Mobile IP Working Group David B. Johnson
INTERNET-DRAFT Carnegie Mellon University
Charles Perkins
Sun Microsystems
30 July 1997
Mobility Support in IPv6
<draft-ietf-mobileip-ipv6-03.txt>
Status of This Memo
This document is a submission by the Mobile IP Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the Working Group mailing list at "mobile-ip@SmallWorks.COM".
Distribution of this memo is unlimited.
This document is an Internet-Draft. Internet-Drafts are working
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To view the entire list of current Internet-Drafts, please check
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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 packets destined for the mobile
node directly to it at this care-of address.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Terminology 2
2.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 2
2.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 3
2.3. Specification Language . . . . . . . . . . . . . . . . . 4
3. Overview of Mobile IPv6 Operation 6
3.1. Protocol Summary . . . . . . . . . . . . . . . . . . . . 6
3.2. Comparison with Mobile IP for IPv4 . . . . . . . . . . . 11
4. New IPv6 Destination Options 12
4.1. Binding Update Option . . . . . . . . . . . . . . . . . . 12
4.2. Binding Acknowledgement Option . . . . . . . . . . . . . 16
4.3. Binding Request Option . . . . . . . . . . . . . . . . . 20
4.4. Home Address Option . . . . . . . . . . . . . . . . . . . 21
5. Requirements for IPv6 Nodes 23
6. Correspondent Node Operation 25
6.1. Receiving Packets from a Mobile Node . . . . . . . . . . 25
6.2. Receiving Binding Updates . . . . . . . . . . . . . . . . 25
6.3. Requests to Cache a Binding . . . . . . . . . . . . . . . 26
6.4. Requests to Delete a Binding . . . . . . . . . . . . . . 27
6.5. Sending Binding Acknowledgements . . . . . . . . . . . . 27
6.6. Cache Replacement Policy . . . . . . . . . . . . . . . . 28
6.7. Receiving ICMP Error Messages . . . . . . . . . . . . . . 28
6.8. Sending Packets to a Mobile Node . . . . . . . . . . . . 29
7. Home Agent Operation 31
7.1. Dynamic Home Agent Address Discovery . . . . . . . . . . 31
7.2. Primary Care-of Address Registration . . . . . . . . . . 31
7.3. Primary Care-of Address De-registration . . . . . . . . . 33
7.4. Tunneling Intercepted Packets to a Mobile Node . . . . . 34
7.5. Renumbering the Home Subnet . . . . . . . . . . . . . . . 35
8. Mobile Node Operation 37
8.1. Sending Packets While Away from Home . . . . . . . . . . 37
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8.2. Movement Detection . . . . . . . . . . . . . . . . . . . 38
8.3. Forming New Care-of Addresses . . . . . . . . . . . . . . 40
8.4. Sending Binding Updates to the Home Agent . . . . . . . . 41
8.5. Sending Binding Updates to Correspondent Nodes . . . . . 42
8.6. Sending Binding Updates to the Previous Default Router . 45
8.7. Retransmitting Binding Updates . . . . . . . . . . . . . 45
8.8. Rate Limiting for Sending Binding Updates . . . . . . . . 46
8.9. Receiving Binding Acknowledgements . . . . . . . . . . . 46
8.10. Using Multiple Care-of Addresses . . . . . . . . . . . . 47
8.11. Returning Home . . . . . . . . . . . . . . . . . . . . . 47
9. Routing Multicast Packets 49
10. Constants 50
11. Security Considerations 51
11.1. Binding Updates, Acknowledgements, and Requests . . . . . 51
11.2. Home Address Options . . . . . . . . . . . . . . . . . . 51
11.3. General Mobile Computing Issues . . . . . . . . . . . . . 52
Appendix A. Changes from Previous Draft 53
Acknowledgements 54
References 55
Chair's Address 57
Authors' Addresses 58
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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 IPv6 subnet, since routing is based on the network prefix in a
packet's destination IP address. In order continue communication
in spite of its movement, a mobile node could change its IP address
each time it moves to a new IPv6 subnet, 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 IPv6 subnet to another without changing
the mobile node's IP address. A mobile node is always addressable
by its "home address", the IP address assigned to the mobile node
within its home IPv6 subnet. 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 subnet. The movement of a mobile node away from its home
subnet 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 amongst wireless transceivers,
each of which covers only a very small geographic area -- are
possibly more suited to other solutions. For example, as long as
node movement does not occur between link-level points of attachment
on different IPv6 subnets, link-layer mobility support offered by a
number of current wireless LAN products is 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 handoff, but such extensions are beyond the sope
of this document.
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2. Terminology
2.1. 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.
network prefix
A bit string that consists of some number of initial bits of an
IP address.
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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2.2. Mobile IPv6 Terms
home address
An IP address assigned to a mobile node within its home subnet.
The network prefix in a mobile node's home address is equal to
the network prefix of the home subnet.
home subnet
The IP subnet indicated by a mobile node's home address.
