding Error (BE) message is used by the correspondent node to
signal an error related to mobility, such as an inappropriate attempt
to use the Home Address destination option without an existing
binding; see <xref target='sending-binding-error' /> for details.
</t>
<t>The Binding Error message uses the MH Type value 7. When this
value is indicated in the MH Type field, the format of the
Message Data field in the Mobility Header is as follows:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Status'>
<vspace blankLines='1' /> 8-bit unsigned integer indicating the reason for this message.
The following values are currently defined:
<list style='empty'>
<list style='hanging'>
<t hangText=' 1'>
Unknown binding for Home Address destination option
</t>
<t hangText=' 2'>
Unrecognized MH Type value
</t>
</list>
</list>
</t>
<t hangText='Reserved'>
<vspace blankLines='1' /> A 8-bit field reserved for future use. The value
MUST be initialized to zero by the sender,
and MUST be ignored by the receiver.
</t>
<t hangText='Home Address'>
<vspace blankLines='1' /> The home address that was contained in the Home Address
destination option. The mobile node uses this information to
determine which binding does not exist, in cases where the
mobile node has several home addresses.
</t>
<t hangText='Mobility Options'>
<vspace blankLines='1' /> Variable-length field of such length that the
complete Mobility Header is an integer multiple
of 8 octets long. This field contains zero or more TLV-encoded
mobility options. The receiver MUST ignore and skip any
options which it does not understand.
<vspace blankLines='1' />There MAY be additional information, associated with this Binding Error
message that need not be present in all Binding Error messages sent.
Mobility options allow future extensions to the
format of the Binding Error message to be defined. The encoding
and format of defined options are described in
<xref target='params' />. This specification does not define any
options valid for the Binding Error message.
</t>
</list>
<t>If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 2.
</t>
</section>
</section>
<section anchor='params' title='Mobility Options'>
<t>Mobility messages can include zero or more mobility options.
This allows optional fields that may not be needed in every
use of a particular Mobility Header, as well as future extensions to
the format of the messages.
Such options are included in the Message Data field of the message
itself, after the fixed portion of the message data specified
in the message subsections of <xref target='mobility-header' />.
</t>
<t>The presence of such options will be indicated by the Header Len of
the Mobility Header. If included, the Binding Authorization Data
option (<xref target='auth-option' />) MUST be the last option and
MUST NOT have trailing padding. Otherwise, options can be placed in
any order.
</t>
<section title='Format'>
<t>Mobility options are encoded within the remaining space of the
Message Data field of a mobility message, using a type-length-value
(TLV) format as follows:
</t>
<figure><artwork>
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Option Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Option Type'>
<vspace blankLines='1' /> 8-bit identifier of the type of mobility option. When processing
a Mobility Header containing an option for
which the Option Type value is not recognized by the
receiver, the receiver MUST quietly ignore and skip over the
option, correctly handling any remaining options in the
message.
</t>
<t hangText='Option Length'>
<vspace blankLines='1' /> 8-bit unsigned integer, representing the length in octets of the
mobility option, not including the Option Type and
Option Length fields.
</t>
<t hangText='Option Data'>
<vspace blankLines='1' /> A variable length field that contains data specific to the option.
</t>
</list>
<t>The following subsections specify the Option types which are
currently defined for use in the Mobility Header.
</t>
<t>Implementations MUST silently ignore any mobility options that they do not
understand.
</t>
<t>Mobility options may have alignment requirements. Following the
convention in IPv6, these options are aligned in a packet so that
multi-octet values within the Option Data field of each option fall on
natural boundaries (i.e., fields of width n octets are placed at an
integer multiple of n octets from the start of the header, for n = 1,
2, 4, or 8) <xref target='RFC2460' />.
</t>
</section>
<section anchor='pad1-option' title='Pad1'>
<t>The Pad1 option does not have any alignment requirements. Its
format is as follows:
</t>
<figure><artwork>
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>NOTE! the format of the Pad1 option is a special
case - it has neither Option Length nor Option Data
fields.
</t>
<t>The Pad1 option is used to insert one octet of padding
in the Mobility Options area of a Mobility Header. If more
than one octet of padding is required, the PadN option,
described next, should be used rather than multiple Pad1
options.
</t>
</section>
<section anchor='padn-option' title='PadN'>
<t>The PadN option does not have any alignment requirements. Its
format is as follows:
</t>
<figure><artwork>
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 1 | Option Length | Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
</artwork></figure>
<t>The PadN option is used to insert two or more octets of padding
in the Mobility Options area of a mobility message. For N octets
of padding, the Option Length field contains the value N-2, and the
Option Data consists of N-2 zero-valued octets. PadN Option data
MUST be ignored by the receiver.
</t>
</section>
<section anchor='refresh-option' title='Binding Refresh Advice'>
<t>The Binding Refresh Advice option has an alignment requirement of
2n. Its format is as follows:
</t>
<figure><artwork>
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 = 2 | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Refresh Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>The Binding Refresh Advice option is only valid in the
Binding Acknowledgement, and only on Binding Acknowledgements
sent from the mobile node's home agent in reply to a home
registration. The Refresh Interval
is measured in units of four seconds, and indicates remaining time until
the mobile node SHOULD send a new home registration to the
home agent. The Refresh Interval MUST be set to indicate a smaller
time interval than the Lifetime value of the Binding Acknowledgement.
</t>
</section>
<section anchor='alt-option' title='Alternate Care-of Address'>
<t>The Alternate Care-of Address option has an alignment requirement of
8n+6. Its format is as follows:
</t>
<figure><artwork>
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 = 3 | Length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Alternate Care-of Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>Normally, a Binding Update specifies the desired care-of address in
the Source Address field of the IPv6 header. However, this is not
possible in some cases, such as when the mobile node wishes to
indicate a care-of address which it cannot use as a topologically
correct source address (<xref target='bind-upd' /> and <xref target='update-corresp' />) or
when the used security mechanism does not protect the
IPv6 header (<xref target='update-home' />).
</t>
<t>The Alternate Care-of Address option is provided for these
situations. This option is valid only in Binding Update. The
Alternate Care-of Address field contains an address to use as the
care-of address for the binding, rather than using the Source Address
of the packet as the care-of address.
</t>
</section>
<section anchor='nonce-option' title='Nonce Indices'>
<t>The Nonce Indices option has an alignment requirement of 2n.
Its format is as follows:
</t>
<figure><artwork>
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 = 4 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index | Care-of Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>The Nonce Indices option is valid only in the Binding Update message
sent to a correspondent node, and only when present together with a
Binding Authorization Data option. When the correspondent node
authorizes the Binding Update, it needs to produce home and care-of
keygen tokens from its stored random nonce values.
</t>
<t>The Home Nonce Index field tells the correspondent node which nonce
value to use when producing the home keygen token.
</t>
<t>The Care-of Nonce Index field is ignored in requests to delete a
binding. Otherwise, it tells the correspondent node which nonce value
to use when producing the care-of keygen token.
</t>
</section>
<section anchor='auth-option' title='Binding Authorization Data'>
<t>The Binding Authorization Data option does not have alignment
requirements as such. However, since this option must be the last
mobility option, an implicit alignment requirement is 8n + 2. The format
of this option is as follows:
</t>
<figure><artwork>
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 = 5 | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Authenticator |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>The Binding Authorization Data option is valid in the Binding
Update and Binding Acknowledgement.
</t>
<t>The Option Length field contains the length of the authenticator in
octets.
</t>
<t>The Authenticator field contains a cryptographic value which can be
used to determine that the message in question comes from the right
authority. Rules for calculating this value depends on the used
authorization procedure.
</t>
<t>For the return routability procedure, this option
can appear in the Binding Update and Binding Acknowledgements.
Rules for calculating the Authenticator value are the following:
</t>
<figure><artwork>
Mobility Data = care-of address | correspondent | MH Data
Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data))
</artwork></figure>
<t>Where | denotes concatenation. "Care-of address" is the care-of
address which will be registered for the mobile node if the Binding
Update succeeds, or the home address of the mobile node if this
option is used in de-registration. Note also that this address might
be different from the source address of the Binding Update message,
if the Alternative Care-of Address mobility option is used, or when
the lifetime of the binding is set to zero.<vspace blankLines='1'/>
The "correspondent" is the IPv6
address of the correspondent node.
Note that, if the message is sent to a destination which is itself
mobile, the "correspondent" address may not be the address found
in the Destination Address field of the IPv6 header; instead the
home address from the type 2 Routing header should
be used.
</t>
<t>"MH Data"
is the content of the Mobility Header, excluding the Authenticator
field itself. The Authenticator value is calculated as if the Checksum
field in the Mobility Header was zero. The Checksum in the transmitted
packet is still calculated in the usual manner, with the calculated
Authenticator being a part of the packet protected by the Checksum.
Kbm is the binding management key, which is typically created using
nonces provided by the correspondent node (see <xref target='processing-rr' />).
Note that while the contents of a potential Home Address destination
option are not covered in this formula, the rules for the calculation
of the Kbm do take the home address in account. This ensures that
the MAC will be different for different home addresses.
</t>
<t>The first 96 bits from the MAC result are used as the Authenticator field.
</t>
</section>
</section>
<section anchor='hao' title='Home Address Option'>
<t>The Home Address option is carried by the Destination
Option extension header (Next Header value = 60).
It is used in a packet sent by a mobile node while away from home, to
inform the recipient of the mobile node's home address.
</t>
<t>The Home Address option
is encoded in type-length-value (TLV) format as follows:
</t>
<figure><artwork>
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Option Type'>
<vspace blankLines='1' /> 201 = 0xC9
</t>
<t hangText='Option Length'>
<vspace blankLines='1' /> 8-bit unsigned integer.
Length of the option, in octets,
excluding the Option Type and Option Length fields.
This field MUST be set to 16.
</t>
<t hangText='Home Address'>
<vspace blankLines='1' /> The home address of the mobile node sending the packet.
This address MUST be a unicast routable address.
</t>
</list>
<t>The <xref target='RFC2460'>alignment requirement</xref> for the Home Address option
is 8n+6.
</t>
<t>The three highest-order bits of the Option Type field are encoded to
indicate <xref target='RFC2460'>specific processing of the option</xref>;
for the Home Address option,
these three bits are set to 110.
This indicates the following processing requirements:
</t>
<list style='symbols'>
<t> Any IPv6 node that does not recognize the Option Type
must discard the packet, and
if the packet's Destination Address was not a
multicast address, return an ICMP Parameter Problem,
Code 2, message to the packet's Source Address.
The Pointer
field in the ICMP message SHOULD point at the Option Type field.
Otherwise, for multicast addresses, the ICMP
message MUST NOT be sent.
</t>
<t> The data within the option cannot change en route
to the packet's final destination.
</t>
</list>
<t>The Home Address option MUST be placed as follows:
</t>
<list style='symbols'>
<t> After the routing header, if that header is present
</t>
<t> Before the Fragment Header, if that header is present
</t>
<t> Before the AH Header or ESP Header, if either one of
those headers are present
</t>
</list>
<t>For each IPv6 packet header, the Home Address Option MUST NOT appear
more than once. However, an <xref target='RFC2473'>encapsulated packet</xref>
MAY contain a separate Home Address option associated with each
encapsulating IP header.
</t>
<t>The inclusion of a Home Address destination option in a packet affects
the receiving node's processing of only this single packet.
No state is created or modified in the receiving node
as a result of receiving a Home Address option in a packet.
In particular,
the presence of a Home Address option in a received packet
MUST NOT alter the contents of the receiver's Binding Cache
and MUST NOT cause any changes in the routing of subsequent packets
sent by this receiving node.
</t>
</section>
<section anchor='rhtype2' title='Type 2 Routing Header'>
<t>Mobile IPv6 defines a new routing header variant, the type 2 routing
header, to allow the packet to be routed directly from a correspondent
to the mobile node's care-of address.
The mobile node's care-of address
is inserted into the IPv6 Destination Address field.
Once the packet arrives at the care-of address,
the mobile node retrieves its home address from the routing
header, and this is used
as the final destination address for the packet.
</t>
<t>The new routing header uses a different type than defined for "regular"
IPv6 source routing, enabling firewalls to apply different rules to source
routed packets than to Mobile IPv6. This routing header type (type 2)
is restricted to carry only one IPv6 address. All IPv6 nodes which
process this routing header MUST verify that the address contained
within is the node's own home address in order to prevent packets from
being forwarded outside the node. The IP address contained in
the routing header, since it is the mobile node's home address,
MUST be a unicast routable address. Furthermore,
if the scope of the home address is smaller than the scope of the
care-of address, the mobile node MUST discard the packet
(see <xref target='addressability' />).
</t>
<section title='Format'>
<t>The type 2 routing header has the following format:
</t>
<figure><artwork>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=2 | Routing Type=2|Segments Left=1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Next Header'>
<vspace blankLines='1' /> 8-bit selector. Identifies the type of header
immediately following the routing header. Uses
the same values as the IPv6 <xref target='RFC2460'>Next Header field</xref>.
</t>
<t hangText='Hdr Ext Len'>
<vspace blankLines='1' /> 2 (8-bit unsigned integer); length of the routing
header in 8-octet units, not including the first
8 octets.
</t>
<t hangText='Routing Type'>
<vspace blankLines='1' /> 2 (8-bit unsigned integer).
</t>
<t hangText='Segments Left'> <vspace blankLines='1' />
1 (8-bit unsigned integer).
</t>
<t hangText='Reserved'> <vspace blankLines='1' />
32-bit reserved field. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
</t>
<t hangText='Home Address'> <vspace blankLines='1' />
The Home Address of the destination Mobile Node.
</t>
</list>
<t>
For a type 2 routing header, the Hdr Ext Len MUST be 2.
The Segments Left value describes the number of route segments
remaining; i.e., number of explicitly listed intermediate nodes
still to be visited before reaching the final destination.
Segments Left MUST be 1.
The ordering rules for extension headers in an IPv6 packet are
described in Section 4.1 of <xref target='RFC2460'>RFC 2460</xref>.
The type 2 routing header defined for Mobile IPv6 follows the same
ordering as other routing headers.
If another routing header is present along with a type 2 routing header,
the type 2 routing header should follow the other routing header.
A packet containing such nested encapsulation should be created as
if the inner (type 2) routing header was constructed first and
then treated as an original packet by header construction process
for the other routing header.
</t>
<t>In addition, the general procedures defined by IPv6 for routing
headers suggest that a received routing header MAY be automatically
"reversed" to construct a routing header for use in any response
packets sent by upper-layer protocols, if the received packet is
authenticated [6]. This MUST NOT be done automatically for type 2
routing headers.
</t>
</section>
</section>
<section anchor='ha-disc-req' title='ICMP Home Agent Address Discovery Request Message'>
<t>The ICMP Home Agent Address Discovery Request message is
used by a mobile node to initiate the
dynamic home agent address discovery mechanism,
as described in <xref target='mn-ha-disc' />.
The mobile node sends the Home Agent Address Discovery Request
message to the Mobile IPv6 <xref target='RFC2526'>Home-Agents anycast address</xref> for its
own home subnet prefix.
(Note that the currently defined anycast
addresses may not work with all prefix lengths other
than those defined in <xref target='RFC4291'>RFC 4291</xref> <xref target='RFC3627' />.)
</t>
<figure><artwork>
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Type'>
<vspace blankLines='1' /> 144
</t>
<t hangText='Code'>
<vspace blankLines='1' /> 0
</t>
<t hangText='Checksum'>
<vspace blankLines='1' />The <xref target='RFC4443'>ICMP checksum</xref>.
</t>
<t hangText='Identifier'>
<vspace blankLines='1' />An identifier to aid in matching Home Agent Address Discovery Reply
messages to this Home Agent Address Discovery Request message.
</t>
<t hangText='Reserved'>
<vspace blankLines='1' />This field is unused.
It MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
</t>
</list>
<t>The Source Address of the Home Agent Address Discovery Request
message packet is typically one of the mobile node's current care-of
addresses. At the time of performing this dynamic home agent address
discovery procedure, it is likely that the mobile node is not
registered with any home agent. Therefore, neither the nature of
the address nor the identity of the mobile node can be established
at this time. The home agent MUST then return the Home Agent Address
Discovery Reply message directly to the Source Address chosen by the
mobile node.
</t>
</section>
<section anchor='ha-disc-reply' title='ICMP Home Agent Address Discovery Reply Message'>
<t>The ICMP Home Agent Address Discovery Reply message is
used by a home agent to respond to a mobile node that uses the
dynamic home agent address discovery mechanism,
as described in <xref target='ha-disc' />.
</t>
<figure><artwork>
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Home Agent Addresses .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Type'>
<vspace blankLines='1' /> 145
</t>
<t hangText='Code'>
<vspace blankLines='1' /> 0
</t>
<t hangText='Checksum'>
<vspace blankLines='1' />The <xref target='RFC4443'>ICMP checksum</xref>.
</t>
<t hangText='Identifier'>
<vspace blankLines='1' />The identifier from the invoking
Home Agent Address Discovery Request message.
</t>
<t hangText='Reserved'>
<vspace blankLines='1' />This field is unused.
It MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
</t>
<t hangText='Home Agent Addresses'>
<vspace blankLines='1' />A list of addresses of home agents on the home link for the mobile node.
The number of addresses presented in the list is indicated by
the remaining length of the IPv6 packet carrying the
Home Agent Address Discovery Reply message.
</t>
</list>
</section>
<section anchor='renum-mps'
title='ICMP Mobile Prefix Solicitation Message Format'>
<t>
The ICMP Mobile Prefix Solicitation Message is sent by a mobile
node to its home agent while it is away from home.
The purpose of the message is to solicit a Mobile Prefix Advertisement
from the home agent, which will allow the mobile node to gather
prefix information about its home network.
This information can be used to configure and update home address(es)
according to changes in prefix information
supplied by the home agent.
</t>
<figure><artwork>
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>IP Fields:
</t>
<list style='hanging'>
<t hangText='Source Address'> <vspace blankLines='1'/>
The mobile node's care-of address.
</t>
<t hangText='Destination Address'> <vspace blankLines='1'/>
The address of the mobile node's home agent.
This home agent must be on the link that the mobile node wishes
to learn prefix information about.
</t>
<t hangText='Hop Limit'> <vspace blankLines='1'/>
Set to an initial hop limit value, similarly to
any other unicast packet sent by the mobile node.
</t>
<t hangText='Destination Option:'> <vspace blankLines='1' />
A Home Address destination option MUST be included.
</t>
<t hangText='ESP header:'> <vspace blankLines='1' />
IPsec headers MUST be supported and SHOULD be used
as described in <xref target='security-prefix' />.
</t>
</list>
<t>ICMP Fields:
</t>
<list style='hanging'>
<t hangText='Type'> <vspace blankLines='1' />
146
</t>
<t hangText='Code'> <vspace blankLines='1' />
0
</t>
<t hangText='Checksum'> <vspace blankLines='1' />
The <xref target='RFC4443'>ICMP checksum</xref>.
</t>
<t hangText='Identifier'> <vspace blankLines='1' />
An identifier to aid in matching a future Mobile Prefix
Advertisement to this Mobile Prefix Solicitation.
</t>
<t hangText='Reserved'> <vspace blankLines='1' />
This field is unused. It MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
</t>
</list>
<t>
The Mobile Prefix Solicitation messages may have options. These
options MUST use the option format defined in
Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>).
This document does not define any option
types for the Mobile Prefix Solicitation message, but future documents
may define new options. Home agents MUST silently ignore any options
they do not recognize and continue processing the message.
</t>
</section>
<section anchor='renum-mpa'
title='ICMP Mobile Prefix Advertisement Message Format'>
<t>
A home agent will send a Mobile Prefix Advertisement to a mobile
node to distribute prefix information about the home link while the
mobile node is traveling away from the home network. This will occur
in response to a Mobile Prefix Solicitation with an Advertisement, or
by an unsolicited Advertisement sent according to the rules in
<xref target='ha:renum' />.
</t>
<figure><artwork>
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |M|O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>IP Fields:
</t>
<list style='hanging'>
<t hangText='Source Address'> <vspace blankLines='1' />
The home agent's address as the mobile node would
expect to see it (i.e., same network prefix).
</t>
<t hangText='Destination Address'> <vspace blankLines='1' />
If this message is a response to a Mobile Prefix Solicitation,
this field contains the Source Address field from that packet.
For unsolicited messages, the mobile node's care-of address
SHOULD be used. Note that unsolicited messages can only be
sent if the mobile node is currently registered with the home
agent.
</t>
<t hangText='Routing header:'> <vspace blankLines='1' />
A type 2 routing header MUST be included.
</t>
<t hangText='ESP header:'> <vspace blankLines='1' />
IPsec headers MUST be supported and SHOULD be used
as described in <xref target='security-prefix' />.
</t>
</list>
<t>ICMP Fields:
</t>
<list style='hanging'>
<t hangText='Type'> <vspace blankLines='1' />
147
</t>
<t hangText='Code'> <vspace blankLines='1' />
0
</t>
<t hangText='Checksum'> <vspace blankLines='1' />
The <xref target='RFC4443'>ICMP checksum</xref>.
</t>
<t hangText='Identifier'> <vspace blankLines='1' />
An identifier to aid in matching this Mobile Prefix
Advertisement to a previous Mobile Prefix Solicitation.
</t>
<t hangText='M'> <vspace blankLines='1' />
1-bit Managed Address Configuration flag. When set, hosts
use the administered (stateful) protocol for address
autoconfiguration in addition to any addresses autoconfigured
using stateless address autoconfiguration. The use of this
flag is described in
<xref target='RFC4861' /> <xref target='RFC4862' />.
</t>
<t hangText='O'> <vspace blankLines='1' />
1-bit Other Stateful Configuration flag. When set, hosts use
the administered (stateful) protocol for autoconfiguration of
other (non-address) information. The use of this flag is
described in <xref target='RFC4861' /> <xref target='RFC4862' />.
</t>
<t hangText='Reserved'> <vspace blankLines='1' />
This field is unused. It MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
</t>
</list>
<t>
The Mobile Prefix Advertisement messages may have options.
These options MUST use the option format defined in
Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>).
This document defines one option
which may be carried in a Mobile Prefix Advertisement message, but
future documents may define new options. Mobile nodes MUST silently
ignore any
options they do not recognize and continue processing the message.
</t>
<list style='hanging'>
<t hangText='Prefix Information'> <vspace blankLines='1' />
Each message contains one or more Prefix Information options.
Each option carries the prefix(es) that the mobile node should
use to configure its home address(es). <xref target='ha:renum' />
describes which prefixes should be advertised to the mobile node.
<vspace blankLines='1' />
The Prefix Information option is defined in Section 4.6.2 of
Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>), with
modifications defined in
<xref target='global-rtr' /> of this specification.
The home agent MUST use this modified Prefix Information
option to send home network prefixes as
defined in <xref target='ha:renum-aggr' />.
</t>
</list>
<t>
If the Advertisement is sent in response to a Mobile Prefix
Solicitation, the home agent MUST copy the Identifier value
from that message into the Identifier field of the Advertisement.
</t>
<t>
The home agent MUST NOT send more than one Mobile Prefix
Advertisement message per second to any mobile node.
</t>
<t>
The M and O bits MUST be cleared if the Home Agent DHCPv6
support is not provided. If such support is provided then they
are set in concert with the home network's administrative settings.
</t>
</section>
</section>
<section anchor='nd' title='Modifications to IPv6 Neighbor Discovery'>
<section anchor='router-adv'
title='Modified Router Advertisement Message Format'>
<t>
Mobile IPv6 modifies the format of the
<xref target='RFC4861'>Router Advertisement message</xref>
by the addition of a single flag bit
to indicate that the router sending the Advertisement message
is serving as a home agent on this link.
The format of the Router Advertisement message is as follows:
</t>
<figure><artwork>
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cur Hop Limit |M|O|H| Reserved| Router Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reachable Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retrans Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
</artwork></figure>
<t>
This format represents the following changes over that originally
specified for <xref target='RFC4861'>Neighbor Discovery</xref>:
</t>
<list style='hanging'>
<t hangText='Home Agent (H)'> <vspace blankLines='1' />
The Home Agent (H) bit is set in a Router Advertisement to indicate that
the router sending this Router Advertisement is
also functioning as a Mobile IPv6 home agent on this link.
</t>
<t hangText='Reserved'> <vspace blankLines='1' />
Reduced from a 6-bit field to a 5-bit field
to account for the addition of the above bit.
</t>
</list>
</section>
<section anchor='global-rtr' title='Modified Prefix Information Option Format'>
<t>
Mobile IPv6 requires knowledge of a router's global address
in building a Home Agents List
as part of the dynamic home agent address discovery mechanism.
</t>
<t>
However, <xref target='RFC4861'>Neighbor Discovery</xref>
only advertises a router's link-local address,
by requiring this address to be used as the
IP Source Address of each Router Advertisement.
</t>
<t>
Mobile IPv6 extends Neighbor Discovery
to allow a router to advertise its global address,
by the addition of a single flag bit in the format of a
Prefix Information option for use in Router Advertisement messages.
The format of the Prefix Information option is as follows:
</t>
<figure><artwork>
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 | Prefix Length |L|A|R|Reserved1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preferred Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>
This format represents the following changes over that originally
specified for <xref target='RFC4861'>Neighbor Discovery</xref>:
</t>
<list style='hanging'>
<t hangText='Router Address (R)'> <vspace blankLines='1' />
1-bit router address flag. When set, indicates that the Prefix
field contains a complete IP address assigned to the sending router.
The indicated prefix is the first Prefix Length bits of the Prefix
field.
The router IP address has the same scope
and conforms to the same lifetime values as the advertised prefix.
This use of the Prefix field is
compatible with its use in advertising the prefix itself,
since Prefix Advertisement uses only the leading bits.
Interpretation of this flag bit is thus independent of the
processing required for the
On-Link (L) and Autonomous Address-Configuration (A) flag bits.
</t>
<t hangText='Reserved1'> <vspace blankLines='1' />
Reduced from a 6-bit field to a 5-bit field
to account for the addition of the above bit.
</t>
</list>
<t>
In a Router Advertisement, a home agent MUST, and all other routers MAY,
include at least one Prefix Information option with the Router Address
(R) bit set.
Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>)
specifies that, when including all
options in a Router Advertisement causes the size of the Advertisement
to exceed the link MTU, multiple Advertisements can be sent, each
containing a subset of the Neighbor Discovery options.
Also, when sending unsolicited multicast Router Advertisements more
frequently than the limit specified in RFC 4861, the
sending router need not include all options in each of these
Advertisements. However, in both of these cases the router SHOULD
include at least one Prefix Information option with the
Router Address (R) bit set in each such advertisement, if this bit is
set in some advertisement sent by the router.
</t>
<t>
In addition, the following requirement can assist mobile nodes
in movement detection. Barring changes in the prefixes for the
link, routers that send multiple Router Advertisements with
the Router Address (R) bit set in some of the included Prefix
Information options SHOULD provide at least one option and
router address which stays the same in all of the
Advertisements.
</t>
</section>
<section anchor='adv-int' title='New Advertisement Interval Option Format'>
<t>Mobile IPv6 defines a new Advertisement Interval option,
used in Router Advertisement
messages to advertise the interval at which the sending router
sends unsolicited multicast Router Advertisements.
The format of the Advertisement Interval option is as follows:
</t>
<figure><artwork>
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertisement Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Type'>
<vspace blankLines='1' /> 7
</t>
<t hangText='Length'>
<vspace blankLines='1' />8-bit unsigned integer.
The length of the option (including the type and length fields)
is in units of 8 octets.
The value of this field MUST be 1.
</t>
<t hangText='Reserved'>
<vspace blankLines='1' />This field is unused.
It MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
</t>
<t hangText='Advertisement Interval'>
<vspace blankLines='1' />32-bit unsigned integer.
The maximum time, in milliseconds,
between successive unsolicited Router Advertisement messages
sent by this router on this network interface.
Using the conceptual router configuration variables
defined by <xref target='RFC4861'>Neighbor Discovery</xref>,
this field MUST be equal to the value MaxRtrAdvInterval,
expressed in milliseconds.
</t>
</list>
<t>Routers MAY include this option in their Router Advertisements.
A mobile node receiving a Router Advertisement containing this option
SHOULD utilize the specified Advertisement Interval for that
router in its movement detection algorithm,
as described in <xref target='move' />.
</t>
<t>This option MUST be silently ignored for other
Neighbor Discovery messages.
</t>
</section>
<section anchor='ha-info' title='New Home Agent Information Option Format'>
<t>Mobile IPv6 defines a new Home Agent Information option,
used in Router Advertisements sent by a home agent to advertise
information specific to this router's functionality as a home agent.
