Mobile IP Working Group David B. Johnson
INTERNET-DRAFT Carnegie Mellon University
Charles Perkins
IBM Corporation
26 November 1996
Mobility Support in IPv6
<draft-ietf-mobileip-ipv6-02.txt>
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.
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
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
To learn the current status of any Internet-Draft, please check
the "1id-abstracts.txt" listing contained in the Internet-Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
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Contents
Abstract i
Status of This Memo 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
4. New IPv6 Destination Options 11
4.1. Binding Update Option . . . . . . . . . . . . . . . . . . 11
4.2. Binding Acknowledgement Option . . . . . . . . . . . . . 14
4.3. Binding Request Option . . . . . . . . . . . . . . . . . 17
5. Requirements for IPv6 Nodes 18
6. Correspondent Node Operation 20
6.1. Receiving Binding Updates . . . . . . . . . . . . . . . . 20
6.2. Requests to Cache a Binding . . . . . . . . . . . . . . . 21
6.3. Requests to Delete a Binding . . . . . . . . . . . . . . 21
6.4. Sending Binding Acknowledgements . . . . . . . . . . . . 21
6.5. Cache Replacement Policy . . . . . . . . . . . . . . . . 22
6.6. Receiving ICMP Error Messages . . . . . . . . . . . . . . 23
6.7. Sending Packets to a Mobile Node . . . . . . . . . . . . 24
7. Home Agent Operation 26
7.1. Primary Care-of Address Registration . . . . . . . . . . 26
7.2. Primary Care-of Address De-registration . . . . . . . . . 28
7.3. Tunneling Intercepted Packets to a Mobile Node . . . . . 28
7.4. Renumbering the Home Subnet . . . . . . . . . . . . . . . 29
8. Mobile Node Operation 31
8.1. Movement Detection . . . . . . . . . . . . . . . . . . . 31
8.2. Forming New Care-of Addresses . . . . . . . . . . . . . . 33
8.3. Sending Binding Updates to the Home Agent . . . . . . . . 34
8.4. Sending Binding Updates to Correspondent Nodes . . . . . 35
8.5. Sending Binding Updates to the Previous Default Router . 37
8.6. Retransmitting Binding Updates . . . . . . . . . . . . . 37
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8.7. Rate Limiting for Sending Binding Updates . . . . . . . . 38
8.8. Receiving Binding Acknowledgements . . . . . . . . . . . 38
8.9. Using Multiple Care-of Addresses . . . . . . . . . . . . 39
8.10. Returning Home . . . . . . . . . . . . . . . . . . . . . 39
9. Routing Multicast Packets 41
10. Constants 42
11. Security Considerations 43
Acknowledgements 44
A. Open Issues 45
A.1. Session Keys with Local Routers . . . . . . . . . . . . . 45
A.2. Source Address Filtering by Firewalls . . . . . . . . . . 45
A.3. Dynamic Home Agent Address Discovery . . . . . . . . . . 46
A.4. Replay Protection for Binding Updates . . . . . . . . . . 46
References 47
Chair's Address 49
Authors' Addresses 50
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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 it 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 accommodates node movement from an
Ethernet segment to a wireless LAN cell, as long as the mobile node's
IP address remains unchanged after such a 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 addressable 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 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 its 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
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 only while visiting a particular
foreign subnet. The network prefix of a care-of address being used
by a mobile node is equal to the network prefix of the foreign
subnet to which the mobile node is link-level connected, and thus
packets addressed to this care-of address will be routed to the
mobile node's 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 [14] or stateful (e.g., DHCPv6 [3])
address autoconfiguration, according to the methods of IPv6 Neighbor
Discovery [8], although other methods of acquiring a care-of address
are also possible.
While away from home, the mobile node registers one of its binding
with a router in its home subnet, requesting this router to function
as the "home agent" for 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.
Mobile IPv6 provides a mechanism for 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 through the
care-of address indicated in this binding. If, instead, the sending
node has no cached binding for this destination address, the node
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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.