Standard IP routing mechanisms will deliver packets destined
for a mobile node's home address to its home subnet.
mobile node
A node that can change its link-level point of attachment from
one IP subnet 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 link-level connected to the same IP
subnet as it was previously. If a mobile node is not currently
link-level connected to its home subnet, 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
Any IP subnet other than the mobile node's home subnet.
home agent
A router on a mobile node's home subnet 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 subnet destined to the mobile node's home
address, encapsulates them, and tunnels them to the mobile
node's registered care-of address.
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care-of address
An IP address associated with a mobile node while visiting
a foreign subnet, which uses the network prefix of that
foreign subnet. Among the multiple care-of addresses that a
mobile node may have at a time (e.g., with different network
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.
2.3. Specification Language
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST
This word, or the adjective "REQUIRED", means that the
definition is an absolute requirement of the specification.
MUST NOT
This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD
This word, or the adjective "RECOMMENDED", means that there may
exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood
and carefully weighed before choosing a different course.
SHOULD NOT
This phrase means that there may exist valid reasons in
particular circumstances when the particular behavior is
acceptable or even useful, but the full implications should be
understood and the case carefully weighed before implementing
any behavior described with this label.
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MAY
This word, or the adjective "OPTIONAL", means that an item
is truly optional. For example, one vendor may choose to
include the item because a particular marketplace requires
it or because the vendor feels that it enhances the product,
while another vendor may omit the same item. An implementation
which does not include a particular option MUST be prepared to
interoperate with another implementation which does include the
option.
silently discard
The implementation discards the packet without further
processing, and without indicating an error to the sender. The
implementation SHOULD provide the capability of logging the
error, including the contents of the discarded packet, and
SHOULD record the event in a statistics counter.
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3. Overview of Mobile IPv6 Operation
3.1. Protocol Summary
A mobile node is always addressable by its home address, whether it
is currently attached to its home subnet or is away from home. While
a mobile node is at home, packets addressed to the mobile node's
home address are routed to it using conventional Internet routing
mechanisms in the same way as if the node were never mobile. Since
the network prefix of a mobile node's home address is equal to the
network prefix of its home subnet, packets addressed to it will be
routed to its home subnet.
While a mobile node is attached to some foreign subnet 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
subnet. The network prefix of a mobile node's care-of address is
equal to the network prefix of the foreign subnet being visited
by the mobile node; if the mobile node is link-level connected
to this foreign subnet 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 [16] or stateful (e.g., DHCPv6 [3])
address autoconfiguration, according to the methods of IPv6 Neighbor
Discovery [9], although other methods of acquiring a care-of address
are also possible.
While away from home, the mobile node registers one of its bindings
with a router in its home subnet, requesting this router to function
as the "home agent" for the mobile node. This binding registration
is done by the mobile node sending a packet with a "Binding Update"
destination option to the home agent, which replies by returning a
packet containing a "Binding Acknowledgement" destination option to
the mobile node. 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 on the home subnet, and tunnels each
intercepted packet to the mobile node's primary care-of address.
To tunnel each intercepted packet, the home agent encapsulates the
packet using IPv6 encapsulation [4], addressed to the mobile node's
primary care-of address.
The Binding Update and Binding Acknowledgement destination options,
together with a "Binding Request" destination option, are also used
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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. A node
communicating with a mobile node is referred to in this document as a
"correspondent node" of the mobile node.
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 [14] or UDP [13].
- 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 Destination Options header 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 physical source of the packet in the same way as packets
from non-mobile nodes. By also including the Home Address option,
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
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correspondent node as a result of receiving a Home Address option in
a packet.
In summary, 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 an IPv6 Authentication
header [1], providing sender authentication, data integrity
protection, and replay protection. The Binding Update option
is described in detail in Section 4.1.
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 an IPv6 Authentication
header [1], providing sender authentication, data integrity
protection, and replay protection. The Binding Acknowledgement
option is described in detail in Section 4.2.
Binding Request
A Binding Request option is used to request a mobile node
to send a Binding Update to the requesting node, 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 special
authentication is required for the Binding Request option. The
Binding Request option is described in detail in Section 4.3.
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
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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 described in detail in Section 4.4.
Extensions to the format of these options may be included after the
fixed portion of the option data specified in this document. The
presence of such extensions 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 extensions. Currently, no extensions have been
defined.
This document describes the Mobile IPv6 protocol in terms of the
following two conceptual data structures used in the maintenance of
cached bindings:
Binding Cache
A cache, maintained by each IPv6 node, of bindings for other
nodes. 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, keyed by the mobile node's 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. 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 through Neighbor Discovery [9].
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 of the binding 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. Each Binding Update List entry records
the IP address of the node to which the Update was sent, the
home address for which one Binding Update was sent, and the
remaining lifetime of that binding. The Binding Update List
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MAY be implemented in any manner consistent with the external
behavior described in this document.
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.
