IETF Mobile IP Working Group David B. Johnson
INTERNET-DRAFT Carnegie Mellon University
Charles Perkins
Sun Microsystems
25 June 1999
Mobility Support in IPv6
<draft-ietf-mobileip-ipv6-08.txt>
Status of This Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note
that other groups may also distribute working documents as
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document specifies the operation of mobile computers using IPv6.
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 four new IPv6 destination options,
including one that MUST be supported in packets received by any node,
whether mobile or stationary.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Comparison with Mobile IP for IPv4 3
3. Terminology 6
3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 7
3.3. Specification Language . . . . . . . . . . . . . . . . . 8
4. Overview of Mobile IPv6 9
4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 9
4.2. New IPv6 Destination Options . . . . . . . . . . . . . . 11
4.3. Conceptual Data Structures . . . . . . . . . . . . . . . 13
4.4. Binding Management . . . . . . . . . . . . . . . . . . . 17
5. New IPv6 Destination Options 19
5.1. Binding Update Option Format . . . . . . . . . . . . . . 19
5.2. Binding Acknowledgement Option Format . . . . . . . . . . 23
5.3. Binding Request Option Format . . . . . . . . . . . . . . 27
5.4. Home Address Option Format . . . . . . . . . . . . . . . 29
5.5. Mobile IPv6 Destination Option Sub-Options . . . . . . . 31
6. Modifications to IPv6 Neighbor Discovery 33
6.1. Modified Router Advertisement Message Format . . . . . . 33
6.2. Modified Prefix Information Option Format . . . . . . . . 34
6.3. New Advertisement Interval Option Format . . . . . . . . 36
6.4. New Home Agent Information Option Format . . . . . . . . 37
6.5. Changes to Sending Router Advertisements . . . . . . . . 39
6.6. Changes to Sending Router Solicitations . . . . . . . . . 40
7. Requirements for IPv6 Nodes 42
7.1. Requirements for All IPv6 Hosts and Routers . . . . . . . 42
7.2. Requirements for All IPv6 Routers . . . . . . . . . . . . 42
7.3. Requirements for IPv6 Home Agents . . . . . . . . . . . . 42
7.4. Requirements for IPv6 Mobile Nodes . . . . . . . . . . . 43
8. Correspondent Node Operation 45
8.1. Receiving Packets from a Mobile Node . . . . . . . . . . 45
8.2. Receiving Binding Updates . . . . . . . . . . . . . . . . 45
8.3. Requests to Cache a Binding . . . . . . . . . . . . . . . 46
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8.4. Requests to Delete a Binding . . . . . . . . . . . . . . 47
8.5. Sending Binding Acknowledgements . . . . . . . . . . . . 47
8.6. Sending Binding Requests . . . . . . . . . . . . . . . . 48
8.7. Cache Replacement Policy . . . . . . . . . . . . . . . . 48
8.8. Receiving ICMP Error Messages . . . . . . . . . . . . . . 49
8.9. Sending Packets to a Mobile Node . . . . . . . . . . . . 50
9. Home Agent Operation 52
9.1. Receiving Router Advertisement Messages . . . . . . . . . 52
9.2. Dynamic Home Agent Address Discovery . . . . . . . . . . 53
9.3. Primary Care-of Address Registration . . . . . . . . . . 55
9.4. Primary Care-of Address De-registration . . . . . . . . . 57
9.5. Intercepting Packets for a Mobile Node . . . . . . . . . 58
9.6. Tunneling Intercepted Packets to a Mobile Node . . . . . 60
9.7. Renumbering the Home Subnet . . . . . . . . . . . . . . . 61
10. Mobile Node Operation 65
10.1. Sending Packets While Away from Home . . . . . . . . . . 65
10.2. Receiving Packets While Away from Home . . . . . . . . . 67
10.3. Movement Detection . . . . . . . . . . . . . . . . . . . 68
10.4. Forming New Care-of Addresses . . . . . . . . . . . . . . 71
10.5. Sending Binding Updates to the Home Agent . . . . . . . . 72
10.6. Dynamic Home Agent Address Discovery . . . . . . . . . . 73
10.7. Sending Binding Updates to Correspondent Nodes . . . . . 74
10.8. Sending Binding Updates to the Previous Default Router . 76
10.9. Retransmitting Binding Updates . . . . . . . . . . . . . 77
10.10. Rate Limiting for Sending Binding Updates . . . . . . . . 77
10.11. Receiving Binding Acknowledgements . . . . . . . . . . . 77
10.12. Receiving Binding Requests . . . . . . . . . . . . . . . 78
10.13. Receiving ICMP Error Messages . . . . . . . . . . . . . . 79
10.14. Receiving Tunneled Router Advertisements . . . . . . . . 79
10.15. Using Multiple Care-of Addresses . . . . . . . . . . . . 80
10.16. Routing Multicast Packets . . . . . . . . . . . . . . . . 81
10.17. Returning Home . . . . . . . . . . . . . . . . . . . . . 82
11. Constants 83
12. IANA Considerations 84
13. Security Considerations 85
13.1. Binding Updates, Acknowledgements, and Requests . . . . . 85
13.2. Home Address Option . . . . . . . . . . . . . . . . . . . 85
13.3. General Mobile Computing Issues . . . . . . . . . . . . . 86
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Changes from Previous Version of the Draft 88
Acknowledgements 90
References 91
Chair's Address 93
Authors' Addresses 94
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1. Introduction
This document specifies the operation of mobile computers using
Internet Protocol Version 6 (IPv6) [5]. Without specific support
for mobility in IPv6, packets destined to a mobile node (host or
router) would not be able to reach it while the mobile node is away
from its home link (the link on which its home IPv6 subnet prefix is
in use), since routing is based on the subnet prefix in a packet's
destination IP address. 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.
The protocol operation defined here, known as Mobile IPv6, allows a
mobile node to move from one link to another without changing the
mobile node's IP address. A mobile node is always addressable by
its "home address", an IP address assigned to the mobile node within
its home subnet prefix on its home link. Packets may be routed to
the mobile node using this address regardless of the mobile node's
current point of attachment to the Internet, and the mobile node may
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.
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.
One can think of the Mobile IPv6 protocol as solving the "macro"
mobility management problem. More "micro" mobility management
applications -- for example, handoff among wireless transceivers,
each of which covers only a very small geographic area -- are
possibly more suited to other solutions. For example, in many
current wireless LAN products, link-layer mobility mechanisms allow a
"handoff" of a mobile node from one cell to another, reestablishing
link-layer connectivity to the node in each new location. As long
as such handoff occurs only within cells of the mobile node's home
link, such link-layer mobility mechanisms are likely to offer faster
convergence and lower overhead than Mobile IPv6. Extensions to the
Mobile IPv6 protocol are also possible to support a more local,
hierarchical form of mobility management, but such extensions are
beyond the scope of this document.
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The protocol specified in this document solves the problem of
transparently routing packets to and from mobile nodes while away
from home. However, it 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:
- Handling links with partial reachability, such as typical
wireless networks. Some aspects of this problem are addressed
by the movement detection procedure described in Section 10.3,
but no attempt has been made to fully solve this problem in its
general form. Most aspects of this problem can be solved by the
workaround of restricting such networks to only one router per
link, although there are still possible hidden terminal problems
when two nodes on the same link (on opposite sides of the router)
attempt to communicate directly.
- Access control on a link being visited by a mobile node. This
is a general problem any time an untrusted node is allowed
to connect to any link layer. It is independent whether the
connecting node uses Mobile IP, DHCP [2], or just "borrows" an IP
address on the link.
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2. Comparison with Mobile IP for IPv4
The design of Mobile IP support in IPv6 (Mobile IPv6) represents a
natural combination of the experiences gained from the development
of Mobile IP support in IPv4 (Mobile IPv4) [16, 15, 17], together
with the opportunities provided by the design and deployment of a new
version of IP itself (IPv6) and the new protocol features offered
by IPv6. Mobile IPv6 thus shares many features with Mobile IPv4,
but the protocol is now fully integrated into IP and provides many
improvements over Mobile IPv4. This section summarizes the major
differences between Mobile IPv4 and Mobile IPv6:
- Support for what is known in Mobile IPv4 as "Route
Optimization" [18] is now built in as a fundamental part
of the protocol, rather than being added on as a optional
set of extensions that may not be supported by all nodes
as in Mobile IPv4. This integration of Route Optimization
functionality allows direct routing from any correspondent node
to any mobile node, without needing to pass through the mobile
node's home network and be forwarded by its home agent, and thus
eliminates the problem of "triangle routing" present in the base
Mobile IPv4 protocol [16]. This integration also allows the
Mobile IPv4 "registration" functionality and the Mobile IPv4
Route Optimization functionality to be performed by a single
protocol rather than two separate (and different) protocols.
- Support is also integrated into Mobile IPv6 -- and into IPv6
itself -- for allowing mobile nodes and Mobile IP to coexist
efficiently with routers that perform "ingress filtering" [6]. A
mobile node now uses its care-of address as the Source Address in
the IP header of packets it sends, allowing the packets to pass
normally through ingress filtering routers. The home address
of the mobile node is carried in the packet in a Home Address
destination option, allowing the use of the care-of address in
the packet to be transparent above the IP layer. The ability
to correctly process a Home Address option in a received packet
is required in all IPv6 nodes, whether mobile nor stationary,
whether host or router.
- The use of the care-of address as the Source Address in each
packet's IP header also simplifies routing of multicast packets
sent by a mobile node. With Mobile IPv4, the mobile node
had to tunnel multicast packets to its home agent in order to
transparently use its home address as the source of the multicast
packets. With Mobile IPv6, the use of the Home Address option
allows the home address to be used but still be compatible with
multicast routing that is based in part on the packet's Source
Address.
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- There is no longer any need to deploy special routers as
"foreign agents" as are used in Mobile IPv4. In Mobile IPv6,
mobile nodes make use of the enhanced features of IPv6, such
as Neighbor Discovery [14] and Address Autoconfiguration [23],
to operate in any location away from home without any special
support required from its local router.
- Unlike Mobile IPv4, Mobile IPv6 utilizes IPsec [9, 10, 11] for
all security requirements (sender authentication, data integrity
protection, and replay protection) for Binding Updates (which
serve the role of both registration and Route Optimization in
Mobile IPv4). Mobile IPv4 relies on its own security mechanisms
for these functions, based on statically configured "mobility
security associations".
- The movement detection mechanism in Mobile IPv6 provides
bidirectional confirmation of a mobile node's ability to
communicate with its default router in its current location
(packets that the router sends are reaching the mobile node, and
packets that the mobile node sends are reaching the router).
This confirmation provides a detection of the "black hole"
situation that may exist in some wireless environments where the
link to the router does not work equally well in both directions,
such as when the mobile node has moved out of good wireless
transmission range from the router. The mobile node may then
attempt to find a new router and begin using a new care-of
address if its link to its current router is not working well.
In contrast, in Mobile IPv4, only the forward direction (packets
from the router are reaching the mobile node) is confirmed,
allowing the black hole condition to persist.
- Most packets sent to a mobile node while away from home in
Mobile IPv6 are tunneled using an IPv6 Routing header rather than
IP encapsulation, whereas Mobile IPv4 must use encapsulation
for all packets. The use of a Routing header requires less
additional header bytes to be added to the packet, reducing the
overhead of Mobile IP packet delivery. To avoid modifying the
packet in flight, however, packets intercepted and tunneled
by a mobile node's home agent in Mobile IPv6 must still use
encapsulation for tunneling.
- While a mobile node is away from home, its home agent intercepts
any packets for the mobile node that arrive at the home network,
using IPv6 Neighbor Discovery [14] rather than ARP [19] as is
used in Mobile IPv4. The use of Neighbor Discovery improves
the robustness of the protocol (e.g., due to the Neighbor
Advertisement "override" bit) and simplifies implementation
of Mobile IP due to the ability to not be concerned with any
particular link layer as is required in ARP.
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- The use of IPv6 encapsulation (and the Routing header) removes
the need in Mobile IPv6 to manage "tunnel soft state", which was
required in Mobile IPv4 due to limitations in ICMP for IPv4. Due
to the definition of ICMP for IPv6, the use of tunnel soft state
is no longer required in IPv6 for correctly relaying ICMP error
messages from within the tunnel back to the original sender of
the packet.
- The dynamic home agent address discovery mechanism in Mobile IPv6
uses IPv6 anycast [8] and returns a single reply to the mobile
node, rather than the corresponding Mobile IPv4 mechanism that
used IPv4 directed broadcast and returned a separate reply from
each home agent on the mobile node's home link. The Mobile IPv6
mechanism is more efficient and more reliable, since only
one packet need be sent back to the mobile node and since the
mobile node is less likely to lose one of the replies because no
"implosion" of replies is required by the protocol.
- Mobile IPv6 defines an Advertisement Interval option on
Router Advertisements (equivalent to Agent Advertisements in
Mobile IPv4), allowing a mobile node to decide for itself how
many Router Advertisements (Agent Advertisements) it is willing
to miss before declaring its current router unreachable.
- The use of IPv6 destination options allows all Mobile IPv6
control traffic to be piggybacked on any existing IPv6 packets,
whereas in Mobile IPv4 and its Route Optimization extensions,
separate UDP packets were required for each control message.
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3. Terminology
3.1. General Terms
IP
Internet Protocol Version 6 (IPv6).
node
A device that implements IP.
router
A node that forwards IP packets not explicitly addressed to
itself.
host
Any node that is not a router.
link
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.
interface
A node's attachment to a link.
subnet prefix
A bit string that consists of some number of initial bits of an
IP address.
interface identifier
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.
link-layer address
A link-layer identifier for an interface, such as IEEE 802
addresses on Ethernet links.
packet
An IP header plus payload.
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3.2. Mobile IPv6 Terms
home address
An IP address assigned to a mobile node within its home link.
home subnet prefix
The IP subnet prefix corresponding to a mobile node's home
address.
home link
The link on which a mobile node's home subnet prefix is
defined. Standard IP routing mechanisms will deliver packets
destined for a mobile node's home address to its home link.
mobile node
A node that can change its point of attachment from one link to
another, while still being reachable via its home address.
movement
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".
correspondent node
A peer node with which a mobile node is communicating. The
correspondent node may be either mobile or stationary.
foreign subnet prefix
Any IP subnet prefix other than the mobile node's home subnet
prefix.
foreign link
Any link other than the mobile node's home link.
home agent
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
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address, encapsulates them, and tunnels them to the mobile
node's registered care-of address.
care-of address
An IP 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 a time (e.g., with different subnet
prefixes), the one registered with the mobile node's home agent
is called its "primary" care-of address.
binding
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.
3.3. Specification Language
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 RFC 2119 [3].
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4. Overview of Mobile IPv6
4.1. Basic Operation
A mobile node is always addressable by its home address, whether it
is currently attached to its home link or is away from home. While
a mobile node is at home, packets addressed to its home address are
routed to it using conventional Internet routing mechanisms in the
same way as if the node were never mobile. Since the subnet prefix
of a mobile node's home address is the subnet prefix (or one of the
subnet prefixes) on the mobile node's home link (it is the mobile
node's home subnet prefix), packets addressed to it will be routed to
its home link.
While a mobile node is attached to some foreign link away from home,
it is also addressable by one or more care-of addresses, in addition
to its home address. A care-of address is an IP address associated
with a mobile node while visiting a particular foreign link. The
subnet prefix of a mobile node's care-of address is the subnet prefix
(or one of the subnet prefixes) on the foreign link being visited by
the mobile node; if the mobile node is connected to this foreign link
while using that care-of address, packets addressed to this care-of
address will be routed to the mobile node in its location away from
home.
The association between a mobile node's home address and care-of
address is known as a "binding" for the mobile node. A mobile node
typically acquires its care-of address through stateless [23] or
stateful (e.g., DHCPv6 [2]) Address Autoconfiguration, according
to the methods of IPv6 Neighbor Discovery [14]. Other methods
of acquiring a care-of address are also possible, such as static
pre-assignment by the owner or manager of a particular foreign link,
but details of such other methods are beyond the scope of this
document.
While away from home, a mobile node registers one of its care-of
addresses with a router on its home link, requesting this router
to function as the "home agent" for the mobile node. This binding
registration is done by the mobile node sending to the home agent
a packet containing a "Binding Update" destination option; the
home agent then replies to the mobile node by returning a packet
containing a "Binding Acknowledgement" destination option. The
care-of address in this binding registered with its home agent is
known as the mobile node's "primary care-of address". The mobile
node's home agent thereafter uses proxy Neighbor Discovery to
intercept any IPv6 packets addressed to the mobile node's home
address (or home addresses) on the home link, and tunnels each
intercepted packet to the mobile node's primary care-of address.