The format of the Home Agent Information option is as follows:
</t>
<figure><artwork>
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent Preference | Home Agent Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork></figure><list style='hanging'>
<t hangText='Type'>
<vspace blankLines='1' /> 8
</t>
<t hangText='Length'>
<vspace blankLines='1' />8-bit unsigned integer.
The length of the option (including the type and length fields)
in units of 8 octets.
The value of this field MUST be 1.
</t>
<t hangText='Reserved'>
<vspace blankLines='1' />This field is unused.
It MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
</t>
<t hangText='Home Agent Preference'>
<vspace blankLines='1' />16-bit unsigned integer.
The preference for the home agent sending this Router Advertisement,
for use in ordering the addresses returned to a mobile node
in the Home Agent Addresses field of
a Home Agent Address Discovery Reply message.
Higher values mean more preferable.
If this option is not included in a Router Advertisement in which
the Home Agent (H) bit is set,
the preference value for this home agent MUST be considered to be 0.
Greater values indicate a more preferable home agent than lower
values.
<vspace blankLines='1' />The manual configuration of the Home Agent Preference
value is described in <xref target='req:ha' />.
In addition, the sending home agent MAY dynamically set the
Home Agent Preference value, for example basing it on the number
of mobile nodes it is currently serving or
on its remaining resources for serving additional mobile nodes;
such dynamic settings are beyond the scope of this document.
Any such dynamic setting of the Home Agent Preference,
however, MUST set the preference appropriately,
relative to the default Home Agent Preference value of 0
that may be in use by some home agents on this link
(i.e., a home agent not including a Home Agent Information
option in its Router Advertisements will be considered to have
a Home Agent Preference value of 0).
</t>
<t hangText='Home Agent Lifetime'>
<vspace blankLines='1' />16-bit unsigned integer.
The lifetime associated with the home agent in units of seconds.
The default value is the same as the Router Lifetime, as
specified in the main body of the Router Advertisement.
The maximum value corresponds to 18.2 hours.
A value of 0 MUST NOT be used.
The Home Agent Lifetime applies only to
this router's usefulness as a home agent;
it does not apply to information contained in other
message fields or options.
</t>
</list>
<t>Home agents MAY include this option in their Router Advertisements.
This option MUST NOT be included in a Router Advertisement in which
the Home Agent (H) bit (see <xref target='router-adv' />) is not set.
If this option is not included in a Router Advertisement in which
the Home Agent (H) bit is set,
the lifetime for this home agent MUST be considered to be the
same as the Router Lifetime in the Router Advertisement.
If multiple Advertisements are being sent instead of a single larger
unsolicited multicast Advertisement, all of the multiple
Advertisements with the Router Address (R) bit set MUST include
this option with the same contents, otherwise this option MUST be
omitted from all Advertisements.
</t>
<t>This option MUST be silently ignored for other
Neighbor Discovery messages.
</t>
<t>If both the Home Agent Preference and Home Agent Lifetime
are set to their default values specified above,
this option SHOULD NOT be included in the Router Advertisement
messages sent by this home agent.
</t>
</section>
<section anchor='change-adv' title='Changes to Sending Router Advertisements'>
<t>The <xref target='RFC4861'>Neighbor Discovery protocol specification</xref>
limits routers to
a minimum interval of 3 seconds between sending unsolicited multicast
Router Advertisement messages from any given network interface
(limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that:
</t>
<list style='empty'>
<t>"Routers generate Router Advertisements frequently enough
that hosts will learn of their presence within a few
minutes, but not frequently enough to rely on an absence
of advertisements to detect router failure; a separate
Neighbor Unreachability Detection algorithm provides failure
detection."
</t>
</list>
<t>This limitation, however, is not suitable to providing timely movement
detection for mobile nodes. Mobile nodes detect their own movement by
learning the presence of new routers as the mobile node moves into
wireless transmission range of them (or physically connects to a new
wired network), and by learning that previous routers are no longer
reachable. Mobile nodes MUST be able to quickly detect when they move
to a link served by a new router, so that they can acquire a new
care-of address and send Binding Updates to register this care-of
address with their home agent and to notify correspondent nodes as
needed.
</t>
<t>One method which can provide for faster movement detection, is to
increase the rate at which unsolicited Router Advertisements are sent.
Mobile IPv6 relaxes this limit such that routers MAY send unsolicited
multicast Router Advertisements more frequently. This method can be
applied where the router is expecting to provide service to visiting
mobile nodes (e.g., wireless network interfaces), or on which it is
serving as a home agent to one or more mobile nodes (who may return
home and need to hear its Advertisements).
</t>
<t>Routers supporting mobility SHOULD be able to be configured with a
smaller MinRtrAdvInterval value and MaxRtrAdvInterval value to allow
sending of unsolicited multicast Router Advertisements more often.
The minimum allowed values are:
</t>
<list style='symbols'>
<t> MinRtrAdvInterval 0.03 seconds
</t>
<t> MaxRtrAdvInterval 0.07 seconds
</t>
</list>
<t>
In the case where the minimum intervals and delays are used, the mean
time between unsolicited multicast router advertisements is 50ms.
Use of these modified limits MUST be configurable (see also the
configuration variable MinDelayBetweenRas in <xref target='variables'/>
which may also have to be modified accordingly).
Systems where these values are available MUST NOT default to them,
and SHOULD default to values specified in
Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>).
Knowledge of the type of network
interface and operating environment SHOULD be taken into account in
configuring these limits for each network interface. This is important
with some wireless links, where increasing the frequency of multicast
beacons can cause considerable overhead. Routers SHOULD adhere to the
intervals specified in <xref target='RFC4861'>RFC 4861</xref>,
if this overhead is likely to cause service degradation.
</t>
<t>Additionally, the possible low values of MaxRtrAdvInterval may
cause some problems with movement detection in some mobile nodes. To ensure
that this is not a problem, Routers SHOULD add 20ms to any
Advertisement Intervals sent in RAs, which are below 200 ms, in
order to account for scheduling granularities on both the MN and
the Router.
</t>
<t>
Note that multicast Router Advertisements are not always required
in certain wireless networks that have limited bandwidth. Mobility
detection or link changes in such networks may be done at lower
layers. Router advertisements in such networks SHOULD be sent only
when solicited. In such networks it SHOULD be possible to disable
unsolicited multicast Router Advertisements on specific interfaces.
The MinRtrAdvInterval and MaxRtrAdvInterval in such a case can be
set to some high values.
</t>
<t>
Home agents MUST include the Source Link-Layer Address option in all
Router Advertisements they send. This simplifies the process of
returning home, as discussed in <xref target='return-home' />.
</t>
<t>
Note that according to
Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>),
AdvDefaultLifetime is
by default based on the value of MaxRtrAdvInterval. AdvDefaultLifetime
is used in the Router Lifetime field of Router Advertisements. Given
that this field is expressed in seconds, a small MaxRtrAdvInterval
value can result in a zero value for this field. To prevent this,
routers SHOULD keep AdvDefaultLifetime in at least one second, even if
the use of MaxRtrAdvInterval would result in a smaller value.
</t>
</section>
</section>
<section anchor='new-require' title='Requirements for Types of IPv6 Nodes'>
<t>Mobile IPv6 places some special requirements on the functions
provided by different types of IPv6 nodes.
This section summarizes those requirements,
identifying the functionality each requirement is intended to support.
</t>
<t>The requirements are set for the following groups of nodes:
</t>
<list style='symbols'>
<t> All IPv6 nodes.
</t>
<t> All IPv6 nodes with support for route optimization.
</t>
<t> All IPv6 routers.
</t>
<t> All Mobile IPv6 home agents.
</t>
<t> All Mobile IPv6 mobile nodes.
</t>
</list>
<t>It is outside the scope of this specification to specify which of
these groups are mandatory in IPv6. We only describe what is mandatory
for a node that supports, for instance, route optimization. Other
specifications are expected to define the extent of IPv6.
</t>
<section anchor='req:all' title='All IPv6 Nodes'>
<t>Any IPv6 node may at any time be a correspondent node of a mobile
node, either sending a packet to a mobile node or receiving a packet
from a mobile node. There are no Mobile IPv6 specific MUST requirements for
such nodes, and basic IPv6 techniques are sufficient. If a mobile node
attempts to set up route optimization with a node with only basic IPv6
support, an ICMP error will signal that the node does not support such
optimizations (<xref target='recv-icmp-update' />), and communications will flow through the home agent .
</t>
<t>An IPv6 node MUST NOT support the Home Address destination option,
type 2 routing header, or the Mobility Header unless it fully supports
the requirements listed in the next sections for either route
optimization, mobile node, or home agent functionality.
</t>
</section>
<section anchor='req:cn' title='IPv6 Nodes with Support for Route Optimization'>
<t>Nodes that implement route optimization are a subset of all IPv6 nodes
on the Internet. The ability of a correspondent node to participate
in route optimization is essential for the efficient operation of the
IPv6 Internet, for the following reasons:
</t>
<list style='symbols'>
<t> Avoidance of congestion in the home network,
and enabling the use of lower-performance home agent equipment
even for supporting thousands of mobile nodes.
</t>
<t> Reduced network load across the entire Internet, as mobile
devices begin to predominate.
</t>
<t> Reduction of jitter and latency for the communications.
</t>
<t> Greater likelihood of success for QoS signaling as tunneling
is avoided and, again, fewer sources of congestion.
</t>
<t> Improved robustness against network partitions, congestion,
and other problems, since fewer routing path segments are
traversed.
</t>
</list>
<t>These effects combine to enable much better performance and robustness
for communications between mobile nodes and IPv6 correspondent nodes.
Route optimization introduces a small amount of additional state for
the peers, some additional messaging, and up to 1.5 roundtrip delays
before it can be turned on. However, it is believed that the benefits
far outweigh the costs in most cases. <xref target='mobile-sending' />
discusses how mobile nodes may avoid route optimization for some of
the remaining cases, such as very short-term communications.
</t>
<t>The following requirements apply to all correspondent nodes that support
route optimization:
</t>
<list style='symbols'>
<t> The node MUST be able to validate a Home Address option using an
existing Binding Cache entry, as described in <xref target='rxing-hao' />.
</t>
<t> The node MUST be able to insert a type 2 routing header
into packets to be sent to a mobile node, as described
in <xref target='corresp-deliver' />.
</t>
<t> Unless the correspondent node is also acting as a mobile node,
it MUST ignore type 2 routing headers and silently discard all
packets that it has received with such headers.
</t>
<t> The node SHOULD be able to interpret ICMP messages as
described in <xref target='cn:recvicmp' />.
</t>
<t> The node MUST be able to send Binding Error messages
as described in <xref target='sending-binding-error' />.
</t>
<t> The node MUST be able to process Mobility Headers as described in
<xref target='processing-mh' />.
</t>
<t> The node MUST be able to participate in a return routability
procedure (<xref target='processing-rr' />).
</t>
<t> The node MUST be able to process Binding Update
messages (<xref target='processing-bindings' />).
</t>
<t> The node MUST be able to return a Binding Acknowledgement
(<xref target='send-ba' />).
</t>
<t> The node MUST be able to maintain a Binding Cache
of the bindings received in accepted Binding Updates, as
described in <xref target='cn:data' /> and <xref target='cache-mgt' />.
</t>
<t> The node SHOULD allow route optimization to be
administratively enabled or disabled. The default SHOULD be
enabled.
</t>
</list>
</section>
<section anchor='req:routers' title='All IPv6 Routers'>
<t>All IPv6 routers, even those not
serving as a home agent for Mobile IPv6, have an
effect on how well mobile nodes can communicate:
</t>
<list style='symbols'>
<t> Every IPv6 router SHOULD be able to send an Advertisement
Interval option (<xref target='adv-int' />) in each of its
<xref target='RFC4861'>Router Advertisements</xref>,
to aid movement detection by
mobile nodes (as in <xref target='move' />). The use of this
option in Router Advertisements SHOULD be configurable.
</t>
<t> Every IPv6 router SHOULD be able to support sending unsolicited
multicast Router Advertisements at the faster rate described in
<xref target='change-adv' />. If the router supports a faster rate, the
used rate MUST be configurable.
</t>
<t> Each router SHOULD include at least one prefix with the Router
Address (R) bit set and with its full IP address in its Router
Advertisements (as described in <xref target='global-rtr' />).
</t>
<t> Routers supporting filtering packets with routing
headers SHOULD support different rules for type 0 and type 2
routing headers (see <xref target='rhtype2' />) so that filtering
of source routed packets (type 0) will not necessarily limit
Mobile IPv6 traffic which is delivered via type 2 routing
headers.
</t>
</list>
</section>
<section anchor='req:ha' title='IPv6 Home Agents'>
<t>In order for a mobile node to operate correctly while away from
home, at least one IPv6 router on the mobile node's home link
must function as a home agent for the mobile node.
The following additional requirements apply to all IPv6 routers
that serve as a home agent:
</t>
<list style='symbols'>
<t> Every home agent MUST be able to maintain an entry in its
Binding Cache for each mobile node for which it is serving as
the home agent (<xref target='ha:data' /> and <xref target='home-reg' />).
</t>
<t> Every home agent MUST be able to intercept packets (using proxy
<xref target='RFC4861'>Neighbor Discovery</xref>) addressed
to a mobile node
for which it is currently serving as the home agent, on that
mobile node's home link, while the mobile node is away from
home (<xref target='intercept' />).
</t>
<t> Every home agent MUST be able to <xref target='RFC2473'>encapsulate</xref>
such intercepted packets in order to tunnel them to the primary
care-of address for the mobile node indicated in its binding in
the home agent's Binding Cache (<xref target='ha-deliver' />).
</t>
<t> Every home agent MUST support <xref target='RFC2473'>decapsulating</xref>
reverse tunneled packets sent to it from a mobile node's home
address. Every home agent MUST also check that the source
address in the tunneled packets corresponds to the currently
registered location of the mobile node (<xref target='reverse' />).
</t>
<t> The node MUST be able to process Mobility Headers as described in
<xref target='ha-processing-mh' />.
</t>
<t> Every home agent MUST be able to return a Binding
Acknowledgement in response to a Binding Update
(<xref target='home-reg' />).
</t>
<t> Every home agent MUST maintain a separate Home Agents List for
each link on which it is serving as a home agent, as described
in <xref target='ha:data' /> and <xref target='ha-advert' />.
</t>
<t> Every home agent MUST be able to accept packets addressed to
the Mobile IPv6 <xref target='RFC2526'>Home-Agents anycast address</xref> for the subnet
on which it is serving as a home agent, and MUST
be able to participate in dynamic home agent address discovery
(<xref target='ha-disc' />).
</t>
<t> Every home agent SHOULD support a configuration mechanism to
allow a system administrator to manually set the value to be
sent by this home agent in the Home Agent Preference field of
the Home Agent Information Option in Router Advertisements
that it sends (<xref target='ha-info' />).
</t>
<t> Every home agent SHOULD support sending ICMP Mobile Prefix
Advertisements (<xref target='renum-mpa' />), and SHOULD respond
to Mobile Prefix Solicitations (<xref target='renum-mps' />).
If supported, this behavior MUST be configurable, so that
home agents can be configured to avoid sending such
Prefix Advertisements according to the needs of the
network administration in the home domain.
</t>
<t> Every home agent MUST support IPsec ESP for protection of
packets belonging to the return routability procedure
(<xref target='ha:rrprot' />).
</t>
<t> Every home agent SHOULD support the multicast
group membership control protocols as described in
<xref target='ha:multicast' />. If this support is provided, the
home agent MUST be capable of using it to determine which
multicast data packets to forward via the tunnel to the mobile
node.
</t>
<t> Home agents MAY support stateful address autoconfiguration
for mobile nodes as described in <xref target='ha:dhcp' />.
</t>
</list>
</section>
<section anchor='req:mn' title='IPv6 Mobile Nodes'>
<t>Finally, the following requirements apply to all IPv6 nodes
capable of functioning as mobile nodes:
</t>
<list style='symbols'>
<t> The node MUST maintain a
Binding Update List (<xref target='mn:data' />).
</t>
<t> The node MUST support sending packets containing a
Home Address option (<xref target='mobile-sending' />), and follow
the required IPsec interaction (<xref target='outbound-ipsec' />).
</t>
<t> The node MUST be able to perform
<xref target='RFC2473'>IPv6 encapsulation and decapsulation</xref>.
</t>
<t> The node MUST be able to process type 2 routing header as
defined in <xref target='rhtype2' /> and
<xref target='mobile-receiving' />.
</t>
<t> The node MUST support receiving a Binding Error message
(<xref target='mn-bm' />).
</t>
<t> The node MUST support receiving ICMP errors
(<xref target='recv-icmp-update' />).
</t>
<t> The node MUST support movement detection, care-of address
formation, and returning home (<xref target='movement-all' />).
</t>
<t> The node MUST be able to process Mobility Headers
as described in <xref target='mn-processing-mh' />.
</t>
<t> The node MUST support the return routability
procedure (<xref target='mobile-rrp' />).
</t>
<t> The node MUST be able to send Binding Updates,
as specified in <xref target='update-home' />
and <xref target='update-corresp' />.
</t>
<t> The node MUST be able to receive and process
Binding Acknowledgements, as specified in
<xref target='mobile-recv-acks' />.
</t>
<t> The node MUST support receiving a Binding Refresh Request
(<xref target='bind-req' />), by responding with a Binding Update.
</t>
<t> The node MUST support receiving Mobile Prefix Advertisements
(<xref target='mn:get-adv' />) and reconfiguring its home address
based on the prefix information contained therein.
</t>
<t> The node SHOULD support use of the dynamic home agent address
discovery mechanism, as described in <xref target='mn-ha-disc' />.
</t>
<t> The node MUST allow route optimization to be administratively
enabled or disabled. The default SHOULD be enabled.
</t>
<t> The node MAY support the multicast address listener part
of a multicast group membership protocol as described in
<xref target='multicast' />. If this support is provided, the
mobile node MUST be able to receive tunneled multicast
packets from the home agent.
</t>
<t> The node MAY support stateful address autoconfiguration
mechanisms such as <xref target='RFC3315'>DHCPv6</xref> on the
interface represented by the tunnel to the home agent.
</t>
</list>
</section>
</section>
<section anchor='corresp' title='Correspondent Node Operation'>
<section anchor='cn:data' title='Conceptual Data Structures'>
<t>IPv6 nodes with route optimization support maintain a Binding
Cache of bindings for other nodes.
A separate Binding Cache SHOULD be maintained by each IPv6 node
for each of its unicast routable addresses.
The Binding Cache MAY be implemented in any manner consistent with
the external behavior described in this document,
for example by being combined with the node's Destination Cache
as maintained by <xref target='RFC4861'>Neighbor Discovery</xref>.
When sending a packet,
the Binding Cache is searched before the
Neighbor Discovery conceptual <xref target='RFC4861'>Destination Cache</xref>.
</t>
<t>Each Binding Cache entry conceptually contains the following fields:
</t>
<list style='symbols'>
<t> The home address of the mobile node for which this is
the Binding Cache entry.
This field is used as the key for searching the Binding Cache
for the destination address of a packet being sent.
</t>
<t> The care-of address for the mobile node indicated by
the home address field in this Binding Cache entry.
</t>
<t> A lifetime value,
indicating the remaining lifetime for this Binding Cache entry.
The lifetime value is initialized from the Lifetime field
in the Binding Update that created or last modified this
Binding Cache entry. A correspondent node MAY select a
smaller lifetime for the Binding Cache entry, and supply that value
to the mobile node in the Binding Acknowledgment message.
</t>
<t> A flag indicating whether or not this Binding Cache entry is a
home registration entry (applicable only on nodes which
support home agent functionality).
</t>
<t> The maximum value of the Sequence Number field received
in previous Binding Updates for this home address.
The Sequence Number field is 16 bits long.
Sequence Number values MUST be compared
modulo 2**16 as explained in <xref target='recv-updates' />.
</t>
<t> Usage information for this Binding Cache entry.
This is needed to implement the cache replacement policy
in use in the Binding Cache. Recent use of a cache entry
also serves as an indication that a Binding Refresh Request
should be sent when the lifetime of this entry
nears expiration.
</t>
</list>
<t>Binding Cache entries not marked as home registrations MAY be replaced
at any time by any reasonable local cache replacement policy
but SHOULD NOT be unnecessarily deleted.
The Binding Cache for any one of a node's IPv6 addresses
may contain at most one entry for each mobile node home address.
The contents of a node's Binding Cache MUST NOT be
changed in response to a Home Address option in a received packet.
</t>
</section>
<section anchor='processing-mh' title='Processing Mobility Headers'>
<t>Mobility Header processing MUST observe the following rules:
</t>
<list style='symbols'>
<t> The checksum must be verified as per <xref target='mobility-header' />.
If invalid, the node MUST silently discard the message.
</t>
<t> The MH Type field MUST have a known value
(<xref target='mobility-header-format' />).
Otherwise, the node MUST discard the message and issue a
Binding Error message as described in
<xref target='sending-binding-error' />, with Status field set to
2 (unrecognized MH Type value).
</t>
<t> The Payload Proto field MUST be IPPROTO_NONE (59 decimal).
Otherwise, the node MUST discard the message and SHOULD send
ICMP Parameter Problem, Code 0, directly to the Source Address of the
packet as specified in <xref target='RFC4443'>RFC 4443</xref>.
Thus no Binding Cache information is used in sending the
ICMP message. The Pointer field in the ICMP message SHOULD point
at the Payload Proto field.
</t>
<t> The Header Len field in the Mobility Header MUST NOT be less
than the length specified for this particular type of message
in <xref target='mobility-header' />. Otherwise, the node MUST
discard the message and SHOULD send ICMP Parameter Problem,
Code 0, directly to the Source Address of the packet as specified
in <xref target='RFC4443'>RFC 4443</xref>.
(The Binding Cache information is
again not used.)
The Pointer field in the ICMP message SHOULD point
at the Header Len field.
</t>
</list>
<t>Subsequent checks depend on the particular Mobility Header.
</t>
</section>
<section anchor='corresp-receive' title='Packet Processing'>
<t>This section describes how the correspondent node sends packets to the
mobile node, and receives packets from it.
</t>
<section anchor='rxing-hao' title='Receiving Packets with Home Address Option'>
<t>Packets containing a Home Address option MUST be dropped if the
given home address is not a unicast routable address.
</t>
<t>
Mobile nodes can include a Home Address destination option in a
packet if they believe the correspondent node has a Binding Cache
entry for the home address of a mobile node. If the Next Header
value of the Destination Option is one of the following: {50 (ESP),
51 (AH), 135 (Mobility Header)}, the packet SHOULD be processed
normally. Otherwise, the packet MUST be dropped if there is no
corresponding Binding Cache entry. A corresponding Binding
Cache entry MUST have the same home address as appears in
the Home Address destination option, and the currently
registered care-of address MUST be equal to the source address
of the packet.
</t>
<t>If the packet is dropped due to the above tests, the correspondent node
MUST send the Binding Error message as described in
<xref target='sending-binding-error' />. The Status field in this message
should be set to 1 (unknown binding for Home Address destination option).
</t>
<t>The correspondent node
MUST process the option in a manner consistent with exchanging the
Home Address field from the Home Address option into the IPv6
header and replacing the original value of the Source Address field
there. After all IPv6 options have been processed, it MUST be
possible for upper layers to process the packet without the knowledge that it
came originally from a care-of address or that a Home Address
option was used.
</t>
<t>The use of IPsec Authentication Header (AH) for the Home Address option is not required,
except that if the IPv6 header of a packet is covered by AH,
then the authentication MUST also cover the Home Address option;
this coverage is achieved automatically
by the definition of the Option Type code for the Home Address option,
since it indicates that the data within the option cannot change en route
to the packet's final destination,
and thus the option is included in the AH computation.
By requiring that any authentication of the IPv6 header also cover
the Home Address option,
the security of the Source Address field in the IPv6 header is
not compromised by the presence of a Home Address option.
</t>
<t>When attempting to verify AH authentication data in a packet that
contains a Home Address option, the receiving node MUST calculate the AH
authentication data as if the following were true: The Home Address
option contains the care-of address, and the source IPv6 address field
of the IPv6 header contains the home address. This conforms with the
calculation specified in <xref target='outbound-ipsec' />.
</t>
</section>
<section anchor='corresp-deliver' title='Sending Packets to a Mobile Node'>
<t>Before sending any packet, the sending node SHOULD examine its Binding
Cache for an entry for the destination address to which the packet is
being sent.
If the sending node has a Binding Cache entry
for this address, the sending node SHOULD use a type 2 routing header
to route the packet to this mobile node (the destination node) by way
of its care-of address. However, the sending node MUST NOT do this
in the following cases:
</t>
<list style='symbols'>
<t> When sending an IPv6 <xref target='RFC4861'>Neighbor Discovery</xref>
packet.
</t>
<t> Where otherwise noted in <xref target='mobility-header' />.
</t>
</list>
<t>When calculating authentication data in a packet that contains a
type 2 routing header, the correspondent node MUST calculate the
AH authentication data as if the following were true: The routing header
contains the care-of address, the destination IPv6 address field of
the IPv6 header contains the home address, and the Segments Left field
is zero. The IPsec Security Policy Database lookup MUST based
on the mobile node's home address.
</t>
<t>For instance, assuming there are no additional routing headers in this
packet beyond those needed by Mobile IPv6, the correspondent node could set
the fields in the packet's IPv6 header and routing header as follows:
</t>
<list style='symbols'>
<t> The Destination Address in the packet's IPv6 header
is set to the mobile node's home address (the original
destination address to which the packet was being sent).
</t>
<t> The routing header is initialized to contain a single route
segment, containing the mobile node's care-of address
copied from the Binding Cache entry.
The Segments Left field is, however, temporarily set to zero.
</t>
</list>
<t>The IP layer will insert the routing header before performing any necessary IPsec
processing. Once all IPsec processing has been performed, the node
swaps the IPv6 destination field with the Home Address field in the
routing header, sets the Segments Left field to one, and sends the
packet. This ensures the AH calculation is done on the packet in the
form it will have on the receiver after advancing the routing header.
</t>
<t>Following the definition of a type 2 routing header in
<xref target='rhtype2' />, this packet will be routed to the mobile node's
care-of address, where it will be delivered to the mobile node
(the mobile node has associated the care-of address with its
network interface).
</t>
<t>Note that following the above conceptual model in an implementation creates
some additional requirements for path MTU discovery since the layer that
determines the packet size (e.g., TCP and applications using UDP) needs to be
aware of the size of the headers added by the IP layer on the sending node.
</t>
<t>If, instead, the sending node has no Binding Cache entry for
the destination address to which the packet is being sent,
the sending node simply sends the packet normally,
with no routing header.
If the destination node is not a mobile node
(or is a mobile node that is currently at home),
the packet will be delivered directly to this node
and processed normally by it.
If, however, the destination node is a mobile node that is
currently away from home,
the packet will be intercepted by the mobile node's home agent
and tunneled to the mobile node's current primary care-of address.
</t>
</section>
<section anchor='sending-binding-error' title='Sending Binding Error Messages'>
<t><xref target='processing-mh' /> and <xref target='rxing-hao' /> describe error
conditions that lead to a need to send a Binding Error message.
</t>
<t>A Binding Error message is sent directly to the address that appeared
in the IPv6 Source Address field of the offending packet. If the
Source Address field does not contain a unicast address, the Binding
Error message MUST NOT be sent.
</t>
<t>The Home Address field in the Binding Error message MUST be copied
from the Home Address field in the Home Address destination option
of the offending packet, or set to the unspecified address if no
such option appeared in the packet.
</t>
<t>Note that the IPv6 Source Address and Home Address field values
discussed above are the values from the wire, i.e., before any
modifications possibly performed as specified in
<xref target='rxing-hao' />.
</t>
<t>Binding Error messages SHOULD be subject to rate limiting in the same
manner as is done for <xref target='RFC4443'>ICMPv6 messages</xref>.
</t>
</section>
<section anchor='cn:recvicmp' title='Receiving ICMP Error Messages'>
<t>When the correspondent node has a Binding Cache entry for a mobile
node, all traffic destined to the mobile node goes directly to the
current care-of address of the mobile node using a routing
header. Any ICMP error message caused by packets on their way to
the care-of address will be returned in the normal manner to the
correspondent node.
</t>
<t>On the other hand, if the correspondent
node has no Binding Cache entry for the mobile node,
the packet will be routed through the mobile node's home link.
Any ICMP error message caused by the packet on
its way to the mobile node while in the tunnel, will be transmitted
to the mobile node's home agent. By
the definition of <xref target='RFC2473'>IPv6 encapsulation</xref>, the home agent
MUST relay certain ICMP error messages back
to the original sender of the packet, which in this case is the
correspondent node.