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 IP subnet to another. If each wireless
subnet is connected to the Internet through a separate base station,
such that the wireless transmission range from the two base stations
overlap, the mobile node may be able to remain link-level connected
within both subnets while in the area of overlap. In this case, the
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mobile node could acquire a new care-of address in the new subnet
before moving out of transmission range and link-level disconnecting
from the old subnet. 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 in
the new subnet.
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
subnet. In addition, the impact of of any possible failure of the
home agent, the home subnet, or intervening networks leading to or
from the home subnet is reduced, since these nodes and links are not
involved in the delivery of most packets to the mobile node.
3.2. Comparison with Mobile IP for IPv4
[This section will include a comparison between the Mobile IPv6
protocol and the Mobile IPv4 protocol [11, 10, 12]. However, this
comparison has not yet been written. It will be filled in with the
next revsion to this draft.]
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4. New IPv6 Destination Options
4.1. Binding Update Option
The Binding Update destination option is used by a mobile node to
notify a correspondent node or the mobile node's home agent of a new
care-of address.
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|L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Care-of Address +
| (only present if C bit set) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Link-Local Address +
| (only present if L bit set) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
192 ???
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. For the
current definition of the Binding Update option, the minimum
value for this field is 6, for the case in which neither the
Care-of Address Present (C) bit nor the Home Link-Local Address
Present (L) bit are set, and the maximum value is 38, for the
case in which both of these bits are set.
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Acknowledge (A)
The Acknowledge (A) bit is set by the sending node to request a
Binding Acknowledgement (Section 4.2) be returned upon receipt
of the Binding Update option.
Home Registration (H)
The Home Registration (H) bit is set by the sending node to
request the receiving node to act as this node's home agent.
The Destination Address in the IP header of the packet carrying
this option MUST be that of a router sharing the same network
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 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.
Home Link-Local Address Present (L)
The Home Link-Local Address Present (L) bit indicates the
presence of the Home Link-Local Address field in the Binding
Update. This bit is set by the sending node to request
the receiving node to act as a proxy (for participating in
the Neighbor Discovery Protocol) for the node while it is
away from home. This bit MUST NOT be set unless the Home
Registration (H) bit is also set in the Binding Update.
Reserved
Sent as 0; ignored on reception.
Lifetime
16-bit unsigned integer. The number of seconds remaining
before the binding must be considered expired. A value of all
ones (0xffff) indicates infinity. A value of zero indicates
that the Binding Cache entry for the mobile node should be
deleted.
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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). There is no
requirement, however, that the Sequence Number value strictly
increase by 1 with each new Binding Update sent.
Care-of Address
This field in the Binding Update is optional and is only
present when the Care-of Address Present (L) 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.
Home Link-Local Address
This field in the Binding Update is optional and is only
present when the Home Link-Local Address Present (L) bit is
set. If present, it gives the link-local address of the mobile
node used by the mobile node when it was last attached to its
home subnet.
Any packet including 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 include an IPv6
Authentication header [1] in order to protect against forged Binding
Updates. The authentication MUST provide 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 should 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 should
be deleted. In each of these cases, no Binding Cache entry for the
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mobile node should be created in response to receiving the Binding
Update.
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 the
data within the option cannot change en-route to the packet's final
destination, and 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.
Extensions to the Binding Update option format may be included after
the fixed portion of the Binding Update option specified above. The
presence of such extensions will be indicated by the Option Length
field. When the Option Length is greater than the length defined
above, depending on the state of the Care-of Address Present (C)
and Home Link-Local Address Present (L) bits, the remaining octets
are interpreted as extensions. Currently, no extensions have been
defined.
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4.2. Binding Acknowledgement Option
The Binding Acknowledgement destination option is used to acknowledge
receipt of a Binding Update option (Section 4.1). When a node
receives a packet containing a Binding Update option, with this
node being the destination node of the packet, this node MUST
return a Binding Acknowledgement to the source of the packet, if the
Acknowledge (A) bit is set in the Binding Update.
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 | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Refresh | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
193 ???
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. For the
current definition of the Binding Acknowledgement option, this
field MUST be set to 9.
Status
8-bit unsigned integer indicating the disposition of the
Binding Update. Values of the Status field less than 128
indicate that the Binding Update was accepted by the receiving
node. 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
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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
Up-to-date values of the Status field are to be specified in
the most recent "Assigned Numbers" [15].
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, in order to
extend the lifetime.
Refresh
The recommended period 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
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 the mobile node's binding 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.
Sequence Number
The Sequence Number in the Binding Acknowledgement is copied
from the Sequence Number field in the Binding Update option,
for use by the mobile node in matching this Acknowledgement
with an outstanding Binding Update.
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Any packet that includes a Binding Acknowledgement option MUST
include an IPv6 Authentication header [1] in order to protect
against forged Binding Acknowledgements. The authentication MUST
provide 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 6.8.
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
Binding Acknowledgement, as described in Section 6.8, 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.
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 110, indicating
that the data within the option cannot change en-route to the
packet's final destination, and 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.