To tunnel each intercepted packet, the home agent encapsulates the
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packet using IPv6 encapsulation [4], with the outer IPv6 header
addressed to the mobile node's primary care-of address.
Section 10.15 discusses the reasons why it may be desirable for
a mobile node to use more than one care-of address at the same
time. However, a mobile node's primary care-of address is distinct
among these in that the home agent maintains only a single care-of
address registered for each mobile node, and always tunnels a mobile
node's packets intercepted from its home link to this mobile node's
registered primary care-of address. The home agent thus need not
implement any policy to determine which of possibly many care-of
addresses to which to tunnel each intercepted packet, leaving the
mobile node entirely in control of this policy by which of its
care-of addresses it registers with its home agent.
It is possible that while a mobile node is away from home, some nodes
on its home link may be reconfigured, such that the router that was
operating as the mobile node's home agent is replaced by a different
router serving this role. In this case, the mobile node may not
know the IP address of its own home agent. Mobile IPv6 provides a
mechanism, known as "dynamic home agent address discovery", that
allows a mobile node to dynamically discover the IP address of a home
agent on its home link with which it may register its care-of address
while away from home. The mobile node sends a Binding Update to the
"Mobile IPv6 Home-Agents" anycast address for its own home subnet
prefix [8] and thus reaches one of the (possibly many) routers on
its home link currently operating as a home agent. This home agent
rejects the mobile node's Binding Update, but returns in the Binding
Acknowledgement in response a list of all home agents on the home
link. This list of home agents is maintained by each home agent on
the home link through use of the Home Agent (H) bit in each home
agent's periodic unsolicited multicast Router Advertisements.
The Binding Update and Binding Acknowledgement destination options,
together with a "Binding Request" destination option, are also used
to allow IPv6 nodes communicating with a mobile node, to dynamically
learn and cache the mobile node's binding. When sending a packet
to any IPv6 destination, a 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 an IPv6 Routing
header [5] (instead of IPv6 encapsulation) to route the packet to
the mobile node by way of the care-of address indicated in this
binding. If, instead, the sending node has no cached binding for
this destination address, the node sends the packet normally (with
no Routing header), and the packet is subsequently intercepted and
tunneled by the mobile node's home agent as described above. 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.
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Since a Binding Update, Binding Acknowledgement, and Binding Request
are each represented in a packet as an IPv6 destination option [5],
they may be included in any IPv6 packet. Any of these options can be
sent in either of two ways:
- A Binding Update, Binding Acknowledgement, or Binding Request can
be included within any IPv6 packet carrying any payload such as
TCP [21] or UDP [20].
- A Binding Update, Binding Acknowledgement, or Binding Request can
be sent as a separate IPv6 packet containing no payload. In this
case, the Next Header field in the last extension header in the
packet is set to the value 59, to indicate "No Next Header" [5].
Mobile IPv6 also defines one additional IPv6 destination option.
When a mobile node sends a packet while away from home, it will
generally set the Source Address in the packet's IPv6 header to one
of its current care-of addresses, and will also include a "Home
Address" destination option in the packet, giving the mobile node's
home address. Many routers implement security policies such as
"ingress filtering" [6] that do not allow forwarding of packets that
appear to have a Source Address that is not topologically correct.
By using the care-of address as the IPv6 header Source Address,
the packet will be able to pass normally through such routers,
yet ingress filtering rules will still be able to locate the true
topological source of the packet in the same way as packets from
non-mobile nodes. By also including the Home Address option in each
packet, the sending mobile node can communicate its home address to
the correspondent node receiving this packet, allowing the use of
the care-of address to be transparent above the Mobile IPv6 support
level (e.g., at the transport layer). The inclusion of a Home
Address option in a packet affects only the correspondent node's
receipt of this single packet; no state is created or modified in the
correspondent node as a result of receiving a Home Address option in
a packet.
4.2. New IPv6 Destination Options
As discussed in general in Section 4.1, the following four new IPv6
destination options are defined for Mobile IPv6:
Binding Update
A Binding Update option 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". Any packet that includes a
Binding Update option MUST also include either an AH [9] or
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ESP [10] header providing sender authentication, data integrity
protection, and replay protection. The Binding Update option
is described in detail in Section 5.1.
Binding Acknowledgement
A Binding Acknowledgement option is used to acknowledge receipt
of a Binding Update, if an acknowledgement was requested
in the Binding Update. Any packet that includes a Binding
Acknowledgement option MUST also include either an AH [9] or
ESP [10] header providing sender authentication, data integrity
protection, and replay protection. The Binding Acknowledgement
option is described in detail in Section 5.2.
Binding Request
A Binding Request option is used to request a mobile node to
send to the requesting node a Binding Update containing the
mobile node's current binding. This option is typically used
by a correspondent node to refresh a cached binding for a
mobile node, when the cached binding is in active use but the
binding's lifetime is close to expiration. No authentication
is required for the Binding Request option. The Binding
Request option is described in detail in Section 5.3.
Home Address
A Home Address option is used in a packet sent by a mobile
node to inform the recipient of that packet of the mobile
node's home address. For packets sent by a mobile node while
away from home, the mobile node generally uses one of its
care-of addresses as the Source Address in the packet's IPv6
header. By including a Home Address option in the packet, the
correspondent node receiving the packet is able to substitute
the mobile node's home address for this care-of address when
processing the packet, thus making the use of the care-of
address transparent to the correspondent node. If the IP
header of a packet carrying a Home Address option is covered
by authentication, then the Home Address option MUST also be
covered by this authentication, but no other authentication is
required for the Home Address option. The Home Address option
is described in detail in Section 5.4.
Sub-options within the format of these options MAY be included after
the fixed portion of the option data specified in this document. The
presence of such sub-options will be indicated by the Option Length
field within the option. When the Option Length is greater than the
length required for the option specified here, the remaining octets
are interpreted as sub-options. The encoding and format of defined
sub-options are described in Section 5.5.
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4.3. Conceptual Data Structures
This document describes the Mobile IPv6 protocol in terms of the
following three conceptual data structures:
Binding Cache
A cache, maintained by each IPv6 node, of bindings for other
nodes. 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 Neighbor Discovery [14].
When sending a packet, the Binding Cache is searched before the
Neighbor Discovery conceptual Destination Cache [14] (i.e., any
Binding Cache entry for this destination SHOULD take precedence
over any Destination Cache entry for the same destination).
Each Binding Cache entry conceptually contains the following
fields:
- 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. If the destination address of the
packet matches the home address in the Binding Cache entry,
this entry SHOULD be used in routing that packet.
- The care-of address for the mobile node indicated by
the home address field in this Binding Cache entry. If
the destination address of a packet being routed by a
node matches the home address in this entry, the packet
SHOULD be routed to this care-of address, as described in
Section 8.9, for packets originated by this node, or in
Section 9.6, if this node is the mobile node's home agent
and the packet was intercepted by it on the home link.
- 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.
Once the lifetime on this entry expires, the entry MUST be
deleted from the Binding Cache.
- A flag indicating whether or not this Binding Cache entry
is a "home registration" entry.
- A flag indicating whether or not this Binding Cache entry
represents a mobile node that should be advertised as a
router in proxy Neighbor Advertisements sent by this node
on its behalf. This flag is only valid if the Binding
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Cache entry indicates that this is a "home registration"
entry.
- The value of the Prefix Length field received in the
Binding Update that created or last modified this Binding
Cache entry. This field is only valid if the "home
registration" flag is set on this Binding Cache entry.
- The maximum value of the Sequence Number field received
in previous Binding Updates for this mobile node home
address. The Sequence Number field is 16 bits long, and
all comparisons between Sequence Number values MUST be
performed modulo 2**16.
- Recent usage information for this Binding Cache entry, as
needed to implement the cache replacement policy in use in
the Binding Cache and to assist in determining whether a
Binding Request should be sent when the lifetime on this
entry nears expiration.
- The time at which a Binding Request was last sent for this
entry, as needed to implement the rate limiting restriction
for sending Binding Requests.
An entry in a node's Binding Cache for which the node is
serving as a home agent is marked as a "home registration"
entry and SHOULD NOT be deleted by the home agent until the
expiration of its binding lifetime. Other Binding Cache
entries MAY be replaced at any time by any reasonable local
cache replacement policy but SHOULD NOT be unnecessarily
deleted. Any node's Binding Cache 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.
Binding Update List
A list, maintained by each mobile node, recording information
for each Binding Update sent by this mobile node, for which
the Lifetime sent in that Binding Update has not yet expired.
The Binding Update List includes all bindings sent by the
mobile node: those to correspondent nodes, to the mobile
node's home agent, and to a previous default router of the
mobile node. The Binding Update List MAY be implemented in any
manner consistent with the external behavior described in this
document. Each Binding Update List entry conceptually contains
the following fields:
- The IP address of the node to which a Binding Update was
sent. This node might still have a Binding Cache entry
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created or updated from this Binding Update, if the Binding
Update was successfully received by that node (e.g., not
lost by the network) and if that node has not deleted the
entry before its expiration (e.g., to reclaim space in its
Binding Cache for other entries).
- The home address for which that Binding Update was sent.
This will be one of the mobile node's home addresses for
most Binding Updates (Sections 10.5 and 10.7), but will
be the mobile node's previous care-of address for Binding
Updates sent to the mobile node's previous default router
(Section 10.8).
- 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 giving its new care-of address to
this destination after changing its care-of address.
- 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.
- 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.
- 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.
- The state of any retransmissions needed for this Binding
Update, if the Acknowledge (A) bit was set in 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.
- A flag that, when set, indicates that future Binding
Updates should not 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 2,
error message in response to a Binding Update sent to that
destination, as described in Section 10.13.
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Home Agents List
A list, maintained by each home agent, recording information
about each other home agent on a link on which this node
is serving as a home agent; each home agent maintains a
separate Home Agents List for each such link on which it is
serving. This list is used in the dynamic home agent address
discovery mechanism. The information for the list is learned
through receipt of the periodic unsolicited multicast Router
Advertisements from each other home agent on the link, in which
the Home Agent (H) bit is set, in a manner similar to the
Default Router List conceptual data structure maintained by
each host for Neighbor Discovery [14]. The Home Agents List
MAY be implemented in any manner consistent with the external
behavior described in this document. Each Home Agents List
entry conceptually contains the following fields:
- The link-local IP address of another router on the home
link that this node currently believes is operating as
a home agent for this link. A new entry is created or
an existing entry is updated in the Home Agents List in
response to receipt of a valid Router Advertisement in
which the Home Agent (H) bit is set. The link-local
address of the home agent is learned through the Source
Address of the Router Advertisements received from it [14].
- One or more global IP addresses for this home agent,
learned through Prefix Information options with the
Router Address (R) bit is set, 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 [14].
- 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 this home agent
is initialized from the Home Agent Lifetime field in the
option; otherwise, the lifetime is initialized from the
Router Lifetime field in the received Router Advertisement.
The Home Agents List entry lifetime is decremented until it
reaches zero, at which time this entry MUST be deleted from
the Home Agents List.
- The preference for this home agent, for use in ordering the
Home Agents List returned in a Binding Acknowledgement;
higher values indicate a more preferable home agent. The
preference value is taken from the Home Agent Preference
field (a signed, twos-complement integer) in the received
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Router Advertisement, if the Router Advertisement contains
a Home Agent Information Option, and is otherwise set to
the default value of 0.
4.4. Binding Management
When a mobile node configures a new care-of address and decides to
use this new address as its primary care-of address, the mobile
node registers this new binding with its home agent by sending
the home agent a Binding Update. The mobile node indicates
that an acknowledgement is needed for this Binding Update and
continues to periodically retransmit it until acknowledged. The
home agent acknowledges the Binding Update by returning a Binding
Acknowledgement to the mobile node.
When a mobile node receives a packet tunneled to it from its
home agent, the mobile node assumes that the original sending
correspondent node has no Binding Cache entry for the mobile node,
since the correspondent node would otherwise have sent the packet
directly to the mobile node using a Routing header. The mobile node
thus returns a Binding Update to the correspondent node, allowing
it to cache the mobile node's binding for routing future packets to
it. Although the mobile node may request an acknowledgement for
this Binding Update, it need not, since subsequent packets from the
correspondent node will continue to be intercepted and tunneled by
the mobile node's home agent, effectively causing any needed Binding
Update retransmission.
A correspondent node with a Binding Cache entry for a mobile node
may refresh this binding, for example if the binding's lifetime
is near expiration, by sending a Binding Request to the mobile
node. Normally, a correspondent node will only refresh a Binding
Cache entry in this way if it is actively communicating with the
mobile node and has indications, such as an open TCP connection to
the mobile node, that it will continue this communication in the
future. When a mobile node receives a Binding Request, it replies by
returning a Binding Update to the node sending the Binding Request.
A mobile node may use more than one care-of address at the same
time, although only one care-of address may be registered for it at
its home agent as its primary care-of address. The mobile node's
home agent will tunnel all intercepted packets for the mobile node
to its (single) registered primary care-of address, but the mobile
node will accept packets that it receives at any of its current
care-of addresses. Use of more than one care-of address by a mobile
node may be useful, for example, to improve smooth handoff when the
mobile node moves from one wireless link to another. If each of
these wireless links is connected to the Internet through a separate
base station, such that the wireless transmission range from the
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two base stations overlap, the mobile node may be able to remain
connected to both links while in the area of overlap. In this case,
the mobile node could acquire a new care-of address on the new link
before moving out of transmission range and disconnecting from the
old link. The mobile node may thus still accept packets at its
old care-of address while it works to update its home agent and
correspondent nodes, notifying them of its new care-of address on the
new link.
Since correspondent nodes cache bindings, it is expected that
correspondent nodes usually will route packets directly to the mobile
node's care-of address, so that the home agent is rarely involved
with packet transmission to the mobile node. This is essential for
scalability and reliability, and for minimizing overall network load.
By caching the care-of address of a mobile node, optimal routing of
packets can be achieved from the correspondent node to the mobile
node. Routing packets directly to the mobile node's care-of address
also eliminates congestion at the mobile node's home agent and home
link. In addition, the impact of any possible failure of the home
agent, the home link, or intervening networks leading to or from the
home link is reduced, since these nodes and links are not involved in
the delivery of most packets to the mobile node.
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5. New IPv6 Destination Options
5.1. Binding Update Option Format
The Binding Update destination option is used by a mobile node
to notify other nodes of a new care-of address for itself. As a
destination option, it MAY be included in any existing packet being
sent to this same destination or MAY be sent in a packet by itself;
a packet containing a Binding Update is sent in the same way as any
packet sent by a mobile node (Section 10.1).
The Binding Update option is encoded in type-length-value (TLV)
format as follows:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|C|R|Reservd| Prefix Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Care-of Address +
| (only present if C bit set) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Options...
+-+-+-+-+-+-+-+-+-+-+-+-
Option Type
198 = 0xC6
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. This field
MUST be set to 8 (or to 24 if the Care-of Address Present (C)
bit is set), plus the total length of all sub-options present,
including their Sub-Option Type and Sub-Option Len fields.
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Acknowledge (A)
The Acknowledge (A) bit is set by the sending mobile node to
request a Binding Acknowledgement (Section 5.2) be returned
upon receipt of the Binding Update.
Home Registration (H)
The Home Registration (H) bit is set by the sending mobile node
to request the receiving node to act as this node's home agent.
The destination of the packet carrying this option MUST be that
of a router sharing the same subnet prefix as the home address
of the mobile node in the binding (given by the Home Address
field in the Home Address option in the packet).
Care-of Address Present (C)
The Care-of Address Present (C) bit, when set, indicates the
presence of the Care-of Address field in the Binding Update.
The care-of address for this binding is either the address in
the Care-of Address field in the Binding Update, if this bit is
set, or the Source Address in the packet's IPv6 header, if this
bit is not set.
Router (R)
The Router (R) bit, when set, indicates that the sending
mobile node is a router. This bit is only valid when the
Home Registration (H) bit is also set, and MUST NOT be set
otherwise. This bit is saved in the home agent's "home
registration" Binding Cache entry for the mobile node, and
is copied into the corresponding bit in all proxy Neighbor
Advertisement messages sent on behalf of this mobile node by
the home agent using this Binding Cache entry.