</t>
<t>Thus, in all cases, any meaningful ICMP error messages caused by
packets from a correspondent node to a mobile node will be returned to
the correspondent node. If the correspondent node receives persistent
ICMP Destination Unreachable messages after sending packets to a
mobile node based on an entry in its Binding Cache, the correspondent
node SHOULD delete this Binding Cache entry. Note that if the mobile
node continues to send packets with the Home Address destination
option to this correspondent node, they will be dropped due to the
lack of a binding. For this reason it is important that only
persistent ICMP messages lead to the deletion of the Binding Cache
entry.
</t>
</section>
</section>
<section anchor='processing-rr' title='Return Routability Procedure'>
<t>This subsection specifies actions taken by
a correspondent node during the return routability procedure.
</t>
<section anchor='rxing-hoti' title='Receiving Home Test Init Messages'>
<t>Upon receiving a Home Test Init message, the correspondent node
verifies the following:
</t>
<list style='symbols'>
<t> The packet MUST NOT include a Home Address destination option.
</t>
</list>
<t>
Any packet carrying a Home Test Init message which
fails to satisfy this test MUST be silently ignored.
</t>
<t>
Otherwise, in preparation for sending the corresponding
Home Test Message, the correspondent node checks that it has the
necessary material to engage in a return routability procedure,
as specified in <xref target='securing-regs-with-cns' />.
The correspondent node MUST have a secret
Kcn and a nonce. If it does not have this material
yet, it MUST produce it before continuing with the
return routability procedure.
</t>
<t><xref target='txing-hot' /> specifies further processing.
</t>
</section>
<section anchor='rxing-coti' title='Receiving Care-of Test Init Messages'>
<t>Upon receiving a Care-of Test Init message, the correspondent node
verifies the following:
</t>
<list style='symbols'>
<t> The packet MUST NOT include a Home Address destination option.
</t>
</list>
<t>
Any packet carrying a Care-of Test Init message which
fails to satisfy this test MUST be silently ignored.
</t>
<t>
Otherwise, in preparation for sending the corresponding Care-of
Test Message, the correspondent node checks that it has the
necessary material to engage in a return routability procedure
in the manner described in <xref target='rxing-hoti' />.
</t>
<t><xref target='txing-cot' /> specifies further processing.
</t>
</section>
<section anchor='txing-hot' title='Sending Home Test Messages'>
<t>The correspondent node creates a home keygen token and uses the
current nonce index as the Home Nonce Index. It then creates a
Home Test message (<xref target='hot' />) and sends it to the mobile
node at the latter's home address.
</t>
</section>
<section anchor='txing-cot' title='Sending Care-of Test Messages'>
<t>The correspondent node creates a care-of keygen token and uses the current
nonce index as the Care-of Nonce Index. It then creates a Care-of
Test message (<xref target='cot' />) and sends it to the mobile node at
the latter's care-of address.
</t>
</section>
</section>
<section anchor='processing-bindings' title='Processing Bindings'>
<t>This section explains how the correspondent node processes
messages related to bindings. These messages are:
</t>
<list style='symbols'>
<t> Binding Update
</t>
<t> Binding Refresh Request
</t>
<t> Binding Acknowledgement
</t>
<t> Binding Error
</t>
</list>
<section anchor='recv-updates' title='Receiving Binding Updates'>
<t>Before accepting a Binding Update,
the receiving node MUST validate the Binding Update according to
the following tests:
</t>
<list style='symbols'>
<t> The packet MUST contain a unicast routable home address, either
in the Home Address option or in the Source Address, if the
Home Address option is not present.
</t>
<t> The Sequence Number field in the Binding Update is greater
than the Sequence Number received in the previous valid Binding
Update for this home address, if any.
<vspace blankLines='1' /> If the receiving node has no Binding Cache entry for the
indicated home address, it MUST accept any Sequence Number
value in a received Binding Update from this mobile node.
<vspace blankLines='1' /> This Sequence Number
comparison MUST be performed modulo 2**16, i.e., the number
is a free running counter represented modulo 65536. A Sequence
Number in a received Binding Update is considered
less than or equal to the last received number if its value lies in
the range of the last received number and the preceding 32768 values,
inclusive. For example, if the last received sequence number was 15,
then messages with sequence numbers 0 through 15, as well as 32783
through 65535, would be considered less than or equal.
</t>
</list>
<t>When the Home Registration (H) bit is not set, the following are also required:
</t>
<list style='symbols'>
<t> A Nonce Indices mobility option MUST be present, and the
Home and Care-of Nonce Index values in this option MUST be
recent enough to be recognized by the correspondent node.
(Care-of Nonce Index values are not inspected for requests
to delete a binding.)
</t>
<t> The correspondent node MUST re-generate the home keygen token
and the care-of keygen token from the information contained in the
packet. It then generates the binding management key Kbm and uses
it to verify the authenticator field in the Binding Update as
specified in <xref target='bind-upd' />.
</t>
<t> The Binding Authorization Data mobility option MUST be present, and
its contents MUST satisfy rules presented in <xref target='rr-cnbind' />.
Note that a care-of address different from the Source Address
MAY have been specified by including an Alternate Care-of Address
mobility option in the Binding Update. When such a message
is received and the return routability procedure is used as an
authorization method, the correspondent node MUST verify the
authenticator by using the address within the Alternate Care-of
Address in the calculations.
</t>
<t> The Binding Authorization Data mobility option MUST be
the last option and MUST NOT have trailing padding.
</t>
</list>
<t> If the Home Registration (H) bit is set, the Nonce Indices
mobility option MUST NOT be present.
</t>
<t> If the mobile node sends a sequence number which is not greater than
the sequence number from the last valid Binding Update for this home address, then the
receiving node MUST send back a Binding Acknowledgement with status
code 135, and the last accepted sequence number in the Sequence
Number field of the Binding Acknowledgement.
</t>
<t> If a binding already exists for the given home address and the
home registration flag has a different value than the Home
Registration (H) bit in the Binding Update, then the receiving node
MUST send back a Binding Acknowledgement with status code 139 (registration
type change disallowed). The home registration flag stored in the
Binding Cache entry MUST NOT be changed.
</t>
<t> If the receiving node no longer recognizes the Home
Nonce Index value, Care-of Nonce Index value, or both values
from the Binding Update, then the receiving node MUST
send back a Binding Acknowledgement with status
code 136, 137, or 138, respectively.
</t>
<t>
Packets carrying Binding Updates that fail to satisfy all of
these tests for any reason other than insufficiency of the Sequence
Number, registration type change, or expired nonce index values,
MUST be silently discarded.
</t>
<t>
If the Binding Update is valid according to the tests above, then the
Binding Update is processed further as follows:
</t>
<list style='symbols'>
<t> The Sequence Number value received from a mobile node in a
Binding Update is stored by the receiving node in its Binding
Cache entry for the given home address.
</t>
<t>
If the Lifetime specified in the Binding Update is not zero,
then this is a request to cache a binding for the home
address. If the Home Registration (H) bit is set in the
Binding Update, the Binding Update is processed according to the
procedure specified in <xref target='home-reg' />;
otherwise, it is processed according to the procedure
specified in <xref target='cache-update' />.
</t>
<t> If the Lifetime specified in the Binding Update is zero or the
specified care-of address matches the home address for the
binding, then this is a request to delete the
cached binding for the home address. In this case, the Binding
Update MUST include a valid home nonce index, and the care-of
nonce index MUST be ignored by the correspondent node.
The generation of the binding management key depends then
exclusively on the home keygen token (<xref target='rrp' />).
If the Home Registration (H) bit is set in the
Binding Update, the Binding Update is processed according to the
procedure specified in <xref target='home-dereg' />; otherwise,
it is processed according to the procedure specified in
<xref target='cache-delete' />.
</t>
</list>
<t>The specified care-of address MUST be determined as follows:
</t>
<list style='symbols'>
<t> If the Alternate Care-of Address option is present, the
care-of address is the address in that option.
</t>
<t> Otherwise, the care-of address is the Source Address
field in the packet's IPv6 header.
</t>
</list>
<t>The home address for the binding MUST be determined as follows:
</t>
<list style='symbols'>
<t> If the Home Address destination option is present, the home
address is the address in that option.
</t>
<t> Otherwise, the home address is the Source Address field in the
packet's IPv6 header.
</t>
</list>
</section>
<section anchor='cache-update' title='Requests to Cache a Binding'>
<t>This section describes the processing of
a valid Binding Update that requests a node to cache a
binding, for which the Home Registration (H) bit is not set in the
Binding Update.
</t>
<t>In this case, the receiving node SHOULD create a new entry in its
Binding Cache for this home address, or update its existing Binding
Cache entry for this home address, if such an entry already exists.
The lifetime
for the Binding Cache entry is initialized from the Lifetime field
specified in the Binding Update, although this lifetime MAY be
reduced by the node caching the binding; the lifetime for the Binding
Cache entry MUST NOT be greater than the Lifetime value specified in
the Binding Update. Any Binding Cache entry MUST be deleted after
the expiration of its lifetime.
</t>
<t>Note that if the mobile node did not request a Binding
Acknowledgement, then it is not aware of the selected shorter lifetime.
The mobile node may thus use route optimization and send packets with
the Home Address destination option. As discussed in
<xref target='rxing-hao' />, such packets will be dropped if there is no
binding. This situation is recoverable, but can cause temporary packet
loss.
</t>
<t>The correspondent node MAY refuse to accept a new Binding Cache
entry if it does not have sufficient resources. A new entry MAY
also be refused if the correspondent node believes its resources
are utilized more efficiently in some other purpose, such as
serving another mobile node with higher amount of traffic. In both
cases the correspondent node SHOULD return a Binding
Acknowledgement with status value 130.
</t>
</section>
<section anchor='cache-delete' title='Requests to Delete a Binding'>
<t>This section describes the processing of a valid Binding Update that
requests a node to delete a binding when the Home Registration
(H) bit is not set in the Binding Update.
</t>
<t>Any existing binding for the given home address MUST be deleted. A Binding
Cache entry for the home address MUST NOT be created in response to
receiving the Binding Update.
</t>
<t>If the Binding Cache entry was created by use of return routability
nonces, the correspondent node MUST ensure that the same nonces are
not used again with the particular home and care-of address. If
both nonces are still valid, the correspondent node has to remember
the particular combination of nonce indexes, addresses, and
sequence number as illegal until at least one of the nonces has
become too old.
</t>
</section>
<section anchor='send-ba' title='Sending Binding Acknowledgements'>
<t>
A Binding Acknowledgement may be sent to indicate receipt of a
Binding Update as follows:
<list style='symbols'>
<t>
If the Binding Update was discarded as described in
<xref target='processing-mh' /> or <xref target='recv-updates' />,
a Binding Acknowledgement MUST NOT be sent. Otherwise the treatment
depends on the following rules.
</t>
<t>
If the Acknowledge (A) bit is set in the
Binding Update, a Binding Acknowledgement MUST be sent.
Otherwise, the treatment depends on the next rule.
</t>
<t>
If the node rejects the Binding Update due to an expired nonce index,
sequence number being out of window (<xref target='recv-updates' />),
or insufficiency of resources (<xref target='cache-update' />),
a Binding Acknowledgement MUST be sent. If the node accepts the
Binding Update, the Binding Acknowledgement SHOULD NOT be sent.
</t>
</list>
</t>
<t>If the node accepts the Binding Update and creates or
updates an entry for this binding, the Status field in
the Binding Acknowledgement MUST be set to a value less
than 128. Otherwise, the Status field MUST be set
to a value greater than or equal to 128. Values for
the Status field are described in <xref target='bind-ack' /> and
in the IANA registry of <xref target='RFC3232'>assigned numbers</xref>.
</t>
<t>If the Status field in the Binding Acknowledgement contains the
value 136 (expired home nonce index),
137 (expired care-of nonce index), or 138 (expired nonces) then
the message MUST NOT include the Binding Authorization Data
mobility option. Otherwise, the Binding Authorization Data mobility
option MUST be included, and MUST meet the specific authentication
requirements for Binding Acknowledgements as defined in
<xref target='securing-regs-with-cns' />.
</t>
<t>If the Source Address field of the IPv6 header that carried the
Binding Update does not contain a unicast address,
the Binding Acknowledgement MUST NOT be sent and the Binding
Update packet MUST be silently discarded. Otherwise, the
acknowledgement MUST be sent to the Source Address. Unlike
the treatment of regular packets, this addressing procedure
does not use information from the Binding Cache. However,
a routing header is needed in some cases.
If the Source Address is the home address of the mobile node,
i.e., the Binding Update did not contain a Home Address
destination option, then the Binding Acknowledgement MUST be sent to
that address and the routing header MUST NOT be used.
Otherwise, the Binding Acknowledgement MUST be sent using a
type 2 routing header which contains the mobile node's home address.
</t>
</section>
<section anchor='corresp-send-brr' title='Sending Binding Refresh Requests'>
<t>If a Binding Cache entry being deleted is still in active use when
sending packets to a mobile node, then the next packet sent to the mobile
node will be routed normally to the mobile node's home link.
Communication with the mobile node continues, but the tunneling
from the home network creates additional overhead and latency in
delivering packets to the mobile node.
</t>
<t>If the sender knows that the Binding Cache entry is still in active
use, it MAY send a Binding Refresh Request message to the mobile node
in an attempt to avoid this overhead and latency due to deleting and
recreating the Binding Cache entry. This message is always sent to
the home address of the mobile node.
</t>
<t>The correspondent node MAY retransmit Binding Refresh Request messages
as long as the rate limitation is applied. The correspondent node
MUST stop retransmitting when it receives a Binding Update.
</t>
</section>
</section>
<section anchor='cache-mgt' title='Cache Replacement Policy'>
<t>Conceptually, a node maintains a separate timer for each entry in its
Binding Cache. When creating or updating a Binding Cache entry in
response to a received and accepted Binding Update, the node sets the
timer for this entry to the specified Lifetime period. Any entry in
a node's Binding Cache MUST be deleted after the expiration of the
Lifetime specified in the Binding Update from which the entry was
created or last updated.
</t>
<t>Each node's Binding Cache will, by necessity, have a finite size.
A node MAY use any reasonable local policy for managing the space
within its Binding Cache.
</t>
<t>A node MAY choose to drop any entry already in its Binding Cache in
order to make space for a new entry. For example, a "least-recently
used" (LRU) strategy for cache entry replacement among entries should
work well, unless the size of the Binding Cache is
substantially insufficient. When entries are deleted, the
correspondent node MUST follow the rules in <xref target='rrreplay' /> in
order to guard the return routability procedure against replay
attacks.
</t>
<t>If the node sends a packet to a destination for which it has dropped
the entry from its Binding Cache, the packet will be routed through
the mobile node's home link.
The mobile node can detect this and establish a new binding if necessary.
</t>
<t>However, if the mobile node believes that the binding still exists, it
may use route optimization and send packets with the Home Address
destination option. This can create temporary packet loss, as
discussed earlier, in the context of binding lifetime reductions
performed by the correspondent node (<xref target='cache-update' />).
</t>
</section>
</section>
<section anchor='ha-consider' title='Home Agent Operation'>
<section anchor='ha:data' title='Conceptual Data Structures'>
<t>Each home agent MUST maintain a Binding Cache and Home Agents List.
</t>
<t>The rules for maintaining a Binding Cache are the same for home agents and
correspondent nodes and have already been described in
<xref target='cn:data' />.
</t>
<t>The Home Agents List is maintained by each home agent, recording
information about each router on the same link that is acting as
a home agent. This list is used by the
dynamic home agent address discovery mechanism.
A router is known to be acting as a home agent, if it sends a
Router Advertisement in which the Home Agent (H) bit is set.
When the lifetime for a list entry (defined below)
expires, that entry is removed from the Home Agents List.
The Home Agents List is similar to the
Default Router List conceptual data structure
maintained by each host for <xref target='RFC4861'>Neighbor Discovery</xref>.
The Home Agents List MAY be implemented in any manner
consistent with the external behavior described in this document.
</t>
<t>Each home agent maintains a separate Home Agents List for
each link on which it is serving as a home agent.
A new entry is created or an existing entry is updated
in response to receipt of a valid
Router Advertisement in which the Home Agent (H) bit is set.
Each Home Agents List entry conceptually contains the following fields:
</t>
<list style='symbols'>
<t> The link-local IP address of a home agent on the link.
This address is learned through
the Source Address of
the <xref target='RFC4861'>Router Advertisements</xref>
received from the router.
</t>
<t> One or more global IP addresses for this home agent.
Global addresses are learned through Prefix Information
options with the Router Address (R) bit set and received
in Router Advertisements from this link-local address.
Global addresses for the router in a Home Agents List entry
MUST be deleted once the prefix associated with that
address is no longer valid <xref target='RFC4861' />.
</t>
<t> The remaining lifetime of this Home Agents List entry.
If a Home Agent Information Option is present in a
Router Advertisement received from a home agent,
the lifetime of the Home Agents List entry representing that
home agent is initialized from the Home Agent Lifetime field
in the option (if present); otherwise, the lifetime is initialized from the
Router Lifetime field in the received Router Advertisement.
If Home Agents List entry lifetime reaches zero,
the entry MUST be deleted from the Home Agents List.
</t>
<t> The preference for this home agent;
higher values indicate a more preferable home agent.
The preference value is taken from the Home Agent Preference field
in the received Router Advertisement,
if the Router Advertisement contains a Home Agent Information Option
and is otherwise set to the default value of 0.
A home agent uses this preference
in ordering the Home Agents List when it sends
an ICMP Home Agent Address Discovery message.
</t>
</list>
</section>
<section anchor='ha-processing-mh' title='Processing Mobility Headers'>
<t>All IPv6 home agents MUST observe the rules described in
<xref target='processing-mh' /> when processing Mobility Headers.
</t>
</section>
<section title='Processing Bindings'>
<section anchor='home-reg' title='Primary Care-of Address Registration'>
<t>When a node receives a Binding Update, it MUST validate it and determine
the type of Binding Update according to the steps described in
<xref target='recv-updates' />. Furthermore, it MUST authenticate the Binding
Update as described in <xref target='securing-regs-with-home-agents' />.
An authorization step specific for the home agent is also needed to
ensure that only the right node can control a
particular home address. This is provided through the home address unequivocally identifying
the security association that must be used.
</t>
<t>This section describes the processing of a valid and authorized
Binding Update when it requests the registration of the mobile node's
primary care-of address.
</t>
<t>To begin processing the Binding Update,
the home agent MUST perform the following sequence of tests:
</t>
<list style='symbols'>
<t> If the node implements only correspondent node functionality,
or has not been configured to act as a home agent, then the node
MUST reject the Binding Update. The node MUST also
return a Binding Acknowledgement to the mobile node,
in which the Status field is set to 131 (home
registration not supported).
</t>
<t> Else, if the home address for the binding
(the Home Address field in the packet's Home Address option)
is not an on-link IPv6 address with respect to the home
agent's current Prefix List, then the home
agent MUST reject the Binding Update and SHOULD return a
Binding Acknowledgement to the mobile node, in which the
Status field is set to 132 (not home subnet).
</t>
<t> Else, if the home agent chooses to reject the Binding Update
for any other reason
(e.g., insufficient resources to serve another mobile node as a
home agent), then the home agent SHOULD return a Binding
Acknowledgement to the mobile node, in which the Status field is
set to an appropriate value to indicate the reason for the rejection.
</t>
<t> A Home Address destination option MUST be present
in the message. It MUST be validated as described in
<xref target='rxing-hao' /> with the following additional
rule. The Binding Cache entry existence test MUST NOT be done
for IPsec packets when the Home Address option contains an
address for which the receiving node could act as a home
agent.
</t>
</list>
<t>If home agent accepts the Binding Update, it MUST then create
a new entry in its Binding Cache for this mobile node or update
its existing Binding Cache entry, if such an entry already exists.
The Home Address field as received in the Home Address option
provides the home address of the mobile node.
</t>
<t>The home agent MUST mark this Binding Cache entry as
a home registration to indicate that the node is serving as
a home agent for this binding.
Binding Cache entries marked as a home registration
MUST be excluded from the normal cache replacement policy
used for the Binding Cache (<xref target='cache-mgt' />)
and MUST NOT be removed from the Binding Cache until
the expiration of the Lifetime period.
</t>
<t>Unless this home agent already has a binding for the given
home address, the home
agent MUST perform <xref target='RFC4862'>Duplicate Address Detection</xref> on
the mobile node's home link before
returning the Binding Acknowledgement. This ensures that no other node
on the home link was using the mobile node's home address when the
Binding Update arrived.
If this Duplicate Address Detection fails for the given home address
or an associated link local address, then the home agent MUST reject
the complete Binding Update and MUST return a Binding Acknowledgement
to the mobile node, in which the Status field is set to 134 (Duplicate
Address Detection failed). When the home agent sends a successful
Binding Acknowledgement to the mobile node, the home agent assures to
the mobile node that its address(es) will be kept
unique by the home agent for as long as the lifetime was granted for
the binding.
</t>
<t>The specific addresses, which are to be tested before accepting the
Binding Update and later to be defended by performing Duplicate
Address Detection, depend on the setting of the Link-Local Address
Compatibility (L) bit, as follows:
</t>
<list style='symbols'>
<t> L=0: Defend only the given address. Do not derive
a link-local address.
</t>
<t> L=1: Defend both the given non link-local unicast (home)
address and the derived link-local. The link-local address
is derived by replacing the subnet prefix in the mobile node's
home address with the link-local prefix.
</t>
</list>
<t>
The lifetime of the Binding Cache entry depends on a number of
factors:
</t>
<list style='symbols'>
<t> The lifetime for the Binding Cache entry MUST NOT be greater
than the Lifetime value specified in the Binding Update.
</t>
<t> The lifetime for the Binding Cache entry MUST NOT be greater
than the remaining valid lifetime for the subnet prefix in the
mobile node's home address specified with the Binding Update.
The remaining valid lifetime for this prefix is determined by the
home agent based on its own Prefix List entry <xref target='RFC4861'/>.
<vspace blankLines='1' /> The remaining preferred lifetime SHOULD
NOT have any impact on the lifetime for the binding cache entry.
<vspace blankLines='1' /> The home agent MUST remove a binding
when the valid lifetime of the prefix associated with it expires.
</t>
<t> The home agent MAY further decrease the specified lifetime for the
binding, for example based on a local policy. The resulting lifetime is
stored by the home agent in the Binding Cache entry, and this
Binding Cache entry MUST be deleted by the home agent after
the expiration of this lifetime.
</t>
</list>
<t>
Regardless of the setting of the Acknowledge (A) bit in the Binding
Update, the home agent MUST return a Binding Acknowledgement
to the mobile node constructed as follows:
</t>
<list style='symbols'>
<t> The Status field MUST be set to a value indicating success.
The value 1 (accepted but prefix discovery necessary) MUST
be used if the subnet prefix of the specified home address
is deprecated, or becomes deprecated during the lifetime of
the binding, or becomes invalid at the end of the lifetime.
The value 0 MUST be used otherwise. For the purposes of
comparing the binding and prefix lifetimes, the prefix lifetimes
are first converted into units of four seconds by ignoring the
two least significant bits.
</t>
<t> The Key Management Mobility Capability (K) bit is set
if the following conditions are all fulfilled, and
cleared otherwise:
<list style='symbols'>
<t> The Key Management Mobility Capability (K) bit was
set in the Binding Update.
</t>
<t> The IPsec security associations between the mobile
node and the home agent have been established
dynamically.
</t>
<t> The home agent has the capability to update its endpoint
in the used key management protocol to the new care-of
address every time it moves.
</t>
</list>
<vspace blankLines='1' />
Depending on the final value of the bit in the Binding
Acknowledgement, the home agent SHOULD perform the following
actions:
<list style='hanging'>
<t hangText='K = 0'> <vspace blankLines='1' />
Discard key management connections, if any, to the old
care-of address. If the mobile node did not have a
binding before sending this Binding Update, discard
the connections to the home address.
</t>
<t hangText='K = 1'> <vspace blankLines='1' />
Move the peer endpoint of the key management
protocol connection, if any, to the new
care-of address. For an IKE phase 1
connection, this means that any IKE packets
sent to the peer are sent to this address, and
packets from this address with the original
ISAKMP cookies are accepted.
<vspace blankLines='1' />
Note that <xref target='RFC2408'>RFC 2408</xref>
Section 2.5.3 gives specific
rules that ISAKMP cookies must satisfy:
they must depend on specific parties and can
only be generated by the entity
itself. Then it recommends a particular way to do
this, namely a hash of IP addresses. With the K
bit set to 1, the recommended implementation
technique does not work directly. To satisfy the
two rules, the specific parties must be treated as
the original IP addresses, not the ones in use at
the specific moment.
</t>
</list>
</t>
<t> The Sequence Number field MUST be copied from the Sequence
Number given in the Binding Update.
</t>
<t> The Lifetime field MUST be set to the remaining lifetime
for the binding as set by the home agent in its home registration
Binding Cache entry for the mobile node, as described above.
</t>
<t> If the home agent stores the Binding Cache entry in nonvolatile
storage, then the Binding Refresh Advice mobility option MUST be
omitted. Otherwise, the home agent MAY include this option to
suggest that the mobile node refreshes its binding before
the actual lifetime of the binding ends.
<vspace blankLines='1' />
If the Binding Refresh Advice mobility option is present, the
Refresh Interval field in the option MUST be set to a value less
than the Lifetime value being returned in the Binding Acknowledgement.
This indicates that the mobile node SHOULD attempt to refresh
its home registration at the indicated shorter interval.
The home agent MUST still retain the registration for
the Lifetime period, even if the mobile node does not refresh
its registration within the Refresh period.
</t>
</list>
<t>The rules for selecting the Destination IP address (and possibly
routing header construction) for the Binding Acknowledgement to
the mobile node are the same as in <xref target='send-ba' />.
</t>
<t>In addition,
the home agent MUST follow the procedure defined in
<xref target='intercept' /> to intercept packets on the mobile node's
home link addressed to the mobile node,
while the home agent is serving as the home agent for this mobile node.
The home agent MUST also be prepared to accept reverse tunneled
packets from the new care-of address of the mobile node, as described
in <xref target='reverse' />. Finally,
the home agent MUST also propagate new home network prefixes, as
described in <xref target='ha:renum' />.
</t>
</section>
<section anchor='home-dereg' title='Primary Care-of Address De-Registration'>
<t>A binding may need to be de-registered when the mobile node returns
home or when the mobile node knows that it will not have any
care-of addresses in the visited network.
</t>
<t>
A Binding Update is validated and authorized in the manner described
in the previous section; note that when the mobile node de-registers
when it is at home, it MAY choose to omit the Home Address destination
option, in which case the mobile node's home address is the source IP
address of the de-registration Binding Update. This section
describes the processing of a valid Binding Update that requests the
receiving node to no longer serve as its home agent, de-registering
its primary care-of address.
</t>
<t>
To begin processing the Binding Update, the home agent MUST perform
the following test:
</t>
<list style='symbols'>
<t> If the receiving node has no entry marked as a home registration
in its Binding Cache for this mobile node, then this node MUST
reject the Binding Update and SHOULD return a Binding
Acknowledgement to the mobile node, in which the Status field is
set to 133 (not home agent for this mobile node).
</t>
</list>
<t>
If the home agent does not reject the Binding Update as described
above, then the home agent MUST return a Binding Acknowledgement
to the mobile node, constructed as follows:
</t>
<list style='symbols'>
<t> The Status field MUST be set to a value 0, indicating success.
</t>
<t> The Key Management Mobility Capability (K) bit is set or
cleared and actions based on its value are performed as
described in the previous section. The mobile node's
home address is used as its new care-of address for
the purposes of moving the key management connection to
a new endpoint.
</t>
<t> The Sequence Number field MUST be copied from the Sequence
Number given in the Binding Update.
</t>
<t> The Lifetime field MUST be set to zero.
</t>
<t> The Binding Refresh Advice mobility option MUST be omitted.
</t>
</list>
<t>
The rules for selecting the Destination IP address (and, if required,
routing header construction) for the Binding Acknowledgement to the
mobile node are the same as in the previous section. When the Status
field in the Binding Acknowledgement is greater than or equal to 128
and the Source Address of the Binding Update is on the home link,
and the Binding Update came from a mobile node on the same link,
the home agent MUST send it to the mobile node's link layer address
(retrieved either from the Binding Update or through Neighbor
Solicitation).
</t>
<t>
When a mobile node sends a binding update to refresh the binding
from the visited link and soon after moves to the home link and
sends a de-registration binding update, a race condition can
happen if the first binding update gets delayed. The delayed
binding update can cause the home agent to create a new binding
cache entry for a mobile node that had just attached to the home
link and successfully deleted the binding. This would prevent the
mobile node from using its home address from the home link.