Extensions to the Binding Acknowledgement option format may be
included after the fixed portion of the Binding Acknowledgement
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option specified above. The presence of such extensions will be
indicated by the Option Length field. When the Option Length is
greater than 8 octets, the remaining octets are interpreted as
extensions. Currently, no extensions have been defined.
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4.3. Binding Request Option
The Binding Request destination option is used to request a mobile
node's binding from the mobile node. When a mobile node receives
a packet containing a Binding Request option, it SHOULD return a
Binding Update (Section 4.1) 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
194 ???
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. For the
current definition of the Binding Request option, this field
MUST be set to 0.
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 110, indicating that the
data within the option cannot change en-route to the packet's final
destination, and 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.
Extensions to the Binding Request option format may be included after
the fixed portion of the Binding Request option specified above.
The presence of such extensions will be indicated by the Option
Length field. When the Option Length is greater than 0 octets,
the remaining octets are interpreted as extensions. Currently, no
extensions have been defined.
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4.4. Home Address Option
The Home Address destination 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.
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 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
195 ???
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. For the
current definition of the Home Address option, this field MUST
be set to 8.
Home Address
The home address of the mobile node sending the packet.
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
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receiving a Home Address option in a packet. In particular, the
receipt of a packet containing a Home Address option MUST NOT alter
the contents of the receiver's Binding Cache due to the presence of
the Home Address option, and the mapping between the home address
and care-of address indicated by the Home Address option MUST NOT be
used as a basis for routing subsequent packets sent by this receiving
node.
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. 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 11.
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 the data
within the option cannot change en-route to the packet's final
destination, and 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.
Extensions to the Home Address option format may be included after
the fixed portion of the Home Address option specified above.
The presence of such extensions will be indicated by the Option
Length field. When the Option Length is greater than 8 octets,
the remaining octets are interpreted as extensions. Currently, no
extensions have been defined.
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5. Requirements for IPv6 Nodes
Mobile IPv6 places some special requirements on the functions
provided by different 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.
Since any IPv6 node may at any time be a correspondent node of a
mobile node, the following requirements pertain to all IPv6 nodes:
- Every IPv6 node MUST be able to process a Home Address option
received in a 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 requested.
- Every IPv6 node SHOULD be able to maintain a Binding Cache of the
bindings received in accepted Binding Updates.
In order for a mobile node to operate correctly while away from
home, at least one IPv6 router in the mobile node's home subnet must
function as a home agent for the mobile node. The following special
requirements pertain to all IPv6 routers capable of serving as a home
agent:
- 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) on the local subnet addressed to a mobile
node for which it is currently serving as the home agent while
that 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.
- Every home agent MUST be able to return a Binding Acknowledgement
in response to a Binding Update received with the Acknowledge (A)
bit set.
- Every home agent MUST be able to accept packets addressed to the
Home-Agents anycast address for the subnet on which it is serving
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as a home agent, and MUST be able to participate in dynamic home
agent address discovery.
Finally, the following requirements pertain 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 Updates, as
specified in Sections 8.4, 8.5, and 8.6; and MUST be able to
receive and process Binding Acknowledgements, as specified in
Section 8.9.
- Every IPv6 mobile node MUST maintain a Binding Update List in
which it records the IP address of each other node to which it
has sent a Binding Update, for which the Lifetime sent in that
binding has not yet expired.
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6. Correspondent Node Operation
A correspondent node is any node communicating with a mobile node.
The correspondent node, itself, may be fixed 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.
6.1. Receiving Packets from a Mobile Node
Packets sent by a mobile node while away from home generally include
a Home Address option. When a 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. Further processing of the packet (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 thus transparent to
both the mobile node and the correspondent node above the level of
the Home Address option generation and processing.
6.2. 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 an IP Authentication header and the
authentication is valid [1]. The Authentication header MUST
provide sender authentication, integrity protection, and replay
protection.
- The Option Length field in the Binding Update option is greater
than or equal to the length specified in Section 4.1.
- The Sequence Number field in the Binding Update option is greater
than the Sequence Number received in the previous Binding Update
for this home address, if any. The Sequence Number comparison is
performed modulo 2**16.
- 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.
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Any Binding Update 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.
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
Home-Agents anycast address for a local subnet 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 7.1. (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, then this is a request to cache a binding for
the mobile node. Processing for this type of received Binding
Update is described in Section 6.3.
- 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. Processing for this type of received Binding
Update is described in Section 6.4.
6.3. Requests to Cache a Binding
If a node receives a valid Binding Update requesting it to cache a
binding for a mobile node, as specified in Section 6.2, then the node
MUST examine the Home Registration (H) bit in the Binding Update
to determine how to further process the Binding Update. If the
Home Registration (H) bit is set, the Binding Update is processed
according to the procedure specified in Section 7.2.
If the Home Registration (H) bit is not set, then the receiving node
SHOULD 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 new Binding Cache entry records the association between
this address and the care-of address for the binding, as specified
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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.