Reservd
Reserved. Sent as 0; ignored on reception.
Prefix Length
The Prefix Length field is valid only for a "home registration"
Binding Update. This field MUST be zero if the Home
Registration (H) bit is not set in the Binding Update. The
Prefix Length field is set by the sending mobile node to the
(nonzero) length of its subnet prefix in its home address
(given in the Home Address option in the packet) to request
its home agent to use the interface identifier in the mobile
node's home address (the remaining low-order bits after the
indicated subnet prefix) to form all other home addresses for
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the mobile node on the home link. The home agent becomes the
home agent not only for the individual home address given in
this binding, but also for all other home addresses for this
mobile node formed from this interface identifier. That is,
for each on-link prefix on the home link, the home agent uses
the interface identifier to form other valid addresses for the
mobile node on the home link, and acts as a home agent also
for those addresses. In addition, the home agent forms the
link-local address and site-local address corresponding to
this interface identifier, and defends each for purposes of
Duplicate Address Detection. Details of this operation are
described in Section 9.3.
Sequence Number
Used by the receiving node to sequence Binding Updates and by
the sending node to match a returned Binding Acknowledgement
with this Binding Update. Each Binding Update sent by a mobile
node MUST use a Sequence Number greater than the Sequence
Number value sent in the previous Binding Update (if any) to
the same destination address (modulo 2**16).
Lifetime
32-bit unsigned integer. The number of seconds remaining
before the binding must be considered expired. A value of all
one bits (0xffffffff) indicates infinity. A value of zero
indicates that the Binding Cache entry for the mobile node
should be deleted.
Care-of Address
This field in the Binding Update is optional and is only
present when the Care-of Address Present (C) bit is set. If
present, it gives the care-of address of the mobile node for
this binding. For most Binding Updates sent, it is expected
that this field will not be present, and instead that the
care-of address for the binding will be given by the Source
Address field in the packet's IPv6 header.
Sub-Options
Additional information, associated with this Binding Update
option, that need not be present in all Binding Updates sent.
This use of sub-options also allows for future extensions to
the format of the Binding Update option to be defined. The
encoding and format of defined sub-options are described in
Section 5.5. The following sub-options are valid in a Binding
Update option:
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- Unique Identifier Sub-Option
Any packet that includes a Binding Update option MUST also include
a Home Address option. The home address of the mobile node in the
binding given in the Binding Update option is indicated by the Home
Address field in the Home Address option in the packet.
Any packet that includes a Binding Update option MUST also include
either an AH [9] or ESP [10] header providing sender authentication,
data integrity protection, and replay protection.
If the care-of address in the binding (either the Care-of Address
field in the Binding Update option or the Source Address field in
the packet's IPv6 header) is equal to the home address of the mobile
node, the Binding Update option indicates that any existing binding
for the mobile node MUST be deleted. Likewise, if the Lifetime
field in the Binding Update option is equal to 0, the Binding Update
option indicates that any existing binding for the mobile node MUST
be deleted. In each of these cases, a Binding Cache entry for the
mobile node MUST NOT be created in response to receiving the Binding
Update.
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; the last Sequence Number value received from
a mobile node in a Binding Update is stored by a correspondent node
in its Binding Cache entry for that mobile node. Thus, the mobile
node's and the correspondent node's knowledge of the last sequence
number expire at the same time. If the sending mobile node has no
Binding Update List entry, the Sequence Number may start at any
value; if the receiving correspondent node has no Binding Cache entry
for the sending mobile node, it MUST accept any Sequence Number value
in a received Binding Update from this mobile node.
The three highest-order bits of the Option Type are encoded to
indicate specific processing of the option [5]. For the Binding
Update option, these three bits are set to 110, indicating that any
IPv6 node processing this option that does not recognize the Option
Type must discard the packet and, only 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; and that the
data within the option cannot change en-route to the packet's final
destination.
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5.2. Binding Acknowledgement Option Format
The Binding Acknowledgement destination option is used to acknowledge
receipt of a Binding Update option (Section 5.1). When a node
receives a packet containing a Binding Update option, with this
node being the destination of the packet (only the destination node
processes the option since it is a destination option), this node
MUST return a Binding Acknowledgement to the source of the packet,
if the Acknowledge (A) bit is set in the Binding Update. As a
destination option, this node MAY include the Binding Acknowledgement
in any existing packet being sent to the mobile node or MAY send it
in a packet by itself. A packet containing a Binding Acknowledgement
is sent in the same way as any packet to a mobile node, using a
Routing header to route the packet to the mobile node by way of the
care-of address in the binding (Section 8.9).
The Binding Acknowledgement option is encoded in type-length-value
(TLV) format as follows:
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 | Status | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Refresh |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Options...
+-+-+-+-+-+-+-+-+-+-+-+-
Option Type
7
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. This field
MUST be set to 11 plus the total length of all sub-options
present, including their Sub-Option Type and Sub-Option Len
fields.
Status
8-bit unsigned integer indicating the disposition of the
Binding Update. Values of the Status field less than 128
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indicate that the Binding Update was accepted by the receiving
node. The following such Status values are currently defined:
0 Binding Update accepted
Values of the Status field greater than or equal to 128
indicate that the Binding Update was rejected by the receiving
node. The following such Status values are currently defined:
128 Reason unspecified
129 Poorly formed Binding Update
130 Administratively prohibited
131 Insufficient resources
132 Home registration not supported
133 Not home subnet
134 Sequence Number field value too small
135 Dynamic home agent address discovery response
136 Incorrect interface identifier length
137 Not home agent for this mobile node
Up-to-date values of the Status field are to be specified in
the most recent "Assigned Numbers" [22].
Sequence Number
The Sequence Number in the Binding Acknowledgement is copied
from the Sequence Number field in the Binding Update being
acknowledged, for use by the mobile node in matching this
Acknowledgement with an outstanding Binding Update.
Lifetime
The granted lifetime for which this node will attempt to retain
the entry for this mobile node in its Binding Cache. If the
node sending the Binding Acknowledgement is serving as the
mobile node's home agent, the Lifetime period also indicates
the period for which this node will continue this service; if
the mobile node requires home agent service from this node
beyond this period, the mobile node MUST send a new Binding
Update to it before the expiration of this period (even if
it is not changing its primary care-of address), in order to
extend the lifetime. The value of this field is undefined
if the Status field indicates that the Binding Update was
rejected.
Refresh
The recommended interval at which the mobile node SHOULD send
a new Binding Update to this node in order to "refresh" the
mobile node's binding in this node's Binding Cache. This
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refreshing of the binding is useful in case the node fails and
loses its cache state. The Refresh period is determined by
the node sending the Binding Acknowledgement (the node caching
the binding). If this node is serving as the mobile node's
home agent, the Refresh value may be set, for example, based on
whether the node stores its Binding Cache in volatile storage
or in nonvolatile storage. If the node sending the Binding
Acknowledgement is not serving as the mobile node's home agent,
the Refresh period SHOULD be set equal to the Lifetime period
in the Binding Acknowledgement; even if this node loses this
cache entry due to a failure of the node, packets from it can
still reach the mobile node through the mobile node's home
agent, causing a new Binding Update to this node to allow it
to recreate this cache entry. The value of this field is
undefined if the Status field indicates that the Binding Update
was rejected.
Sub-Options
Additional information, associated with this Binding
Acknowledgement option, that need not be present in all Binding
Acknowledgements sent. This use of sub-options also allows for
future extensions to the format of the Binding Acknowledgement
option to be defined. The encoding and format of defined
sub-options are described in Section 5.5. The following
sub-options are valid in a Binding Acknowledgement option:
- Home Agents List Sub-Option
Any packet that includes a Binding Acknowledgement option MUST
also include either an AH [9] or ESP [10] header providing sender
authentication, data integrity protection, and replay protection.
If the node returning the Binding Acknowledgement accepted the
Binding Update for which the Acknowledgement is being returned (the
value of the Status field in the Acknowledgement is less than 128),
this node will have an entry for the mobile node in its Binding Cache
and MUST use this entry (which includes the care-of address received
in the Binding Update) in sending the packet containing the Binding
Acknowledgement to the mobile node. The details of sending this
packet to the mobile node are the same as for sending any packet to a
mobile node using a binding, and are described in Section 8.9. The
packet is sent using a Routing header, routing the packet to the
mobile node by way of its care-of address recorded in the Binding
Cache entry.
If the node returning the Binding Acknowledgement instead
rejected the Binding Update (the value of the Status field in the
Acknowledgement is greater than or equal to 128), this node MUST
similarly use a Routing header in sending the packet containing the
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Binding Acknowledgement, as described in Section 8.9, but MUST NOT
use its Binding Cache in forming the IP header or Routing header
in this packet. Rather, the care-of address used by this node in
sending the packet containing the Binding Acknowledgement MUST be
copied from the care-of address received in the rejected Binding
Update; this node MUST NOT modify its Binding Cache in response
to receiving this rejected Binding Update and MUST ignore its
Binding Cache in sending the packet in which it returns this Binding
Acknowledgement. The packet is sent using a Routing header, routing
the packet to the home address of the rejected Binding Update by
way of the care-of address indicated in the packet containing the
Binding Update. When sending a Binding Acknowledgement to reject a
Binding Update, the Binding Acknowledgement MUST be sent in an IPv6
packet containing no payload (with the Next Header field in the last
extension header in the packet set to indicate "No Next Header" [5]).
The three highest-order bits of the Option Type are encoded to
indicate specific processing of the option [5]. For the Binding
Acknowledgement option, these three bits are set to 000, indicating
that any IPv6 node processing this option that does not recognize the
Option Type must skip over this option and continue processing the
header, and that the data within the option cannot change en-route to
the packet's final destination.
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5.3. Binding Request Option Format
The Binding Request destination option is used to request a mobile
node's binding from the mobile node. As a destination option, it
MAY be included in any existing packet being sent to the mobile
node or MAY be sent in a packet by itself; a packet containing a
Binding Request option is sent in the same way as any packet to a
mobile node (Section 8.9). When a mobile node receives a packet
containing a Binding Request option, it SHOULD return a Binding
Update (Section 5.1) to the source of the Binding Request.
The Binding Request option is encoded in type-length-value (TLV)
format as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Option Type | Option Length | Sub-Options...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Option Type
8
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. This field
MUST be set to 0 plus the total length of all sub-options
present, including their Sub-Option Type and Sub-Option Len
fields.
Sub-Options
Additional information, associated with this Binding Request
option, that need not be present in all Binding Requests sent.
This use of sub-options also allows for future extensions to
the format of the Binding Request option to be defined. The
encoding and format of defined sub-options are described in
Section 5.5. The following sub-options are valid in a Binding
Request option:
- Unique Identifier Sub-Option
The three highest-order bits of the Option Type are encoded to
indicate specific processing of the option [5]. For the Binding
Request option, these three bits are set to 000, indicating that any
IPv6 node processing this option that does not recognize the Option
Type must skip over this option and continue processing the header,
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and that the data within the option cannot change en-route to the
packet's final destination.
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5.4. Home Address Option Format
The Home Address destination option is used in a packet sent by a
mobile node while away from home, to inform the recipient of that
packet of the mobile node's home address. For packets sent by a
mobile node while away from home, the mobile node generally uses
one of its care-of addresses as the Source Address in the packet's
IPv6 header. By including a Home Address option in the packet, the
correspondent node receiving the packet is able to substitute the
mobile node's home address for this care-of address when processing
the packet, thus making the use of the care-of address transparent to
the correspondent node.
The Home Address option is encoded in type-length-value (TLV) format
as follows:
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 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Options...
+-+-+-+-+-+-+-+-+-+-+-+-
Option Type
201 = 0xC9
Option Length
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 plus the total length of all sub-options
present, including their Sub-Option Type and Sub-Option Len
fields.
Home Address
The home address of the mobile node sending the packet.
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Sub-Options
Additional information, associated with this Home Address
option, that need not be present in all Home Address options
sent. This use of sub-options also allows for future
extensions to the format of the Home Address option to be
defined. The encoding and format of defined sub-options are
described in Section 5.5. Currently, no valid sub-options are
defined for use in a Home Address option.
The inclusion of a Home Address 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.
No authentication of the Home Address option is required, except that
if the IPv6 header of a packet is covered by authentication, then
that 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 authentication
computation. If the packet carries no IP authentication, then the
contents of the Home Address option, as well as the Source Address
field or any other field in the IPv6 header, may have been forged or
altered during transit.
Upon receipt of a packet containing a Home Address option, the
receiving node replaces the Source Address in the IPv6 header with
the Home Address in the Home Address option. 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. Security
issues related to the Home Address option are discussed further in
Section 13.
The three highest-order bits of the Option Type are encoded to
indicate specific processing of the option [5]. For the Home Address
option, these three bits are set to 110, indicating that any IPv6
node processing this option that does not recognize the Option Type
must discard the packet and, only 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; and that the data
within the option cannot change en-route to the packet's final
destination.
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5.5. Mobile IPv6 Destination Option Sub-Options
In order to allow optional fields that may not be needed in most uses
of any given Mobile IPv6 destination option, and to allow future
extensions to the format of these destination options to be defined,
any of the Mobile IPv6 destination options defined in this document
MAY include one or more sub-options.
Such sub-options are included in the data portion of the destination
option itself, after the fixed portion of the option data specified
for that particular destination option (Sections 5.1 through 5.4).
The presence of such sub-options will be indicated by the Option
Length field. When the Option Length is greater than the standard
length defined for that destination option, the remaining octets are
interpreted as sub-options.
These sub-options are encoded within the remaining space of the
option data for that option, using a type-length-value (TLV) format
as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type| Sub-Option Len| Sub-Option Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Option Type
8-bit identifier of the type of sub-option. In processing a
Mobile IPv6 destination option containing a sub-option for
which the Sub-Option Type value is not recognized by the
receiver, the receiver SHOULD quietly ignore and skip over the
sub-option, correctly handling any remaining sub-options in the
option.
Sub-Option Length
8-bit unsigned integer. Length of the Sub-Option Data field
of this sub-option, in octets. The Sub-Option Len does not
include the length of the Sub-Option Type and Sub-Option Len
fields.
Sub-Option Data
Variable-length field. Sub-Option-Type-specific data.
Each section above defining the Mobile IPv6 destination options
specifies which of the defined sub-options is valid for that
destination option.
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Currently, the following two sub-option types are defined for use in
Mobile IPv6 destination options:
Unique Identifier Sub-Option
This sub-option is valid only in Binding Request and Binding
Update destination options. The Sub-Option Data contains a
single 16-bit value that serves to uniquely identify a Binding
Request among those sent by this Source Address, and to allow
the Binding Update to identify the specific Binding Request to
which it responds. This matching of Binding Updates to Binding
Requests is required in the procedure for renumbering the home
subnet while a mobile node is away from home (Section 9.7).
The Sub-Option Type and Sub-Option Len fields for this
sub-option MUST be set as follows:
- Sub-Option Type: 1
- Sub-Option Len: 2
Home Agents List Sub-Option
This sub-option is valid only in a Binding Acknowledgement
destination option. The Sub-Option Data contains a list of
home agents on the home link for the mobile node to which
this Binding Acknowledgement is sent. This sub-option MUST
NOT be present unless the Binding Acknowledgement is sent
in response to a "Mobile IPv6 Home-Agents" anycast Binding
Update sent by a mobile node attempting dynamic home agent
address discovery [8]. In this case, the Status field in the
Binding Acknowledgement MUST be set to 135 (dynamic home agent
address discovery response). The specific construction of
the Sub-Option Data field for this sub-option is defined in
Section 9.2. The Sub-Option Type and Sub-Option Len fields for
this sub-option MUST be set as follows:
- Sub-Option Type: 2
- Sub-Option Len: 16 * N, where N is the number of home
agent addresses included in the Sub-Option Data.
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6. Modifications to IPv6 Neighbor Discovery
6.1. Modified Router Advertisement Message Format
Mobile IPv6 modifies the format of the Router Advertisement
message [14] by the addition of a single flag bit for use in the
dynamic home agent address discovery mechanism (Sections 9.2
and 10.6). The format of the Router Advertisement message is
as follows:
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 ...
+-+-+-+-+-+-+-+-+-+-+-+-
This format represents the following changes over that originally
specified for Neighbor Discovery [14]:
Home Agent (H)
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 IP home agent.