A mobile node's home agent and correspondent nodes learn and
cache the mobile node's binding through use of a set of new IPv6
destination options [5] defined for Mobile IPv6. Since an IPv6
Destination Options header containing one or more destination options
can appear in any IPv6 packet, any Mobile IPv6 option can be sent in
either of two ways:
- A Mobile IPv6 option can be included within any IPv6 packet
carrying any payload such as TCP [11] or UDP [10].
- A Mobile IPv6 option 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].
The following three new IPv6 destination options are defined for
Mobile IPv6:
Binding Update
A Binding Update is used by a mobile node to notify a
correspondent node or its home agent of its current binding.
The Binding Update sent to the mobile node's home agent is
marked as a "home registration". Any packet that includes a
Binding Update option MUST also include an IPv6 Authentication
header [1]. The Binding Update option is described in detail
in Section 4.1.
Binding Acknowledgement
A Binding Acknowledgement is used to acknowledge receipt of
a Binding Update, if an acknowledgement was requested in the
Binding Update. Other Binding Updates MAY be acknowledged
but need not be. Any packet that includes a Binding
Acknowledgement option MUST also include an IPv6 Authentication
header [1]. The Binding Acknowledgement option is described in
detail in Section 4.2.
Binding Request
A Binding Request is used to request a mobile node to send a
Binding Update to this node, containing its current binding.
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This option is typically used by a correspondent node to
refresh a cached binding for a mobile node, when the lifetime
on this cached binding is close to expiration. The Binding
Request option is described in detail in Section 4.3.
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, whereas other Binding Cache
entries MAY be replaced at any time by any reasonable local
cache replacement policy. 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 [8].
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 Update has not yet
expired. For each such Binding Update, the Binding Update List
records the IP address of the node to which the Update was
sent, the home address for which the Update was sent, and the
remaining lifetime of the binding. The Binding Update List
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
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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
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 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.
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
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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 the
home subnet is reduced, since these nodes and links are not involved
in the delivery of most packets to the mobile node.
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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 its 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|L| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Care-of Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ 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, this field
MUST be set to 24 if the Home Link-Local Address Present (L)
bit is not set, and MUST otherwise be set to 40.
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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.
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.
Identification
a 32-bit number used by the receiving node to sequence Binding
Updates, and by the sending node to match a returned Binding
Acknowledgement message with this Binding Update.
Care-of Address
The care-of address of the mobile node for this binding. When
set equal to the home address of the mobile node, the Binding
Update option instead indicates that any existing binding for
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the mobile node should be deleted; no binding for the mobile
node should be created in this case.
Home Link-Local Address
The link-local address of the mobile node used by the mobile
node when it was last attached to its home subnet. This field
in the Binding Update is optional and is only present when the
Home Link-Local Address (L) bit is set.
The home address of the mobile node in the binding is indicated by
the Source Address field in the IP header of the packet containing
the Binding Update option.
Any packet that includes a Binding Update option MUST include an IPv6
Authentication header [1] in order to protect against forged Binding
Updates.
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 24 octets if the Home
Link-Local Address (L) bit is not set, or greater than 40 octets if
the Home Link-Local Address (L) bit is set, 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 Binding Update addressed to itself, in which the
Acknowledge (A) bit set, it MUST return a Binding Acknowledgement.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Refresh | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 8.
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
130 Administratively prohibited
131 Insufficient resources
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132 Home registration not supported
133 Not home subnet
134 Identification field mismatch
135 Unknown home agent address
Up-to-date values of the Status field are to be specified in
the most recent "Assigned Numbers" [12].
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, in case
the node fails and loses its cache state. The Refresh period
is determined by the node sending the Binging 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.
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 to extend the
lifetime.
Identification
The acknowledgement Identification is copied from the Binding
Update option, for use by the mobile node in matching the
acknowledgement with an outstanding Binding Update.
Any packet that includes a Binding Acknowledgement option MUST
include an IPv6 Authentication header [1] in order to protect against
forged Binding Acknowledgements.
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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 Cache entry, and are described in
Section 6.7. The packet is sent using a Routing header, routing the
packet to the mobile node through 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 6.7, 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 Source Address of the rejected Binding Update
through the care-of address indicated in 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
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 that node.