6.4. Requests to Delete a Binding
If a node receives a valid Binding Update requesting it to delete a
cached binding for a mobile node, as specified in Section 6.2, then
the node MUST examine the Home Registration (H) bit in the Binding
Update to determine how to further process the Binding Update. If
the Home Registration (H) bit is set, the Binding Update is processed
according to the procedure specified in Section 7.3.
If the Home Registration (H) bit is not set, then 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.
6.5. 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 4.2 and
in the most recent "Assigned Numbers" [15].
As described in Section 4.2, the packet in which the Binding
Acknowledgement is returned MUST include an IPv6 Authentication
header [1] in order to protect against forged Binding
Acknowledgements, 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 not accepted), as
described in Section 6.8. 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
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.
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6.6. 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 was 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 7.2) 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
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 subnet. 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 to its
Binding Cache.
6.7. 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
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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 subnet, where 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
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 Host Unreachable or Network Unreachable error
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 subnet, 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.
6.8. 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
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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.
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. Since the mobile node recognizes its own
home address as one if 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 7.4. The mobile node will then send
a Binding Update to the sending node, as described in Section 8.5,
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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7. Home Agent Operation
7.1. Dynamic Home Agent Address Discovery
If a received Binding Update indicates that the mobile node sending
it is attempting dynamic home agent address discovery, as described
in Section 6.2, then the receiving node MUST process the Binding
Update as specified in this section.
A mobile node attempts dynamic home agent address discovery by
sending its "home registration" Binding Update to the Home-Agents
anycast address for its home IP subnet (the packet MUST also include
a Home Address option, as described in Section 8.4). 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 and giving its
unicast address. The Status field in the Binding Acknowledgement
MUST be set to 135 (dynamic home agent address discovery response).
The mobile node, upon receiving this Binding Acknowledgement, MAY
then resend its Binding Update to the unicast home agent address
given in the Acknowledgement.
7.2. Primary Care-of Address Registration
General processing of a received Binding Update that requests a
binding to be cached, is described in Section 6.3. However, if the
Home Registration (H) bit is set in the Binding Update, then the
receiving node MUST process the Binding Update as specified in this
section, rather than following the general procedure specified in
Section 6.3.
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).
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- 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 or
update the existing entry in its Binding Cache for this mobile node's
home address, as described in Section 6.3. In addition, 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 6.6) and MUST NOT be removed from the Binding
Cache until the expiration of the Lifetime period.
If the home agent was not already serving as a home agent for this
mobile node (the home agent did not already have a Binding Cache
entry for this home address marked as a "home registration"), then
the home agent MUST multicast onto the home subnet (to the all-nodes
multicast address) a Neighbor Advertisement message [9] on behalf
of the mobile node, to advertise the home agent's own link-layer
address for the mobile node's home IP address. The Target Address in
the 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 home agent's link-layer address. 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 subnet receiving this Neighbor Advertisement
message will thus update its Neighbor Cache to associate the mobile
node's home address with the home agent's link layer address, causing
it to transmit any future packets for the mobile node 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 subnet 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 [9].
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In addition, while this node is serving as a home agent for this
mobile node (it still has a "home registration" entry for this mobile
node in its Binding Cache), it MUST act as a proxy for this mobile
node to reply to any received Neighbor Solicitation messages for
it. When a home 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". If such an entry exists
in its 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. Likewise, if the mobile node included
its home link-local address and set the Home Link-Local Address
Present (L) bit in its Binding Update with which it established
this "home registration" with its home agent, its home agent MUST
also similarly act as a proxy for the mobile node's home link-local
address while it has this "home registration" entry in its Binding
Cache. Acting as a proxy in this way allows other nodes on the
mobile node's home subnet to resolve the mobile node's IPv6 home
address and IPv6 link-local address, and allows the home agent to
to defend these addresses on the home subnet for Duplicate Address
Detection [9].
7.3. Primary Care-of Address De-registration
General processing of a received Binding Update that requests a
binding to be deleted, is described in Section 6.4. However, if the
Home Registration (H) bit is set in the Binding Update, then the
receiving node MUST process the Binding Update as specified in this
section, rather than following the general procedure specified in
Section 6.4.
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 it 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).
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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.
In addition, in this case, the home agent MUST multicast a Neighbor
Advertisement message (to the all-nodes multicast address), giving
the mobile node's home address as the Target Address, and specifying
the mobile node's link-layer address in a Target Link-layer
Address option in the Neighbor Advertisement message. The home
agent MAY retransmit this Neighbor Advertisement message up to
MAX_ADVERT_REXMIT times to increase its reliability; any nodes on the
home subnet that miss all of these Neighbor Advertisements can also
eventually detect the link-layer address change for the mobile node's
home address, through use of Neighbor Unreachability Detection [9].
7.4. 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 6.8 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).
In addition, 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 subnet addressed to the mobile node's
home address, as described in Section 7.2. 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 Authentication header present in the
packet [1].
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 mobile node itself (using its
primary care-of address as registered with the home agent). When a
home agent encapsulates an intercepted packet for forwarding to the
mobile node, the home agent sets the Source 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]
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will result in decapsulation and processing of the original packet by
the mobile node.