</t>
<t>
In order to prevent this, the home agent SHOULD NOT remove the
binding cache entry immediately after receiving the deregistration
binding update from the mobile node. It SHOULD mark the binding
cache entry as invalid, and MUST stop intercepting packets on the
mobile node's home link that are addressed to the mobile node
(<xref target='intercept' />). The home agent should wait
for MAX_DELETE_BCE_TIMEOUT (<xref target='constants' />)
seconds before removing the binding cache entry completely. In the
scenario described above, if the home agent receives the delayed
binding update that the mobile node sent from the visited link, it
would reject the message since the sequence number would be less
than the last received deregistration binding update from the home
link. The home agent would then send a Binding Acknowledgment with
status '135' (Sequence number out of window) to the care of address
on the visited link. The mobile node can continue using the home
address from the home link.
</t>
</section>
</section>
<section title='Packet Processing'>
<section anchor='intercept' title='Intercepting Packets for a Mobile Node'>
<t>
While a node is serving as the home agent for a mobile node
it MUST attempt to intercept packets on the mobile node's
home link that are addressed to the mobile node.
</t>
<t>
In order to do this, when a node begins serving as the home agent
it MUST have performed Duplicate Address Detection (as specified
in <xref target='home-reg'/>), and subsequently
it MUST multicast onto the home link a
<xref target='RFC4861'>Neighbor Advertisement message</xref>
on behalf of the mobile node. For the home address specified
in the Binding Update, the home agent sends a
<xref target='RFC4861'>Neighbor Advertisement message</xref>
to the all-nodes multicast address on the home link
to advertise the home agent's own link-layer address
for this IP address on behalf of the mobile node. If
the Link-Layer Address Compatibility (L) flag has been
specified in the Binding Update, the home agent MUST
do the same for the link-local address of the mobile node.
</t>
<t>
All fields in each Neighbor Advertisement message SHOULD
be set in the same way they would be set by the mobile node if it
was sending this <xref target='RFC4861'>Neighbor Advertisement</xref>
while at home, with the following exceptions:
</t>
<list style='symbols'>
<t> The Target Address in the Neighbor Advertisement
MUST be set to the specific IP address for
the mobile node.
</t>
<t> The Advertisement MUST include a Target Link-layer
Address option specifying the home agent's link-layer
address.
</t>
<t> The Router (R) bit in the Advertisement MUST be
set to zero.
</t>
<t> The Solicited Flag (S) in the Advertisement MUST NOT
be set, since it was not solicited by any Neighbor
Solicitation.
</t>
<t> The Override Flag (O) in the Advertisement MUST be set,
indicating that the Advertisement SHOULD override any
existing Neighbor Cache entry at any node receiving it.
</t>
<t> The Source Address in the IPv6 header MUST be set to the
home agent's IP address on the interface used to send the advertisement.
</t>
</list>
<t>Any node on the home link that receives one of the Neighbor Advertisement
messages (described above)
will update its Neighbor Cache to associate the
mobile node's address with the home agent's link layer address,
causing it to transmit any future packets normally destined to the
mobile node to the mobile node's home agent.
Since multicasting on the local link (such as Ethernet) is
typically not guaranteed to be reliable,
the home agent MAY retransmit this Neighbor Advertisement message
up to MAX_NEIGHBOR_ADVERTISEMENT (see <xref target='RFC4861' />)
times to increase its reliability.
It is still possible that some nodes on the home link will
not receive any of the Neighbor Advertisements,
but these nodes will eventually be able to
detect the link-layer address change for the mobile node's address
through use of <xref target='RFC4861'>Neighbor Unreachability Detection</xref>.
</t>
<t>While a node is serving as a home agent for some mobile node,
the home agent uses IPv6 <xref target='RFC4861'>Neighbor Discovery</xref>
to intercept unicast packets on the home link
addressed to the mobile node.
In order to intercept packets in this way,
the home agent MUST act as a proxy for this mobile node
and reply to any received Neighbor Solicitations for it.
When a home agent receives a Neighbor Solicitation,
it MUST check if the Target Address specified in the message
matches the address of any mobile node for which it
has a Binding Cache entry marked as a home registration.
</t>
<t>If such an entry exists in the home agent's Binding Cache,
the home agent MUST reply to the Neighbor Solicitation
with a Neighbor Advertisement
giving the home agent's own link-layer address as
the link-layer address for the specified Target Address.
In addition, the Router (R) bit in the Advertisement MUST be set to zero.
Acting as a proxy in this way allows other nodes on the
mobile node's home link to resolve the mobile node's address
and for the home agent to defend these addresses
on the home link
for <xref target='RFC4861'>Duplicate Address Detection</xref>.
</t>
</section>
<section anchor='ha-deliver' title='Processing Intercepted Packets'>
<t>For any packet sent to a mobile node from the mobile node's home agent
(in which the home agent is the original sender of the packet),
the home agent is operating as a correspondent node of
the mobile node for this packet
and the procedures described in <xref target='corresp-deliver' /> apply.
The home agent then uses a routing header to route the packet
to the mobile node by way of the primary care-of address in the
home agent's Binding Cache.
</t>
<t>While the mobile node is away from home, the home agent
intercepts any packets on the
home link addressed to the mobile node's home address, as described in <xref target='intercept' />.
In order to forward each intercepted packet to the mobile node,
the home agent MUST tunnel the packet
to the mobile node using <xref target='RFC2473'>IPv6 encapsulation</xref>.
When a home agent
encapsulates an intercepted packet for forwarding to the mobile node,
the home agent sets the Source Address in the new
tunnel IP header to the home agent's own IP address
and sets the Destination Address in the tunnel IP header
to the mobile node's primary care-of address.
When received by the mobile node,
normal processing of the <xref target='RFC2473'>tunnel header</xref>
will result in decapsulation and processing
of the original packet by the mobile node.
</t>
<t>However, packets addressed to the mobile node's link-local address MUST NOT
be tunneled to the mobile node.
Instead, these packets MUST be discarded
and the home agent SHOULD return an
ICMP Destination Unreachable, Code 3, message
to the packet's Source Address
(unless this Source Address is a multicast address).
</t>
<t>Interception and tunneling of the following multicast addressed packets
on the home network are only done if the home agent supports multicast
group membership control messages from the mobile node as described in the
next section.
Tunneling of multicast packets to a mobile node
follows similar limitations to those defined above for unicast
packets addressed to the mobile node's link-local address.
Multicast packets addressed to a multicast address with
<xref target='RFC4291'>link-local scope</xref>,
to which the mobile node is subscribed,
MUST NOT be tunneled to the mobile node.
These packets SHOULD be silently discarded
(after delivering to other local multicast recipients).
Multicast packets addressed to a multicast address with
a scope larger than link-local, but smaller than global
(e.g., <xref target='RFC4291'>site-local and organization-local</xref>),
to which the mobile node is subscribed,
SHOULD NOT be tunneled to the mobile node.
Multicast packets addressed with a global scope, to which the mobile node
has successfully subscribed, MUST be tunneled to the mobile node.
</t>
<t>Before tunneling a packet to the mobile node, the home agent
MUST perform any IPsec processing as indicated by the security
policy data base.
</t>
</section>
<section anchor='ha:multicast' title='Multicast Membership Control'>
<t>This section is a prerequisite for the multicast data packet
forwarding, described in the previous section. If this support is not
provided, multicast group membership control messages are silently
ignored.
</t>
<t>
In order to forward multicast data packets from the home
network to all the proper mobile nodes, the home agent SHOULD be
capable of receiving tunneled multicast group membership control
information from the mobile node in order to determine which
groups the mobile node has subscribed to. These multicast group
membership messages are Listener Report messages specified in
<xref target='RFC2710'>MLD</xref> or in other protocols such
<xref target='RFC3810'>as</xref>.
</t>
<t>
The messages are issued by the mobile node, but sent through the
reverse tunnel to the home agent. These messages are issued whenever
the mobile node decides to enable reception of packets for a
multicast group or in response to an MLD Query from the home agent.
The mobile node will also issue multicast group control messages to
disable reception of multicast packets when it is no longer
interested in receiving multicasts for a particular group.
</t>
<t>To obtain the mobile node's current multicast group membership the
home agent must periodically transmit MLD Query messages through the
tunnel to the mobile node. These MLD periodic transmissions
will ensure the home agent has an accurate record of the groups in
which the mobile node is interested despite packet losses of the
mobile node's MLD group membership messages.
</t>
<t>All MLD packets are sent directly between the mobile node and the home
agent. Since all of these packets are destined to a link-scope multicast address and have a hop limit
of 1, there is no direct forwarding of such packets between the home
network and the mobile node. The MLD packets between the mobile node
and the home agent are encapsulated within the same tunnel header used
for other packet flows between the mobile node and home agent.
</t>
<t>Note that at this time, even though a link-local source is used on MLD
packets, no functionality depends on these addresses being unique, nor
do they elicit direct responses. All MLD messages are sent to
multicast destinations. To avoid ambiguity on the home agent, due to
mobile nodes which may choose identical link-local source addresses
for their MLD function, it is necessary for the home agent to identify
which mobile node was actually the issuer of a particular MLD message.
This may be accomplished by noting which tunnel such an MLD arrived
by, which IPsec SA was used, or by other distinguishing means.
</t>
<t>This specification puts no requirement on how the functions in this
section and the multicast forwarding in <xref target='ha-deliver' /> are
to be achieved. At the time of this writing it was thought that a
full IPv6 multicast router function would be necessary on the home
agent, but it may be possible to achieve the same effects through a
"proxy MLD" application coupled with kernel multicast forwarding.
This may be the subject of future specifications.
</t>
</section>
<section anchor='ha:dhcp' title='Stateful Address Autoconfiguration'>
<t>This section describes how home agents support the use of stateful
address autoconfiguration mechanisms such as
<xref target='RFC3315'>DHCPv6</xref> from the mobile nodes. If this
support is not provided, then the M and O bits must remain cleared on
the Mobile Prefix Advertisement Messages. Any mobile node which sends
DHCPv6 messages to the home agent without this support will not
receive a response.
</t>
<t>If DHCPv6 is used, packets are sent with link-local source addresses
either to a link-scope multicast address or a link-local address.
Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel
standard DHCPv6 packets to the home agent. Since these link-scope
packets cannot be forwarded onto the home network, it is necessary for
the home agent to either implement a DHCPv6 relay agent or a DHCPv6
server function itself. The arriving tunnel or IPsec SA of DHCPv6
link-scope messages from the mobile node must be noted so that DHCPv6
responses may be sent back to the appropriate mobile node. DHCPv6
messages sent to the mobile node with a link-local destination must be
tunneled within the same tunnel header used for other packet flows.
</t>
</section>
<section anchor='reverse' title='Handling Reverse Tunneled Packets'>
<t>Unless a binding has been established between the mobile node and a
correspondent node, traffic from the mobile node to the correspondent
node goes through a reverse tunnel. Home agents MUST support reverse
tunneling as follows:
</t>
<list style='symbols'>
<t> The tunneled traffic arrives to the home agent's address using
<xref target='RFC2473'>IPv6 encapsulation</xref>.
</t>
<t> Depending on the security policies used by the home agent, reverse
tunneled packets MAY be discarded unless accompanied by a valid
ESP header. The support for authenticated reverse tunneling allows the
home agent to protect the home network and correspondent nodes from
malicious nodes masquerading as a mobile node.
</t>
<t> Otherwise, when a home agent decapsulates
a tunneled packet from the mobile node, the home
agent MUST verify that the Source Address
in the tunnel IP header is the mobile node's primary care-of
address. Otherwise, any node in the Internet could send
traffic through the home agent and escape ingress filtering
limitations. This simple check forces the attacker to
know the current location of the real mobile node
and be able to defeat ingress filtering. This check is not necessary
if the reverse-tunneled packet is protected by ESP in tunnel mode.
</t>
</list>
</section>
<section anchor='ha:rrprot' title='Protecting Return Routability Packets'>
<t>The return routability procedure, described in <xref target='rrp' />,
assumes that the confidentiality of the Home Test Init and Home
Test messages is protected as they are tunneled between the
home agent and the mobile node.
Therefore, the home agent MUST support tunnel mode IPsec ESP for
the protection of packets belonging to the return routability procedure.
Support for a non-null encryption transform and authentication
algorithm MUST be available.
It is not necessary to distinguish between different kinds of
packets during the return routability procedure.
</t>
<t>Security associations are needed to provide this protection. When the
care-of address for the mobile node changes as
a result of an accepted Binding Update, special treatment is
needed for the next packets sent using these security associations.
The home agent MUST set the new care-of address as the destination
address of these packets, as if the outer header destination address in the
security association had <xref target='RFC3776'>changed</xref>.
</t>
<t>The above protection SHOULD be used with all mobile
nodes. The use is controlled by configuration of the IPsec security
policy database both at the mobile node and at the home agent.
</t>
<t>As described earlier, the Binding Update and Binding Acknowledgement
messages require protection between the home agent and the mobile
node. The Mobility Header protocol carries both these messages
as well as the return routability messages. From the point
of view of the security policy database these messages are
indistinguishable.
When IPsec is used to protect return routability signaling
or payload packets, this protection MUST only be applied
to the return routability packets entering the IPv6
encapsulated tunnel interface between the mobile node and
the home agent. This can be achieved, for instance, by
defining the security policy database entries
specifically for the tunnel interface.
That is, the policy entries are
not generally applied on all traffic on the physical interface(s) of
the nodes, but rather only on traffic that enters the tunnel.
This makes use of per-interface security policy database
<xref target='RFC2401'>entries</xref> specific to the tunnel interface
<xref target='RFC2460'>(the node's attachment to the tunnel</xref>).
</t>
</section>
</section>
<section anchor='ha-disc' title='Dynamic Home Agent Address Discovery'>
<t>
This section describes an optional mechanism by which a home agent can
help mobile nodes to discover the addresses of other home agents on
the mobile node's home network. The home agent keeps track of the
other home agents on the same link and responds to queries sent by
the mobile node.
</t>
<section anchor='ha-advert' title='Receiving Router Advertisement Messages'>
<t>For each link on which a router provides service as a home agent,
the router maintains a Home Agents List
recording information about all other home agents on that link.
This list is used in the
dynamic home agent address discovery mechanism;
the mobile node uses the list as described in <xref target='mn-ha-disc' />.
The information for the list is learned through receipt of
the periodic unsolicited multicast Router Advertisements,
in a manner similar to the Default Router List conceptual data structure
maintained by each host for <xref target='RFC4861'>Neighbor Discovery</xref>.
In the construction of the Home Agents List, the
Router Advertisements are from each (other) home agent on the link
and the Home Agent (H) bit is set in them.
</t>
<t>On receipt of a valid Router Advertisement,
as defined in the processing algorithm specified for
<xref target='RFC4861'>Neighbor Discovery</xref>,
the home agent performs the following steps
in addition to any steps already required of it by Neighbor Discovery:
</t>
<list style='symbols'>
<t> If the Home Agent (H) bit in the Router Advertisement is not
set, delete the sending node's entry in the current Home Agents List
(if one exists). Skip all the following steps.
</t>
<t> Otherwise, extract the Source Address from the IP header of
the Router Advertisement.
This is the link-local IP address on this link of the home
agent sending this <xref target='RFC4861'>Advertisement</xref>.
</t>
<t> Determine
the preference for this home agent.
If the Router Advertisement contains a Home Agent Information
Option, then the preference is taken from the Home Agent
Preference field in the option;
otherwise, the default preference of 0 MUST be used.
</t>
<t> Determine
the lifetime for this home agent.
If the Router Advertisement contains a Home Agent Information
Option, then the lifetime is taken from the Home Agent Lifetime
field in the option;
otherwise, the lifetime specified by the Router Lifetime
field in the Router Advertisement SHOULD be used.
</t>
<t> If the link-local address of the home agent sending this
Advertisement is already present in this home agent's
Home Agents List and the received home agent lifetime value is
zero, immediately delete this entry in the Home Agents List.
</t>
<t> Otherwise, if the link-local address of the home agent sending
this Advertisement is already present in the receiving home
agent's Home Agents List, reset its lifetime and preference
to the values determined above.
</t>
<t> If the link-local address of the home agent sending this
Advertisement is not already present in the Home Agents List
maintained by the receiving home agent, and the lifetime
for the sending home agent is non-zero,
create a new entry in the list, and initialize its lifetime
and preference to the values determined above.
</t>
<t> If the Home Agents List entry for the link-local address of
the home agent sending this Advertisement was not deleted
as described above,
determine any global address(es) of the home agent based on each
Prefix Information option received in this Advertisement
in which the Router Address (R) bit is set (<xref target='global-rtr' />).
Add all such global addresses to the list of global addresses
in this Home Agents List entry.
</t>
</list>
<t>A home agent SHOULD maintain an entry in its Home Agents List
for each valid home agent address until that entry's
lifetime expires,
after which time the entry MUST be deleted.
</t>
<t>As described in <xref target='mn-ha-disc' />,
a mobile node attempts dynamic home agent address discovery by
sending an ICMP Home Agent Address Discovery Request message to the
Mobile IPv6 <xref target='RFC2526'>Home-Agents anycast address</xref>
for its home IP subnet prefix.
A home agent receiving a Home Agent Address Discovery Request
message that serves this subnet SHOULD return an ICMP Home Agent
Address Discovery Reply message to the mobile node
with the Source Address of the Reply packet
set to one of the global unicast addresses of the home agent.
The Home Agent Addresses field in the Reply message is
constructed as follows:
</t>
<list style='symbols'>
<t> The Home Agent Addresses field SHOULD contain all global IP
addresses for each home agent currently listed in this home
agent's own Home Agents List (<xref target='ha:data' />).
</t>
<t> The IP addresses in the Home Agent Addresses field SHOULD
be listed in order of decreasing preference values, based
either on the respective advertised preference from a Home
Agent Information option or on the default preference of 0 if
no preference is advertised (or on the configured home agent
preference for this home agent itself).
</t>
<t> Among home agents with equal preference, their IP addresses
in the Home Agent Addresses field SHOULD be listed in an order
randomized with respect to other home agents with equal
preference every time a Home Agent Address Discovery Reply
message is returned by this home agent.
</t>
<t> If more than one global IP address is associated with a
home agent, these addresses SHOULD be listed in a randomized
order.
</t>
<t> The home agent SHOULD reduce the number of home agent IP
addresses so that the packet fits within the minimum IPv6
<xref target='RFC2460'>MTU</xref>. The home agent addresses selected for
inclusion in the packet SHOULD be those from the complete list
with the highest preference. This limitation avoids the danger
of the Reply message packet being fragmented (or rejected by
an intermediate router with an
<xref target='RFC4443'>ICMP Packet Too Big message</xref>).
</t>
</list>
</section>
</section>
<section anchor='ha:renum' title='Sending Prefix Information to the Mobile Node'>
<section anchor='ha:renum-aggr' title='List of Home Network Prefixes'>
<t>Mobile IPv6 arranges to propagate relevant prefix information to the
mobile node when it is away from home, so that it may be used in
mobile node home address configuration and in network renumbering.
In this mechanism, mobile nodes away from home receive Mobile Prefix
Advertisements messages. These messages include Prefix Information
Options for the prefixes configured on the home
subnet interface(s) of the home agent.
</t>
<t>
If there are multiple home agents, differences in the advertisements
sent by different home agents can lead to an inability to use a
particular home address when changing to another home agent. In order
to ensure that the mobile nodes get the same information from
different home agents, it is preferred that all of the home agents
on the same link be configured in the same manner.
</t>
<t>
To support this, the home agent monitors prefixes advertised by
itself and other home agents on the home link.
In Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>) it
is acceptable for two routers to advertise different sets of prefixes
on the same link. For home agents, the differences should be detected
for a given home address because the mobile node communicates only
with one home agent
at a time and the mobile node needs to know the full set of
prefixes assigned to the home link. All other comparisons of Router
Advertisements are as specified in Section 6.2.7 of RFC 4861.
</t>
</section>
<section anchor='ha:renum-send' title='Scheduling Prefix Deliveries'>
<t>A home agent serving a mobile node will schedule the
delivery of the new prefix information to that mobile node
when any of the following conditions occur:
</t>
<t>MUST:
</t>
<list style='symbols'>
<t> The state of the flags changes for the prefix of the mobile node's
registered home address.
</t>
<t> The valid or preferred lifetime is reconfigured or changes for any
reason other than advancing real time.
</t>
<t> The mobile node requests the information with
a Mobile Prefix Solicitation (see <xref target='mn:send-sol' />).
</t>
</list>
<t>SHOULD:
</t>
<list style='symbols'>
<t> A new prefix is added to the home subnet interface(s) of
the home agent.
</t>
</list>
<t>MAY:
</t>
<list style='symbols'>
<t> The valid or preferred lifetime or the state of the flags changes
for a prefix which is not used in any Binding Cache entry for this
mobile node.
</t>
</list>
<t>The home agent uses the following algorithm to determine
when to send prefix information to the mobile node.
</t>
<list style='symbols'>
<t> If a mobile node sends a solicitation, answer right away.
</t>
<t> If no Mobile Prefix Advertisement has been sent to the mobile
node in the last MaxMobPfxAdvInterval seconds (see <xref target='variables' />),
then ensure that a transmission is scheduled. The actual
transmission time is randomized as described below.
</t>
<t> If a prefix matching the mobile node's home registration is
added on the home subnet interface or if its information changes in any
way that does not deprecate the mobile node's address, ensure
that a transmission is scheduled. The actual transmission time
is randomized as described below.
</t>
<t> If a home registration expires, cancel any scheduled
advertisements to the mobile node.
</t>
</list>
<t>The list of prefixes is sent in its entirety in all cases.
</t>
<t>If the home agent has already scheduled the transmission of a Mobile
Prefix Advertisement to the mobile node, then the home agent will replace the
advertisement with a new one to be sent at the scheduled time.
</t>
<t>Otherwise, the home agent computes
a fresh value for RAND_ADV_DELAY which offsets from the current
time for the scheduled transmission.
First calculate the maximum delay for the scheduled Advertisement: </t><t> </t><t>
</t>
<figure><artwork>
MaxScheduleDelay = min (MaxMobPfxAdvInterval, Preferred Lifetime),
</artwork></figure>
<t>where MaxMobPfxAdvInterval is as defined in <xref target='constants' />.
Then compute the final delay for the advertisement: </t><t> </t><t>
</t>
<figure><artwork>
RAND_ADV_DELAY = MinMobPfxAdvInterval +
(rand() % abs(MaxScheduleDelay - MinMobPfxAdvInterval))
</artwork></figure>
<t>Here rand() returns a random integer value in the range of 0 to the
maximum possible integer value.
This computation is expected to alleviate bursts of advertisements
when prefix information changes.
In addition, a home agent MAY
further reduce the rate of packet transmission by further delaying
individual advertisements, when necessary to avoid overwhelming
local network resources.
The home agent SHOULD periodically continue to retransmit an
unsolicited Advertisement to the mobile node, until it is
acknowledged by the receipt of a Mobile Prefix Solicitation
from the mobile node.
</t>
<t>The home agent MUST wait PREFIX_ADV_TIMEOUT (see <xref target='constants' />)
before the first retransmission and double the retransmission wait
time for every succeeding retransmission until a maximum number of
PREFIX_ADV_RETRIES attempts (see <xref target='constants' />) has been tried.
If the mobile node's bindings
expire before the matching Binding Update has been received,
then the home agent MUST NOT attempt any more retransmissions,
even if not all PREFIX_ADV_RETRIES have been retransmitted.
In the mean time, if the mobile node sends another Binding Update without
returning home, then the home agent SHOULD begin
transmitting
the unsolicited Advertisement again.
</t>
<t>If some condition, as described above, occurs on the home link and
causes another Prefix Advertisement to be sent to the mobile node,
before the mobile node acknowledges a previous transmission,
the home agent SHOULD combine any Prefix Information options in
the unacknowledged Mobile Prefix Advertisement into a new Advertisement.
The home agent then discards the old Advertisement.
</t>
</section>
<section anchor='ha:renum-mpa' title='Sending Advertisements'>
<t>When sending a Mobile Prefix Advertisement to the mobile node,
the home agent MUST construct the packet as follows:
</t>
<list style='symbols'>
<t> The Source Address in the packet's IPv6 header
MUST be set to the home agent's IP address
to which the mobile node addressed its current
home registration or its default global home agent
address if no binding exists.
</t>
<t> If the advertisement was solicited, it MUST be destined
to the source address of the solicitation. If it was
triggered by prefix changes or renumbering, the
advertisement's destination will be the mobile node's home
address in the binding which triggered the rule.
</t>
<t> A type 2 routing header MUST be included with the mobile
node's home address.
</t>
<t> IPsec headers MUST be supported and SHOULD be used.
</t>
<t> The home agent MUST send the packet as it would any other
unicast IPv6 packet that it originates.
</t>
<t> Set the Managed Address Configuration (M) flag if the
corresponding flag has been set in any of the Router
Advertisements from which the prefix information has been
learned (including the ones sent by this home agent).
</t>
<t> Set the Other Stateful Configuration (O) flag if the
corresponding flag has been set in any of the Router
Advertisements from which the prefix information has been
learned (including the ones sent by this home agent).
</t>
</list>
</section>
<section anchor='ha:renum-life' title='Lifetimes for Changed Prefixes'>
<t>As described in <xref target='home-reg' />,
the lifetime returned by the home agent
in a Binding Acknowledgement
MUST NOT be greater than the remaining valid lifetime for the
subnet prefix in the mobile node's home address.
This limit on the binding lifetime
serves to prohibit use of a mobile node's home address
after it becomes invalid.
</t>
</section>
</section>
</section>
<section anchor='mobile' title='Mobile Node Operation'>
<section anchor='mn:data' title='Conceptual Data Structures'>
<t>Each mobile node MUST maintain a Binding Update List.
</t>
<t>The Binding Update List records
information for each Binding Update sent by this mobile node, in
which the lifetime of the binding has not yet expired.
The Binding Update List includes all bindings sent by the mobile node
either to its home agent or correspondent nodes.
It also contains Binding Updates which are waiting for the completion
of the return routability procedure before they can be sent.
However, for multiple Binding
Updates sent to the same destination address, the Binding Update List
contains only the most recent Binding Update (i.e., with the greatest
Sequence Number value) sent to that destination. The Binding Update
List MAY be implemented in any manner consistent with the external
behavior described in this document.
</t>
<t>Each Binding Update List entry conceptually contains the following fields:
</t>
<list style='symbols'>
<t> The IP address of the node to which a Binding Update was sent.
</t>
<t> The home address for which that Binding Update was sent.
</t>
<t> The care-of address sent in that Binding Update.
This value is necessary for the mobile node to determine if it
has sent a Binding Update while giving its new care-of address to this
destination after changing its care-of address.
</t>
<t> The initial value of the Lifetime field sent in that Binding Update.
</t>
<t> The remaining lifetime of that binding.
This lifetime is initialized from the Lifetime value sent in
the Binding Update and is decremented until it reaches zero,
at which time this entry MUST be deleted from the Binding Update List.
</t>
<t> The maximum value of the Sequence Number field sent
in previous Binding Updates to this destination.
The Sequence Number field is 16 bits long and all comparisons
between Sequence Number values MUST be performed modulo 2**16
(see <xref target='recv-updates' />).
</t>
<t> The time at which a Binding Update was last sent to this
destination, as needed to implement the rate limiting
restriction for sending Binding Updates.
</t>
<t> The state of any retransmissions needed for this Binding Update.
This state includes the time remaining until the next retransmission
attempt for the Binding Update
and the current state of the exponential back-off mechanism
for retransmissions.
</t>
<t> A flag specifying whether or not future Binding Updates
should be sent to this destination.
The mobile node sets this flag in the Binding Update List entry
when it receives an ICMP Parameter Problem, Code 1, error message
in response to a return routability message or Binding Update sent
to that destination, as described in <xref target='recv-icmp-update' />.
</t>
</list>
<t>The Binding Update List is used to determine whether a particular
packet is sent directly to the correspondent node or tunneled via
the home agent (see <xref target='mobile-sending' />).
</t>
<t>The Binding Update list also conceptually contains the following data
related to running the return routability procedure. This data is
relevant only for Binding Updates sent to correspondent nodes.
</t>
<list style='symbols'>
<t> The time at which a Home Test Init or Care-of Test Init message was
last sent to this destination, as needed to implement the rate
limiting restriction for the return routability procedure.
</t>
<t> The state of any retransmissions needed for this return routability
procedure. This state includes the time remaining until the next
retransmission attempt and the current state of the exponential
back-off mechanism for retransmissions.
</t>
<t> Cookie values used in the Home Test Init and Care-of Test Init
messages.
</t>
<t> Home and care-of keygen tokens received from the correspondent node.