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 Acknowledgement 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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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 SHOULD be able to process a received Binding
Update option, and to return a Binding Acknowledgement message 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 a registry of mobile
node bindings, recording each mobile node's primary care-of
address, for those mobile nodes for which it is serving as the
home agent.
- 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 Binding Acknowledgements
in response to Binding Updates received from a mobile node.
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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 8.3, 8.4, and 8.5; and MUST be able to
receive and process Binding Acknowledgements, as specified in
Section 8.8.
- 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 Binding Updates
Upon receiving a Binding Update option in some packet, the receiving
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 is
assumed to provide both authentication and integrity protection.
- The Option Length field in the option is greater than or equal to
24 octets if the Home Link-Local Address (L) bit is not set, or
greater or equal to 40 octets if the Home Link-Local Address (L)
bit is set.
- The Identification field is valid.
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 Lifetime specified in the Binding Update is nonzero and
the specified Care-of Address is not equal to the Source Address
in the IP header of the packet carrying the Binding Update,
then this is a request to cache a binding for the mobile node
(the home address of the mobile node is specified by the Source
Address field in the packet's IP header). Processing for this
type of received Binding Update is described in Section 6.2.
- If the Lifetime specified in the Binding Update is zero or the
specified Care-of Address matches the Source Address field in the
IP header of the packet carrying the Binding Update, then this is
a request to delete the mobile node's binding (as above, the home
address of the mobile node is specified by the Source Address
field in the packet's IP header). Processing for this type of
received Binding Update is described in Section 6.3.
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6.2. 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 6.1, 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 7.1.
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 Source Address field in the packet's IP header.
The new Binding Cache entry records the association between this
address and the Care-of Address specified in the Binding Update.
The node must also begin a timer to delete this Binding Cache entry
after the expiration of the Lifetime period specified in the Binding
Update.
6.3. Requests to Delete a Binding
If a node receives a valid Binding Update requesting it to delete
a binding for a mobile node, as specified in Section 6.1, 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
MUST delete any existing entry in its Binding Cache for this mobile
node. The home address of the mobile node is taken from the Source
Address field in the packet's IP header.
6.4. 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 message acknowledging receipt of the Binding
Update. If the node accepts the Binding Update and adds the binding
contained in it to its Binding Cache, 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 add the binding
contained in it to its Binding Cache, the Status field in the Binding
Acknowledgement MUST be set to a value greater than or equal to 128.
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Specific values for the Status field are described in Section 4.2 and
in the most recent "Assigned Numbers" [12].
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 through the care-of address contained in the Binding Update
being acknowledged. This 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.
6.5. 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 7.1) 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 message
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 is
likely to work well.
If a packet is sent by a node to a destination for which it has
dropped the cache 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
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mobile node sending a Binding Update to this sending node, allowing
this node to add an entry again for this destination to its Binding
Cache.
6.6. 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
mobile node's address (its home address), then the correspondent
node uses a Routing header to deliver the packet to the mobile node
through the care-of address 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 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 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.
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6.7. 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) through
the care-of address 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 following fields in the packet's IP header and
Routing header as indicated below:
- The Destination Address in the packet's IP 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 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 IP
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 7.3. The mobile node will then send
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a Binding Update to the sending node, as described in Section 8.4,
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. Primary Care-of Address Registration
General processing of a received Binding Update that requests a
binding to be cached, is described in Section 6.2. 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 6.2.
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 message 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 in the Binding Update
(the Source Address in the packet's IP header) 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 message to the mobile
node, in which the Status field is set to 133 (Not home subnet).
- 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 message 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 6.2. 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 6.5) 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
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entry for this 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 [8] 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 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 [8].
In addition, while this node is serving as a home agent to 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 (L) bit
in its Binding Update with which it registered 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 a "home registration"
entry in its Binding Cache for the mobile node. 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 [8].