7.5. Renumbering the Home Subnet
Neighbor Discovery [9] specifies a mechanism by which all nodes on a
subnet can gracefully autoconfigure new addresses, say by each node
combining a new routing prefix with its existing link-layer address.
As currently specified, this mechanism works when the nodes are on
the same link as the router issuing the necessary multicast packets
to advertise the new routing prefix(es) appropriate for the link.
However, for mobile nodes away from home, special care must be taken
to allow the mobile nodes to renumber gracefully. The most direct
method of ensuring this is for the home agent to encapsulate and
tunnel the multicast packets to the primary care-of address of each
mobile node for which it is serving as the home agent. The rules for
this are as follows:
- A mobile node assumes that its routing prefix has not changed
unless it receives an authenticated Router Advertisement message
from its home agent that the prefix has changed.
- When the mobile node is at home, the home agent does not tunnel
Router Advertisements to it.
- The mobile node's home agent serves as a proxy for the mobile
node's home address and link-local address, including defending
these addresses for Duplicate Address Detection, while the mobile
node is registered with the home agent away from home.
- When a home subnet prefix changes, the home agent tunnels Router
Advertisement packets to each mobile node registered with it that
is currently away from home and using a home address with the
affected routing prefix. Such tunneled Router Advertisements
MUST be authenticated [1].
- When a mobile node receives a tunneled Router Advertisement
containing a new routing prefix, it MUST perform the standard
autoconfiguration operation to create its new address.
- When a mobile node returns to its home subnet, it must again
perform Duplicate Address Detection at the earliest possible
moment after it has deleted its "home registration" binding with
its home agent.
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- A mobile node MAY send a Router Solicitation to its home agent at
any time, within the constraints imposed by rate control defined
by Neighbor Discovery [9].
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8. Mobile Node Operation
8.1. 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:
- For most packets, the mobile node will generally use its home
address as the source of the packet. Doing so makes its 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. 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.
- For short-term communication, particularly for communication
that may easily be retried if it fails, the mobile node MAY
choose to use one of its care-of addresses as the source of the
packet. An example of this type of communication might be DNS
queries sent by the mobile node [7, 8]. 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 type of communication, however, the
mobile node SHOULD use its home address.
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
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 the mobile node uses its home address as the
source of a packet while away from home, the mobile node SHOULD
construct the packet as follows:
- The Source Address field in the packet's IPv6 header is set to
one of the mobile node's care-of addresses.
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- A Home Address option is included in the packet, with the Home
Address field set to the mobile node's home address.
Without this use of the care-of address in the IPv6 header, with the
mobile node's home address instead in the Home Address option, the
packet will likely be discarded by any router implementing ingress
filtering [6].
8.2. Movement Detection
A mobile node MAY use any combination of mechanisms available to
it to detect when its link-level point of attachment has moved
from one IP subnet 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 [9].
Mobile nodes SHOULD use Router Discovery to discover new routers and
on-link network prefixes; a mobile node MAY send Router Solicitation
messages, or MAY wait for unsolicited (periodic) Router Advertisement
messages, as specified for Router Discovery [9]. 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 network 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 network
prefix advertised by that router from its Prefix List to use as
the network prefix in its primary care-of address. A mobile node
MAY also have associated additional care-of addresses, using other
network prefixes from its Prefix List. The method by which a mobile
node selects and forms a care-of address from the available network
prefixes is described in Section 8.3. The mobile node registers
its primary care-of address with its home agent, as described in
Section 8.4.
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
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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 the 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 [9], while the mobile node is actively sending
packets to (or through) its default router, the mobile node can
detect that the router 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 form its default router in response to an explicit Neighbor
Solicitation messages to it. Note that although this mechanism only
detects that the mobile node's default router has become unreachable
to the mobile node 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 to its
default router, however, 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 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.
On some types of network interfaces, the mobile node MAY also
supplement this 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. 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 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
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mobile node receives no packets from the router for a period of
time), then the mobile node SHOULD 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's link-layer connection to a new IP subnet, such that the
mobile node would need to switch to a new default router and primary
care-of address. Upon lower-layer indication of link-layer mobility,
the mobile node SHOULD send Router Solicitation messages to determine
if new routers (and new on-link network 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
care-of addresses it has formed from the on-link network prefixes
currently available through different default 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 default routers to decide when to
switch to a new primary care-of address using that default 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.
8.3. Forming New Care-of Addresses
After detecting that its link-layer point of attachment has moved
from one IPv6 subnet 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 network 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 (not more often than once per
MAX_UPDATE_RATE seconds).
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In addition, after discovering a new on-link network prefix, a
mobile node MAY form a new (non-primary) care-of address using that
network 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 network 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 8.10.
As described in Section 3.1, in order to form a new care-of address,
a mobile node MAY use either stateless [16] or stateful (e.g.,
DHCPv6 [3]) 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 subnet. For example, a mobile node may be
statically configured with an IPv6 address assigned by the system
administrator of some foreign subnet, for its use while visiting that
subnet. If so, rather than using address autoconfiguration to form
a new care-of address using this network prefix, the mobile node
MAY use its own pre-assigned address as its care-of address on this
subnet.