</t>
<t> Home and care-of nonce indices received from the correspondent node.
</t>
<t> The time at which each of the tokens and nonces were received from
the correspondent node, as needed to implement reuse while moving.
</t>
</list>
</section>
<section anchor='mn-processing-mh' title='Processing Mobility Headers'>
<t>All IPv6 mobile nodes MUST observe the rules described in
<xref target='processing-mh' /> when processing Mobility Headers.
</t>
</section>
<section title='Packet Processing'>
<section anchor='mobile-sending' title='Sending Packets While Away from Home'>
<t>While a mobile node is away from home,
it continues to use its home address, as well as also using
one or more care-of addresses.
When sending a packet while away from home,
a mobile node MAY choose among these in selecting the
address that it will use as the source of the packet, as follows:
</t>
<list style='symbols'>
<t> Protocols layered over IP will generally treat the mobile node's
home address as its IP source address for most packets.
For packets sent that are
part of transport-level connections established while the mobile node
was at home, the mobile node MUST use its home address. Likewise, for
packets sent that are part of transport-level connections that the
mobile node may still be using after moving to a new location, the
mobile node SHOULD use its home address in this way.
If a binding exists, the mobile
node SHOULD send the packets directly to the correspondent node.
Otherwise, if a binding
does not exist, the mobile node MUST use reverse tunneling.
</t>
<t> The mobile node MAY choose to directly use one of its care-of
addresses as the source of the packet, not requiring the use of a
Home Address option in the packet. This is particularly useful
for short-term communication that may easily be retried if it
fails. Using the mobile node's care-of address as the source for
such queries will generally have a lower overhead than using the
mobile node's home address, since no extra options need to be used
in either the query or its reply. Such packets can be routed
normally, directly between their source and destination without
relying on Mobile IPv6. If application running on the mobile node
has no particular knowledge that the communication being sent fits
within this general type of communication, however, the mobile
node should not use its care-of address as the source of the
packet in this way.
<vspace blankLines='1' />
The choice of the most efficient communications method is
application specific, and outside the scope of this specification.
The APIs necessary for controlling the choice are also out of scope.
One example of such an API is described in
the <xref target='RFC5014'>IPv6 Socket API for
Source Address Selection specification</xref>.
</t>
<t> While not at its home link, the mobile node MUST NOT use the
Home Address destination option when communicating
with link-local peers.
<vspace blankLines='1' /> Similarly, the mobile node MUST NOT use
the Home Address destination option for
IPv6 <xref target='RFC4861'>Neighbor Discovery</xref> packets.
</t>
</list>
<t>Detailed operation of these cases is described later in this section
and also discussed in <xref target='RFC3484' />.
</t>
<t>For packets sent by a mobile node while it is at home,
no special Mobile IPv6 processing is required.
Likewise,
if the mobile node uses any address other than one of its home addresses
as the source of a packet sent while away from home,
no special Mobile IPv6 processing is required.
In either case,
the packet is simply addressed and transmitted in the same way
as any normal IPv6 packet.
</t>
<t>For packets sent by the mobile node sent while away from home
using the mobile node's home address as the source,
special Mobile IPv6 processing of the packet is required.
This can be done in the following two ways:
</t>
<list style='hanging'>
<t hangText='Route Optimization'>
<vspace blankLines='1' />This manner of delivering packets does not require going through
the home network, and typically will enable faster and more reliable
transmission.
<vspace blankLines='1' />The mobile node needs to ensure that a Binding Cache
entry exists for its home address so that the correspondent node can process
the packet (<xref target='rxing-hao' /> specifies the rules for Home
Address Destination Option Processing at a correspondent node). The
mobile node SHOULD examine its Binding Update List for an entry
which fulfills the following conditions:
<list style='symbols'>
<t> The Source Address field of the packet being sent is
equal to the home address in the entry.
</t>
<t> The Destination Address field of the packet being sent
is equal to the address of the correspondent node
in the entry.
</t>
<t> One of the current care-of addresses of the mobile node
appears as the care-of address in the entry.
</t>
<t> The entry indicates that a binding has been successfully
created.
</t>
<t> The remaining lifetime of the binding is greater than zero.
</t>
</list>
<vspace blankLines='1' />When these conditions are met, the mobile node knows that the
correspondent node has a suitable Binding Cache entry.
<vspace blankLines='1' />A mobile node SHOULD arrange to supply the home address in a Home
Address option, and MUST set the IPv6 header's Source Address field to
the care-of address which the mobile node has registered to be used
with this correspondent node.
The correspondent node will then use the address supplied in the
Home Address option to serve the function traditionally done
by the Source IP address in the IPv6 header.
The mobile node's home address is then supplied to
higher protocol layers and applications.
<vspace blankLines='1' />Specifically:
<list style='symbols'>
<t> Construct the packet using the mobile node's
home address as the packet's Source Address,
in the same way as if the mobile node were at home.
This includes the calculation
of upper layer checksums using the home address as the value
of the source.
</t>
<t> Insert a Home Address option into the packet
with the Home Address field copied from the original value of
the Source Address field in the packet.
</t>
<t> Change the Source Address field in the packet's IPv6 header
to one of the mobile node's care-of addresses.
This will typically be the mobile node's current primary care-of
address, but MUST be an address assigned to the interface on the link
being used.
</t>
</list>
<vspace blankLines='1' />By using the care-of address as the Source Address in the IPv6 header,
with the mobile node's home address instead in the Home Address option,
the packet will be able to safely pass through any router
implementing <xref target='RFC2827'>ingress filtering</xref>.
</t>
<t hangText='Reverse Tunneling'>
<vspace blankLines='1' />
<vspace blankLines='1' />This is the mechanism which tunnels the packets via the home agent.
It is not as efficient as the above mechanism, but is needed if there
is no binding yet with the correspondent node.
<vspace blankLines='1' />This mechanism is used for packets that have the mobile node's home
address as the Source Address in the IPv6 header, or with multicast
control protocol packets as described in
<xref target='multicast' />. Specifically:
<list style='symbols'>
<t> The packet is sent to the home agent using
<xref target='RFC2473'>IPv6 encapsulation</xref>.
</t>
<t> The Source Address in the tunnel packet is the primary
care-of address as registered with the home agent.
</t>
<t> The Destination Address in the tunnel packet is the
home agent's address.
</t>
</list>
<vspace blankLines='1' />Then, the home agent will pass the encapsulated packet to the
correspondent node.
</t>
</list>
</section>
<section anchor='outbound-ipsec' title='Interaction with Outbound IPsec Processing'>
<t>This section sketches the interaction between outbound Mobile IPv6
processing and outbound IP Security (IPsec) processing for packets
sent by a mobile node while away from home. Any specific
implementation MAY use algorithms and data structures other than those
suggested here, but its processing MUST be consistent with the effect
of the operation described here and with the relevant IPsec
specifications. In the steps described below, it is assumed that IPsec
is being used in <xref target='RFC2401'>transport mode</xref> and that the mobile node
is using its home address as the source for the packet (from the point
of view of higher protocol layers or applications, as described in
<xref target='mobile-sending' />):
</t>
<list style='symbols'>
<t> The packet is created by higher layer protocols and applications
(e.g., by TCP) as if the mobile node were at home and Mobile IPv6
were not being used.
</t>
<t> Determine the outgoing interface for the packet. (Note that
the selection between reverse tunneling and route optimization
may imply different interfaces, particularly if tunnels are
considered interfaces as well.)
</t>
<t> As part of outbound packet processing in IP, the packet is
compared against the IPsec security policy database to
determine what processing is required for the packet <xref target='RFC2401' />.
</t>
<t> If IPsec processing is required, the packet is either mapped to
an existing Security Association (or SA bundle), or a new SA (or
SA bundle) is created for the packet, according to the procedures
defined for IPsec.
</t>
<t> Since the mobile node is away from home, the mobile is either
using reverse tunneling or route optimization to reach the
correspondent node.
<vspace blankLines='1' />If reverse tunneling is used, the packet is constructed in the normal
manner and then tunneled through the home agent.
<vspace blankLines='1' />If route optimization is in use, the mobile node inserts a Home Address
destination option into the packet, replacing the Source Address in the
packet's IP header with the care-of address used with this correspondent node,
as described in <xref target='mobile-sending' />.
The Destination Options header in which the Home Address destination
option is inserted MUST appear in the packet after the routing header,
if present, and before the IPsec <xref target='RFC2402'>(AH</xref> or
<xref target='RFC2406'>ESP</xref>) header, so that the Home Address destination option
is processed by the destination node before the IPsec header is processed.
<vspace blankLines='1' />Finally, once the packet is fully assembled, the necessary IPsec
authentication (and encryption, if required) processing is performed
on the packet, initializing the Authentication Data in the IPsec
header.
<vspace blankLines='1' />RFC 2402 treatment of destination options is extended as follows. The AH
authentication data MUST be calculated as if the following were true:
<list style='symbols'>
<t> the IPv6 source address in the IPv6 header contains the
mobile node's home address,
</t>
<t> the Home Address field of the Home Address destination option
(<xref target='hao' />) contains the new care-of address.
</t>
</list>
</t>
<t> This allows, but does not require, the receiver of the packet
containing a Home Address destination option to exchange the two
fields of the incoming packet to reach the above situation, simplifying processing for all
subsequent packet headers. However, such an exchange is not
required, as long as
the result of the authentication calculation remains the same.
</t>
</list>
<t>When an automated key management protocol is used to create new
security associations for a peer, it is important to ensure that
the peer can send the key management protocol packets to the mobile
node. This may not be possible if the peer is the home agent of the
mobile node and the purpose of the security associations would be to
send a Binding Update to the home agent. Packets addressed to the
home address of the mobile node cannot be used before the Binding
Update has been processed. For the default case
of using <xref target='RFC2409'>IKE</xref> as the automated key management
protocol, such problems can be avoided by
the following requirements when communicating with its home agent:
</t>
<list style='symbols'>
<t> When the mobile node is away from home, it MUST use its care-of
address as the Source Address of all packets it sends as part of
the key management protocol (without use of Mobile IPv6 for these
packets, as suggested in <xref target='mobile-sending' />).
</t>
<t> In addition, for all security associations bound to the mobile
node's home address established by IKE, the mobile node MUST
include an <xref target='RFC2408'>ISAKMP Identification Payload</xref> in the IKE phase 2
exchange, giving the mobile node's home address as the initiator
of the <xref target='RFC2407'>Security Association</xref>.
</t>
</list>
<t>The Key Management Mobility Capability (K) bit in Binding Updates and
Acknowledgements can be used to avoid the need to rerun IKE upon movements.
</t>
</section>
<section anchor='mobile-receiving' title='Receiving Packets While Away from Home'>
<t>While away from home, a mobile node will receive packets addressed to
its home address, by one of two methods:
</t>
<list style='symbols'>
<t> Packets sent by a correspondent node, that does not have a
Binding Cache entry for the mobile node, will be sent to the
home address, captured by the home agent and tunneled to the mobile node.
</t>
<t> Packets sent by a correspondent node that has a Binding Cache
entry for the mobile node that contains the mobile node's current
care-of address, will be sent by the correspondent node using
a type 2 routing header. The packet will be addressed to the mobile
node's care-of address, with the final hop in the routing header
directing the packet to the mobile node's home address; the
processing of this last hop of the routing header is entirely
internal to the mobile node, since the care-of address and home
address are both addresses within the mobile node.
</t>
</list>
<t>For packets received by the first method, the mobile node
MUST check that the IPv6 source address of the tunneled packet is
the IP address of its home agent. In this method, the mobile node may
also send a Binding Update to the original sender of the packet as described in
<xref target='update-corresp' /> and subject to the rate
limiting defined in <xref target='rate-limit' />. The mobile
node MUST also process the received packet in the manner defined for
<xref target='RFC2473'>IPv6 encapsulation</xref>, which will result in the
encapsulated (inner) packet being processed normally by upper-layer
protocols within the mobile node as if it had been addressed (only)
to the mobile node's home address.
</t>
<t>For packets received by the second method, the following rules will
result in the packet being processed normally by upper-layer protocols
within the mobile node as if it had been addressed to the mobile
node's home address.
</t>
<t>A node receiving a packet addressed to itself (i.e., one of the node's
addresses is in the IPv6 destination field) follows the next header chain
of headers and processes them. When it encounters a type 2 routing header
during this processing, it performs the following checks.
If any of these checks fail, the node MUST silently discard the packet.
</t>
<list style='symbols'>
<t> The length field in the routing header is exactly 2.
</t>
<t> The segments left field in the routing header is 1 on the wire.
(But implementations may process
the routing header so that the value may become 0 after the
routing header
has been processed, but before the rest of the packet is processed.)
</t>
<t> The Home Address field in the routing header is one of the node's
home addresses, if the segments left field was 1. Thus, in
particular the address field is required to be a unicast routable
address.
</t>
</list>
<t>Once the above checks have been performed, the node swaps the
IPv6 destination field with the Home Address field in the routing header,
decrements segments left by one from the value it had on the wire, and resubmits the packet to IP for processing
the next header.
Conceptually, this follows the same model as in RFC 2460.
However, in the case of type 2 routing header this can be simplified since
it is known that the packet will not be forwarded to a different node.
</t>
<t>The definition of AH requires the sender to calculate the
AH integrity check value of a routing header in the same way it appears in
the receiver after it has processed the header.
Since IPsec headers follow the routing header, any IPsec processing
will operate on the packet with the home address in the IP destination
field and segments left being zero. Thus, the AH calculations at the
sender and receiver will have an identical view of the packet.
</t>
</section>
<section anchor='multicast' title='Routing Multicast Packets'>
<t>A mobile node that is connected to its home link
functions in the same way as any other (stationary) node.
Thus, when it is at home, a mobile node functions identically
to other multicast senders and receivers.
Therefore, this section describes the behavior of a
mobile node that is not on its home link.
</t>
<t>In order to receive packets sent to some multicast group, a mobile
node must join that multicast group. One method, in which a mobile
node MAY join the group, is via a (local) multicast router on the
foreign link being visited. In this case, the mobile node MUST
use its care-of address and MUST NOT use the Home Address destination
option when sending <xref target='RFC2710'>MLD packets</xref>.
</t>
<t>Alternatively, a mobile node MAY join multicast groups via a
bi-directional tunnel to its home agent. The mobile node tunnels its
multicast group membership control packets (such as those defined
in <xref target='RFC2710' /> or in <xref target='RFC3810' />) to its home agent,
and the home agent forwards multicast packets down the tunnel to the
mobile node. A mobile node MUST NOT tunnel multicast group membership
control packets until (1) the mobile node has a binding in place at
the home agent, and (2) the latter sends at least one multicast
group membership control packet via the tunnel. Once this condition is
true, the mobile node SHOULD assume it does not change as long as the
binding does not expire.
</t>
<t>A mobile node that wishes to send packets to a multicast group also
has two options:
</t>
<list style='numbers'>
<t> Send directly on the foreign link being visited.
<vspace blankLines='1' />
To do this, the application uses the care-of address as a source
address for multicast traffic, just as it would use a stationary
address. This requires that the application either knows the
care-of address, or uses an API such as
the <xref target='RFC5014'>IPv6 Socket API for
Source Address Selection specification</xref>
to request that the care-of address be used as the source address
in transmitted packets. The
mobile node MUST NOT use Home Address destination option in such
traffic.
</t>
<t> Send via a tunnel to its home agent.
<vspace blankLines='1' />Because multicast routing in general depends upon
the Source Address used in the IPv6 header of the multicast packet,
a mobile node that tunnels a multicast packet to its
home agent MUST use its home address as the IPv6 Source Address of the
inner multicast packet.
</t>
</list>
<t>Note that direct sending from the foreign link is only
applicable while the mobile node is at that foreign link. This
is because the associated multicast tree is specific to
that source location and any change of location and source
address will invalidate the source specific tree or branch
and the application context of the other multicast
group members.
</t>
<t>This specification does not provide mechanisms to enable such
local multicast session to survive hand-off and to seamlessly
continue from a new care-of address on each new foreign link. Any
such mechanism, developed as an extension to this
specification, needs to take into account the impact of
fast moving mobile nodes on the Internet multicast
routing protocols and their ability to maintain the
integrity of source specific multicast trees and branches.
</t>
<t>While the use of bidirectional tunneling can ensure that multicast trees are
independent of the mobile nodes movement, in some case such tunneling
can have adverse affects. The latency of specific types of multicast
applications (such as multicast based discovery protocols) will be
affected when the round-trip time between the foreign subnet and the
home agent is significant compared to that of the topology to be
discovered. In addition, the delivery tree from the home agent in such
circumstances relies on unicast encapsulation from the agent to the
mobile node. Therefore, bandwidth usage is inefficient compared to the native
multicast forwarding in the foreign multicast system.
</t>
</section>
<section anchor='recv-icmp-update' title='Receiving ICMP Error Messages'>
<t>Any node that does not recognize the Mobility header will
return an ICMP Parameter Problem, Code 1, message to
the sender of the packet. If the mobile node receives
such an ICMP error message in response to a return routability
procedure or Binding Update, it SHOULD record in its
Binding Update List that future Binding Updates SHOULD NOT be sent to
this destination.
Such Binding Update List
entries SHOULD be removed after a period of time in order to allow
for retrying route optimization.
</t>
<t>New Binding Update List entries MUST NOT be created as a result of
receiving ICMP error messages.
</t>
<t>Correspondent nodes that have participated in the return routability
procedure MUST implement the ability to correctly process received
packets containing a Home Address destination option. Therefore,
correctly implemented correspondent nodes should always be able to
recognize Home Address options. If a mobile node receives an ICMP
Parameter Problem, Code 2, message from some node indicating that it
does not support the Home Address option, the mobile node SHOULD log
the error and then discard the ICMP message.
</t>
</section>
<section anchor='mn-bm' title='Receiving Binding Error Messages'>
<t>
When a mobile node receives a packet containing a Binding Error
message, it should first check if the mobile node has a Binding Update
List entry for the source of the Binding Error message. If the
mobile node does not have such an entry, it MUST ignore the message.
This is necessary to prevent a waste of resources on, e.g., return
routability procedure due to spoofed Binding Error messages.
</t>
<t>
Otherwise, if the message Status field was 1 (unknown binding for Home
Address destination option), the mobile node should perform one of the
following three actions:
</t>
<list style='symbols'>
<t>
If the Binding Error Message was sent by the Home Agent, the
Mobile Node SHOULD send a Binding Update to the Home Agent
according to <xref target='update-home' />.
</t>
<t>
If the mobile node
has recent upper layer progress information, which indicates that
communications with the correspondent node are progressing, it MAY
ignore the message. This can be done in order to limit the damage that
spoofed Binding Error messages can cause to ongoing communications.
</t>
<t>
If the mobile node has no upper layer progress information, it MUST
remove the entry and route further communications through the
home agent. It MAY also optionally start a return routability
procedure (see <xref target='securing-regs-with-cns' />).
</t>
</list>
<t>If the message Status field was 2 (unrecognized MH Type value),
the mobile node should perform one of the following two actions:
</t>
<list style='symbols'>
<t> If the mobile node is not expecting an acknowledgement or
response from the correspondent node, the mobile node SHOULD ignore
this message.
</t>
<t> Otherwise, the mobile node SHOULD cease the use
of any extensions to this specification. If no extensions had been
used, the mobile node should cease the attempt to use route
optimization.
</t>
</list>
</section>
</section>
<section title='Home Agent and Prefix Management'>
<section anchor='mn-ha-disc' title='Dynamic Home Agent Address Discovery'>
<t>Sometimes
when the mobile node needs to send a
Binding Update to its home agent to register its new
primary care-of address,
as described in <xref target='update-home' />,
the mobile node may not know the address of any router
on its home link that can serve as a home agent for it.
For example,
some nodes on its home link may have been reconfigured
while the mobile node has been away from home,
such that the router that was operating as the mobile node's home agent
has been replaced by a different router serving this role.
</t>
<t>In this case, the mobile node MAY attempt to
discover the address of a suitable home agent on its home link.
To do so,
the mobile node sends an
ICMP Home Agent Address Discovery Request message
to the Mobile IPv6 <xref target='RFC2526'>Home-Agents anycast address</xref>
for its home subnet prefix.
As described in <xref target='ha-disc' />,
the home agent on its home link that receives this Request message
will return an ICMP Home Agent Address Discovery Reply message.
This message gives the
addresses for the home agents operating on the home link.
</t>
<t>The mobile node, upon receiving this Home Agent Address Discovery
Reply message, MAY then send its home registration Binding Update
to any of the unicast IP addresses listed in the Home Agent
Addresses field in the Reply. For example, the mobile node MAY
attempt its home registration to each of these addresses, in turn,
until its registration is accepted. The mobile node sends a Binding
Update to an address and waits for the matching Binding
Acknowledgement, moving on to the next address if there is no
response.
The mobile node MUST, however, wait at least
InitialBindackTimeoutFirstReg seconds (see <xref target='variables' />)
before sending a Binding Update to the next home agent.
In trying each of the returned home agent addresses,
the mobile node SHOULD try each of them in the order they appear in
the Home Agent Addresses field in the received
Home Agent Address Discovery Reply message.
In order to do this, the mobile node SHOULD store the list of
home agents for later use in case the home agent currently
managing the mobile node's care-of address forwarding should
become unavailable. The list MAY be stored, along with any
available lifetime information for the home agent addresses,
in nonvolatile memory to survive reboots by the mobile node.
</t>
<t>If the mobile node has a current registration with some home agent
(the Lifetime for that registration has not yet expired),
then the mobile node MUST attempt any new registration
first with that home agent.
If that registration attempt fails (e.g., timed out or rejected),
the mobile node SHOULD then reattempt this registration
with another home agent.
If the mobile node knows of no other suitable home agent,
then it MAY attempt the dynamic home agent address discovery
mechanism described above.
</t>
<t>If, after a mobile node transmits a Home Agent Address
Discovery Request message to the Home Agents Anycast address,
it does not receive a corresponding Home Agent Address
Discovery Reply message within INITIAL_DHAAD_TIMEOUT
(see <xref target='constants' />)
seconds, the mobile node MAY retransmit the same
Request message to the same anycast address. This
retransmission MAY be repeated up to a maximum
of DHAAD_RETRIES (see <xref target='constants' />) attempts.
Each retransmission MUST be delayed by twice the time interval
of the previous retransmission.
</t>
</section>
<section anchor='mn:send-sol' title='Sending Mobile Prefix Solicitations'>
<t>When a mobile node has a home address that is about to become
invalid, it SHOULD send a Mobile Prefix Solicitation to its home
agent in an attempt to acquire fresh routing prefix information.
The new information also enables the mobile node to participate in
renumbering operations affecting the home network, as described in
<xref target='ha:renum' />.
</t>
<t>The mobile node MUST use the Home Address destination option to carry
its home address. The mobile node MUST support and SHOULD use IPsec
to protect the solicitation. The mobile node MUST set the Identifier
field in the ICMP header to a random value.
</t>
<t>As described in <xref target='update-corresp' />,
Binding Updates sent by the mobile node to other nodes
MUST use a lifetime no greater than the remaining lifetime
of its home registration of its primary care-of address.
The mobile node SHOULD further limit the lifetimes that it sends on
any Binding Updates to be within the remaining valid lifetime
(see <xref target='ha:renum-send' />) for the prefix in its home address.
</t>
<t>When the lifetime for a changed prefix decreases, and the
change would cause cached bindings at correspondent nodes
in the Binding Update List to be stored past the newly shortened
lifetime, the mobile node MUST issue a Binding Update to
all such correspondent nodes.
</t>
<t>These limits on the binding lifetime
serve to prohibit use of a mobile node's home address
after it becomes invalid.
</t>
</section>
<section anchor='mn:get-adv' title='Receiving Mobile Prefix Advertisements'>
<t><xref target='ha:renum' /> describes the operation of a home agent
to support boot time configuration and renumbering a mobile node's
home subnet while the mobile node is away from home.
The home agent sends
Mobile Prefix Advertisements to the mobile node while away from home,
giving "important" Prefix Information options that describe changes
in the prefixes in use on the mobile node's home link.
</t>
<t>The Mobile Prefix Solicitation is similar to the Router Solicitation
used in <xref target='RFC4861'>Neighbor Discovery</xref>, except it
is routed from
the mobile node on the visited network to the home agent on the home
network by usual unicast routing rules.
</t>
<t>When a mobile node receives a Mobile Prefix Advertisement,
it MUST validate it according to the following test:
</t>
<list style='symbols'>
<t> The Source Address of the IP packet carrying the Mobile
Prefix Advertisement
is the same as the home agent address to which the mobile node
last sent an accepted home registration Binding Update to
register its primary care-of address.
Otherwise, if no such registrations have been made, it SHOULD
be the mobile node's stored home agent address, if one exists.
Otherwise, if the mobile node has not yet discovered its home
agent's address, it MUST NOT accept Mobile Prefix Advertisements.
</t>
<t> The packet MUST have a type 2 routing header and SHOULD
be protected by an IPsec header as described in
<xref target='security-prefix' /> and <xref target='renum-mpa' />.
</t>
<t> If the ICMP Identifier value matches the ICMP Identifier value
of the most recently sent Mobile Prefix Solicitation and no
other advertisement has yet been received for this value, then the
advertisement is considered
to be solicited and will be processed further.
<vspace blankLines='1' /> Otherwise, the advertisement is unsolicited, and MUST be
discarded. In this case the mobile node SHOULD send a
Mobile Prefix Solicitation.
</t>
</list>
<t>Any received Mobile Prefix Advertisement not meeting these tests
MUST be silently discarded.
</t>
<t>
For an accepted Mobile Prefix Advertisement, the mobile node MUST
process Managed Address Configuration (M), Other Stateful
Configuration (O), and the Prefix Information Options as if they
arrived in a Router <xref target='RFC4861'>Advertisement</xref> on
the mobile node's home link.
(This specification does not, however, describe how to acquire
home addresses through stateful protocols.)
Such processing may result in the mobile node
configuring a new home address, although due to separation between
preferred lifetime and valid lifetime,
such changes should not affect most communications by the mobile node,
in the same way as for nodes that are at home.
</t>
<t>
This specification assumes that any security associations and
security policy entries that may be needed for new prefixes have
been pre-configured in the mobile node. Note that while dynamic key
management avoids the need to configure new security associations,
it is still necessary to add policy entries to protect the
communications involving the home address(es). Mechanisms for
setting up these entries are outside the scope of this specification.
</t>
</section>
</section>
<section anchor='movement-all' title='Movement'>
<section anchor='move' title='Movement Detection'>
<t>
The primary goal of movement detection is to detect L3 handovers.
This section does not attempt to specify a fast movement detection
algorithm which will function optimally for all types of
applications, link-layers and deployment scenarios; instead, it
describes a generic method that uses the facilities of IPv6 Neighbor
Discovery, including Router Discovery and Neighbor Unreachability
Detection. At the time of this writing, this method is considered
well enough understood to recommend for standardization, however it
is expected that future versions of this specification or other
specifications may contain updated versions of the movement
detection algorithm that have better performance.
</t>
<t>
Generic movement detection uses Neighbor Unreachability Detection
to detect when the default router is no longer bi-directionally
reachable, in which case the mobile node must discover a new default
router (usually on a new link). However, this detection only occurs
when the mobile node has packets to send, and in the absence of
frequent Router Advertisements or indications from the link-layer,
the mobile node might become unaware of an L3 handover that
occurred. Therefore, the mobile node should supplement this method
with other information whenever it is available to the mobile node
(e.g., from lower protocol layers).
</t>
<t>When the mobile node detects an L3 handover, it performs Duplicate
Address <xref target='RFC4862'>Detection</xref> on its link-local address,
selects a new
default router as a consequence of Router Discovery, and then
performs Prefix Discovery with that new router to form new care-of
address(es) as described in <xref target='new-coa' />. It then registers its
new primary care-of address with its home agent as described in
<xref target='update-home' />. After updating its home registration, the mobile
node then updates associated mobility bindings in correspondent
nodes that it is performing route optimization with as specified in
<xref target='update-corresp' />.
</t>
<t>Due to the temporary packet flow disruption and signaling overhead
involved in updating mobility bindings, the mobile node should avoid
performing an L3 handover until it is strictly necessary.
Specifically, when the mobile node receives a Router Advertisement
from a new router that contains a different set of on-link
prefixes, if the mobile node detects that the currently selected
default router on the old link is still bi-directionally reachable,
it should generally continue to use the old router on the old link
rather than switch away from it to use a new default router.
</t>
<t>Mobile nodes can use the information in received Router
Advertisements to detect L3 handovers. In doing so the mobile node
needs to consider the following issues:
</t>
<list style='symbols'>
<t> There might be multiple routers on the same link, thus hearing a
new router does not necessarily constitute an L3 handover.