Reserved
Reduced from a 6-bit field to a 5-bit field to account for the
addition of the Home Agent (H) bit.
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6.2. Modified Prefix Information Option Format
Mobile IPv6 requires knowledge of a router's global address for two
reasons:
- To allow a home agent (a router) to learn the address of all
other home agents on the link for which it is providing home
agent service, for use in building its Home Agents List as
part of the dynamic home agent address discovery mechanism
(Sections 9.2 and 10.6).
- To allow a mobile node to send a Binding Update to its previous
default router, after moving to a new subnet and acquiring a new
care-of address (Section 10.8).
However, Neighbor Discovery [14] only advertises a router's
link-local address, by requiring this address to be used as the IP
Source Address of each Router Advertisement.
Mobile IPv6 extends Neighbor Discovery to allow a router to easily
and efficiently 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:
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 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This format represents the following changes over that originally
specified for Neighbor Discovery [14]:
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Router Address (R)
1-bit router address flag. When set, indicates that the
Prefix field, in addition to advertising the indicated prefix,
contains a complete IP address assigned to the sending router.
This 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 number Prefix bits specified by the Prefix Length
field. Interpretation of this flag bit is thus independent
of the processing required for the On-Link (L) and Autonomous
Address-Configuration (A) flag bits.
Reserved1
Reduced from a 6-bit field to a 5-bit field to account for the
addition of the Router Address (R) bit.
In a solicited Router Advertisement, a router MUST include at least
one Prefix Information option with the Router Address (R) bit set.
Neighbor Discovery specifies that, if 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 options [14]. In this case, at least one of these
multiple Advertisements being sent instead of a single larger
solicited Advertisement, MUST include a Prefix Information option
with the Router Address (R) bit set.
All routers SHOULD include at least on Prefix Information option with
the Router Address (R) bit set, in each unsolicited multicast Router
Advertisement that they send. If multiple Advertisements are being
sent instead of a single larger unsolicited multicast Advertisement,
at least one of these multiple Advertisements SHOULD include a Prefix
Information option with the Router Address (R) bit set.
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6.3. New Advertisement Interval Option Format
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:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
7
Length
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.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Advertisement Interval
32-bit unsigned integer. The maximum time, in milliseconds,
between successive unsolicited router Router Advertisement
messages sent by this router on this network interface. Using
the conceptual router configuration variables defined by
Neighbor Discovery [14], this field MUST be equal to the value
MaxRtrAdvInterval, expressed in milliseconds.
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 Section 10.3.
This option MUST be silently ignored for other Neighbor Discovery
messages.
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6.4. New Home Agent Information Option Format
Mobile IPv6 defines a new Home Agent Information option, used in
Router Advertisement messages 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:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
8
Length
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.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Home Agent Preference
16-bit signed, twos-complement integer. The preference for
the home agent sending this Router Advertisement, for use
in ordering the addresses contained in the Home Agents List
Sub-Option returned in a Binding Acknowledgement; 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 SHOULD be considered
to be 0. Values greater than 0 indicate a home agent more
preferable than this default value, and values less than 0
indicate a less preferable home agent. In addition to the
manual configuration of the Home Agent Preference value as
described in Section 7.3, the Home Agent Preference sent by
a home agent could, for example, be set dynamically by the
sending home agent based on the number of mobile nodes it is
currently serving or on its remaining resources for serving
additional mobile nodes, but such dynamic settings are beyond
the scope of this document.
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Home Agent Lifetime
16-bit unsigned integer. The lifetime associated with the home
agent in units of seconds. 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. 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 SHOULD be considered to be the
same as the Router Lifetime specified in the main body of the
Router Advertisement message.
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 (Section 6.1) is not set.
This option MUST be silently ignored for other Neighbor Discovery
messages.
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.
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6.5. Changes to Sending Router Advertisements
The Neighbor Discovery protocol specification [14] 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:
"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."
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.
Thus, to provide good support for mobile nodes, Mobile IPv6 relaxes
this limit such that routers MAY send unsolicited multicast Router
Advertisements more frequently. In particular, on network interfaces
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), the home agent SHOULD be configured
with a smaller MinRtrAdvInterval value and MaxRtrAdvInterval value,
to allow sending of unsolicited multicast Router Advertisements more
often. Recommended values for these limits are:
- MinRtrAdvInterval 0.5 seconds
- MaxRtrAdvInterval 1.5 seconds
Use of these modified limits MUST be configurable, and specific
knowledge of the type of network interface in use SHOULD be taken
into account in configuring these limits for each network interface.
When sending unsolicited multicast Router Advertisements more
frequently than the standard limit on unsolicited multicast
Advertisement frequency, the sending router need not include all
options in each of these Advertisements, but it SHOULD include at
least one Prefix Information option with the Router Address (R) bit
set (Section 6.2) in each.
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6.6. Changes to Sending Router Solicitations
In addition to the limit on routers sending unsolicited multicast
Router Advertisement messages (Section 6.5), Neighbor Discovery
defines limits on nodes sending Router Solicitation messages, such
that a node SHOULD send no more than 3 Router Solicitations, and that
these 3 transmissions SHOULD be spaced at least 4 seconds apart.
However, these limits prevent a mobile node from finding a new
default router (and thus a new care-of address) quickly as it moves
about.
Mobile IPv6 relaxes this limit such that, while a mobile node is away
from home, it MAY send Router Solicitations more frequently. The
following limits for sending Router Solicitations are recommended for
mobile nodes while away from home:
- A mobile node that is not configured with any current care-of
address (e.g., the mobile node has moved since its previous
care-of address was configured), MAY send more than the defined
Neighbor Discovery limit of MAX_RTR_SOLICITATIONS Router
Solicitations.
- The rate at which a mobile node sends Router Solicitations MUST
be limited, although a mobile node MAY send Router Solicitations
more frequently than the defined Neighbor Discovery limit of
RTR_SOLICITATION_INTERVAL seconds. The minimum interval MUST
be configurable, and specific knowledge of the type of network
interface in use SHOULD be taken into account in configuring this
limit for each network interface. A recommended minimum interval
is 1 second.
- After sending at most MAX_RTR_SOLICITATIONS Router Solicitations,
a mobile node MUST reduce the rate at which it sends subsequent
Router Solicitations. Subsequent Router Solicitations SHOULD
be sent using a binary exponential backoff mechanism, doubling
the interval between consecutive Router Solicitations, up to a
maximum interval. The maximum interval MUST be configurable and
SHOULD be chosen appropriately based on the characteristics of
the type of network interface in use.
- While still searching for a new default router and care-of
address, a mobile node MUST NOT increase the rate at which it
sends Router Solicitations unless it has received a positive
indication (such as from lower network layers) that it has moved
to a new link. After successfully acquiring a new care-of
address, the mobile node SHOULD also increase the rate at which
it will send Router Solicitations when it next begins searching
for a new default router and care-of address.
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- A mobile node that is currently configured with a care-of
address SHOULD NOT send Router Solicitations, until its movement
detection algorithm (Section 10.3) determines that it has moved
and that its current care-of address might no longer be valid.
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7. Requirements for IPv6 Nodes
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. Further details on this functionality is
provided in the following sections.
7.1. Requirements for All IPv6 Hosts and Routers
Since 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, the following requirements apply to ALL
IPv6 nodes (whether host or router, whether mobile or stationary):
- Every IPv6 node MUST be able to process a Home Address option
received in any IPv6 packet.
- Every IPv6 node SHOULD be able to process a Binding Update option
received in a packet, and to return a Binding Acknowledgement
option if the Acknowledge (A) bit is set in the received Binding
Update.
- Every IPv6 node SHOULD be able to maintain a Binding Cache of the
bindings received in accepted Binding Updates.
7.2. Requirements for All IPv6 Routers
The following requirements apply to all IPv6 routers, even those not
serving as a home agent for Mobile IPv6:
- Every IPv6 router SHOULD be able to send an Advertisement
Interval option in its Router Advertisements, to aid movement
detection by mobile nodes. The use of this option in Router
Advertisements MUST be configurable.
- Every IPv6 router SHOULD be able to support sending unsolicited
multicast Router Advertisements at the faster rate described in
Section 6.5. The use of this faster rate MUST be configurable.
7.3. Requirements for IPv6 Home Agents
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 capable of serving as a home
agent:
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- 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. Each such Binding Cache entry records the mobile node's
binding with its primary care-of address and is marked as a "home
registration".
- Every home agent MUST be able to intercept packets (using proxy
Neighbor Discovery) 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.
- Every home agent MUST be able to encapsulate 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.
- Every home agent MUST be able to return a Binding Acknowledgement
option in response to a Binding Update option received with the
Acknowledge (A) bit set.
- Every home agent MUST be able to accept packets addressed
to the "Mobile IPv6 Home-Agents" anycast address for the
subnet on which it is serving as a home agent [8], and MUST be
able to participate in dynamic home agent address discovery
(Section 9.2).
- 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.
7.4. Requirements for IPv6 Mobile Nodes
Finally, the following requirements apply to all IPv6 nodes capable
of functioning as mobile nodes:
- Every IPv6 mobile node MUST be able to perform IPv6
decapsulation [4].
- Every IPv6 mobile node MUST support sending Binding Update
options, as specified in Sections 10.5, 10.7, and 10.8; and MUST
be able to receive and process Binding Acknowledgement options,
as specified in Section 10.11.
- Every IPv6 mobile node MUST support use of the dynamic home agent
address discovery mechanism, as described in Section 10.6.
- Every IPv6 mobile node MUST maintain a Binding Update List in
which it records the IP address of each other node to which it
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has sent a Binding Update, for which the Lifetime sent in that
binding has not yet expired.
- Every IPv6 mobile node MUST support receiving a Binding Request
option, by responding with a Binding Update option.
- Every IPv6 mobile node MUST support sending packets containing a
Home Address option; this option MUST be included in all packets
sent while away from home, if the packet would otherwise have
been sent with the mobile node's home address as the IP Source
Address.
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8. Correspondent Node Operation
A correspondent node is any node communicating with a mobile node.
The correspondent node, itself, may be stationary or mobile, and may
possibly also be functioning as a home agent for Mobile IPv6. The
procedures in this section thus apply to all IPv6 nodes.
8.1. Receiving Packets from a Mobile Node
Packets sent by a mobile node while away from home generally include
a Home Address option. When any node receives a packet containing
a Home Address option, it MUST process the option in a manner
consistent with copying the Home Address field from the Home Address
option into the IPv6 header, replacing the original value of the
Source Address field there. However, any actual modifications to
the Source Address field in the packet's IPv6 header MUST not be
performed until after all processing of other options contained in
this same Destination Options extension header is completed.
Further processing of such a packet after option processing (e.g.,
at the transport layer) thus need not know that the original Source
Address was a care-of address, or that the Home Address option was
used in the packet. Since the sending mobile node uses its home
address at the transport layer when sending such a packet, the use of
the care-of address and Home Address option is transparent to both
the mobile node and the correspondent node above the level of the
Home Address option generation and processing.
8.2. Receiving Binding Updates
Upon receiving a Binding Update option in some packet, the receiving
node MUST validate the Binding Update according to the following
tests:
- The packet contains a valid AH [9] or ESP [10] header that
provides sender authentication, integrity protection, and replay
protection.
- The packet MUST contain a valid Home Address option. The home
address for the binding is specified by the Home Address field of
the Home Address option.
- The Option Length field in the Binding Update option is greater
than or equal to the length specified in Section 5.1.
- The Sequence Number field in the Binding Update option is greater
than the Sequence Number received in the previous Binding Update
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for this home address, if any. The Sequence Number comparison is
performed modulo 2**16.
Any Binding Update not satisfying all of these tests MUST be
silently ignored, and the packet carrying the Binding Update MUST be
discarded.
If the Binding Update is valid according to the tests above, then the
Binding Update is processed further as follows:
- If the Destination Address in the packet's IPv6 header is the
"Mobile IPv6 Home-Agents" anycast address [8] for an on-link
prefix and this address is assigned to one of this node's network
interfaces, then the mobile node sending this Binding Update is
attempting dynamic home agent address discovery. Processing for
this type of received Binding Update is described in Section 9.2.
(If the Destination Address is not assigned to one of this node's
network interfaces, then the packet would have been forwarded as
a normal packet and the Binding Update, as a destination option,
would not be processed in any way by this node.)
- If the Lifetime specified in the Binding Update is nonzero and
the specified Care-of Address is not equal to the home address
for the binding (as given in the Home Address option in the
packet), then this is a request to cache a binding for the
mobile node. If the Home Registration (H) bit is set in the
Binding Update, the Binding Update is processed according to the
procedure specified in Section 9.3; otherwise, it is processed
according to the procedure specified in Section 8.3.
- 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 mobile node's
cached binding. If the Home Registration (H) bit is set in the
Binding Update, the Binding Update is processed according to the
procedure specified in Section 9.4; otherwise, it is processed
according to the procedure specified in Section 8.4.
8.3. Requests to Cache a Binding
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 8.2. This section describes the processing of a valid
Binding Update that requests a node to cache a mobile node's binding,
for which the Home Registration (H) bit is not set in the Binding
Update.
In this case, the receiving node SHOULD create a new entry in its
Binding Cache for this mobile node (or update its existing Binding
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Cache entry for this mobile node, if such an entry already exists).
The home address of the mobile node is taken from the Home Address
field in the packet's Home Address option. The new Binding Cache
entry records the association between this home address and the
care-of address for the binding, as specified in either the Care-of
Address field of the Binding Update or in the Source Address field
in the packet's IPv6 header. Any Binding Cache entry created or
updated in response to processing this Binding Update MUST be deleted
after the expiration of the Lifetime period specified in the Binding
Update.
8.4. Requests to Delete a Binding
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 8.2. This section describes the processing of a valid
Binding Update that requests a node to delete a mobile node's binding
from its Binding Cache, for which the Home Registration (H) bit is
not set in the Binding Update.
In this case, the receiving node MUST delete any existing entry in
its Binding Cache for this mobile node. The home address of the
mobile node is taken from the Home Address field in the packet's Home
Address option.
8.5. Sending Binding Acknowledgements
When any node receives a packet containing a Binding Update option
in which the Acknowledge (A) bit is set, it SHOULD return a Binding
Acknowledgement option acknowledging receipt of the Binding Update.
If the node accepts the Binding Update and creates or updates an
entry in its Binding Cache for this binding, the Status field in
the Binding Acknowledgement MUST be set to a value less than 128;
if the node rejects the Binding Update and does not create or
update an entry for this binding, the Status field in the Binding
Acknowledgement MUST be set to a value greater than or equal to 128.
Specific values for the Status field are described in Section 5.2 and
in the most recent "Assigned Numbers" [22].
As described in Section 5.2, the packet in which the Binding
Acknowledgement is returned MUST include either an AH [9] or ESP [10]
header providing sender authentication, data integrity protection,
and replay protection; and the packet MUST be sent using a Routing
header in the same way as any other packet sent to a mobile node
using a care-of address (even if the binding was rejected), as
described in Section 8.9. The packet is routed first to the care-of
address contained in the Binding Update being acknowledged, and
then to the mobile node's home address. This use of the Routing
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header ensures that the Binding Acknowledgement will be routed to the
current location of the node sending the Binding Update, whether the
Binding Update was accepted or rejected.
8.6. Sending Binding Requests
Entries in a node's Binding Cache MUST be deleted when their lifetime
expires. If such an entry is still in active use in sending packets
to a mobile node, the next packet sent to the mobile node will be
routed normally to the mobile node's home link, where it will be
intercepted and tunneled to the mobile node. The mobile node will
then return a Binding Update to the sender, allowing it to create
a new Binding Cache entry for sending future packets to the mobile
node. Communication with the mobile node continues uninterrupted,
but the forwarding of this packet through the mobile node's home
agent creates additional overhead and latency in delivering packets
to the mobile node.
If the sender knows that the Binding Cache entry is still in active
use, it MAY send a Binding Request option to the mobile node in
an attempt to avoid this overhead and latency due to deleting and
recreating the Binding Cache entry. Since a Binding Request is a
destination option, it may, for example, be included in any packet
already being sent to the mobile node, such as a packet that is part
of ongoing TCP communication with the mobile node. When the mobile
node receives a packet from some sender containing a Binding Request
option, it returns a Binding Update option to that sender, giving its
current binding and a new lifetime.