8.4. Sending Binding Updates to the Home Agent
After deciding to change its primary care-of address as described
in Sections 8.2 and 8.3, 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.
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- 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 binding MUST be specified in the Care-of Address
field in the Binding Update.
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 4.2, 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).
It is possible that when the mobile node needs to send such a Binding
Update to its home agent, that the mobile node does not know the
address of any router in its home subnet that can serve as a home
agent for it. In this case, the mobile node SHOULD use the dynamic
home agent address resolution procedure to find the address of a
suitable home agent in its home subnet. To do so, the mobile node
sends the packet, as described above, with the Destination Address in
the packet's IPv6 header set the Home-Agents anycast address for its
home subnet. The home agent in its home subnet that receives this
Binding Update will reject the Update, returning to the mobile node
the home agent's unicast IP address. The mobile node SHOULD then
retransmit its Binding Update to this home agent using the provided
unicast address.
If the mobile node has a current registration with some home agent
in its home subnet (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 in its home
subnet. If the mobile node knows of no other suitable home agent,
then it MAY attempt the dynamic home agent address resolution
procedure described above.
8.5. Sending Binding Updates to Correspondent Nodes
A mobile node MAY send a Binding Update to any correspondent node at
any time to allow it to cache its current care-of address (subject
to the rate limiting defined in Section 8.8). 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
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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. 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 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. 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
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 8.4), the mobile node will also send a Binding Update to each
correspondent node for which an entry exists in the mobile node's
Binding Update List. Thus, correspondent 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
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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. 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 the mobile node's home address. If the original packet
contains a Routing header, the final Address indicated in the
Routing header should be used in this comparison rather than the
Destination Address in the original IPv6 header.
- 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 tests above, is the
Source Address in the original (inner) IPv6 header of the 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 added to 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
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Section 4.1. No other IPv6 nodes are authorized to send Binding
Updates on behalf of a mobile node.
8.6. 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 SHOULD send a Binding Update
to its previous default router, giving its new care-of address. 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. Note that the previous
router does not necessarily know the mobile node's (permanent) home
address as part of this registration.
If any subsequent packets arrive at this previous router for
forwarding to the mobile node's old primary care-of address,
the router SHOULD encapsulate each such packet (using IPv6
encapsulation [4]) and tunnel it to the mobile node at its new
primary care-of address. Moreover, for the lifetime of the "home
registration" Binding Cache entry for the mobile node at this
router, this router MUST act as a proxy for the mobile node's
previous care-of address, for purposes of participation in Neighbor
Discovery [9], in the same way as any home agent does for a mobile
node's home address (Section 7.2). This allows the router to
intercept packets addressed to the mobile node's previous care-of
address, and to encapsulate and tunnel them to the mobile node's new
care-of address, as described in Section 7.4.
8.7. 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 he
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
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upon each retransmission until either the node receives a
Binding Acknowledgement or the timeout period reaches the value
MAX_BINDACK_TIMEOUT.
8.8. Rate Limiting for Sending Binding Updates
A mobile node MUST NOT send Binding Updates more often than once per
MAX_UPDATE_RATE seconds to any node. 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 the
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.
8.9. 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 an IP Authentication header and the
authentication is valid [1]. The Authentication header MUST
provide both sender authentication, integrity protection, and
replay protection.
- The Option Length field in the option is greater than or equal to
9 octets.
- 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
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List to indicate that the Binding Update has been acknowledged.
The mobile node MUST thus stop retransmitting the Binding Update.
- If the Status field indicates that the Binding Update was not
accepted (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 8.8.
8.10. Using Multiple Care-of Addresses
As described in Section 8.3, a mobile node MAY have 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 link-level points of attachment at the same time (e.g.,
with overlapping wireless cells), on which different on-link network
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 network prefixes, based on the movement detection mechanism
in use, as described in Section 8.2. When the mobile node selects
a new primary care-of address, it MUST register it with its home
agent through a Binding Update with the Home Registration (H) and
Acknowledge (A) bits set, as described in Section 8.4.
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 [3], the
mobile node may not wish to release the address immediately upon
switching to a new primary care-of address. The stateful address
autoconfiguration server will allow mobile nodes to acquire new
addresses while still using previously allocated addresses.
8.11. Returning Home
A mobile node detects that it has returned to its home subnet through
the movement detection algorithm in use (Section 8.2), when the
mobile node detects that the network prefix of its home subnet is
again on-link. The mobile node SHOULD then send a Binding Update to
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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 (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 subnet
(to the all-nodes multicast address) a Neighbor Advertisement
message [9], 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. Similarly,
the mobile node MUST multicast a Neighbor Advertisement message to
advertise its link-layer address for its IPv6 link-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 subnet will not receive any of these Neighbor
Advertisements, but these nodes will eventually be able to recover
through use of Neighbor Unreachability Detection [9].