</t>
<t> When there are multiple routers on the same link they might
advertise different prefixes. Thus even hearing a new router
with a new prefix might not be a reliable indication of an L3
handover.
</t>
<t> The link-local addresses of routers are not globally unique,
hence after completing an L3 handover the mobile node might
continue to receive Router Advertisements with the same
link-local source address. This might be common if routers use
the same link-local address on multiple interfaces. This issue
can be avoided when routers use the Router Address (R) bit, since
that provides a global address of the router.
</t>
</list>
<t>
In addition, the mobile node should consider the following events as
indications that an L3 handover may have occurred. Upon receiving
such indications, the mobile node needs to perform Router Discovery
to discover routers and prefixes on the new link, as described
in Section 6.3.7 of
Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>).
</t>
<list style='symbols'>
<t> If Router Advertisements that the mobile node receives include
an Advertisement Interval option, the mobile node may use its
Advertisement Interval field as an indication of the frequency
with which it should expect to continue to receive future
Advertisements from that router. This field specifies the
minimum rate (the maximum amount of time between successive
Advertisements) that the mobile node should expect. If this
amount of time elapses without the mobile node receiving any
Advertisement from this router, the mobile node can be sure that
at least one Advertisement sent by the router has been lost.
The mobile node can then implement its own policy to determine
how many lost Advertisements from its current default router
constitute an L3 handover indication.
</t>
<t> Neighbor Unreachability Detection determines that the default
router is no longer reachable.
</t>
<t> With some types of networks, notification that a L2 handover has
occurred might be obtained from lower layer protocols or
device driver software within the mobile node. While further
details around handling L2 indications as movement hints is an
item for further study, at the time of writing this
specification the following is considered reasonable:
<vspace blankLines='1' /> A L2 handover indication may or may not imply L2 movement and
L2 movement may or may not imply L3 movement; the correlations
might be a function of the type of L2 but might also be a
function of actual deployment of the wireless topology.
<vspace blankLines='1' /> Unless it is well-known that a L2 handover indication is
likely to imply L3 movement, instead of immediately
multicasting a router solicitation it may be better to attempt
to verify whether the default router is still bi-directionally
reachable. This can be accomplished by sending a unicast
Neighbor Solicitation and waiting for a Neighbor Advertisement
with the solicited flag set. Note that this is similar to
Neighbor Unreachability detection but it does not have the
same state machine, such as the STALE state.
<vspace blankLines='1' /> If the default router does not respond to the Neighbor
Solicitation it makes sense to proceed to multicasting a
Router Solicitation.
</t>
</list>
</section>
<section anchor='home-detect' title='Home Link Detection'>
<t>
When an MN detects that it has arrived on a new link using the
movement detection algorithm in use (<xref target='move' />,)
or on bootstrapping, it performs the following steps to determine
if it is on the home link.
</t>
<list style='symbols'>
<t>
The MN performs the procedure described in <xref target='new-coa' />
and configures an address. It also keeps track of all the on-link
prefix(es) received in the RA along with their prefix lengths.
</t>
<t>
If the home prefix has not been statically configured the MN uses
some form of bootstrapping procedure
(e.g. <xref target='RFC5026'>RFC5026 </xref>) to determine
the home prefix.
</t>
<t>
Given the availability of the home prefix, the MN checks whether or
not the home prefix matches one of the prefixes received in the RA.
If it does, the MN concludes that it is connected to the home link.
<!--
Please see <xref target='seccons:nd'/> for information regarding the
security vulnerability associated with this determination.
-->
</t>
</list>
</section>
<section anchor='new-coa' title='Forming New Care-of Addresses'>
<t>After detecting that it has moved a mobile node SHOULD generate a new
primary care-of address using normal IPv6 mechanisms. This SHOULD also
be done when the current primary care-of address becomes deprecated.
A mobile node MAY form a new primary care-of address
at any time, but a mobile node MUST NOT send a Binding Update
about a new care-of address to its home agent
more than MAX_UPDATE_RATE times within a second.
</t>
<t>In addition, a mobile node MAY form new non-primary care-of
addresses even when it has not switched to a new default router.
A mobile node can have only one primary care-of address at a time
(which is registered with its home agent),
but it MAY have an additional care-of address for
any or all of the prefixes on its current link.
Furthermore, since a wireless network interface may actually allow
a mobile node to be reachable on more than one link at a time
(i.e., within wireless transmitter range of routers on more
than one separate link),
a mobile node MAY have care-of addresses on more than one link at a time.
The use of more than one care-of address at a time
is described in <xref target='multiple' />.
</t>
<t>As described in <xref target='overview' />,
in order to form a new care-of address,
a mobile node MAY use either <xref target='RFC4862'>stateless</xref>
or stateful (e.g., <xref target='RFC3315'>DHCPv6</xref>)
Address Autoconfiguration.
If a mobile node needs to use a source address (other than the
unspecified address) in packets sent as a part of address
autoconfiguration, it MUST use an IPv6 link-local address rather than
its own IPv6 home address.
</t>
<t><xref target='RFC4862'>RFC 4862</xref> specifies that in normal
processing for
Duplicate Address Detection, the node SHOULD delay sending the initial
Neighbor Solicitation message by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY. Since delaying DAD can result in
significant delays in configuring a new care-of address when the
Mobile Node moves to a new link, the Mobile Node preferably SHOULD NOT
delay DAD when configuring a new care-of address. The Mobile Node
SHOULD delay according to the mechanisms specified in RFC 4862 unless
the implementation has a behavior that desynchronizes the steps that
happen before the DAD in the case that multiple nodes experience
handover at the same time. Such desynchronizing behaviors might be due
to random delays in the L2 protocols or device drivers, or due to the
movement detection mechanism that is used.
</t>
</section>
<section anchor='multiple' title='Using Multiple Care-of Addresses'>
<t>As described in <xref target='new-coa' />,
a mobile node MAY use more than one care-of address at a time.
Particularly in the case of many wireless networks,
a mobile node effectively might be reachable
through multiple links at the same time
(e.g., with overlapping wireless cells),
on which different on-link subnet prefixes may exist.
The mobile node MUST ensure that its primary care-of address always has a
prefix that is
advertised by its current default router.
After selecting a new primary care-of address,
the mobile node MUST send a Binding Update
containing that care-of address to its home agent.
The Binding Update MUST have the Home Registration (H) and
Acknowledge (A) bits set its home agent, as described on
<xref target='update-home' />.
</t>
<t>To assist with smooth handovers, a mobile node
SHOULD retain its previous primary care-of address as a
(non-primary) care-of address,
and SHOULD still accept packets at this address,
even after registering its new primary care-of address
with its home agent.
This is reasonable, since the mobile node could only receive
packets at its previous primary care-of address if it were indeed
still connected to that link.
If the previous primary care-of address was allocated using stateful
Address <xref target='RFC3315'>Autoconfiguration</xref>,
the mobile node may not wish to release the
address immediately upon switching to a new primary care-of address.
</t>
<t>Whenever a mobile node determines that it is no longer reachable
through a given link, it SHOULD invalidate all care-of addresses
associated with address prefixes that it discovered from routers on
the unreachable link which are not in the current set of address
prefixes advertised by the (possibly new) current default router.
</t>
</section>
<section anchor='return-home' title='Returning Home'>
<t>
A mobile node detects that it has returned to its home link through
the movement detection algorithm in use (<xref target='home-detect' />),
when the mobile node detects that its home subnet prefix is again
on-link. To be able to send and receive packets using its home
address from the home link, the mobile node MUST send a Binding
Update to its home agent to instruct its home agent to no longer
intercept or tunnel packets for it. Until the mobile node sends
such a de-registration Binding Update, it MUST NOT attempt to
send and receive packets using its home address from the home link.
The home agent will continue to intercept all packets sent to the
mobile's home address and tunnel them to the previously registered
care-of address.
</t>
<t>
In this home registration, the mobile node MUST set the Acknowledge (A)
and Home Registration (H) bits, set the Lifetime field to zero,
and set the care-of address for the
binding to the mobile node's own home address. The mobile
node MUST use its home address as the source address in the
Binding Update.
</t>
<t>
When sending this Binding Update to its home agent,
the mobile node must be careful in how it uses
<xref target='RFC4861'>Neighbor Solicitation</xref> (if needed)
to learn the home agent's link-layer address,
since the home agent will be currently configured to
intercept packets to the mobile node's home address using
Proxy Neighbor Discovery (Proxy ND).
In particular, the mobile node is unable to use its
home address as the Source Address in the
Neighbor Solicitation until the home agent stops
defending the home address.
</t>
<t>Neighbor Solicitation by the mobile node for the home agent's address
will normally not be necessary, since the mobile node has already
learned the home agent's link-layer address from a Source Link-Layer
Address option in a Router Advertisement. However, if there are
multiple home agents it may still be necessary to send a
solicitation. In this special case of the mobile node returning home,
the mobile node MUST multicast the packet, and in addition set the
Source Address of this Neighbor Solicitation to the unspecified
address (0:0:0:0:0:0:0:0).
The target of the Neighbor Solicitation
MUST be set to the mobile node's home address. The destination
IP address MUST be set to the Solicited-Node multicast <xref target='RFC4291'>address</xref>.
The home agent will send a multicast Neighbor Advertisement back to
the mobile node with the Solicited flag (S) set to zero.
In any case, the mobile node SHOULD record the information
from the Source Link-Layer Address option or from the advertisement,
and set the state of the Neighbor Cache entry for the home agent to
REACHABLE.
</t>
<t>The mobile node then sends its Binding Update to the
home agent's link-layer address,
instructing its home agent to no longer serve as a home agent for it.
By processing this Binding Update,
the home agent will cease defending the mobile node's
home address for Duplicate Address Detection
and will no longer respond to Neighbor Solicitations
for the mobile node's home address.
The mobile node is then the only node on the link
receiving packets at the mobile node's home address.
In addition, when returning home prior to the expiration of a current
binding for its home address, and configuring its home address on
its network interface on its home link,
the mobile node MUST NOT perform Duplicate Address Detection
on its own home address,
in order to avoid confusion or conflict with its
home agent's use of the same address.
This rule also applies to the derived link-local address of the mobile
node, if the Link
Local Address Compatibility (L) bit was set when the binding was created.
If the mobile node returns home after the bindings for all
of its care-of addresses have expired, then it SHOULD perform DAD.
</t>
<t>After the Mobile Node sends the Binding Update, it MUST be
prepared to reply to Neighbor Solicitations for its home address.
Such replies MUST be sent using a unicast Neighbor Advertisement
to the sender's link-layer address. It is necessary to reply,
since sending the Binding Acknowledgement from the home agent may
require performing Neighbor Discovery, and the mobile node may not
be able to distinguish Neighbor Solicitations coming from the home
agent from other Neighbor Solicitations. Note that a race condition
exists where both the mobile node and the home agent respond to
the same solicitations sent by other nodes; this will be only
temporary, however, until the Binding Update is accepted.
</t>
<t>After receiving the Binding Acknowledgement for its
Binding Update to its home agent,
the mobile node MUST multicast onto the home link
(to the all-nodes multicast address)
a Neighbor <xref target='RFC4861'>Advertisement</xref>,
to advertise the mobile node's own link-layer address
for its own home address.
The Target Address in this Neighbor Advertisement
MUST be set to the mobile node's home address,
and the Advertisement MUST include a Target Link-layer Address option
specifying the mobile node's link-layer address.
The mobile node MUST multicast such
a Neighbor Advertisement for each of its home addresses,
as defined by the current on-link prefixes,
including its link-local address.
The Solicited Flag (S) in these Advertisements MUST NOT be set,
since they were not solicited by any Neighbor Solicitation.
The Override Flag (O) in these Advertisements MUST be set,
indicating that the Advertisements SHOULD override any existing
Neighbor Cache entries at any node receiving them.
</t>
<t>Since multicasting on the local link (such as Ethernet) is
typically not guaranteed to be reliable,
the mobile node MAY retransmit these
<xref target='RFC4861'>Neighbor Advertisements</xref>
up to MAX_NEIGHBOR_ADVERTISEMENT times to increase their reliability.
It is still possible that some nodes on the home link will
not receive any of these Neighbor Advertisements,
but these nodes will eventually be able to
recover through use of
<xref target='RFC4861'>Neighbor Unreachability Detection</xref>.
</t>
<t>Note that the tunnel via the home agent typically stops operating at the same time
that the home registration is deleted.</t>
</section>
</section>
<section anchor='mobile-rrp' title='Return Routability Procedure'>
<t>This section defines the rules that the mobile node must follow when
performing the return routability
procedure. <xref target='update-corresp' /> describes the rules when the
return routability procedure needs to be initiated.
</t>
<section anchor='update-corresp-rr' title='Sending Test Init Messages'>
<t>A mobile node that initiates a return routability procedure MUST send
(in parallel) a Home Test Init message and a Care-of Test Init
messages. However, if the mobile node has recently received (see <xref target='tokenupdate' />) one or
both home or care-of keygen tokens, and associated nonce indices for
the desired addresses, it MAY reuse them. Therefore, the return
routability procedure
may in some cases be completed with only one message pair.
It may even be completed without any messages at all, if the mobile
node has a recent home keygen token and
has previously visited the same care-of address so that it also
has a recent care-of keygen token.
If the mobile node intends to send a Binding Update with
the Lifetime set to zero and the care-of address
equal to its home address - such as when
returning home - sending a Home Test Init message is sufficient.
In this case, generation of the binding management key depends
exclusively on the home keygen token (<xref target='rrp' />).
</t>
<t>A Home Test Init message MUST be created as described in <xref target='hoti' />.
</t>
<t>A Care-of Test Init message MUST be created as described in <xref target='coti' />.
When sending a Home Test Init or Care-of Test Init
message the mobile node MUST record in its Binding Update List the
following fields from the messages:
</t>
<list style='symbols'>
<t> The IP address of the node to which the message was sent.
</t>
<t> The home address of the mobile node. This value will appear in
the Source Address field of the Home Test Init message. When
sending the Care-of Test Init message, this address does not
appear in the message, but represents the home address for which
the binding is desired.
</t>
<t> The time at which each of these messages was sent.
</t>
<t> The cookies used in the messages.
</t>
</list>
<t>Note that a single Care-of Test Init message may be sufficient even
when there are multiple home addresses. In this case the mobile node
MAY record the same information in multiple Binding Update List entries.
</t>
</section>
<section anchor='mobile-recv-rr' title='Receiving Test Messages'>
<t>Upon receiving a packet carrying a Home Test message, a mobile
node MUST validate the packet according to the following tests:
</t>
<list style='symbols'>
<t> The Source Address of the packet belongs to a correspondent
node for which the mobile node has a Binding Update List entry
with a state indicating that return routability procedure is in
progress. Note that there may be multiple such entries.
</t>
<t> The Binding Update List indicates that no home keygen token
has been received yet.
</t>
<t> The Destination Address of the packet has the home address
of the mobile node, and the packet has been received in a tunnel
from the home agent.
</t>
<t> The Home Init Cookie field in the message matches the value
stored in the Binding Update List.
</t>
</list>
<t>Any Home Test message not satisfying all of these tests MUST be silently
ignored. Otherwise, the mobile node MUST record the Home Nonce Index
and home keygen token in the Binding Update List.
If the Binding Update List entry does not have a care-of keygen token,
the mobile node SHOULD continue waiting for the Care-of Test message.
</t>
<t>Upon receiving a packet carrying a Care-of Test message, a mobile
node MUST validate the packet according to the following tests:
</t>
<list style='symbols'>
<t> The Source Address of the packet belongs to a correspondent
node for which the mobile node has a Binding Update List entry with a
state indicating that return routability procedure is in progress.
Note that there may be multiple such entries.
</t>
<t> The Binding Update List indicates that no care-of keygen token
has been received yet.
</t>
<t> The Destination Address of the packet is the current
care-of address of the mobile node.
</t>
<t> The Care-of Init Cookie field in the message matches the value
stored in the Binding Update List.
</t>
</list>
<t>Any Care-of Test message not satisfying all of these tests MUST be silently
ignored. Otherwise, the mobile node MUST record the Care-of Nonce Index
and care-of keygen token in the Binding Update List.
If the Binding Update List entry does not have a home keygen token,
the mobile node SHOULD continue waiting for the Home Test message.
</t>
<t>If after receiving either the Home Test or the Care-of Test message and
performing the above actions, the Binding Update List entry has both
the home and the care-of keygen tokens, the return routability procedure
is complete. The mobile node SHOULD then proceed with sending
a Binding Update as described in <xref target='update-corresp' />.
</t>
<t>Correspondent nodes from the time before this specification was
published may not support the Mobility Header protocol. These nodes
will respond to Home Test Init and Care-of Test Init messages with an
ICMP Parameter Problem code 1. The mobile node SHOULD take such messages
as an indication that the correspondent node cannot provide route
optimization, and revert back to the use of bidirectional tunneling.
</t>
</section>
<section title='Protecting Return Routability Packets'>
<t>The mobile node MUST support the protection of Home Test and Home Test
Init messages as described in <xref target='ha:rrprot' />.
</t>
<t>When IPsec is used to protect return routability signaling or
payload packets, the mobile node MUST set the source address it uses
for the outgoing tunnel packets to the current primary care-of
address. The mobile node starts to use a new primary care-of address
immediately after sending a Binding Update to the home agent to
register this new address.
</t>
</section>
</section>
<section title='Processing Bindings'>
<section anchor='update-home' title='Sending Binding Updates to the Home Agent'>
<t>In order to change its primary care-of address as described in
<xref target='move' /> and <xref target='new-coa' />, a mobile node MUST
register this care-of address with its home agent in order to make
this its primary care-of address.
</t>
<t>Also, if the mobile node wants the services of the home agent beyond
the current registration period, the mobile node should send a new
Binding Update to it well before the expiration of this period, even
if it is not changing its primary care-of address. However, if the
home agent returned a Binding Acknowledgement for the current
registration with Status field set to 1 (accepted but prefix discovery
necessary), the mobile node should not try to
register again before it has learned the validity of its home prefixes
through mobile prefix discovery. This is typically necessary every time
this Status value is received, because information learned
earlier may have changed.
</t>
<t>To register a care-of address or to extend the lifetime of an existing
registration, the mobile node sends a packet to its home agent
containing a Binding Update, with the packet constructed as follows:
</t>
<list style='symbols'>
<t> The Home Registration (H) bit MUST be set in the Binding Update.
</t>
<t> The Acknowledge (A) bit MUST be set in the Binding Update.
</t>
<t> The packet MUST contain a Home Address destination option,
giving the mobile node's home address for the binding.
</t>
<t> The care-of address for the binding MUST be used as the Source
Address in the packet's IPv6 header, unless an Alternate
Care-of Address mobility option is included in the Binding
Update. This option MUST be included in all home
registrations, as the ESP protocol will not be able to protect
care-of addresses in the IPv6 header. (Mobile IPv6
implementations that know they are using IPsec AH to protect a
particular message might avoid this option. For brevity
the usage of AH is not discussed in this document.)
</t>
<t> If the mobile node's link-local address has the same
interface identifier as the home address for which it
is supplying a new care-of address, then the mobile node
SHOULD set the Link-Local Address Compatibility (L) bit.
</t>
<t> If the home address was generated
using <xref target='RFC4941'>RFC 4941</xref>,
then the link local address is unlikely to have a compatible
interface identifier. In this case, the mobile node MUST
clear the Link-Local Address Compatibility (L) bit.
</t>
<t> If the IPsec security associations between the mobile node
and the home agent have been established dynamically, and
the mobile node has the capability to update its endpoint
in the used key management protocol to the new care-of
address every time it moves, the mobile node SHOULD set the
Key Management Mobility Capability (K) bit in the Binding Update.
Otherwise, the mobile node
MUST clear the bit.
</t>
<t> The value specified in the Lifetime field MUST be non-zero and SHOULD be less than
or equal to the remaining valid lifetime of the home address and the
care-of address specified for the binding.
<vspace blankLines='1' /> Mobile nodes that use dynamic home agent address discovery should
be careful with long lifetimes. If the mobile node loses the knowledge
of its binding with a specific home agent, registering a new binding
with another home agent may be impossible as the previous home agent
is still defending the existing binding. Therefore, to ensure that mobile nodes
using home agent address discovery do not lose information about their
binding, they SHOULD de-register before losing this
information, or use small lifetimes.
</t>
</list>
<t>The Acknowledge (A) bit in the Binding Update requests the home agent
to return a Binding Acknowledgement in response to this Binding
Update. As described in <xref target='bind-ack' />, the mobile node
SHOULD retransmit this Binding Update to its home agent until it
receives a matching Binding Acknowledgement. Once reaching a
retransmission timeout period of MAX_BINDACK_TIMEOUT, the mobile node
SHOULD restart the process of delivering the Binding Update, but
trying instead the next home agent returned during dynamic home agent
address discovery (see <xref target='mn-ha-disc' />).
If there was only one home agent, the mobile node instead SHOULD continue
to periodically retransmit the Binding Update at this rate until
acknowledged (or until it begins attempting to register a different
primary care-of address). See <xref target='rate-limit' /> for
information about retransmitting Binding Updates.
</t>
<t>With the Binding Update, the mobile node requests the home agent to
serve as the home agent for the given home address. Until the
lifetime of this registration expires, the home agent considers itself
the home agent for this home address.
</t>
<t>Each Binding Update MUST be authenticated as coming from the right
mobile node, as defined in <xref target='securing-regs-with-home-agents' />.
The mobile node MUST use its home address - either in the Home
Address destination option or in the Source Address field of
the IPv6 header - in Binding Updates sent to the home agent.
This is necessary in order to allow the IPsec policies to be matched
with the correct home address.
</t>
<t>When sending a Binding Update to its home agent, the mobile node MUST
also create or update the corresponding Binding Update List entry, as
specified in <xref target='update-corresp' />.
</t>
<t>The last Sequence Number value sent to the home agent in a Binding
Update is stored by the mobile node. If the sending mobile node has no
knowledge of the correct Sequence Number value, it may start at any
value. If the home agent rejects the value, it sends back a Binding
Acknowledgement with a status code 135, and the last accepted sequence
number in the Sequence Number field of the Binding Acknowledgement. The
mobile node MUST store this information and use the next Sequence Number
value for the next Binding Update it sends.
</t>
<t>If the mobile node has additional home addresses,
then the mobile node SHOULD send an additional
packet containing a Binding Update to its home agent to register the
care-of address for each such other home address.
</t>
<t>The home agent will only perform DAD for the mobile node's home
address when the mobile node has supplied a valid binding between
its home address and a care-of address. If some time elapses during
which the mobile node has no binding at the home agent, it might be
possible for another node to autoconfigure the mobile node's home
address. Therefore, the mobile node MUST treat the creation of a new
binding with the home agent using an existing home address, the same
as creation of a new home address. In the unlikely event that the
mobile node's home address is autoconfigured as the IPv6 address
of another network node on the home network, the home agent will
reply to the mobile node's subsequent Binding Update with a Binding
Acknowledgement containing a Status of 134 (Duplicate Address Detection
failed). In this case, the mobile node MUST NOT attempt to re-use
the same home address. It SHOULD continue to register the care-of
addresses for its other home addresses, if any.
Mechanisms outlined in
<xref target='RFC5026'>"Mobile IPv6 Bootstrapping in Split Scenario"</xref>
allow mobile nodes to acquire new home addresses to replace the one for which
Status 134 was received.
</t>
</section>
<section anchor='update-corresp' title='Correspondent Registration'>
<t>When the mobile node is assured that its home address is valid, it can
initiate a correspondent registration with the purpose of
allowing the correspondent node to cache the mobile node's current
care-of address. This procedure consists of the return routability
procedure followed by a registration.
</t>
<t>This section defines when the correspondent registration is to be initiated and the rules to follow while it is being performed.
</t>
<t>After the mobile node has sent a Binding Update to its home agent,
registering a new primary care-of address (as described in
<xref target='update-home' />), the mobile node SHOULD initiate a
correspondent registration for each node that already appears in
the mobile node's Binding Update List. The initiated procedures can
be used to either update or delete binding information in the
correspondent node.
</t>
<t>For nodes that do not appear in the mobile node's Binding Update List,
the mobile node MAY initiate a correspondent registration at any
time after sending the Binding Update to its home
agent. Considerations regarding when (and if) to initiate the
procedure depend on the specific movement and traffic patterns of the
mobile node and are outside the scope of this document.
</t>
<t>In addition, the mobile
node MAY initiate the correspondent registration in response to receiving a packet that
meets all of the following tests:
</t>
<list style='symbols'>
<t> The packet was tunneled using IPv6 encapsulation.
</t>
<t> The Destination Address in the tunnel (outer) IPv6 header is
equal to any of the mobile node's care-of addresses.
</t>
<t> The Destination Address in the original (inner) IPv6 header
is equal to one of the mobile node's home addresses.
</t>
<t> The Source Address in the tunnel (outer) IPv6 header differs from
the Source Address in the original (inner) IPv6 header.
</t>
<t> The packet does not contain a Home Test, Home Test Init,
Care-of Test, or Care-of Test Init message.
</t>
</list>
<t>If a mobile node has multiple home addresses, it becomes important
to select the right home address to use in the correspondent
registration. The used home address MUST be the Destination
Address of the original (inner) packet.
</t>
<t>The peer address used in the procedure MUST be determined as follows:
</t>
<list style='symbols'>
<t> If a Home Address destination option is present in the original
(inner) packet, the address from this option is used.
</t>
<t> Otherwise, the Source Address in the original (inner) IPv6 header
of the packet is used.
</t>
</list>
<t>Note that the validity of the original packet is checked before
attempting to initiate a correspondent registration. For
instance, if a Home Address destination option appeared in the
original packet, then rules in <xref target='rxing-hao' /> are followed.
</t>
<t>A mobile node MAY also choose to keep its topological location private from
certain correspondent nodes, and thus need not initiate the
correspondent registration.
</t>
<t>Upon successfully completing the return routability procedure, and
after receiving a successful Binding Acknowledgement from the Home
Agent, a Binding Update MAY be sent to the correspondent node.
</t>
<t>In any Binding Update sent by a mobile node, the care-of address
(either the Source Address in the packet's IPv6 header or the Care-of
Address in the Alternate Care-of Address mobility option of the
Binding Update) MUST be set to one of the care-of addresses currently
in use by the mobile node or to the mobile node's home address. A
mobile node MAY set the care-of address differently for sending
Binding Updates to different correspondent nodes.
</t>
<t>A mobile node MAY also send a Binding Update to such a correspondent
node, instructing it to delete any existing binding for the mobile node
from its Binding Cache, as described in <xref target='bind-upd' />. Even
in this case a successful completion of the return routability
procedure is required first.
</t>
<t>If the care-of address is not set to the mobile node's home address, the
Binding Update requests that the correspondent node create or update an
entry for the mobile node in the correspondent node's Binding Cache.
This is done in
order to record a care-of address for use in sending future packets
to the mobile node. In this case, the value specified in the Lifetime
field sent in the Binding Update SHOULD be less than or equal to the
remaining lifetime of the home registration and the care-of address
specified for the binding. The care-of address given in the Binding
Update MAY differ from the mobile node's primary care-of address.
</t>
<t>If the Binding Update is sent to the
correspondent node, requesting the deletion of any existing Binding Cache entry it
has for the mobile node, the care-of address is set to the mobile node's home address and the
Lifetime field set to zero. In this case, generation of the binding
management key depends exclusively on the home keygen token
(<xref target='rrp' />). The care-of nonce index SHOULD be set to zero in
this case. In keeping with the Binding Update creation rules below,
the care-of address MUST be set to the home address if the mobile node
is at home, or to the current care-of address if it is away from home.
</t>
<t>If the mobile node wants to ensure that its new care-of address has
been entered into a correspondent node's Binding Cache, the mobile
node needs to request an acknowledgement by setting the Acknowledge (A)
bit in the Binding Update.
</t>
<t>A Binding Update is created as follows:
</t>
<list style='symbols'>
<t> The current care-of address of the mobile
node MUST be sent either in the Source Address of the IPv6 header, or
in the Alternate Care-of Address mobility option.
</t>
<t> The Destination Address of the IPv6 header MUST contain the address
of the correspondent node.
</t>
<t> The Mobility Header is constructed according to rules in
<xref target='bind-upd' /> and <xref target='rr-cnbind' />, including the
Binding Authorization Data (calculated as defined in
<xref target='auth-option' />) and possibly the Nonce Indices
mobility options.
</t>
<t> The home address of the mobile node MUST be added to the packet
in a Home Address destination option, unless the Source Address
is the home address.