8.7. Cache Replacement Policy
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. 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. When a Binding Cache entry's timer
expires, the node deletes the entry.
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, except that any entry marked as a "home
registration" (Section 9.3) MUST NOT be deleted from the cache until
the expiration of its lifetime period. When attempting to add a
new "home registration" entry in response to a Binding Update with
the Home Registration (H) bit set, if insufficient space exists (or
can be reclaimed) in the node's Binding Cache, the node MUST reject
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the Binding Update and SHOULD return a Binding Acknowledgement to
the sending mobile node, in which the Status field is set to 131
(insufficient resources). When otherwise attempting to add a new
entry to its Binding Cache, a node MAY, if needed, choose to drop any
entry already in its Binding Cache, other than a "home registration"
entry, in order to make space for the new entry. For example, a
"least-recently used" (LRU) strategy for cache entry replacement
among entries not marked as a "home registration" is likely to work
well.
Any binding dropped from a node's Binding Cache due to lack of cache
space will be rediscovered and a new cache entry created, if the
binding is still in active use by the node for sending packets. 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 normally,
leading to the mobile node's home link. There, the packet will be
intercepted by the mobile node's home agent and tunneled to the
mobile node's current primary care-of address. As when a Binding
Cache entry is initially created, this indirect routing to the mobile
node through its home agent will result in the mobile node sending
a Binding Update to this sending node when it receives the tunneled
packet, allowing it to add an entry again for this destination mobile
node to its Binding Cache.
8.8. Receiving ICMP Error Messages
When a correspondent node sends a packet to a mobile node, if the
correspondent node has a Binding Cache entry for the destination
address of the packet, then the correspondent node uses a Routing
header to deliver the packet to the mobile node through the care-of
address in the binding recorded in the Binding Cache entry. Any ICMP
error message caused by the packet on its way to the mobile node will
be returned normally to the correspondent node.
On the other hand, if the correspondent node has no Binding Cache
entry for the mobile node, the packet will be routed to the mobile
node's home link. There, it will be intercepted by the mobile node's
home agent, encapsulated, and tunneled to the mobile node's primary
care-of address. Any ICMP error message caused by the packet on
its way to the mobile node while in the tunnel, will be returned to
the mobile node's home agent (the source of the tunnel). By the
definition of IPv6 encapsulation [4], this encapsulating node MUST
relay certain ICMP error messages back to the original sender of the
packet, which in this case is the correspondent node.
Likewise, if a packet for a mobile node arrives at the mobile node's
previous default router (e.g., the mobile node moved after the packet
was sent), the router will encapsulate and tunnel the packet to the
mobile node's new care-of address (if it has a Binding Cache entry
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for the mobile node). As above, any ICMP error message caused by the
packet while in this tunnel will be returned to the previous default
router (the source of the tunnel), which MUST relay certain ICMP
error messages back to the correspondent node [4].
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. If the correspondent node subsequently transmits another
packet to the mobile node, the packet will be routed to the mobile
node's home link, intercepted by the mobile node's home agent, and
tunneled to the mobile node's primary care-of address using IPv6
encapsulation. The mobile node will then return a Binding Update to
the correspondent node, allowing it to recreate a (correct) Binding
Cache entry for the mobile node.
8.9. Sending Packets to a Mobile Node
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 Routing header to
route the packet to this mobile node (the destination node) by way
of the care-of address in the binding recorded in that Binding Cache
entry. For example, assuming use of a Type 0 Routing header [5], if
no other use of a Routing header is involved in the routing of this
packet, the mobile node sets the fields in the packet's IPv6 header
and Routing header as follows:
- The Destination Address in the packet's IPv6 header is set to
the mobile node's care-of address copied from the Binding Cache
entry.
- The Routing header is initialized to contain a single route
segment, with an Address of the mobile node's home address (the
original destination address to which the packet was being sent).
Following the definition of a Type 0 Routing header [5], 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). Normal processing of
the Routing header by the mobile node will then proceed as follows:
- The mobile node swaps the Destination Address in the packet's
IPv6 header and the Address specified in the Routing header.
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This results in the packet's IP Destination Address being set to
the mobile node's home address.
- The mobile node then resubmits the packet to its IPv6 module for
further processing, "looping back" the packet inside the mobile
node. Since the mobile node recognizes its own home address as
one of its current IP addresses, the packet is processed further
within the mobile node, in the same way then as if the mobile
node was at home.
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 (using
IPv6 encapsulation [4]) to the mobile node's current primary care-of
address, as described in Section 9.6. The mobile node will then send
a Binding Update to the sending node, as described in Section 10.7,
allowing the sending node to create a Binding Cache entry for its use
in sending subsequent packets to this mobile node.
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9. Home Agent Operation
9.1. Receiving Router Advertisement Messages
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, described in Section 9.2.
The information for the list is learned through receipt of the
periodic unsolicited multicast Router Advertisements from each other
home agent on the link, in which the Home Agent (H) bit is set, in a
manner similar to the Default Router List conceptual data structure
maintained by each host for Neighbor Discovery [14].
On receipt of a valid Router Advertisement, as defined in the
processing algorithm specified for Neighbor Discovery [14], the home
agent performs the following steps, in addition to any steps already
required of it by Neighbor Discovery:
- If the Home Agent (H) bit in the Router Advertisement is not set,
skip all of the following steps. There are no special processing
steps required by Mobile IP for this Router Advertisement, since
the Advertisement was not sent by a home agent.
- 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 Advertisement [14].
- Determine from the Router Advertisement 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 SHOULD be used.
- Determine from the Router Advertisement 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.
- 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.
- 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.
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- If the link-local address of the home agent sending this
Advertisement, as determined above, is not already present in
the Home Agents List maintained by the receiving home agent, and
the lifetime for the sending home agent, as determined above,
is non-zero, create a new entry in the list, and initialize its
lifetime and preference to the values determined above.
- 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
(Section 6.2). For each such global address determined from this
Advertisement, add this global address to the list of global
addresses for this home agent in this Home Agents List entry.
A home agent SHOULD maintain an entry in its Home Agents List for
each such valid home agent address until that entry's lifetime
expires, after which time the entry MUST be deleted.
9.2. Dynamic Home Agent Address Discovery
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 8.2. This section describes the processing of a valid
Binding Update that indicates that the mobile node sending it is
attempting dynamic home agent address discovery.
As described in Section 10.6, a mobile node attempts dynamic home
agent address discovery by sending its "home registration" Binding
Update to the "Mobile IPv6 Home-Agents" anycast address [8] for its
home IP subnet prefix (the packet MUST also include a Home Address
option). A home agent receiving such a Binding Update that is
serving this subnet (the home agent is configured with this anycast
address on one of its network interfaces) MUST reject the Binding
Update and SHOULD return a Binding Acknowledgement indicating this
rejection, with the Source Address of the packet carrying the Binding
Acknowledgement set to one of the global unicast addresses of the
home agent. The Status field in the Binding Acknowledgement MUST be
set to 135 (dynamic home agent address discovery response).
In this Binding Acknowledgement rejecting the dynamic home agent
address discovery Binding Update, this home agent SHOULD include a
Home Agents List Sub-Option as follows:
- The Home Agents List Sub-Option in this Binding Acknowledgement
SHOULD contain one global IP address for each home agent
currently listed in this home agent's own Home Agents List
(Section 4.3). However, if this home agent's own global IP
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address would be placed in the list (as described below) as the
first entry in the list, then this home agent SHOULD NOT include
its own address in the list in the sub-option in the Binding
Acknowledgement. Not placing this home agent's own IP address in
the list will cause the receiving mobile node to consider this
home agent as the most preferred home agent; otherwise, this home
agent will be considered to be preferred in its order given by
its place in the list returned.
- The IP addresses in the Home Agents List should be placed in
the Home Agents List Sub-Option in the Binding Acknowledgement
in order of decreasing preference value, 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). The home agent with the highest preference
SHOULD be listed first, and the home agent with the lowest
preference SHOULD be listed last.
- Among home agents with equal preference, their IP addresses in
the Home Agents List SHOULD be listed in an order randomized with
respect to other home agents with equal preference, each time a
Binding Acknowledgement with a Home Agents List Sub-Option is
returned by this home agent.
- For each entry in this home agent's Home Agents List, if more
than one global IP address is associated with this list entry,
then one of these global IP addresses SHOULD be selected to
include in the Home Agents List Sub-Option to be returned in the
Binding Acknowledgement. As described in Section 4.3, one Home
Agents List entry, identified by the home agent's link-local
address, exists for each home agent on the link; associated with
that list entry is one or more global IP addresses for this
home agent, learned through Prefix Information options with the
Router Address (R) bit is set, received in Router Advertisements
from this link-local address. The selected global IP address
for each home agent to include in forming the Home Agents List
Sub-Option to be returned in the Binding Acknowledgement MUST
be the global IP address of the respective home agent sharing a
prefix with the mobile node's home address for which the Binding
Acknowledgement is being returned; if no such global IP address
is known for some home agent, an entry for that home agent MUST
NOT be included in the Home Agents List Sub-Option returned in
the Binding Acknowledgement.
- In order to avoid the possibility of the packet carrying the
Binding Acknowledgement being fragmented, if the resulting
total packet size containing the complete Home Agents List
Sub-Option would exceed the minimum IPv6 MTU [5], the home agent
SHOULD reduce the number of home agent IP addresses returned
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in the packet to the number of addresses that will fit without
exceeding this limit. The home agent addresses returned in the
packet SHOULD be those from the complete list with the highest
preference.
The mobile node, upon receiving this Binding Acknowledgement, MAY
then resend its Binding Update to the home agent address given as the
IP Source Address of the packet carrying the Binding Acknowledgement
or to any of the unicast IP addresses listed in the Home Agents List
Sub-Option in the Acknowledgement. For example, the mobile node may
re-attempt its home registration with each of these home agents in
turn, by sending each a Binding Update and waiting for the matching
Binding Acknowledgement, until its registration is accepted by one
of these home agents. In trying each of the returned home agent
addresses, the mobile node SHOULD try each in the order listed in the
Home Agents List Sub-Option in the Binding Acknowledgement. If the
home agent identified by the Source Address field in the IP header
of the packet carrying the Binding Acknowledgement is not listed in
the Home Agents List Sub-Option, it SHOULD be tried before the first
address given in the list; otherwise, it SHOULD be tried in the in
its listed order.
9.3. Primary Care-of Address Registration
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 8.2. This section describes the processing of a valid
Binding Update that requests the receiving node to serve as its home
agent, registering its primary care-of address.
To begin processing the Binding Update, the home agent MUST perform
the following sequence of tests:
- If the node is not a router that implements home agent
functionality, then the node MUST reject the Binding Update and
SHOULD return a Binding Acknowledgement to the mobile node, in
which the Status field is set to 132 (home registration not
supported).
- 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 133 (not home subnet).
- Else, if the Prefix Length field is nonzero in the Binding Update
and this length differs from the length of the home agent's own
knowledge of the corresponding subnet prefix on the home link,
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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 136 (incorrect subnet prefix length).
- 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.
If the home agent does not reject the Binding Update as described
above, then it becomes the home agent for the mobile node. The new
home agent (the receiving node) MUST then create a new entry in its
Binding Cache for this mobile node (or update its existing Binding
Cache entry for this mobile node, if such an entry already exists)
The home address of the mobile node is taken from the Home Address
field in the packet's Home Address option. The care-of address for
this Binding Cache entry is taken from the Care-of Address field of
the Binding Update (if the Care-of Address Present (C) bit is set in
the Binding Update) or from the Source Address field in the packet's
IPv6 header (otherwise).
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 (Section 8.7) and MUST NOT be
removed from the Binding Cache until the expiration of the Lifetime
period.
In addition, the home agent MUST copy the Router (R) bit from the
Binding Update into the corresponding bit in this Binding Cache entry
for this mobile node.
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 for this prefix [14]. Furthermore,
if the Prefix Length field in the Binding Update is nonzero, then
the lifetime for the Binding Cache entry MUST NOT be greater than
the minimum remaining valid lifetime for all subnet prefixes on
the mobile node's home link. If the value of the Lifetime field
specified by the mobile node in its Binding Update is greater than
this prefix lifetime, the home agent MUST decrease the binding
lifetime to less than or equal to the prefix valid lifetime. The
home agent MAY further decrease the specified lifetime for the
binding, for example based on a local policy implemented by the home
agent. The resulting lifetime is stored by the home agent in the
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Binding Cache entry, and this Binding Cache entry MUST be deleted by
the home agent after the expiration of this lifetime.
The Prefix Length in the Binding Update MUST also be saved in the
Binding Cache entry.
If the Acknowledge (A) bit is set in the Binding Update (it SHOULD
be), then the home agent MUST return a Binding Acknowledgement to the
mobile node, constructed as follows:
- The Status field MUST be set to a value indicating success (the
value MUST be less than 128). The only currently defined success
Status value is 0, indicating simply that the Binding Update was
accepted.
- The Sequence Number field MUST be copied from the Sequence Number
given in the Binding Update.
- 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,
this lifetime MUST NOT be greater than the remaining valid
lifetime for the subnet prefix in the mobile node's home address.
- The Refresh field MUST be set to a value less than or equal to
the Lifetime value being returned in the Binding Update. If the
home agent stores the Binding Cache entry in nonvolatile storage
(that survives the crash or other failure of the home agent),
then the Refresh field SHOULD be set to the same value as the
Lifetime field; otherwise, the home agent MAY set the Refresh
field to a value less than the Lifetime field, to indicate that
the mobile node SHOULD attempt to refresh its home registration
at the indicated shorter interval (although the home agent will
still retain the registration for the Lifetime period, even if
the mobile node does not refresh its registration within the
Refresh period).
In addition, the home agent MUST follow the procedure defined in
Section 9.5 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.
9.4. Primary Care-of Address De-registration
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 8.2. 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.
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To begin processing the Binding Update, the home agent MUST perform
the following test:
- If the receiving node has no entry in its Binding Cache for this
mobile node that is marked as a "home registration", 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 137 (not home agent for this mobile node).
If the home agent does not reject the Binding Update as described
above, then it MUST delete any existing entry in its Binding Cache
for this mobile node.
If the Acknowledge (A) bit is set in the Binding Update (it SHOULD
be), then the home agent MUST return a Binding Acknowledgement to the
mobile node, constructed as follows:
- The Status field MUST be set to a value indicating success (the
value MUST be less than 128). The only currently defined success
Status value is 0, indicating simply that the Binding Update was
accepted.
- The Sequence Number field MUST be copied from the Sequence Number
given in the Binding Update.
- The Lifetime field MUST be set to zero.
- The Refresh field MUST be set to zero.
In addition, the home agent MUST stop intercepting packets on the
mobile node's home link addressed to the mobile node (Section 9.5).
9.5. Intercepting Packets for a Mobile Node
While a node is serving as the home agent for mobile node (while the
node has an entry in its Binding Cache for this mobile node that is
marked as a "home registration"), this node MUST attempt to intercept
packets on the mobile node's home link addressed to the mobile node,
and MUST tunnel each intercepted packet to the mobile node using
using IPv6 encapsulation [4].
In order to intercept such packets on the home link, when a node
becomes the home agent for some mobile node (it did not already
have a Binding Cache entry for this mobile node marked as a "home
registration"), then the home agent MUST multicast onto the home link
a "gratuitous" Neighbor Advertisement message [14] on behalf of the
mobile node. Specifically, the home agent performs the following
steps:
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- The home agent examines the value of the Prefix Length field
in the new "home registration" Binding Cache entry. If this
value is zero, the following step is carried out only for the
individual home address specified for this binding. If, instead,
this field is nonzero, then the following step is carried out
for each address for the mobile node formed from the interface
identifier in the mobile node's home address in this binding
(the remaining low-order bits in the address after the indicated
subnet prefix), together with each one of the subnet prefixes
currently considered by the home agent to be on-link (including
both the link-local and site-local prefix).
- For each specific IP address for the mobile node determined
in the first step above, the home agent multicasts onto the
home link (to the all-nodes multicast address) a Neighbor
Advertisement message [14] on behalf of the mobile node, to
advertise the home agent's own link-layer address for this IP
address. The Target Address in the Neighbor Advertisement
message MUST be set to this IP address for the mobile node, and
the Advertisement MUST include a Target Link-layer Address option
specifying the home agent's link-layer address. In addition,
the Router (R) bit in the Advertisement MUST be copied from the
corresponding bit in the home agent's Binding Cache entry for
the mobile node. The Solicited Flag (S) in the Advertisement
MUST NOT be set, since it was not solicited by any Neighbor
Solicitation message. 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.