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7.2. Primary Care-of Address De-registration
General processing of a received Binding Update that requests a
binding to be deleted, is described in Section 6.3. 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 message 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 in the Binding Update
(the Source Address in the packet's IP header) 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 message to the mobile node, in which the
Status field is set to 133 (Not home subnet).
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, 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 [8].
7.3. 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 6.7
apply. The home agent uses a Routing header to route the packet
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to the mobile node through 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 7.1. 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 its own IP address, and sets the Destination
Address in the tunnel IP header to the mobile node's primary care-of
address. When received by the mobile node (using its primary care-of
address), normal processing of the tunnel header [4] will result in
decapsulation and processing of the original packet by the mobile
node.
7.4. Renumbering the Home Subnet
Neighbor Discovery [8] 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 authenticated Router Advertisement messages
from its home agent that the prefix has changed.
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- 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 which 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 registered with its home agent.
- A mobile node may send a Router Solicitation to its home agent at
any time, within the constraints imposed by rate control in the
Neighbor Discovery specification [8]
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8. Mobile Node Operation
8.1. 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 [8].
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 [8]. 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 8.2. The mobile node registers
its primary care-of address with its home agent, as described in
Section 8.3.
While away from home and using some router as its default router,
it is important for a mobile node to be able to quickly detect when
that router becomes unreachable, so that it can switch to a new
default router and to a new primary care-of address. Since some
links (notably wireless) do not necessarily work equally well in both
directions, it is likewise important for the mobile node to detect
when it becomes unreachable to 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
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Neighbor Discovery [8], 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 to 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
mobile node receives no packets form 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
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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.
8.2. 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).
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 (registered with its home agent), but it MAY have an
additional care-of address for each network prefix on its current
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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 8.9.
As described in Section 3, in order to form a new care-of address,
a mobile node MAY use either stateless [14] 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 IP 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
SHOULD use its own pre-assigned address as its care-of address on
this subnet.
8.3. Sending Binding Updates to the Home Agent
After deciding to change its primary care-of address as described
in Sections 8.1 and 8.2, 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 Home Registration (H)
bit is set in the Binding Update. The mobile node also sets the
Acknowledge (A) bit in the Binding Update, requesting the home
agent to return a Binding Acknowledgement message 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 message. 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).
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8.4. Sending Binding Updates to Correspondent Nodes
A mobile node MAY send a Binding Update to any correspondent node at
any time (subject to the rate limiting defined in Section 8.7). In
any Binding Update sent by a mobile node, the Care-of Address field
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
a 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 field
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 field 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 of 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 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 8.3), the mobile node will also typically 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 and can send almost all packets directly to
the mobile node using the mobile node's current binding.
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
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in any packet sent by a mobile node, the mobile node MAY delay
sending a new Binding Update to any correspondent node for a
short period of time, in hopes that the needed Binding Update
can be included in some packet that the mobile node sends to that
correspondent node for some other reason (for example, as part of
some TCP connection in use). In this case, when sending a packet
to some correspondent node, the mobile node SHOULD check in its
Binding Update List to determine if a new Binding Update to this
correspondent node is needed, and SHOULD include the new Binding
Update in this packet as necessary.
In addition, when a mobile node receives a packet for which the
mobile node can deduce that the original sender of the packet has no
Binding Cache entry for the mobile node, or for which the mobile node
can deduce that the original sender of the packet has an out-of-date
care-of address in its Binding Cache entry for the mobile node, 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) IP header is equal
to any of the mobile node's care-of addresses.
- The Destination Address in the original (inner) IP 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 IP header.
- The Source Address in the tunnel (outer) IP header differs from
the Source Address in the original (inner) IP 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) IP 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.
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A mobile node MAY choose to keep its location private from certain
correspondent nodes, and thus need not send new Binding Updates to
those correspondents. A mobile node MAY also send a Binding Update
to such a correspondent node to instruct it to delete any existing
binding for the mobile node from its Binding Cache, as described in
Section 4.1. No other IPv6 nodes are authorized to send Binding
Updates on behalf of a mobile node.