</t>
</list>
<t>Each Binding Update MUST have a Sequence Number greater than the Sequence
Number value sent in the previous Binding Update to the same
destination address (if any). The sequence numbers are compared modulo
2**16, as described in <xref target='recv-updates' />. There is no
requirement, however, that the Sequence Number value strictly increase
by 1 with each new Binding Update sent or received, as long as the
value stays within the window. The last Sequence Number value sent to
a destination in a Binding Update is stored by the mobile node in its
Binding Update List entry for that destination. If the sending mobile
node has no Binding Update List entry, the Sequence Number SHOULD
start at a random value. The mobile node MUST NOT use the same
Sequence Number in two different Binding Updates to the same
correspondent node, even if the Binding Updates provide different
care-of addresses.
</t>
<t>The mobile node is responsible for the completion of the correspondent
registration, as well as any retransmissions that may be needed
(subject to the rate limitation defined in <xref target='rate-limit' />).
</t>
</section>
<section anchor='mobile-recv-acks' title='Receiving Binding Acknowledgements'>
<t>Upon receiving a packet carrying a Binding Acknowledgement, a mobile
node MUST validate the packet according to the following tests:
</t>
<list style='symbols'>
<t> The packet meets the authentication requirements for Binding
Acknowledgements defined in <xref target='bind-ack' />
and <xref target='overview:security' />. That is, if the Binding Update was sent
to the home agent, underlying IPsec protection is used. If the Binding
Update was sent to the correspondent node, the Binding Authorization
Data mobility option
MUST be present and have a valid value.
</t>
<t> The Binding Authorization Data mobility option, if present, MUST be
the last option and MUST NOT have trailing padding.
</t>
<t> The Sequence Number field matches the Sequence Number sent by the
mobile node to this destination address in an outstanding Binding
Update, and the Status field is not 135.
</t>
</list>
<t>
Any Binding Acknowledgement not satisfying all of these tests MUST be
silently ignored.
</t>
<t>
When a mobile node receives a packet carrying a valid Binding
Acknowledgement, the mobile node MUST examine the Status field as
follows:
</t>
<list style='symbols'>
<t> If the Status field indicates that the Binding Update was
accepted (the Status field is less than 128), then the mobile
node MUST update the corresponding entry in its Binding Update
List to indicate that the Binding Update has been acknowledged;
the mobile node MUST then stop retransmitting the Binding Update.
In addition, if the value specified in the Lifetime field in the
Binding Acknowledgement is less than the Lifetime value sent
in the Binding Update being acknowledged, the mobile node
MUST subtract the difference between these two Lifetime values
from the remaining lifetime for the binding as maintained in the
corresponding Binding Update List entry (with a minimum value
for the Binding Update List entry lifetime of 0). That is, if
the Lifetime value sent in the Binding Update was L_update, the
Lifetime value received in the Binding Acknowledgement was L_ack,
and the current remaining lifetime of the Binding Update List
entry is L_remain, then the new value for the remaining lifetime
of the Binding Update List entry should be
<list style='empty'>
<t> max((L_remain - (L_update - L_ack)), 0)
</t>
</list>
<vspace blankLines='1' /> where max(X, Y) is the maximum of X and Y. The effect of this
step is to correctly manage the mobile node's view of the
binding's remaining lifetime (as maintained in the corresponding
Binding Update List entry) so that it correctly counts down from
the Lifetime value given in the Binding Acknowledgement, but with
the timer countdown beginning at the time that the Binding Update
was sent.
<vspace blankLines='1' /> Mobile nodes SHOULD send a new Binding Update well before the
expiration of this period in order to extend the lifetime.
This helps to avoid disruptions in communications which might
otherwise be caused by network delays or clock drift.
</t>
<t> If the Binding Acknowledgement correctly passes
authentication and the Status field value is 135 (Sequence Number
out of window), then the mobile node MUST update its binding
sequence number appropriately to match the sequence number given
in the Binding Acknowledgement. Otherwise, if the Status field
value is 135 but the Binding Acknowledgement does not pass
authentication, the message MUST be silently ignored.
</t>
<t> If the Status field value is 1 (accepted but
prefix discovery necessary), the mobile node
SHOULD send a Mobile Prefix Solicitation message to update its
information about the available prefixes.
</t>
<t> If the Status field indicates that the Binding Update was
rejected (the Status field is greater than or equal to 128),
then the mobile node can take steps to correct the
cause of the error and retransmit the Binding Update (with a new
Sequence Number value), subject to the rate limiting restriction
specified in <xref target='rate-limit' />. If this is not done or
it fails, then the mobile node SHOULD record in its Binding Update List that future
Binding Updates SHOULD NOT be sent to this destination.
</t>
</list>
<t>The treatment of a Binding Refresh Advice mobility option within
the Binding Acknowledgement depends on where the
acknowledgement came from. This option MUST be ignored if the
acknowledgement came from a correspondent node. If it came from
the home agent, the mobile node uses the Refresh Interval field in
the option as a suggestion that it SHOULD attempt to refresh its
home registration at the indicated shorter interval.
</t>
<t>If the acknowledgement came from the home agent, the mobile node
examines the value of the Key Management Mobility Capability (K) bit.
If this bit is not set, the mobile node SHOULD discard key management
protocol connections, if any, to the home agent. The mobile node MAY
also initiate a new key management connection.
</t>
<t>If this bit is set, the mobile node SHOULD move its own endpoint in
the key management protocol connections to the home agent, if any.
The mobile node's new endpoint should be the new care-of address. For
an IKE phase 1 connection, this means that packets sent to this address
with the original ISAKMP cookies are accepted.
</t>
</section>
<section anchor='receive-req' title='Receiving Binding Refresh Requests'>
<t>
When a mobile node receives a packet containing a Binding Refresh
Request message, if the mobile node has a Binding Update List entry for
the source of the Binding Refresh Request, and the mobile node wants
to retain its binding cache entry at the correspondent node, then the
mobile node should start a return routability procedure. If the mobile
node wants to have its binding cache entry removed, it can either
ignore the Binding Refresh Request and wait for the binding to time
out, or at any time, it can delete its binding from a correspondent
node with an explicit binding update with a zero lifetime and the
care-of address set to the home address. If the mobile
node does not know if it needs the binding cache entry, it can make
the decision in an implementation dependent manner, such as based on
available resources.
</t>
<t>Note that the mobile node should be careful to
not respond to Binding Refresh Requests for addresses not in
the Binding Update List to avoid being subjected to a
denial of service attack.
</t>
<t>If the return routability procedure completes successfully, a Binding
Update message SHOULD be sent, as described in <xref target='update-corresp' />.
The Lifetime field in this Binding Update SHOULD be set to a new
lifetime, extending any current lifetime remaining from a previous
Binding Update sent to this node (as indicated in any existing Binding
Update List entry for this node), and the lifetime SHOULD again be
less than or equal to the remaining lifetime of the home registration and
the care-of address specified for the binding.
When sending this Binding
Update, the mobile node MUST update its Binding Update List in the
same way as for any other Binding Update sent by the mobile node.
</t>
</section>
</section>
<section anchor='rate-limit' title='Retransmissions and Rate Limiting'>
<t>The mobile node is responsible for retransmissions and
rate limiting in the return routability procedure,
registrations,
and in solicited prefix discovery.
</t>
<t>When the mobile node sends a Mobile Prefix Solicitation,
Home Test Init, Care-of Test Init or
Binding Update for which it expects a response,
the mobile node has to determine a value for the initial
retransmission timer:
</t>
<list style='symbols'>
<t> If the mobile node is sending a Mobile Prefix Solicitation,
it SHOULD use an initial retransmission interval of
INITIAL_SOLICIT_TIMER (see <xref target='constants' />).
</t>
<t> If the mobile node is sending a Binding Update and does not
have an existing binding at the home agent, it SHOULD use
InitialBindackTimeoutFirstReg (see <xref target='variables' />) as a
value for the initial retransmission timer. This long
retransmission interval will allow the home agent to complete
the Duplicate Address Detection procedure mandated in
this case, as detailed in <xref target='update-home' />.
</t>
<t> Otherwise, the mobile node should use the specified value
of INITIAL_BINDACK_TIMEOUT for the initial retransmission timer.
</t>
</list>
<t>If the mobile node fails to receive a valid matching response within the
selected initial retransmission interval, the mobile node SHOULD
retransmit the message until a response is received.
</t>
<t>The retransmissions by the mobile node MUST use an exponential
back-off process in which the timeout period is doubled upon
each retransmission, until either the node receives a response
or the timeout period reaches the value MAX_BINDACK_TIMEOUT.
The mobile node MAY continue to send these messages at
this slower rate indefinitely.
</t>
<t>The mobile node SHOULD start a separate back-off process for different
message types, different home addresses and different care-of
addresses. However, in addition an overall rate limitation applies for
messages sent to a particular correspondent node. This ensures that
the correspondent node has a sufficient amount of time to respond when
bindings for multiple home addresses are registered, for instance.
The mobile node MUST NOT send Mobility Header messages of a particular
type to a particular correspondent node more than
MAX_UPDATE_RATE times within a second.
</t>
<t>Retransmitted Binding Updates MUST use a Sequence Number
value greater than that used for the previous transmission of
this Binding Update. Retransmitted Home Test Init and Care-of Test Init
messages MUST use new cookie values.
</t>
</section>
</section>
<section anchor='constants' title='Protocol Constants'>
<figure><artwork>
DHAAD_RETRIES 4 retransmissions
INITIAL_BINDACK_TIMEOUT 1 second
INITIAL_DHAAD_TIMEOUT 3 seconds
INITIAL_SOLICIT_TIMER 3 seconds
MAX_BINDACK_TIMEOUT 32 seconds
MAX_DELETE_BCE_TIMEOUT 10 seconds
MAX_NONCE_LIFETIME 240 seconds
MAX_TOKEN_LIFETIME 210 seconds
MAX_RO_FAILURE 3 retries
MAX_RR_BINDING_LIFETIME 420 seconds
MAX_UPDATE_RATE 3 times
PREFIX_ADV_RETRIES 3 retransmissions
PREFIX_ADV_TIMEOUT 3 seconds
</artwork></figure></section>
<section anchor='variables' title='Protocol Configuration Variables'>
<figure><artwork>
MaxMobPfxAdvInterval Default: 86,400 seconds
MinDelayBetweenRAs Default: 3 seconds,
Min: 0.03 seconds
MinMobPfxAdvInterval Default: 600 seconds
InitialBindackTimeoutFirstReg Default: 1.5 seconds
</artwork></figure>
<t>
Home agents MUST allow the first three variables to be configured by
system management, and mobile nodes MUST allow the last variable to be
configured by system management.
</t>
<t>
The default value for InitialBindackTimeoutFirstReg has been
calculated as 1.5 times the default value of RetransTimer,
as specified in Neighbor Discovery
(<xref target='RFC4861'>RFC 4861</xref>)
times the default value of DupAddrDetectTransmits,
as specified in Stateless Address Autoconfiguration
(<xref target='RFC4862'>RFC 4862</xref>)
</t>
<t>
The value MinDelayBetweenRAs overrides the value of the
protocol constant MIN_DELAY_BETWEEN_RAS, as specified in
Neighbor Discovery (<xref target='RFC4861'>RFC 4861</xref>).
This variable SHOULD be set to MinRtrAdvInterval, if
MinRtrAdvInterval is less than 3 seconds.
</t>
</section>
<section anchor='iana' title='IANA Considerations'>
<t>This document defines a new IPv6 protocol, the Mobility Header,
described in <xref target='mobility-header' />. This protocol has been
assigned protocol number 135.
</t>
<t>This document also creates a new name space "Mobility Header Type",
for the MH Type field in the Mobility Header.
The current
message types are described starting from <xref target='bind-req' />,
and are the following:
</t>
<list style='empty'>
<list style='hanging'>
<t hangText='0'>
Binding Refresh Request
</t>
<t hangText='1'>
Home Test Init
</t>
<t hangText='2'>
Care-of Test Init
</t>
<t hangText='3'>
Home Test
</t>
<t hangText='4'>
Care-of Test
</t>
<t hangText='5'>
Binding Update
</t>
<t hangText='6'>
Binding Acknowledgement
</t>
<t hangText='7'>
Binding Error
</t>
</list>
</list>
<t>Future values of the MH Type can be allocated using
<xref target='RFC5226'>Standards Action or IESG Approval</xref>.
</t>
<t>Furthermore, each mobility message may contain mobility options
as described in <xref target='params' />. This document defines a new
name space "Mobility Option" to identify these options. The current mobility options
are defined starting from <xref target='pad1-option' /> and are the following:
</t>
<list style='empty'>
<list style='hanging'>
<t hangText='0'>
Pad1
</t>
<t hangText='1'>
PadN
</t>
<t hangText='2'>
Binding Refresh Advice
</t>
<t hangText='3'>
Alternate Care-of Address
</t>
<t hangText='4'>
Nonce Indices
</t>
<t hangText='5'>
Authorization Data
</t>
</list>
</list>
<t>Future values of the Option Type can be allocated using
<xref target='RFC5226'>Standards Action or IESG Approval</xref>.
</t>
<t>Finally, this document creates a third new name space "Status Code"
for the Status field in the Binding Acknowledgement message. The
current values are described in Section 6.1.8, and are the following:</t>
<list style='hanging'>
<t hangText='0'>Binding Update accepted </t>
<t hangText='1'>Accepted but prefix discovery necessary </t>
<t hangText='128'>Reason unspecified </t>
<t hangText='129'>Administratively prohibited </t>
<t hangText='130'>Insufficient resources </t>
<t hangText='131'>Home registration not supported </t>
<t hangText='132'>Not home subnet </t>
<t hangText='133'>Not home agent for this mobile node </t>
<t hangText='134'>Duplicate Address Detection failed </t>
<t hangText='135'>Sequence number out of window </t>
<t hangText='136'>Expired home nonce index </t>
<t hangText='137'>Expired care-of nonce index </t>
<t hangText='138'>Expired nonces </t>
<t hangText='139'>Registration type change disallowed </t>
</list>
<t>Future values of the Status field can be allocated using Standards
Action or <xref target="RFC5226">IESG Approval</xref>. </t>
<t>All fields labeled "Reserved" are only to be assigned through
Standards Action or IESG Approval.</t>
<t>This document also defines a new IPv6 destination option,
the Home Address option, described in <xref target='hao' />.
This option has been assigned the Option Type value 0xC9.
</t>
<t>This document also defines a new IPv6 type 2 routing header,
described in <xref target='rhtype2' />. The value 2 has been allocated
by IANA.
</t>
<t>In addition, this document defines four ICMP message types, two used
as part of the dynamic home agent address discovery mechanism, and two
used in lieu of Router Solicitations and Advertisements when the
mobile node is away from the home link.
These messages have been assigned ICMPv6 type numbers from the
informational message range:
</t>
<list style='symbols'>
<t> The Home Agent Address Discovery Request message, described in
<xref target='ha-disc-req' />;
</t>
<t> The Home Agent Address Discovery Reply message, described in
<xref target='ha-disc-reply' />;
</t>
<t> The Mobile Prefix Solicitation, described in
<xref target='renum-mps' />; and
</t>
<t> The Mobile Prefix Advertisement, described in
<xref target='renum-mpa' />.
</t>
</list>
<t>This document also defines two
new <xref target='RFC4861'>Neighbor Discovery</xref>
options, which have been assigned Option Type values within the option
numbering space for Neighbor Discovery messages:
</t>
<list style='symbols'>
<t> The Advertisement Interval option,
described in <xref target='adv-int' />; and
</t>
<t> The Home Agent Information option,
described in <xref target='ha-info' />.
</t>
</list>
</section>
<section anchor='security' title='Security Considerations'>
<section anchor='seccons:threats' title='Threats'>
<t>
Any mobility solution must protect itself against misuses of the
mobility features and mechanisms. In Mobile IPv6, most of the
potential threats are concerned with false Bindings, usually resulting
in Denial-of-Service attacks. Some of the threats also pose potential
for Man-in-the-Middle, Hijacking, Confidentiality, and Impersonation
attacks. The main threats this protocol protects against are the
following:
</t>
<list style='symbols'>
<t> Threats involving Binding Updates sent to home agents and
correspondent nodes. For instance, an attacker might claim
that a certain mobile node is currently at a different
location than it really is. If a home agent accepts such
spoofed information sent to it, the mobile node might not get
traffic destined to it. Similarly, a malicious (mobile) node
might use the home address of a victim node in a forged
Binding Update sent to a correspondent node.
<vspace blankLines='1' />
These pose threats against confidentiality, integrity, and
availability. That is, an attacker might learn the contents of
packets destined to another node by redirecting the traffic to
itself. Furthermore, an attacker might use the redirected
packets in an attempt to set itself as a Man-in-the-Middle
between a mobile and a correspondent node. This would allow
the attacker to impersonate the mobile node, leading to
integrity and availability problems.
<vspace blankLines='1' />
A malicious (mobile) node might also send Binding Updates in
which the care-of address is set to the address of a victim
node. If such Binding Updates were accepted, the malicious
node could lure the correspondent node into sending
potentially large amounts of data to the victim; the
correspondent node's replies to messages sent by the malicious
mobile node will be sent to the victim host or network. This
could be used to cause a Distributed Denial-of-Service
attack. For example, the correspondent node might be a site
that will send a high-bandwidth stream of video to anyone who
asks for it. Note that the use of flow-control protocols such
as TCP does not necessarily defend against this type of
attack, because the attacker can fake the
acknowledgements. Even keeping TCP initial sequence numbers
secret does not help, because the attacker can receive the
first few segments (including the ISN) at its own address, and
only then redirect the stream to the victim's address. These
types of attacks may also be directed to networks instead
of nodes. Further variations of this threat are described
elsewhere <xref target='I-D.aura-mipv6-bu-attacks' />
<xref target='I-D.roe-mobileip-updateauth' />.
<vspace blankLines='1' />
An attacker might also attempt to disrupt a mobile node's
communications by replaying a Binding Update that the node had sent
earlier. If the old Binding Update was accepted, packets destined for
the mobile node would be sent to its old location as opposed to its
current location.
<vspace blankLines='1' />
A malicious mobile node associated to multiple home agents could
create a routing loop amongst them. This can be achieved when a
mobile node binds one home address located on a first home agent
to another home address on a second home agent. This type of
binding will force the home agents to route the same packet among
each other without knowledge that a routing loop has been created.
Such looping problem is limited to cases where a mobile node has
multiple home agents and is permitted to be associated with the
multiple home agents. For the single home agent case, a policy at
the home agent would prevent the binding of one home address to
another home address hosted by the same home agent.
<vspace blankLines='1' />
The potential problems caused by such routing loops in this scenario
can be substantially reduced by use of the Tunnel-Limit Option
specified in RFC 2473 <xref target='RFC2473' />.
<vspace blankLines='1' />
In conclusion, there are Denial-of-Service, Man-in-the-Middle,
Confidentiality, and Impersonation threats against the parties
involved in sending legitimate Binding Updates, the threat of
routing loops when there are multiple home agents, and
Denial-of-Service threats against any other party.
</t>
<t> Threats associated with payload packets: Payload
packets exchanged with mobile nodes are exposed to similar
threats as that of regular IPv6 traffic. However, Mobile IPv6
introduces the Home Address destination option, a new routing
header type (type 2), and uses tunneling headers in the
payload packets. The protocol must protect against potential
new threats involving the use of these mechanisms.
<vspace blankLines='1' />
Third parties become exposed to a reflection threat via
the Home Address destination option, unless appropriate
security precautions are followed. The Home Address
destination option could be used to direct response traffic
toward a node whose IP address appears in the option. In this
case, ingress filtering would not catch the forged "return
address" <xref target='I-D.savola-ipv6-rh-ha-security' />
<xref target='RFC4225' />.
<vspace blankLines='1' />
A similar threat exists with the tunnels between the
mobile node and the home agent. An attacker might forge tunnel
packets between the mobile node and the home agent, making it
appear that the traffic is coming from the mobile node when it
is not. Note that an attacker who is able to forge tunnel
packets would typically also be able to forge packets that appear
to come directly from the mobile node. This is not a new
threat as such. However, it may make it easier for attackers
to escape detection by avoiding ingress filtering and packet
tracing mechanisms. Furthermore, spoofed tunnel packets might
be used to gain access to the home network.
<vspace blankLines='1' />
Finally, a routing header could also be used in reflection
attacks, and in attacks designed to bypass firewalls. The
generality of the regular routing header would allow
circumvention of IP-address based rules in firewalls. It
would also allow reflection of traffic to other nodes. These
threats exist with routing headers in general, even if the
usage that Mobile IPv6 requires is safe.
</t>
<t> Threats associated with dynamic home agent and mobile prefix discovery.
</t>
<t> Threats against the Mobile IPv6 security mechanisms
themselves: An attacker might, for instance, lure the
participants into executing expensive cryptographic operations
or allocating memory for the purpose of keeping state. The
victim node would have no resources left to handle other
tasks.
</t>
</list>
<t>
As a fundamental service in an IPv6 stack, Mobile IPv6 is expected
to be deployed in most nodes of the IPv6 Internet. The above threats
should therefore be considered as being applicable to the
whole Internet.
</t>
<t>
It should also be noted that some additional threats result from
movements as such, even without the involvement of mobility protocols.
Mobile nodes must be capable to defend themselves in the networks that
they visit, as typical perimeter defenses applied in the home network
no longer protect them.
</t>
</section>
<section anchor='seccons:features' title='Features'>
<t>This specification provides a series of features designed to mitigate
the risk introduced by the threats listed above. The main security
features are the following:
</t>
<list style='symbols'>
<t> Reverse Tunneling as a mandatory feature.
</t>
<t> Protection of Binding Updates sent to home agents.
</t>
<t> Protection of Binding Updates sent to correspondent nodes.
</t>
<t> Protection against reflection
attacks that use the Home Address destination option.
</t>
<t> Protection of tunnels between the mobile node and the home
agent.
</t>
<t> Closing routing header vulnerabilities.
</t>
<t> Mitigating Denial-of-Service threats to the Mobile IPv6 security
mechanisms themselves.
</t>
</list>
<t>The support for encrypted reverse tunneling (see
<xref target='mobile-sending' />) allows mobile nodes to defeat certain
kinds of traffic analysis.
</t>
<t>Protecting those Binding Updates that are sent to home agents and
those that are sent to arbitrary correspondent nodes requires very
different security solutions due to the different situations. Mobile
nodes and home agents are naturally expected to be subject to the
network administration of the home domain.
</t>
<t>Thus, they can and are supposed to have a security
association that can be used to reliably authenticate the exchanged
messages. See <xref target='securing-regs-with-home-agents' /> for the
description of the protocol mechanisms, and
<xref target='seccons:homeupd' /> below for a discussion of the resulting
level of security.
</t>
<t>It is expected that Mobile IPv6 route optimization will be used on a
global basis between nodes belonging to different administrative
domains.
It would be a very demanding task to build an authentication
infrastructure on this scale.
Furthermore, a traditional authentication infrastructure cannot be
easily used to authenticate IP addresses because IP addresses can change often.
It is not sufficient to just authenticate the mobile nodes;
Authorization to claim the right to use an address is needed as well.
Thus, an "infrastructureless" approach is necessary.
The chosen infrastructureless method is described in
<xref target='securing-regs-with-cns' />, and
<xref target='security-cn-updates' /> discusses the resulting security
level and the design rationale of this approach.
</t>
<t>Specific rules guide the use of the Home Address destination option, the
routing header, and the tunneling headers in the payload packets. These
rules are necessary to
remove the vulnerabilities associated with their unrestricted use.
The effect of the rules is discussed in
<xref target='seccons:tunnels' />, <xref target='seccons:hao' />,
and <xref target='seccons:type2rh' />.
</t>
<t>Denial-of-Service threats against Mobile IPv6 security mechanisms
themselves concern mainly the Binding Update procedures with
correspondent nodes. The protocol has been designed to limit the effects
of such attacks, as will be described in <xref target='seccons:prevdos' />.
</t>
</section>
<section anchor='seccons:homeupd' title='Binding Updates to Home Agent'>
<t>Signaling between the mobile node and the home agent requires message
integrity. This is necessary to assure the home agent that a Binding
Update is from a legitimate mobile node. In addition, correct ordering
and anti-replay protection are optionally needed.
</t>
<t>IPsec ESP protects the integrity of the Binding Updates and Binding
Acknowledgements by securing mobility messages between the
mobile node and the home agent.
</t>
<t>IPsec can provide anti-replay protection only if dynamic keying is
used (which may not always be the case). IPsec does not guarantee
correct ordering of packets, only that they have not been
replayed. Because of this, sequence numbers within the Mobile IPv6
messages are used to ensure correct ordering (see
<xref target='securing-regs-with-home-agents' />). However, if the 16 bit
Mobile IPv6 sequence number space is cycled through, or the home agent
reboots and loses its state regarding the sequence numbers, replay and
reordering attacks become possible. The use of dynamic keying, IPsec
anti-replay protection, and the Mobile IPv6 sequence numbers can
together prevent such attacks. It is also recommended that use of
non-volatile storage be considered for home agents, to avoid losing
their state.
</t>
<t>A sliding window scheme is used for the sequence numbers. The
protection against replays and reordering attacks without a key
management mechanism works when the attacker remembers up to a maximum
of 2**15 Binding Updates.
</t>
<t>The above mechanisms do not show that the care-of address given in the
Binding Update is correct. This opens the possibility for
Denial-of-Service attacks against third parties. However, since the
mobile node and home agent have a security association, the home agent
can always identify an ill-behaving mobile node. This allows the home
agent operator to discontinue the mobile node's service, and possibly
take further actions based on the business relationship with the
mobile node's owner.
</t>
<t>Note that the use of a single pair of manually keyed security
associations conflicts with the generation of a new home
<xref target='RFC4941'>address</xref> for the mobile node,
or with the adoption of a new home subnet prefix.
This is because IPsec security associations are bound to the used
addresses. While certificate-based automatic keying alleviates this
problem to an extent, it is still necessary to ensure that a given
mobile node cannot send Binding Updates for the address of another
mobile node. In general, this leads to the inclusion of home addresses
in certificates in the Subject AltName field. This again limits
the introduction of new addresses without either manual or automatic
procedures to establish new certificates. Therefore, this specification
restricts the generation of new home addresses (for any reason) to
those situations where a security association or certificate for the new
address already exists.
</t>
<t>Support for IKE has been specified as optional. The following
should be observed about the use of manual keying:
</t>
<list style='symbols'>
<t> As discussed above, with manually keyed IPsec, only a limited
form of protection exists against replay and reordering
attacks. A vulnerability exists if either the sequence number
space is cycled through, or if the home agent reboots and
forgets its sequence numbers (and uses volatile memory to
store the sequence numbers).
<vspace blankLines='1' /> Assuming the mobile node moves continuously every 10
minutes, it takes roughly 455 days before the sequence
number space has been cycled through. Typical movement
patterns rarely reach this high frequency today.
</t>
<t> A mobile node and its home agent belong to the same domain. If
this were not the case, manual keying would not be <xref target='RFC4107'>possible</xref>,
but in Mobile IPv6 only these two parties need to know the
manually configured keys. Similarly, we note that Mobile IPv6
employs standard block ciphers in IPsec, and is not vulnerable
to problems associated with stream ciphers and manual keying.
</t>
<t> It is expected that the owner of the mobile node and the
administrator of the home agent agree on the used keys and
other parameters with some off-line mechanism.
</t>
</list>
<t>The use of IKEv1 with Mobile IPv6 is documented in more detail
in <xref target='RFC3776' />. The following should be observed from the use
of IKEv1:
</t>
<list style='symbols'>
<t> It is necessary to prevent a mobile node from claiming another
mobile node's home address. The home agent must verify that
the mobile node trying to negotiate the SA for a particular
home address is authorized for that home address. This implies
that even with the use of IKE, a policy entry needs to be
configured for each home address served by the home agent.
<vspace blankLines='1' /> It may be possible to include home addresses in the Subject
AltName field of certificate to avoid this. However,
implementations are not guaranteed to support the use of a
particular IP address (care-of address) while another address
(home address) appears in the certificate. In any case, even
this approach would require user-specific tasks in the
certificate authority.
</t>
<t> If preshared secret authentication is used, IKEv1 main mode
cannot be used. Aggressive mode or group preshared secrets
need to be used with corresponding security
implications instead.
<vspace blankLines='1' /> Note that, like many other issues, this is a general IKEv1
issue related to the ability to use different IP addresses,
and not specifically related to Mobile IPv6. For further
information, see Section 4.4 in <xref target='RFC3776' />.
</t>
<t> Due to the problems outlined in <xref target='outbound-ipsec' />,
IKE phase 1 between the mobile node and its home agent is
established using the mobile node's current care-of address.