Any node on the home link receiving one of the Neighbor Advertisement
messages described above will thus 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 for the
mobile node normally destined to this address instead to the mobile
node's home agent. Since multicasts on the local link (such as
Ethernet) are typically not guaranteed to be reliable, the home
agent MAY retransmit this Neighbor Advertisement message up to
MAX_ADVERT_REXMIT times to increase its 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 detect the link-layer address change for the mobile node's
home address, through use of Neighbor Unreachability Detection [14].
While a node is serving as a home agent for some mobile node (it
still has a "home registration" entry for this mobile node in its
Binding Cache), the home agent uses IPv6 Neighbor Discovery [14]
to intercept unicast packets on the home link addressed the mobile
node's home address. In order to intercept packets in this way,
the home agent MUST act as a proxy for this mobile node to reply to
any received Neighbor Solicitation messages for it. When a home
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agent receives a Neighbor Solicitation message, it MUST check if the
Target Address specified in the message matches the home address
of any mobile node for which it has a Binding Cache entry marked
as a "home registration". This check MUST include all possible
home addresses for the mobile node, based on the subnet prefixes
currently considered to be on-link by the home agent (including the
corresponding link-local address and site-local address), if the
Prefix Length in the Binding Cache entry for this mobile node (from
the Binding Update that created this Cache entry) is nonzero.
If such an entry exists in the home agent's Binding Cache, the home
agent MUST reply to the Neighbor Solicitation message with a Neighbor
Advertisement message, 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 copied from
the corresponding bit in the home agent's Binding Cache entry for the
mobile node. Acting as a proxy in this way allows other nodes on
the mobile node's home link to resolve the mobile node's IPv6 home
address, and allows the home agent to to defend these addresses on
the home link for Duplicate Address Detection [14].
9.6. Tunneling Intercepted Packets to a Mobile Node
For any packet sent to a mobile node from the mobile node's home
agent (for 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
Section 8.9 apply. The home agent (as a correspondent node) uses a
Routing header to route the packet to the mobile node by way of the
care-of address in the home agent's Binding Cache (the mobile node's
primary care-of address, in this case).
While the mobile node is away from home and this node is acting
as the mobile node's home agent, the home agent intercepts any
packets on the home link addressed to the mobile node's home address
(including addresses formed from other on-link prefixes, if the
Prefix Length field was nonzero in the Binding Update), as described
in Section 9.5. The home agent cannot use a Routing header to
forward these intercepted packets to the mobile node, since it cannot
modify the packet in flight without invalidating any existing IPv6
AH [9] or ESP [10] header present in the packet.
For forwarding each intercepted packet to the mobile node, the
home agent MUST tunnel the packet to the mobile node using IPv6
encapsulation [4]; the tunnel entry point node is the home agent,
and the tunnel exit point node is the primary care-of address as
registered with the home agent (which is an address of the mobile
node itself). When a home agent encapsulates an intercepted packet
for forwarding to the mobile node, the home agent sets the Source
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Address in the prepended 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 (using its primary care-of address), normal processing of
the tunnel header [4] will result in decapsulation and processing of
the original packet by the mobile node.
However, packets addressed to the mobile node's link-local address
MUST NOT be tunneled to the mobile node. Instead, such a packet 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). Packets addressed to
the mobile node's site-local address SHOULD be tunneled to the mobile
node by default, but this behavior MUST be configurable to disable
it; currently, the exact definition and semantics of a "site" and a
site-local address are undefined in IPv6, and this default behavior
might change at some point in the future.
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 and site-local addresses. Multicast
packets addressed to a multicast address with link-local scope [7],
to which the mobile node is subscribed, MUST NOT be tunneled
to the mobile node; such packets SHOULD be silently discarded
(after delivering to other local multicast recipients). Multicast
packets addressed to a multicast address with scope larger
than link-local but smaller than global (e.g., site-local and
organization-local) [7], to which the mobile node is subscribed,
SHOULD be tunneled to the mobile node by default, but this behavior
MUST be configurable to disable it; this default behavior might
change at some point in the future as the definition of these scopes
become better defined in IPv6.
9.7. Renumbering the Home Subnet
IPv6 provides mechanisms through Neighbor Discovery [14] and Address
Autoconfiguration [23] to aid in renumbering a subnet, such as when a
site switches to a new network service provider. In renumbering, new
prefixes and addresses can be introduced for the subnet and old ones
can be deprecated and removed. These mechanisms are defined to work
while all nodes using the old prefixes are at home, connected to the
link using these prefixes. Mobile IPv6 extends these mechanisms for
the case in which one or more mobile nodes using the old prefixes are
away from home while the renumbering takes place.
The IPv6 renumbering mechanisms are based on nodes on the link
receiving Prefix Information options in Router Advertisement messages
giving the valid lifetime and preferred lifetime for different
prefixes on the link [14]. Mobile IPv6 arranges to tunnel certain
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Router Advertisements giving "important" Prefix Information options
to mobile nodes while away from home. To avoid the need to tunnel
all Router Advertisements from the home link to a mobile node away
from home, those Router Advertisements that are tunneled to the
mobile node are retransmitted until acknowledged. To avoid possible
security attacks from forged Router Advertisements tunneled to
the mobile node, all such tunneled Router Advertisements must be
authenticated to the mobile node by its home agent using AH [9] or
ESP [10].
Specifically, a home agent serving some mobile node SHOULD construct
and tunnel to the mobile node a new Router Advertisement when any of
the following conditions occur:
- The preferred or valid lifetime for an existing prefix on the
home link is reduced.
- A new prefix is introduced on the home link.
- The state of the home agent's AdvManagedFlag flag [14] changes
from FALSE to TRUE or from TRUE to FALSE.
The home agent determines these conditions based on its own
configuration as a router and based on the Router Advertisements
that it receives on the home link. The home agent constructs a new
Router Advertisement message containing no options other than the
Prefix Information options describing the prefixes for which one of
the conditions above has occurred since the last Router Advertisement
tunneled to and acknowledged by the mobile node. When multiple
conditions occur at or near the same time, the home agent SHOULD
attempt to combine them into a single Router Advertisement message to
the mobile node.
In tunneling each such Router Advertisement to the mobile node, the
home agent MUST construct the packet as follows:
- 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.
- The packet MUST include either an AH [9] or ESP [10] header
providing sender authentication, data integrity protection, and
replay protection.
- The packet MUST include a Binding Request destination option.
- The Binding Request destination option MUST include a Unique
Identifier Sub-Option (Section 5.5), with the unique identifier
in the sub-option data set to a value different than that in
any other Binding Request sent recently by this node. The word
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"recently" here means within the maximum likely lifetime of a
packet, including transit time from source to destination and
time spent awaiting reassembly with other fragments of the same
packet, if fragmented. However, it is not required that a source
node know the maximum packet lifetime. Rather, it is assumed
that the requirement can be met by maintaining a simple 16-bit
"wrap-around" counter to generate unique identifiers for Binding
Requests that contain a Unique Identifier Sub-Option, incremented
each time a Binding Request containing a Unique Identifier
Sub-Option is sent.
- The packet MUST be tunneled to the mobile node's primary care-of
address using a Routing header, in the same way as any packet
sent to the mobile node originated by the home agent (rather than
using IPv6 encapsulation, as would be used by the home agent for
intercepted packets).
The home agent SHOULD periodically continue to retransmit this
tunneled packet to the mobile node, until it is acknowledged by
the receipt from the mobile node of a Binding Update matching
the Binding Request in the packet (i.e., with matching Sequence
Number). A Binding Update matches a Binding Request if it specifies
a binding for the mobile node to which the Binding Request was sent
and contains a Unique Identifier Sub-Option matching the unique
identifier sent in the Unique Identifier Sub-Option in the Binding
Request.
If while the home agent is still retransmitting a Router
Advertisement to the mobile node, another condition as described
above occurs on the home link causing another Router Advertisement
to be tunneled to the mobile node, the home agent SHOULD combine any
Prefix Information options in the unacknowledged Router Advertisement
into the new Router Advertisement and then begin retransmitting the
new Router Advertisement rather than the old one. When tunneling
a new Router Advertisement, even if it contains Prefix Information
options sent previously in an unacknowledged tunneled Router
Advertisement, the home agent MUST generate a new unique identifer
for use in the Unique Identifier Sub-Option in the Binding Request
tunneled with the new Router Advertisement.
In addition, as described in Section 9.3, the lifetime returned by a
mobile node's home agent in its Binding Acknowledgement in response
to registration of a new primary care-of address by the mobile node
MUST be no greater than the remaining valid lifetime for the subnet
prefix in the mobile node's home address. Furthermore, as described
in Section 10.7, 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. These limits
on a binding lifetimes ensure that no node uses a mobile node's home
address beyond the time that it becomes invalid. The mobile node
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SHOULD further limit the lifetimes that it sends on any Binding
Updates to be within the remaining preferred lifetime for the prefix
in its home address.
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10. Mobile Node Operation
10.1. Sending Packets While Away from Home
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:
- From the point of view of protocol layers and applications
above Mobile IP (e.g., transport protocols), the mobile node
will generally use its home address as the source of the packet
for most packets, even while away from home, since Mobile IP
is designed to make mobility transparent to such software.
Doing so also makes the node's mobility and the fact that it is
currently away from home transparent to the correspondent nodes
with which it communicates. 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 in this
way. 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. When sending such packets, Mobile IP will modify
the packet to move the home address into a Home Address option
and will set the IPv6 header's Source Address field to one of
the mobile node's care-of addresses; these modifications to
the packet are then reversed in the node receiving the packet,
restoring the mobile node's home address to be the packet's
Source Address before processing by higher protocol layers and
applications.
- For short-term communication, particularly for communication that
may easily be retried if it fails, the mobile node MAY choose
to directly use one of its care-of addresses as the source of
the packet, thus not requiring the use of a Home Address option
in the packet. An example of this type of communication might
be DNS queries sent by the mobile node [12, 13]. 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 be used in either the
query or its reply, and all packets can be routed normally,
directly between their source and destination without relying
on Mobile IP. If 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.
If the mobile node uses one of its care-of addresses as the source
of some packet while away from home, no special Mobile IP processing
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is required for sending this packet. The packet is simply addressed
and transmitted in the same way as any normal IPv6 packet, setting
the Source Address field in the packet's IPv6 header to this care-of
address.
On the other hand, if while away from home, the mobile node uses its
home address as the source of a packet from the point of view of
higher protocol layers or applications as described above, special
Mobile IP processing of this packet is required for the insertion of
the Home Address option. Specifically:
- Since Mobile IP is transparent to higher protocol layers (e.g.,
to TCP), the packet is initially constructed 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.
- If the mobile node is at home, no special Mobile IP processing
for this packet is required. The packet is sent normally and the
following additional steps are not performed.
- Likewise, if the Source Address field in the packet's IPv6 header
is not the mobile node's home address, no special Mobile IP
processing for this packet is required. The packet is sent
normally and the following additional steps are not performed.
- Otherwise, 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.
- 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 a care-of address with a subnet prefix that is on-link on the
network interface on which the mobile node will transmit the
packet.
This addition of the Home Address option to a packet MUST be
performed before outgoing IPsec processing, such as the addition of
an AH [9] or ESP [10] header to the packet, is performed. Likewise,
IPsec processing for a received packet containing a Home Address
option MUST be performed before the packet is possibly modified as
part of processing the Home Address option. 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 ingress
filtering [6].
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10.2. Receiving Packets While Away from Home
While away from home, a mobile node will receive packets addressed to
its home address, by one of three methods:
- Packets sent by a correspondent node that does not have a
Binding Cache entry for the mobile node, will be sent by the
correspondent node in the same way as any normal IP packet. Such
packets will then be intercepted by the mobile node's home agent,
encapsulated using IPv6 encapsulation [4], and tunneled to the
mobile node's primary care-of address.
- 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 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.
- Packets sent by a correspondent node that has a Binding Cache
entry for the mobile node that contains an out-of-date care-of
address for the mobile node, will be sent by the correspondent
node using a Routing header, as described above. If the mobile
node sent a Binding Update to its previous default router when
moving from this care-of address to another, and if the Binding
Cache entry that was created from this Binding Update is still
present in this router's Binding Cache, then such a packet
will be intercepted by this router, encapsulated using IPv6
encapsulation [4], and tunneled to the mobile node's primary
care-of address (registered with this router, acting as a home
agent for this out-of-date care-of address).
For packets received by either the first or last of these three
methods, the mobile node SHOULD send a Binding Update to the original
sender of the packet, as described in Section 10.7, subject to the
rate limiting defined in Section 10.10. The mobile node SHOULD
also process the received packet in the manner defined for IPv6
encapsulation [4], 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.
For packets received by the second method above (using a Routing
header), the mobile node SHOULD process the received packet in the
manner defined for the type of IPv6 Routing header used [5], which
will result in the packet being processed normally by upper-layer
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protocols within the mobile node, as if it had been addressed (only)
to the mobile node's home address.
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 [5]. If this is done for upper-layer protocol response
packets sent by a mobile node while away from home, the mobile
node SHOULD NOT include its own care-of address, which appears in
the Routing header of the received packet, in the reversed route
for the response packet. If the received Routing header contained
no additional hops (other than the mobile node's home address and
care-of address), then any upper-layer protocol response packet
SHOULD NOT include a Routing header.
10.3. Movement Detection
A mobile node MAY use any combination of mechanisms available to it
to detect when it has moved from one link to another. The primary
movement detection mechanism for Mobile IPv6 defined here uses the
facilities of IPv6 Neighbor Discovery, including Router Discovery and
Neighbor Unreachability Detection. The description here is based on
the conceptual model of the organization and data structures defined
by Neighbor Discovery [14].
Mobile nodes SHOULD use Router Discovery to discover new routers and
on-link subnet prefixes; a mobile node MAY send Router Solicitation
messages, or MAY wait for unsolicited (periodic) multicast Router
Advertisement messages, as specified for Router Discovery [14].
Based on received Router Advertisement messages, a mobile node (in
the same way as any other node) maintains an entry in its Default
Router List for each router, and an entry in its Prefix List for each
subnet prefix, that it currently considers to be on-link. Each entry
in these lists has an associated invalidation timer value (extracted
from the Router Advertisement) used to expire the entry when it
becomes invalid.
While away from home, a mobile node SHOULD select one router from
its Default Router List to use as its default router, and one subnet
prefix advertised by that router from its Prefix List to use as
the subnet prefix in its primary care-of address. A mobile node
MAY also have associated additional care-of addresses, using other
subnet prefixes from its Prefix List. The method by which a mobile
node selects and forms a care-of address from the available subnet
prefixes is described in Section 10.4. The mobile node registers
its primary care-of address with its home agent, as described in
Section 10.5.
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While a mobile node is away from home and using some router as its
default router, it is important for the mobile node to be able to
quickly detect when that router becomes unreachable, so that it can
switch to a new default router and to a new primary care-of address.
Since some links (notably wireless) do not necessarily work equally
well in both directions, it is likewise important for the mobile
node to detect when it becomes unreachable to packets sent from its
default router, so that the mobile node can take steps to ensure that
any correspondent nodes attempting to communicate with it can still
reach it through some other route.
To detect when its default router becomes unreachable, a mobile
node SHOULD use Neighbor Unreachability Detection. As specified in
Neighbor Discovery [14], while the mobile node is actively sending
packets to (or through) its default router, the mobile node can
detect that the router (as its neighbor) is still reachable either
through indications from upper layer protocols on the mobile node
that a connection is making "forward progress" (e.g., receipt of TCP
acknowledgements for new data transmitted), or through receipt of a
Neighbor Advertisement message from its default router in response
to an explicit Neighbor Solicitation messages to it. Note that
although this mechanism detects that the mobile node's default router
has become unreachable to the mobile node only while the mobile node
is actively sending packets to it, this is the only time that this
direction of reachability confirmation is needed. Confirmation
that the mobile node is still reachable from the router is handled
separately, as described below.
For a mobile node to detect when it has become unreachable from its
default router, the mobile node cannot efficiently rely on Neighbor
Unreachability Detection alone, since the network overhead would be
prohibitively high in many cases for a mobile node to continually
probe its default router with Neighbor Solicitation messages even
when it is not otherwise actively sending packets to it. Instead,
a mobile node SHOULD consider receipt of any IPv6 packets from its
current default router as an indication that it is still reachable
from the router. Both packets from the router's IP address and
(IPv6) packets from its link-layer address (e.g., those forwarded but
not originated by the router) SHOULD be considered.