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9. Routing Multicast Packets
A mobile node that is connected to its home subnet 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 subnet.
In order receive packets sent to some multicast group, a mobile node
must join the that multicast group. One method by which a mobile
node MAY join the group is via a (local) multicast router on the
foreign subnet being visited. This option assumes that there is a
multicast router present on the foreign subnet. The mobile node
SHOULD use its care-of address sharing a network 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, assuming that its home agent
is a multicast router. The mobile node tunnels the appropriate
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 subnet
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
sends multicast packets directly on the foreign subnet MUST use its
care-of address as the IPv6 Source Address of each multicast packet.
Similarly, 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. This second option assumes that the home
agent is a multicast router.
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10. 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
MAX_ADVERT_REXMIT 3
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11. Security Considerations
11.1. 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 [2].
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. This 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.
11.2. Home Address Options
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. 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 IPv6 header,
including the Source Address field, and any other parts of the
packet, including the Home Address option, can be forged or modified
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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 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.
11.3. 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 see
should use mechanisms outside the scope of this document (such as
encryption) to provide appropriate protection. 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 pinpoint the location of the mobile node. Such
mechanisms are all beyond the scope of this document.
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Appendix A. Changes from Previous 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-ipv6-02.txt:
- Added a comparison to Mobile IP for IPv4 and added this section
listing changes from the previous version of this draft.
- Introduced the Home Address destination option, to allow packets
sent by a mobile node while away from home to pass normally
through routers implementing ingress filtering.
- Added the requirement that all IPv6 nodes MUST be able to
correctly process a Home Address destination option in a received
packet.
- Changed the interpretation of the Binding Update option such
that the home address in the binding is the address in the Home
Address option, not the Source Address in the IPv6 header.
- Made the Care-of Address field in the Binding Update optional,
controlled by whether or not the new Care-of Address Present (C)
bit is set in the option. With the new use of the Home Address
option, the care-of address for a binding will usually be
specified by the Source Address field in the packet's IPv6
header, but by retaining this field (and making it optional),
it is possible to send a binding update using a Source Address
different from the care-of address for the binding.
- Changed the 32-bit Identification field in the Binding Update and
Binding Acknowledgement to a 16-bit Sequence Number field, and
clarified the use of this field. Replay protection for Binding
Updates and Binding Acknowledgements is provided by the IPsec
authentication in the packet, but this replay protection does
not provide sequencing due to the use of the replay protection
window. This field satisfies that the additional sequencing
requirement.
- Added a description of the dynamic home agent address discovery
procedure and the use of the new Home-Agents anycast address.
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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 Thomas Narten and Erik Nordmark 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] Randall Atkinson. IP Authentication header. RFC 1826, August
1995.
[2] S. M. Bellovin. Security problems in the TCP/IP protocol suite.
ACM Computer Communications Review, 19(2), March 1989.
[3] Jim Bound and Charles Perkins. Dynamic Host Configuration
Protocol for IPv6 (DHCPv6). Internet-Draft,
draft-ietf-dhc-dhcpv6-10.txt, May 1997. Work in progress.
[4] Alex Conta and Stephen Deering. Generic packet
tunneling in IPv6 specification. Internet-Draft,
draft-ietf-ipngwg-ipv6-tunnel-07.txt, December 1996.
Work in progress.
[5] Stephen E. Deering and Robert M. Hinden. Internet Protocol
version 6 (IPv6) specification. RFC 1883, December 1995.
[6] Paul Ferguson, editor. Network ingress filtering: Defeating
IP source address spoofing denial of service attacks.
Internet-Draft, draft-ferguson-ingress-filtering-02.txt, July
1997. Work in progress.
[7] P. Mockapetris. Domain Names---concepts and facilities.
RFC 1034, November 1987.
[8] P. Mockapetris. Domain Names---implementation and
specification. RFC 1035, November 1987.
[9] Thomas Narten, Erik Nordmark, and William Allen Simpson.
Neighbor Discovery for IP version 6 (IPv6). RFC 1970, August
1996.
[10] Charles Perkins. IP encapsulation within IP. RFC 2003, October
1996.
[11] Charles Perkins, editor. IP mobility support. RFC 2002,
October 1996.
[12] Charles Perkins. Minimal encapsulation within IP. RFC 2004,
October 1996.
[13] J. B. Postel. User Datagram Protocol. RFC 768, August 1980.
[14] J. B. Postel, editor. Transmission Control Protocol. RFC 793,
September 1981.
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[15] Joyce K. Reynolds and Jon Postel. Assigned numbers. RFC 1700,
October 1994.
[16] Susan Thomson and Thomas Narten. IPv6 stateless address
autoconfiguration. RFC 1971, August 1996.
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Chair's Address
The Working Group can be contacted via its current chairs:
Jim Solomon
Motorola, Inc.
1301 E. Algonquin Rd.
Schaumburg, IL 60196
USA
Phone: +1 847 576-2753
E-mail: solomon@comm.mot.com
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
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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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