This implies that when the mobile node moves to a new location, it may
have to re-establish phase 1. A Key Management Mobility Capability (K)
flag is provided for implementations that can update the
IKE phase 1 endpoints without re-establishing phase 1, but the
support for this behavior is optional.
</t>
<t> When certificates are used, IKE fragmentation can occur
as discussed in Section 7 in <xref target='RFC3776' />.
</t>
<t> Nevertheless, even if per-mobile node configuration is required
with IKE, an important benefit of IKE is that it
automates the negotiation of cryptographic parameters,
including the SPIs, cryptographic algorithms, and so on.
Thus, less configuration information is needed.
</t>
<t> The frequency of movements in some link layers
or deployment scenarios may be high enough to
make replay and reordering attacks possible, if only
manual keying is used. IKE SHOULD be used in such
cases. Potentially vulnerable
scenarios involve continuous movement through small cells,
or uncontrolled alternation between available network
attachment points.
</t>
<t> Similarly, in some deployment scenarios the number
of mobile nodes may be very large. In these cases, it can be necessary
to use automatic mechanisms to reduce the management
effort in the administration of cryptographic parameters,
even if some per-mobile node configuration is always
needed. IKE SHOULD also be used in such cases.
</t>
<t> Other automatic key management mechanisms exist beyond
IKEv1, but this document does not address the issues
related to them. We note, however, that most of the
above discussion applies to <xref target='RFC4306'>IKEv2</xref> as well,
at least as it is currently specified.
</t>
</list>
</section>
<section anchor='security-cn-updates' title='Binding Updates to Correspondent Nodes'>
<t>The motivation for designing the return routability procedure was to
have sufficient support for Mobile IPv6, without creating significant new
security problems. The goal for this procedure was not to protect
against attacks that were already possible before the introduction of
Mobile IPv6.
</t>
<t>The next sections will describe the security properties of the used
method, both from the point of view of possible on-path attackers who
can see those cryptographic values that have been sent in the clear
(<xref target='rr-achieved' /> and <xref target='rr-comparison' />) and from
the point of view of other attackers (<xref target='seccons:keylens' />).
</t>
<section anchor='security-cn-updates-overview' title='Overview'>
<t>The chosen infrastructureless method verifies that the mobile node is
"live" (that is, it responds to probes) at its home and care-of
addresses. <xref target='securing-regs-with-cns' /> describes the return
routability procedure in detail. The procedure uses the following
principles:
</t>
<list style='symbols'>
<t> A message exchange verifies that the mobile node is reachable
at its addresses, i.e., is at least able to transmit and receive
traffic at both the home and care-of addresses.
</t>
<t> The eventual Binding Update is cryptographically bound to the
tokens supplied in the exchanged messages.
</t>
<t> Symmetric exchanges are employed to avoid the
use of this protocol in reflection attacks. In a symmetric
exchange, the responses are always sent to the same address
the request was sent from.
</t>
<t> The correspondent node operates in a stateless manner until it
receives a fully authorized Binding Update.
</t>
<t> Some additional protection is provided
by encrypting the tunnels between the mobile node and home
agent with IPsec ESP. As the tunnel also transports the nonce
exchanges, the ability of attackers to see these
nonces is limited. For instance, this prevents attacks from being launched from
the mobile node's current foreign link, even when no link-layer
confidentiality is available.
<vspace blankLines='1' />
The resulting level of security is in theory the same even
without this additional protection: the return routability
tokens are still exposed only to one path within the whole
Internet. However, the mobile nodes are often found on an
insecure link, such as a public access Wireless LAN. Thus,
in many cases, this addition makes a practical difference.
</t>
</list>
<t>
For further information about the design rationale of the return
routability procedure, see <xref target='I-D.aura-mipv6-bu-attacks' />
<xref target='I-D.roe-mobileip-updateauth' />
<xref target='I-D.nordmark-mobileip-bu3way' />
<xref target='RFC4225' />.
The mechanisms used have been adopted from these documents.
</t>
</section>
<section anchor='rr-achieved' title='Achieved Security Properties'>
<t>The return routability procedure protects Binding Updates against all attackers who
are unable to monitor the path between the home agent and the
correspondent node. The procedure does not defend against attackers
who can monitor this path. Note that such attackers are in any case
able to mount an active attack against the mobile node when it is
at its home location. The possibility of such attacks is not an
impediment to the deployment of Mobile IPv6 because these attacks are
possible regardless of whether or not Mobile IPv6 is in use.
</t>
<t>This procedure also protects against Denial-of-Service attacks in
which the attacker pretends to be mobile, but uses the victim's
address as the care-of address. This would cause the correspondent
node to send the victim some unexpected traffic. This procedure defends
against these attacks by requiring at least the passive presence of the
attacker at the care-of address or on the path from the correspondent
to the care-of address. Normally, this will be the mobile node.
</t>
</section>
<section anchor='rr-comparison' title='Comparison to Regular IPv6 Communications'>
<t>This section discusses the protection offered by the return
routability method by comparing it to the security of regular IPv6
communications. We will divide vulnerabilities into three classes: (1)
those related to attackers on the local network of the mobile node,
home agent, or the correspondent node, (2) those related to attackers
on the path between the home network and the correspondent
node, and (3) off-path attackers, i.e., the rest of the Internet.
</t>
<t>We will now discuss the vulnerabilities of regular IPv6
communications. The on-link vulnerabilities of IPv6 communications
include Denial-of-Service, Masquerading, Man-in-the-Middle,
Eavesdropping, and other attacks. These attacks can be launched through
spoofing Router Discovery, Neighbor Discovery and other IPv6
mechanisms. Some of these attacks can be prevented with the use
of cryptographic protection in the packets.
</t>
<t>A similar situation exists with on-path attackers. That is, without
cryptographic protection, the traffic is completely vulnerable.
</t>
<t>Assuming that attackers have not penetrated the security of the
Internet routing protocols, attacks are much harder to launch from
off-path locations. Attacks that can be launched from these locations
are mainly Denial-of-Service attacks, such as flooding and/or
reflection attacks. It is not possible for an off-path attacker to
become a Man-in-the-Middle.
</t>
<t>Next, we will consider the vulnerabilities that exist when IPv6 is
used together with Mobile IPv6 and the return routability procedure.
On the local link, the vulnerabilities are the same as those in IPv6,
but Masquerade and Man-in-the-Middle attacks can now also be launched against
future communications, and not just against current communications. If
a Binding Update was sent while the attacker was present on the link,
its effects remain for the lifetime of the binding. This happens even
if the attacker moves away from the link. In contrast, an attacker
who uses only plain IPv6 generally has to stay on the link in order to continue the
attack. Note that in order to launch these new attacks, the IP address
of the victim must be known. This makes this attack feasible, mainly in
the context of well-known interface IDs, such as those already
appearing in the traffic on the link or registered in the DNS.
</t>
<t>On-path attackers can exploit similar vulnerabilities as in
regular IPv6. There are some minor differences, however. Masquerade,
Man-in-the-Middle, and Denial-of-Service attacks can be launched with just the interception of a
few packets, whereas in regular IPv6 it is necessary to intercept
every packet. The effect of the attacks is the same regardless of the
method, however. In any case, the most difficult task an attacker faces in
these attacks is getting on the right path.
</t>
<t>The vulnerabilities for off-path attackers are the same as in regular
IPv6. Those nodes that are not on the path between the home agent and
the correspondent node will not be able to receive the home address
probe messages.
</t>
<t>In conclusion, we can state the following main results from
this comparison:
</t>
<list style='symbols'>
<t> Return routability prevents
any off-path attacks beyond those that are already possible in
regular IPv6. This is the most important result, preventing
attackers on the Internet from exploiting any
vulnerabilities.
</t>
<t> Vulnerabilities to attackers on the home agent link, the
correspondent node link, and the path between them are roughly
the same as in regular IPv6.
</t>
<t> However, one
difference is that in basic IPv6 an on-path attacker must be
constantly present on the link or the path, whereas with
Mobile IPv6, an attacker can leave a binding behind after
moving away.
<vspace blankLines='1' /> For this reason, this specification limits the creation of
bindings to at most MAX_TOKEN_LIFETIME seconds after the
last routability check has been performed, and limits the
duration of a binding to at most MAX_RR_BINDING_LIFETIME
seconds. With these limitations, attackers cannot take
any practical advantages of this vulnerability.
</t>
<t> There are some other minor differences, such as an effect to
the Denial-of-Service vulnerabilities. These can be considered to be
insignificant.
</t>
<t> The path between the home agent and a correspondent node is
typically easiest to attack on the links at either end, in
particular if these links are publicly accessible wireless LANs.
Attacks against the routers or switches on the path are typically
harder to accomplish. The security on layer 2 of the links
plays then a major role in the resulting overall network
security. Similarly, security of IPv6 Neighbor and Router
Discovery on these links has a large impact. If these were
secured using some new technology in the future, this could
change the situation regarding the easiest point of attack.
</t>
</list>
<t> For a more in-depth discussion of these issues,
see <xref target='RFC4225' />.
</t>
</section>
<section title='Replay Attacks'>
<t>The return routability procedure also protects the participants
against replayed Binding Updates. The attacker is unable replay the
same message due to the sequence number which is a part of the Binding
Update. It is also unable to modify the Binding Update since the MAC
verification would fail after such a modification.
</t>
<t>Care must be taken when removing bindings at the correspondent node,
however. If a binding is removed while the nonce used in its
creation is still valid, an attacker could replay the old Binding
Update. Rules outlined in <xref target='rrreplay' /> ensure that this cannot
happen.
</t>
</section>
<section anchor='seccons:prevdos' title='Denial-of-Service Attacks'>
<t>The return routability procedure has protection
against resource exhaustion Denial-of-Service attacks.
The correspondent nodes do not retain any state about individual
mobile nodes until an authentic Binding Update arrives.
This is achieved through the construct of keygen tokens from the nonces
and node keys that are not specific to individual mobile nodes.
The keygen tokens can be reconstructed by the correspondent node, based
on the home and care-of address information that arrives with the Binding
Update.
This means that the
correspondent nodes are safe against memory exhaustion attacks except
where on-path attackers are concerned. Due to the use of symmetric
cryptography, the correspondent nodes are relatively safe against CPU
resource exhaustion attacks as well.
</t>
<t>Nevertheless, as <xref target='I-D.aura-mipv6-bu-attacks' /> describes, there are situations in
which it is impossible for the mobile and correspondent nodes to
determine if they actually need a binding or whether they just have
been fooled into believing so by an attacker. Therefore, it is
necessary to consider situations where such attacks are being made.
</t>
<t>Even if route optimization is a very important optimization, it is
still only an optimization. A mobile node can communicate with a
correspondent node even if the correspondent refuses to accept any
Binding Updates. However, performance will suffer because packets from
the correspondent node to the mobile node will be routed via the
mobile's home agent rather than a more direct route. A correspondent
node can protect itself against some of these resource exhaustion
attacks as follows. If the correspondent node is flooded with a large
number of Binding Updates that fail the cryptographic integrity
checks, it can stop processing Binding Updates. If a correspondent
node finds that it is spending more resources on checking bogus
Binding Updates than it is likely to save by accepting genuine Binding
Updates, then it may silently discard some or all Binding Updates without
performing any cryptographic operations.
</t>
<t>Layers above IP can usually provide additional information to help
determine whether there is a need to establish a binding with a specific
peer. For example, TCP knows if the node has a queue of data that it
is trying to send to a peer. An implementation of this specification
is not required to make use of information from higher protocol
layers, but some implementations are likely to be able to manage
resources more effectively by making use of such information.
</t>
<t>We also require that all implementations be capable of administratively
disabling route optimization.
</t>
</section>
<section anchor='seccons:keylens' title='Key Lengths'>
<t>Attackers can try to break the return routability procedure in many
ways. <xref target='rr-achieved' /> discusses the situation where the
attacker can see the cryptographic values sent in the clear, and
<xref target='rr-comparison' /> discusses the impact this has on IPv6
communications. This section discusses whether attackers can guess the
correct values without seeing them.
</t>
<t>While the return routability procedure is in progress, 64 bit cookies
are used to protect spoofed responses. This is believed to be
sufficient, given that to blindly spoof a response a very large number
of messages would have to be sent before success would be probable.
</t>
<t>The tokens used in the return routability procedure provide together
128 bits of information. This information is used internally as
input to a hash function to produce a 160 bit quantity suitable for
producing the keyed hash in the Binding Update using the HMAC_SHA1
algorithm. The final keyed hash length is 96 bits. The limiting
factors in this case are the input token lengths and the final keyed
hash length. The internal hash function application does not reduce
the entropy.
</t>
<t>The 96 bit final keyed hash is of typical size and is believed to be
secure. The 128 bit input from the tokens is broken in two pieces, the
home keygen token and the care-of keygen token. An attacker can try to
guess the correct cookie value, but again this would require a large
number of messages (an the average 2**63 messages for one or 2**127
for two). Furthermore, given that the cookies are valid only for a
short period of time, the attack has to keep a high constant message
rate to achieve a lasting effect. This does not appear practical.
</t>
<t>When the mobile node is returning home, it is allowed to use just the
home keygen token of 64 bits. This is less than 128 bits, but
attacking it blindly would still require a large number of messages to
be sent. If the attacker is on the path and capable of seeing the
Binding Update, it could conceivably break the keyed hash with brute
force. However, in this case the attacker has to be on the path, which
appears to offer easier ways for denial-of-service than preventing
route optimization.
</t>
</section>
</section>
<section anchor='cons:dynha-disc' title='Dynamic Home Agent Address Discovery'>
<t>The dynamic home agent address discovery function could be used to learn
the addresses of home agents in the home network.
</t>
<t> The ability to learn addresses of nodes may be useful to attackers
because brute-force scanning of the address space is not practical
with IPv6. Thus, they could benefit from any means which make
mapping the networks easier. For example, if a security threat
targeted at routers or even home agents is discovered, having a
simple ICMP mechanism to easily find out possible targets may prove
to be an additional (though minor) security risk.
</t>
<t> This document does not define any authentication mechanism for
dynamic home agent address discovery messages. Therefore the home
agent cannot verify the home address of the mobile node that
requested the list of home agents.
</t>
<t> Apart from discovering the address(es) of home agents, attackers will
not be able to learn much from this information, and mobile nodes
cannot be tricked into using wrong home agents, as all other
communication with the home agents is secure.
</t>
<t> In cases where additional security is needed, one may consider
instead the use of MIPv6 <xref target='RFC5026'>bootstrapping</xref>,
(based on <xref target='RFC2782'>DNS SRV Resource Records</xref>)
in conjunction with security mechanisms suggested in these specifications.
In that solution, security is provided by
the <xref target='RFC4033'>DNSSEC</xref> framework. The needed
pre-configured data on the mobile node for this mechanism is the
domain name of the mobile service provider, which is marginally better
than the home subnet prefix. For the security, a trust anchor which
dominates the domain is needed.
</t>
</section>
<section anchor='cons:pref-disc' title='Mobile Prefix Discovery'>
<t>
The mobile prefix discovery function may leak interesting information
about network topology and prefix lifetimes to eavesdroppers;
for this reason, requests for this information have to be
authenticated. Responses and unsolicited prefix information needs
to be authenticated to prevent the mobile nodes from being tricked
into believing false information about the prefixes and possibly
preventing communications with the existing addresses. Optionally,
encryption may be applied to prevent leakage of the prefix
information.
</t>
</section>
<section anchor='seccons:tunnels' title='Tunneling via the Home Agent'>
<t>Tunnels between the mobile node and the home agent can be protected by
ensuring proper use of source addresses, and optional cryptographic
protection. These procedures are discussed in
<xref target='overview:security-tunnel' />.
</t>
<t>Binding Updates to the home agents are secure. When receiving
tunneled traffic, the home agent verifies that the outer IP address
corresponds to the current location of the mobile node.
This acts as a weak form of protection against spoofing packets
that appear to come from the mobile node. This is particularly
useful, if no end-to-end security is being applied between the
mobile and correspondent nodes.
The outer IP address check prevents
attacks where the attacker is controlled by ingress filtering. It
also prevents attacks when the attacker does not know the current care-of
address of the mobile node. Attackers who know the care-of address and
are not controlled by ingress filtering could still send traffic
through the home agent. This includes attackers on the same local link
as the mobile node is currently on. But such attackers could
send packets that appear to come from the mobile node without
attacking the tunnel; the attacker could simply send packets with
the source address set to the mobile node's home address. However,
this attack does not work if the final destination of the packet is
in the home network, and some form of perimeter defense is being
applied for packets sent to those destinations. In such cases it is recommended that
either end-to-end security or additional tunnel protection be
applied, as is usual in remote access situations.
</t>
<t>Home agents and mobile nodes may use IPsec ESP to protect payload
packets tunneled between themselves. This is useful for protecting
communications against attackers on the path of the tunnel.
</t>
<t> When a Unique-Local Address (ULA) <xref target='RFC4193'>RFC4193</xref>
is used as a home address, reverse tunneling can be
used to send local traffic from another location. Administrators
should be aware of this when allowing such home addresses. In
particular, the outer IP address check described above is not
sufficient against all attackers. The use of encrypted tunnels is
particularly useful for these kinds of home addresses.
</t>
</section>
<section anchor='seccons:hao' title='Home Address Option'>
<t>When the mobile node sends packets directly to the correspondent node, the
Source Address field of the packet's IPv6 header is the care-of address.
Therefore, <xref target='RFC2827'>ingress filtering</xref> works in the usual manner even
for mobile nodes, as the Source Address is topologically correct.
The Home Address option is used to inform the correspondent node of the
mobile node's home address.
</t>
<t>However, the care-of address in the Source Address field does not
survive in replies sent by the correspondent node unless it has a
binding for this mobile node. Also, not all attacker tracing
mechanisms work when packets are being reflected through correspondent
nodes using the Home Address option. For these reasons, this
specification restricts the use of the Home Address option. It may
only be used when a binding has already been established with the
participation of the node at the home address, as described in
<xref target='overview:security-tunnel' /> and <xref target='hao' />. This
prevents reflection attacks through the use of the Home Address
option. It also ensures that the correspondent nodes reply to the same
address that the mobile node sends traffic from.
</t>
<t>No special authentication of the Home Address option is required
beyond the above, but note that if the IPv6 header of a packet is
covered by IPsec Authentication Header, then that authentication covers
the Home Address option as well. Thus, even when
authentication is used in the IPv6 header, the security of the Source
Address field in the IPv6 header is not compromised by the presence of
a Home Address option. Without authentication of the packet, any
field in the IPv6 header, including the Source Address field or any
other part of the packet and the Home Address option can be
forged or modified in transit. In this case, the contents of the Home
Address option is no more suspect than any other part of the packet.
</t>
</section>
<section anchor='seccons:type2rh' title='Type 2 Routing Header'>
<t>
The definition of the type 2 routing header is described in
<xref target='rhtype2' />. This definition and the associated
processing rules have been chosen so that the header cannot be
used for what is traditionally viewed as source routing.
In particular, the Home Address in the routing header will always
have to be assigned to the home address of the receiving node;
otherwise the packet will be dropped.
</t>
<t>
Generally, source routing has a number of security concerns. These
include the automatic reversal of unauthenticated source routes (which
is an issue for IPv4, but not for IPv6). Another concern is the
ability to use source routing to "jump" between nodes inside, as well
as outside a firewall. These security concerns are not issues in
Mobile IPv6, due to the rules mentioned above.
</t>
<t>
In essence the semantics of the type 2 routing header is the same as a
special form of IP-in-IP tunneling where the inner and outer source
addresses are the same.
</t>
<t>
This implies that a device which implements the filtering of packets
should be able to distinguish between a type 2 routing header and
other routing headers, as required in <xref target='req:routers' />.
This is necessary in order to allow Mobile IPv6 traffic while still
having the option of filtering out other uses of routing headers.
</t>
</section>
<!--
<section anchor='seccons:nd' title='Vulnerabilities from Neighbor Discovery'>
<t>
The ``Home Link Detection'' mechanism (<xref target='home-detect' />)
allows the MN to detect when it is at home.
When a MN detects it is at home, it is expected to deregister, and
also (if in use) to stop IPsec protection for data traffic exchanged
over the tunnel to its Home Agent.
</t>
<t>
Unfortunately, Neighbor Discovery is not a secure protocol.
It is possible that some networks may harbor malicious routing
agents that might advertise false information which would lead
a mobile node to erroneously determine that it had returned to
its home network.
</t>
<t>
A draft <xref target='I-D.ebalard-mext-hld-security' />
has been written that discusses the possible threats and security impacts
associated with the use of this insecure NDP-based mechanism as a
trigger to drop IPsec protection of data traffic for the MN. It also
provides some results on the implementation of the attacks against an
existing MIPv6 module. Possible solutions are suggested.
</t>
</section>
-->
</section>
<section title='Contributors'>
<t>
Work done by Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander,
Erik Nordmark, and Michael Thomas shaped the return routability
protocols described in <xref target='I-D.roe-mobileip-updateauth'/>.
</t>
<t>
Significant contributions were made by members of the Mobile IPv6
Security Design Team, including (in alphabetical order) Gabriel
Montenegro, Erik Nordmark and Pekka Nikander.
</t>
</section>
<section title='Acknowledgements'>
<t>
We would like to thank the members of the Mobile IP and IPng
Working Groups for their comments and suggestions on this work.
We would particularly like to thank (in alphabetical order)
Fred Baker,
Josh Broch,
Samita Chakrabarti,
Robert Chalmers,
Noel Chiappa,
Jean-Michel Combes,
Greg Daley,
Vijay Devarapalli,
Rich Draves,
Francis Dupont,
Ashutosh Dutta,
Arnaud Ebalard,
Wesley Eddy,
Thomas Eklund,
Jun-Ichiro Itojun Hagino,
Brian Haley,
Marc Hasson,
John Ioannidis,
James Kempf,
Rajeev Koodli,
Suresh Krishnan,
Krishna Kumar,
T.J. Kniveton,
Joe Lau,
Aime Le Rouzic,
Jiwoong Lee,
Benjamin Lim,
Vesa-Matti Mantyla,
Kevin Miles,
Glenn Morrow,
Ahmad Muhanna,
Thomas Narten,
Karen Nielsen,
Simon Nybroe,
David Oran,
Mohan Parthasarathy,
Basavaraj Patil,
Brett Pentland,
Lars Henrik Petander,
Alexandru Petrescu,
Mattias Petterson,
Ken Powell,
Ed Remmell,
Phil Roberts,
Patrice Romand,
Luis A. Sanchez,
Pekka Savola,
Jeff Schiller,
Arvind Sevalkar,
Keiichi Shima,
Tom Soderlund,
Hesham Soliman,
Jim Solomon,
Tapio Suihko,
Dave Thaler,
Pascal Thubert,
Benny Van Houdt,
Jon-Olov Vatn,
Ryuji Wakikawa,
Kilian Weniger,
Carl E. Williams,
Vladislav Yasevich,
Alper Yegin,
and Xinhua Zhao,
for their detailed reviews of earlier versions of this document.
Their suggestions have helped to improve both the
design and presentation of the protocol.
</t>
<t>
We would also like to thank the participants of the Mobile IPv6
testing event (1999), implementors who participated in Mobile IPv6
interoperability testing at Connectathons (2000, 2001, 2002, and
2003), and the participants at the ETSI interoperability testing
(2000, 2002). Finally, we would like to thank the TAHI project who has
provided test suites for Mobile IPv6.
</t>
</section>
</middle>
<back>
<references title='Normative References'>
<?rfc include='reference.RFC.2119.xml'?>
<?rfc include='reference.RFC.2401.xml'?>
<?rfc include='reference.RFC.2402.xml'?>
<?rfc include='reference.RFC.2406.xml'?>
<?rfc include='reference.RFC.2407.xml'?>
<?rfc include='reference.RFC.2408.xml'?>
<?rfc include='reference.RFC.2409.xml'?>
<?rfc include='reference.RFC.2460.xml'?>
<?rfc include='reference.RFC.2473.xml'?>
<?rfc include='reference.RFC.2526.xml'?>
<?rfc include='reference.RFC.2710.xml'?>
<?rfc include='reference.RFC.2782.xml'?>
<?rfc include='reference.RFC.3232.xml'?>
<?rfc include='reference.FIPS.180-1.1995.xml'?>
<?rfc include='reference.RFC.3776.xml'?>
<?rfc include='reference.RFC.4033.xml'?>
<?rfc include='reference.RFC.4086.xml'?>
<?rfc include='reference.RFC.4291.xml'?>
<?rfc include='reference.RFC.4443.xml'?>
<?rfc include='reference.RFC.4861.xml'?>
<?rfc include='reference.RFC.4862.xml'?>
<?rfc include='reference.RFC.4193.xml'?>
<?rfc include='reference.RFC.4941.xml'?>
<?rfc include='reference.RFC.5014.xml'?>
<?rfc include='reference.RFC.5026.xml'?>
<?rfc include='reference.RFC.5226.xml'?>
</references>
<references title='Informative References'>
<?rfc include='reference.RFC.2003.xml'?>
<?rfc include='reference.RFC.2004.xml'?>
<?rfc include='reference.RFC.2104.xml'?>
<?rfc include='reference.RFC.2827.xml'?>
<?rfc include='reference.I-D.aura-mipv6-bu-attacks.xml'?>
<?rfc include='reference.RFC.3315.xml'?>
<?rfc include='reference.RFC.3344.xml'?>
<?rfc include='reference.RFC.3484.xml'?>
<?rfc include='reference.I-D.nordmark-mobileip-bu3way.xml'?>
<?rfc include='reference.I-D.roe-mobileip-updateauth.xml'?>
<?rfc include='reference.I-D.ietf-mip6-bootstrapping-integrated-dhc.xml'?>
<?rfc include='reference.RFC.3627.xml'?>
<?rfc include='reference.I-D.savola-ipv6-rh-ha-security.xml'?>
<!--
<?rfc include='reference.I-D.ebalard-mext-hld-security.xml'?>
-->
<?rfc include='reference.RFC.3753.xml'?>
<?rfc include='reference.RFC.3810.xml'?>
<?rfc include='reference.RFC.4107.xml'?>
<?rfc include='reference.RFC.4225.xml'?>
<?rfc include='reference.RFC.4306.xml'?>
<?rfc include='reference.RFC.4877.xml'?>
<?rfc include='reference.RFC.5095.xml'?>
</references>
<section title='Future Extensions'>
<section anchor='future:piggybacking' title='Piggybacking'>
<t>This document does not specify how to piggyback payload packets on the
binding related messages. However, it is envisioned that this can be
specified in a separate document when issues such
as the interaction between piggybacking and IPsec are fully resolved
(see also <xref target='future:newauthz' />). The return routability messages
can indicate support for piggybacking with a new mobility option.
</t>
</section>
<section title='Triangular Routing'>
<t>Due to the concerns about opening reflection attacks with the
Home Address destination option, this specification requires that
this option be verified against the Binding Cache, i.e., there
must be a Binding Cache entry for the Home Address and Care-of Address.
</t>
<t>Future extensions may be specified that allow the use of unverified Home
Address destination options in ways that do not introduce security issues.
</t>
</section>
<section anchor='future:newauthz' title='New Authorization Methods'>
<t>While the return routability procedure provides a good level of security,
there exist methods that have even higher levels of security. Secondly,
as discussed in <xref target='security-cn-updates' />, future enhancements of
IPv6 security may cause a need to also improve the security of the return
routability procedure. Using
IPsec as the sole method for authorizing Binding Updates to
correspondent nodes is also possible. The protection of the Mobility
Header for this purpose is easy, though one must ensure that the IPsec
SA was created with appropriate authorization to use the home address
referenced in the Binding Update. For instance, a certificate used by
IKE to create the security association might contain the home address.
A future specification may specify how this is done.
</t>
</section>
<section anchor='future:nd' title='Neighbor Discovery Extensions'>
<t>
Future specifications may improve the efficiency of Neighbor Discovery
tasks, which could be helpful for fast movements. One factor is
currently being looked at: the delays caused by the Duplicate
Address Detection mechanism. Currently, Duplicate Address Detection
needs to be performed for every new care-of address as the mobile node
moves, and for the mobile node's link-local address on every new
link.
In particular, the need and the trade-offs of re-performing
Duplicate Address Detection for the link-local address every time
the mobile node moves on to new links will need to be examined.
Improvements in this area are, however, generally applicable and
progress independently from the Mobile IPv6 specification.
</t>
<t>
Future functional improvements may also be relevant for Mobile IPv6
and other applications. For instance, mechanisms that would allow
recovery from a Duplicate Address Detection collision would be useful
for link-local, care-of, and home addresses.
</t>
</section>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 02:45:04 |