Since the router SHOULD be sending periodic unsolicited multicast
Router Advertisement messages, the mobile node will have frequent
opportunity to check if it is still reachable from its default
router, even in the absence of other packets to it from the router.
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
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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. It is thus possible for the mobile node to implement
its own policy for determining the number of Advertisements from
its current default router it is willing to tolerate losing before
deciding to switch to a different router from which it may currently
be correctly receiving Advertisements.
On some types of network interfaces, the mobile node MAY also
supplement this monitoring of Router Advertisements, by setting its
network interface into "promiscuous" receive mode, so that it is able
to receive all packets on the link, including those not link-level
addressed to it (i.e., disabling link-level address filtering). The
mobile node will then be able to detect any packets sent by the
router, in order to to detect reachability from the router. This
use of promiscuous mode may be useful on very low bandwidth (e.g.,
wireless) links, but its use MUST be configurable on the mobile node.
If the above means do not provide indication that the mobile node is
still reachable from its current default router (i.e., the mobile
node receives no packets from the router for a period of time), then
the mobile node SHOULD attempt to actively probe the router with
Neighbor Solicitation messages, even if it is not otherwise actively
sending packets to the router. If it receives a solicited Neighbor
Advertisement message in response from the router, then the mobile
node can deduce that it is still reachable. It is expected that the
mobile node will in most cases be able to determine its reachability
from the router by listening for packets from the router as described
above, and thus, such extra Neighbor Solicitation probes should
rarely be necessary.
With some types of networks, it is possible that additional
indications about link-layer mobility can be obtained from
lower-layer protocol or device driver software within the mobile
node. However, a mobile node MUST NOT assume that all link-layer
mobility indications from lower layers indicate a movement of the
mobile node to a new link, such that the mobile node would need to
switch to a new default router and primary care-of address. For
example, movement of a mobile node from one cell to another in many
wireless LANs can be made transparent to the IP level through use of
a link-layer "roaming" protocol, as long as the different wireless
LAN cells all operate as part of the same IP link with the same
subnet prefix. Upon lower-layer indication of link-layer mobility,
the mobile node MAY send Router Solicitation messages to determine if
new routers (and new on-link subnet prefixes) are present on its new
link.
Such lower-layer information might also be useful to a mobile node in
deciding to switch its primary care-of address to one of the other
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care-of addresses it has formed from the on-link subnet prefixes
currently available through different routers from which the mobile
node is reachable. For example, a mobile node MAY use signal
strength or signal quality information (with suitable hysteresis) for
its link with the available routers to decide when to switch to a new
primary care-of address using that router rather than its current
default router (and current primary care-of address). Even though
the mobile node's current default router may still be reachable in
terms of Neighbor Unreachability Detection, the mobile node MAY use
such lower-layer information to determine that switching to a new
default router would provide a better connection.
10.4. Forming New Care-of Addresses
After detecting that it has moved from one link to another (i.e., its
current default router has become unreachable and it has discovered
a new default router), a mobile node SHOULD form a new primary
care-of address using one of the on-link subnet prefixes advertised
by the new router. A mobile node MAY form a new primary care-of
address at any time, except that it MUST NOT do so too frequently.
Specifically, a mobile node MUST NOT send a Binding Update about a
new care-of address to its home agent (which is required to register
the new address as its primary care-of address) more often than once
per MAX_UPDATE_RATE seconds.
In addition, after discovering a new on-link subnet prefix, a mobile
node MAY form a new (non-primary) care-of address using that subnet
prefix, 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 Section 10.15.
As described in Section 4, in order to form a new care-of address,
a mobile node MAY use either stateless [23] or stateful (e.g.,
DHCPv6 [2]) Address Autoconfiguration. If a mobile node needs to
send packets as part of the method of address autoconfiguration,
it MUST use an IPv6 link-local address rather than its own IPv6
home address as the Source Address in the IPv6 header of each such
autoconfiguration packet.
In some cases, a mobile node may already know a (constant) IPv6
address that has been assigned to it for its use only while
visiting a specific foreign link. For example, a mobile node may be
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statically configured with an IPv6 address assigned by the system
administrator of some foreign link, for its use while visiting that
link. If so, rather than using Address Autoconfiguration to form a
new care-of address using this subnet prefix, the mobile node MAY use
its own pre-assigned address as its care-of address on this link.
10.5. Sending Binding Updates to the Home Agent
After deciding to change its primary care-of address as described
in Sections 10.3 and 10.4, a mobile node MUST register this care-of
address with its home agent in order to make this its primary care-of
address. To do so, the mobile node sends a packet to its home agent
containing a Binding Update option, with the packet constructed as
follows:
- The Home Registration (H) bit MUST be set in the Binding Update.
- The Acknowledge (A) bit MUST be set in the Binding Update.
- The packet MUST contain a Home Address option, giving the mobile
node's home address for the binding.
- The care-of address for the binding MUST be used as the Source
Address in the packet's IPv6 header, or the Care-of Address
Present (C) bit MUST be set in the Binding Update and the care-of
address for the binding MUST be specified in the Care-of Address
field in the Binding Update.
- The Prefix Length field SHOULD be set to the length of the mobile
node's subnet prefix in its home address, to request the mobile
node's home agent to serve as a home agent for all home addresses
for the mobile node based on all on-link subnet prefixes on the
home link. Otherwise, this field MUST be set to zero.
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 Section 5.2, 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
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).
The Prefix Length field in the Binding Update allows the mobile node
to request its home agent to serve all home addresses for the mobile
node, as indicated by the interface identifier in the mobile node's
home address (the remaining low-order bits after the indicated subnet
prefix), together with each on-link subnet prefix on the home link.
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Until the lifetime of this registration expires, the home agent
considers itself the home agent for each such home address of the
mobile node. As the set of on-link subnet prefixes on the home link
changes over time, the home agent changes the set of home addresses
for this mobile node for which it is serving as the home agent.
If the mobile node has additional home addresses using a different
interface identifier, 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 (or set of
home addresses sharing an interface identifier). These additional
Binding Updates MUST each be sent as a separate packet, since each
MUST contain an AH [9] or ESP [10] header to authenticate the Binding
Update as coming from the home address being bound.
10.6. Dynamic Home Agent Address Discovery
It is possible that 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 Section 10.5, 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.
In this case, the mobile node SHOULD use the dynamic home agent
address discovery procedure to find the address of a suitable home
agent on its home link. To do so, the mobile node sends the packet,
as described above, with the Destination Address in the packet's
IPv6 header set to the "Mobile IPv6 Home-Agents" anycast address [8]
for its home subnet prefix. As described in Section 9.2, the
home agent on its home link that receives this Binding Update will
reject the Update, returning to the mobile node the home agent's own
global unicast IP address along with a list of the global unicast IP
addresses of each other home agent operating on the home link. The
mobile node SHOULD then retransmit its Binding Update to one of these
homes agent using the provided global unicast address; the mobile
node MAY re-attempt this home registration with each of these home
agents in turn, by sending each a Binding Update and waiting for the
matching Binding Acknowledgement, until its registration is accepted
by one of these home agents.
If the mobile node has a current registration with some home agent
on its home link (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., times out or is rejected), the mobile node SHOULD then
reattempt this registration with another home agent on its home link.
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If the mobile node knows of no other suitable home agent, then it MAY
attempt the dynamic home agent address discovery procedure described
above.
10.7. Sending Binding Updates to Correspondent Nodes
A mobile node MAY send a Binding Update to any correspondent node at
any time to allow the correspondent node to cache the mobile node's
current care-of address (subject to the rate limiting defined in
Section 10.10). 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 field in 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.
If set to one of the mobile node's current care-of addresses (the
care-of address given MAY differ from the mobile node's primary
care-of address), the Binding Update requests the correspondent node
to create or update an entry for the mobile node in the correspondent
node's Binding Cache to record this care-of address for use in
sending future packets to the mobile node. In this case, the
Lifetime value sent in the Binding Update MUST be no greater than
the remaining lifetime of the mobile node's home registration of its
primary care-of address at its home agent.
If, instead, the care-of address is set to the mobile node's home
address, the Binding Update requests the correspondent node to delete
any existing Binding Cache entry that it has for the mobile node.
A mobile node MAY set the care-of address differently for sending
Binding Updates to different correspondent nodes.
When sending any Binding Update, the mobile node MUST record in its
Binding Update List the following fields from the Binding Update:
- The IP address of the node to which the Binding Update was sent.
- The home address for which the Binding Update was sent (the value
in the Home Address option in the packet carrying the Binding
Update).
- The remaining lifetime of the binding, initialized from the
Lifetime field sent in the Binding Update.
The mobile node MUST retain in its Binding Update List information
about all Binding Updates sent, for which the lifetime of the binding
has not yet expired. However, when sending a Binding Update, if an
entry already exists in the mobile node's Binding Update List for
an earlier Binding Update sent to that same destination node, the
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existing Binding Update List entry is updated to reflect the new
Binding Update rather than creating a new Binding Update List entry.
In general, when a mobile node sends a Binding Update to its home
agent to register a new primary care-of address (as described in
Section 10.5), the mobile node will also send a Binding Update to
each other node for which an entry exists in the mobile node's
Binding Update List. Thus, other relevant nodes are generally kept
updated about the mobile node's binding and can send packets directly
to the mobile node using the mobile node's current care-of address.
The mobile node, however, need not send these Binding Updates
immediately after configuring a new care-of address. For example,
since the Binding Update is a destination option and can be included
in any packet sent by a mobile node, the mobile node MAY delay
sending a new Binding Update to any correspondent node for a
short period of time, in hopes that the needed Binding Update
can be included in some packet that the mobile node sends to that
correspondent node for some other reason (for example, as part of
some TCP connection in use). In this case, when sending a packet
to some correspondent node, the mobile node SHOULD check in its
Binding Update List to determine if a new Binding Update to this
correspondent node is needed, and SHOULD include the new Binding
Update in this packet as necessary.
In addition, when a mobile node receives a packet for which the
mobile node can deduce that the original sender of the packet has
no Binding Cache entry for the mobile node, or for which the mobile
node can deduce that the original sender of the packet has an
out-of-date care-of address for the mobile node in its Binding Cache,
the mobile node SHOULD return a Binding Update to the sender giving
its current care-of address (subject to the rate limiting defined
in Section 10.10). In particular, the mobile node SHOULD return a
Binding Update in response to receiving a packet that meets all of
the following tests:
- The packet was tunneled using IPv6 encapsulation.
- The Destination Address in the tunnel (outer) IPv6 header is
equal to any of the mobile node's care-of addresses.
- The Destination Address in the original (inner) IPv6 header
is equal to one of the mobile node's home addresses; or this
Destination Address is equal to one of the mobile node's previous
care-of addresses, if the mobile node has an entry in its Binding
Update List representing an unexpired Binding Update sent to
a previous default router for this previous care-of address
(Section 10.8).
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- The Source Address in the tunnel (outer) IPv6 header differs from
the Source Address in the original (inner) IPv6 header.
The destination address to which the Binding Update should be sent
in response to receiving a packet meeting all of the above tests is
the Source Address in the original (inner) IPv6 header of the packet.
The home address for which this Binding Update is sent should be the
Destination Address of the original (inner) packet.
Binding Updates sent to correspondent nodes are not generally
required to be acknowledged. However, if the mobile node wants
to be sure that its new care-of address has been entered into a
correspondent node's Binding Cache, the mobile node MAY request an
acknowledgement by setting the Acknowledge (A) bit in the Binding
Update. In this case, however, the mobile node SHOULD NOT continue
to retransmit the Binding Update once the retransmission timeout
period has reached MAX_BINDACK_TIMEOUT.
A mobile node MAY choose to keep its location private from certain
correspondent nodes, and thus need not send new Binding Updates to
those correspondents. A mobile node MAY also send a Binding Update
to such a correspondent node to instruct it to delete any existing
binding for the mobile node from its Binding Cache, as described in
Section 5.1. No other IPv6 nodes are authorized to send Binding
Updates on behalf of a mobile node.
10.8. Sending Binding Updates to the Previous Default Router
After switching to a new default router (and thus also changing its
primary care-of address), a mobile node MAY send a Binding Update to
its previous default router, giving its new care-of address. The
packet carrying the Binding Update MUST be addressed to the mobile
node's previous default router's global unicast address, learned
by the mobile node based on Prefix Information options received in
Router Advertisements from it in which the Router Address (R) bit is
set (Sections 4.3 and 6.2).
If the mobile node sends such a Binding Update, the home address
for the binding, specified in the Home Address option included in
the packet carrying this Binding Update, MUST be set the mobile
node's old primary care-of address (that it used while using this
default router), and the care-of address for the binding (either the
Source Address in the packet's IPv6 header or the Care-of Address
field in the Binding Update) MUST be set to the mobile node's new
primary care-of address. In addition, the Home Registration (H)
bit MUST also be set in this Binding Update, to request the mobile
node's previous default router to temporarily act as a home agent
for the mobile node's old primary care-of address. The previous
default router will thus tunnel packets for the mobile node to its
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new care-of address. All of the procedures defined for home agent
operation must be followed by this previous default router for this
registration. Note that the previous router does not necessarily
know the mobile node's (permanent) home address as part of this
registration.
10.9. Retransmitting Binding Updates
If, after sending a Binding Update in which the Acknowledge (A) bit
is set, a mobile node fails to receive a Binding Acknowledgement
within INITIAL_BINDACK_TIMEOUT seconds, the mobile node SHOULD
retransmit the Binding Update until a Binding Acknowledgement
is received. Such a retransmitted Binding Update MUST use the
same Sequence Number value as the original transmission. 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
Binding Acknowledgement or the timeout period reaches the value
MAX_BINDACK_TIMEOUT.
10.10. Rate Limiting for Sending Binding Updates
A mobile node MUST NOT send Binding Updates about the same binding
to any node more often than once per MAX_UPDATE_RATE seconds. After
sending MAX_FAST_UPDATES consecutive Binding Updates to a particular
node with the same care-of address, the mobile node SHOULD reduce
its rate of sending Binding Updates to that node, to the rate of
SLOW_UPDATE_RATE per second. The mobile node MAY continue to send
Binding Updates at this slower rate indefinitely, in hopes that the
node will eventually be able to process a Binding Update and begin
to route its packets directly to the mobile node at its new care-of
address.
10.11. Receiving Binding Acknowledgements
Upon receiving a packet carrying a Binding Acknowledgement, a mobile
node MUST validate the packet according to the following tests:
- The packet contains a valid AH [9] or ESP [10] header providing
sender authentication, data integrity protection, and replay
protection.
- The Option Length field in the option is greater than or equal to
11 octets.
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- The Sequence Number field matches the Sequence Number sent by the
mobile node to this destination address in an outstanding Binding
Update.
Any Binding Acknowledgement not satisfying all of these tests MUST be
silently ignored, although the remainder of the packet (i.e., other
options, extension headers, or payload) SHOULD be processed normally
according to any procedure defined for that part of the packet.
When a mobile node receives a packet carrying a valid Binding
Acknowledgement, the mobile node MUST examine the Status field as
follows:
- 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.
- 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 MUST delete the corresponding Binding Update List
entry (and MUST also stop retransmitting the Binding Update).
Optionally, the mobile node MAY then 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 Section 10.10. In particular, if the Status field
is equal to 135 (dynamic home agent address discovery response),
then the mobile node MAY reattempt its home registration with
the home agent address given in the Source Address field of the
packet carrying the Binding Acknowledgement or with any of the
home agent IP addresses listed in the Home Agents List Sub-Option
in the Binding Acknowledgement. If any of these addresses is not
a global unicast address or does not have a subnet prefix equal
to the mobile node's own subnet prefix, then that particular
address MUST be ignored and the mobile node MUST NOT reattempt
its home registration with that home agent.
10.12. Receiving Binding Requests
When a mobile node receives a packet containing a Binding Request,
it SHOULD return to the sender a packet containing a Binding Update.
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), except that this lifetime
MUST NOT exceed the remaining lifetime for the mobile node's primary
care-of address registration at its home agent. When sending this
Binding Update, the mobile node MUST update its Binding Update List
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in the same way as for any other Binding Update sent by the mobile
node.
Note, however, that the mobile node MAY choose to keep its current
binding private from the sender of the Binding Request. In this
case, the mobile node instead SHOULD returns a Binding Update to the
sender, in which the Lifetime field is set to zero and the care-of
address is set to the mobile node's home address.
If the Binding Request for which the Binding Update is being returned
contains a Unique Identifer Sub-Option, the Binding Update MUST also
include a Unique Identifier Sub-Option. The unique identifier in the
SUb-Option Data field of the Unique Identifier Sub-Option MUST be
copied from the unique identifier carried in the Binding Request.
10.13. Receiving ICMP Error Messages
The Option Type value for a Binding Update option specifies that
any node receiving this option that does not recognize the Option
Type SHOULD return an ICMP Parameter Problem, Code 2, message to
the sender of the packet containing the Binding Update option. If
a node sending a Binding Update receives such an ICMP error message
in response, it should record in its Binding Update List that future
Binding Updates should not be sent to this destination.
Likewise, although ALL IPv6 nodes (whether host or router, whether
mobile or stationary) MUST implement the ability to correctly process
received packets containing a Home Address option, all Option Type
values in IPv6 include a specification of the behavior that a node
receiving a packet containing this option performs if it does not
implement receipt of that type of option. For the Home Address
option, the Option Type value specifies that any node receiving
this option that does not recognize the Option Type SHOULD return
an ICMP Parameter Problem, Code 2, message to the sender of the
packet containing the Home Address option. If a mobile node receives
such an ICMP error message from some node indicating that it does
not recognize the mobile node's Home Address option, the mobile
node SHOULD log the error and then discard the ICMP message; this
error message indicates that the node to which the original packet
was addressed (the node returning the ICMP error message) does not
correctly implement this required part of the IPv6 protocol.
10.14. Receiving Tunneled Router Advertisements
Section 9.7 describes the operation of a home agent to support
renumbering a mobile node's home subnet while the mobile node is
away from home. The home agent tunnels certain Router Advertisement
messages to the mobile node while away from home, giving "important"
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Prefix Information options that describe changes in the prefixes in
use on the mobile node's home link.
When a mobile node receives a tunneled Router Advertisement, it MUST
validate it according to the following tests:
- The Source Address of the IP packet carrying the Router
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.
- The packet contains either an AH [9] or ESP [10] header providing
sender authentication, data integrity protection, and replay
protection.
- The packet contains a Binding Request destination option.
- The Binding Request option contains a Unique Identifier
Sub-Option.
Any received tunneled Router Advertisement not meeting all of these
tests MUST be silently discarded.
If a received tunneled Router Advertisement is not discarded
according to the tests listed above, the mobile node MUST process the
Router Advertisement as if it were connected to its home link [14].
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 communication by
the mobile node, in the same way as for nodes that are at home.
In addition, in processing the packet containing this Router
Advertisement, the mobile node SHOULD return to the home agent a
Binding Update in response to the Binding Request carried in the
packet. The correct formation of this Binding Update by the mobile
node and processing of it by the home agent will be viewed by the
home agent as an acknowledgement of this Router Advertisement,
confirming to it that this Router Advertisement was received by the
mobile node.
10.15. Using Multiple Care-of Addresses
As described in Section 10.4, 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. A
mobile node SHOULD select a primary care-of address from among those
care-of addresses it has formed using any of these subnet prefixes,
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based on the movement detection mechanism in use, as described in
Section 10.3. When the mobile node selects a new primary care-of
address, it MUST register it with its home agent by sending it a
Binding Update with the Home Registration (H) and Acknowledge (A)
bits set, as described in Section 10.5.
To assist with smooth handoffs, 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 Autoconfiguration [2], the mobile
node may not wish to release the address immediately upon switching
to a new primary care-of address.
10.16. Routing Multicast Packets
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. This section therefore describes the behavior of a mobile
node that is not on its home link.
In order to receive packets sent to some multicast group, a mobile
node must join that multicast group. One method by which a mobile
node MAY join the group is via a (local) multicast router on the
foreign link being visited. The mobile node SHOULD use its care-of
address sharing a subnet prefix with the multicast router, as
the source IPv6 address of its multicast group membership control
messages.
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 to its home agent, and the
home agent forwards multicast packets down the tunnel to the mobile
node.
A mobile node that wishes to send packets to a multicast group
also has two options: (1) send directly on the foreign link being
visited; or (2) send via a tunnel to its home agent. 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.
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10.17. Returning Home
A mobile node detects that it has returned to its home link through
the movement detection algorithm in use (Section 10.3), when the
mobile node detects that its home subnet prefix is again on-link.
The mobile node SHOULD then send a Binding Update to its home agent,
to instruct its home agent to no longer intercept or tunnel packets
for it. In this Binding Update, the mobile node MUST set the care-of
address for the binding (the Source Address field in the packet's
IPv6 header) to the mobile node's own home address. As with other
Binding Updates sent to register with its home agent, the mobile
node MUST set the Acknowledge (A) and Home Registration (H) bits,
and SHOULD retransmit the Binding Update until a matching Binding
Acknowledgement is received.
In addition, the mobile node MUST multicast onto the home link
(to the all-nodes multicast address) a Neighbor Advertisement
message [14], to advertise the mobile node's own link-layer address
for its own home address. The Target Address in this Neighbor
Advertisement message 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 message for each of its
home addresses, as defined by the current on-link prefixes, including
its link-local address and site-local address. The Solicited
Flag (S) in these Advertisements MUST NOT be set, since they were
not solicited by any Neighbor Solicitation message. 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.
Since multicasts on the local link (such as Ethernet) are typically
not guaranteed to be reliable, the mobile node MAY retransmit these
Neighbor Advertisement messages up to MAX_ADVERT_REXMIT 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
Neighbor Unreachability Detection [14].
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11. Constants
INITIAL_BINDACK_TIMEOUT 1 second
MAX_BINDACK_TIMEOUT 256 seconds
MAX_UPDATE_RATE once per second
SLOW_UPDATE_RATE once per 10 seconds
MAX_FAST_UPDATES 5 transmissions
MAX_ADVERT_REXMIT 3 transmissions
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12. IANA Considerations
This document defines four new types of IPv6 destination options,
each of which must be assigned an Option Type value:
- The Binding Update option, described in Section 5.1
- The Binding Acknowledgement option, described in Section 5.2
- The Binding Request option, described in Section 5.3
- The Home Address option, described in Section 5.4
In addition, this document defines two new Neighbor Discovery [14]
options, which must be assigned Option Type values within the option
numbering space for Neighbor Discovery messages:
- The Advertisement Interval option, described in Section 6.3.
- The Home Agent Information option, described in Section 6.4.
Finally, this document defines a new type of anycast address, which
must be assigned a reserved value for use with any subnet prefix to
define this anycast address on each subnet:
- The "Mobile IPv6 Home-Agents" anycast address [8], used in the
dynamic home agent address discovery procedure described in
Sections 9.2 and 10.6.
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13. Security Considerations
13.1. Binding Updates, Acknowledgements, and Requests
The Binding Update option described in this document will result
in packets addressed to a mobile node being delivered instead to
its care-of address. This ability to change the routing of these
packets could be a significant vulnerability if any packet containing
a Binding Update option was not authenticated. Such use of "remote
redirection", for instance as performed by the Binding Update option,
is widely understood to be a security problem in the current Internet
if not authenticated [1].
The Binding Acknowledgement option also requires authentication,
since, for example, an attacker could otherwise trick a mobile node
into believing a different outcome from a registration attempt with
its home agent.
No authentication is required for the Binding Request option, since
the use of this option does not modify or create any state in either
the sender or the receiver. The Binding Request option does open
some issues with binding privacy, but those issues can be dealt with
either through existing IPsec encryption mechanisms or through use of
firewalls.
The existing IPsec replay protection mechanisms allow a "replay
protection window" to support receiving packets out of order.
Although appropriate for many forms of communication, Binding Updates
MUST be applied only in the order sent. The Binding Update option
thus includes a Sequence Number field to provide this necessary
sequencing. The use of this Sequence Number together with IPsec
replay protection is similar in many ways, for example, to the the
sequence number in TCP. IPsec provides strong replay protection but
no ordering, and the sequence number provides ordering but need not
worry about replay protection such as through the sequence number
wrapping around.
13.2. Home Address Option
No special authentication of the Home Address option is required,
except that if the IPv6 header of a packet is covered by
authentication, then that 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
(Section 5.4), since it indicates that the option is included in the
authentication computation. 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, then any field in the
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IPv6 header, including the Source Address field, and any other parts
of the packet, including 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.
The use of the Home Address option allows packets sent by a
mobile node to pass normally through routers implementing ingress
filtering [6]. Since the care-of address used in the Source Address
field of the packet's IPv6 header is topologically correct for the
sending location of the mobile node, ingress filtering can trace the
location of the mobile node in the same way as can be done with any
sender when ingress filtering is in use.
However, if a node receiving a packet that includes a Home Address
option implements the processing of this option by physically
copying the Home Address field from the option into the IPv6 header,
replacing the Source Address field there, then the ability to
trace the true location of the sender is removed once this step
in the processing is performed. This diminishing of the power of
ingress filtering only occurs once the packet has been received at
its ultimate destination, and does not affect the capability of
ingress filtering while the packet is in transit. Furthermore, this
diminishing can be entirely eliminated by appropriate implementation
techniques in the receiving node. For example, the original contents
of the Source Address field (the sending care-of address) could be
saved elsewhere in memory with the packet, until all processing of
the packet is completed.
13.3. General Mobile Computing Issues
The mobile computing environment is potentially very different from
the ordinary computing environment. In many cases, mobile computers
will be connected to the network via wireless links. Such links
are particularly vulnerable to passive eavesdropping, active replay
attacks, and other active attacks. Furthermore, mobile computers
are more susceptible to loss or theft than stationary computers.
Any secrets such as authentication or encryption keys stored on the
mobile computer are thus subject to compromise in ways generally not
common in the non-mobile environment.
Users who have sensitive data that they do not wish others to have
access to should use additional mechanisms (such as encryption) to
provide privacy protection, but such mechanisms are beyond the scope
of this document. Users concerned about traffic analysis should
consider appropriate use of link encryption. If stronger location
privacy is desired, the mobile node can create a tunnel to its home
agent. Then, packets destined for correspondent nodes will appear
to emanate from the home subnet, and it may be more difficult to
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pinpoint the location of the mobile node. Such mechanisms are all
beyond the scope of this document.
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Changes from Previous Version of the Draft
This appendix briefly lists some of the major changes in this
draft relative to the previous version of this same draft,
draft-ietf-mobileip-ipv6-07.txt:
- Changed the listed Option Type codes for the destination options
and Type codes Router Advertisement options to their official
values, now that IPv6 has finally officially assigned these.
- Added a Router (R) bit in the Binding Update option format, for
use in "home registration" Binding Updates. This flag bit tells
the home agent that the mobile node is a router, so that the
home agent can set the corresponding bit correctly in all proxy
Neighbor Advertisements that it sends on behalf of this mobile
node. Also added a bit to save this flag in the definition of
the Binding Cache entry format, and defined how it is used in
sending proxy Neighbor Advertisements.
- Added a bit of text in a few places to clarify the meaning of
MAX_UPDATE_RATE.
- Added to Section 7.3 a requirement that 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.
- Added a suggestion in Section 6.4 that the Home Agent Preference
field in a Home Agent Information option being sent by a home
agent could be set dynamically by the sending home agent based
on factors such as the number of mobile nodes it is currently
serving or the remaining resources it has for serving additional
mobile nodes. However, the details of such dynamic settings
are beyond the scope of this document since they do not involve
protocol standardization issues.
- Normalized all references to the "Mobile IPv6 Home-Agents"
anycast address to use the full name of this specific anycast
address and to point to the new definition of reserved IPv6
subnet anycast addresses, RFC 2526.
- Updated all references to their most recent versions, including
changing old Internet-Draft references into new RFC references
where available. Changed remaining specific Internet-Draft
references to be generic "Work in progress" references.
- Updated the "Status of This Memo" section to the new official
version of this text.
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- Corrected a few minor typographical errors in places.
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Acknowledgements
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) Josh Broch
(Carnegie Mellon University), Thomas Narten (IBM), Erik Nordmark (Sun
Microsystems), Simon Nybroe (Telebit Communications), Patrice Romand
(Bull S.A.), Tom Soderlund (Nokia Research), and Jim Solomon (RedBack
Networks) for their detailed reviews of earlier versions of this
draft. Their suggestions have helped to improve both the design and
presentation of the protocol.
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References
[1] S. M. Bellovin. Security problems in the TCP/IP protocol suite.
ACM Computer Communications Review, 19(2), March 1989.
[2] Jim Bound and Charles Perkins. Dynamic Host Configuration
Protocol for IPv6 (DHCPv6), February 1999. Work in progress.
[3] Scott Bradner. Key words for use in RFCs to indicate
requirement levels. RFC 2119, March 1997.
[4] Alex Conta and Stephen Deering. Generic packet tunneling in
IPv6 specification. RFC 2473, December 1998.
[5] Stephen E. Deering and Robert M. Hinden. Internet Protocol
version 6 (IPv6) specification. RFC 2460, December 1998.
[6] Paul Ferguson and Daniel Senie. Network ingress filtering:
Defeating denial of service attacks which employ IP source
address spoofing. RFC 2267, January 1998.
[7] Robert M. Hinden and Stephen E. Deering. IP Version 6
addressing architecture. RFC 2373, July 1998.
[8] David B. Johnson and Stephen E. Deering. Reserved ipv6 subnet
anycast addresses. RFC 2526, March 1999.
[9] Stephen Kent and Randall Atkinson. IP Authentication header.
RFC 2402, November 1998.
[10] Stephen Kent and Randall Atkinson. IP Encapsulating Security
Payload (ESP). RFC 2406, November 1998.
[11] Stephen Kent and Randall Atkinson. Security architecture for
the Internet Protocol. RFC 2401, November 1998.
[12] P. Mockapetris. Domain Names -- concepts and facilities.
RFC 1034, November 1987.
[13] P. Mockapetris. Domain Names -- implementation and
specification. RFC 1035, November 1987.
[14] Thomas Narten, Erik Nordmark, and William Allen Simpson.
Neighbor Discovery for IP version 6 (IPv6). RFC 2461, December
1998.
[15] Charles Perkins. IP encapsulation within IP. RFC 2003, October
1996.
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[16] Charles Perkins, editor. IP mobility support. RFC 2002,
October 1996.
[17] Charles Perkins. Minimal encapsulation within IP. RFC 2004,
October 1996.
[18] Charles Perkins and David B. Johnson. Route optimization in
Mobile IP, February 1999. Work in progress.
[19] David C. Plummer. An Ethernet address resolution protocol:
Or converting network protocol addresses to 48.bit Ethernet
addresses for transmission on Ethernet hardware. RFC 826,
November 1982.
[20] J. B. Postel. User Datagram Protocol. RFC 768, August 1980.
[21] J. B. Postel, editor. Transmission Control Protocol. RFC 793,
September 1981.
[22] Joyce K. Reynolds and Jon Postel. Assigned numbers. RFC 1700,
October 1994.
[23] Susan Thomson and Thomas Narten. IPv6 stateless address
autoconfiguration. RFC 2462, December 1998.
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Chair's Address
The Working Group can be contacted via its current chairs:
Erik Nordmark
Sun Microsystems, Inc.
2550 Garcia Avenue
Mt. View, CA 94041
USA
Phone: +1 415 786-5166
Fax: +1 415 786-5896
E-mail: nordmark@sun.com
Basavaraj Patil
Nortel Networks, Inc.
2201 Lakeside Blvd.
Richardson, TX 75082-4399
USA
Phone: +1 972 684-1489
E-mail: bpatil@nortelnetworks.com
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Authors' Addresses
Questions about this document can also be directed to the authors:
David B. Johnson
Carnegie Mellon University
Computer Science Department
5000 Forbes Avenue
Pittsburgh, PA 15213-3891
USA
Phone: +1 412 268-7399
Fax: +1 412 268-5576
E-mail: dbj@cs.cmu.edu
Charles Perkins
Sun Microsystems, Inc.
Technology Development Group
Mail Stop MPK15-214
Room 2682
901 San Antonio Road
Palo Alto, CA 94303
USA
Phone: +1 415 786-6464
Fax: +1 415 786-6445
E-mail: cperkins@eng.sun.com
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