8.5. 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 message to its previous default router, giving its new care-of
address. If the mobile node sends such a Binding Update, the Source
Address 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 field 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 home address as part of this sequence of events.
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 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 [8], in the
same way as any home agent does for a mobile node's home address
(Section 7.1). 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 7.3.
8.6. Retransmitting Binding Updates
If, after sending a Binding Update in which the Acknowledge (A)
bit is set, a mobile node fails to receive an acceptable 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 Identification value as the original
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transmission. The retransmissions by the mobile node MUST use
an exponential back-off process, in which timeout period is
doubled upon each retransmission until either the node receives a
Binding Acknowledgement or the timeout period reaches the value
MAX_BINDACK_TIMEOUT.
8.7. 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 correspondent node. After sending 5
consecutive Binding Updates to a particular correspondent node with
the same care-of address, the mobile node SHOULD reduce its rate
of sending Binding Updates to that correspondent 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 correspondent 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.
8.8. 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 is
assumed to provide both authentication and integrity protection.
- The Option Length field in the option is greater than or equal to
8 octets.
- The Identification field is valid.
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. Optionally, the mobile node MAY take steps to correct the
cause of the error and retransmit the Binding Update, subject to
the rate limiting restriction specified in Section 8.7.
8.9. Using Multiple Care-of Addresses
As described in Section 8.2, 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 may 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 (Section 8.1). When the mobile node selects a new
primary care-of address, it MUST register it with its home agent
through a Binding Update message with the Home Registration (H) and
Acknowledge (A) bits set, as described in Section 8.3.
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.
8.10. Returning Home
A mobile node detects that it has returned to its home subnet through
the movement detection algorithm in use (Section 8.1), 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
its home agent, to instruct its home agent to no longer intercept
or tunnel packets for it. In this Binding Update, the mobile node
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MUST set the Care-of Address field to its own IPv6 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 message is received.
In addition, the mobile node MUST multicast onto the home subnet
(to the all-nodes multicast address) a Neighbor Advertisement
message [8], to advertise its link-layer address for its own IPv6
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 [8].
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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 message packets.
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. The home agent MUST tunnel the packet directly to the mobile
node's primary care-of address.
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 IP 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 both
the (inner) multicast packet and the (outer) encapsulating 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_ADVERT_REXMIT 3
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11. Security Considerations
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 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.
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 absolute 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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Acknowledgements
We would like to thank the members of the Mobile IP and IPng Working
Groups for their comments and suggestions on this draft. 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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A. Open Issues
A.1. Session Keys with Local Routers
In the IPv4 route optimization proposal [7], a mechanism is outlined
whereby a session key can be established between foreign agents
and mobile nodes, without requiring any pre-established security
relationship between them. A similar mechanism could be defined for
IPv6, to avoid the need for a possibly time-consuming negotiation
between routers and mobile nodes for the purpose of obtaining the
session key, which under many circumstances would only be used once.
This mechanism, if needed, can be specified completely outside
the Mobile IPv6 protocol and would amount to a way of creating a
dynamic security association between two nodes which do not share an
existing trust relationship, but which need to agree on a key for
some particular purpose (here, allowing the future authentication of
a Binding Update). Hopefully, the work of the IP Security Working
Group will allow this function to be performed appropriately for
mobile nodes, say by a Diffie-Hellman key exchange.
A.2. Source Address Filtering by Firewalls
The current specification does nothing to permit mobile nodes to
send their packets through firewalls which filter out packets with
the "wrong" source IPv6 addresses in the IPv6 packet header. The
mobile node's home address may be unlikely to fall within the ranges
required to satisfy the firewall's criteria for further delivery.
As indicated by recent discussion, firewalls are unlikely to
disappear. Any standardized solution [13] to the firewall problem
based on hiding the non-local source address outside the source
address field of the IP header is likely to fail. Any vendor or
facilities administrator wanting to filter based on the address in
the IPv6 source address field would also quickly begin filtering on
hidden source addresses.
Assume, for the moment, that a mobile node is able to establish a
secure tunnel through a firewall protecting the domain in which
a correspondent node is located. The mobile node could then
encapsulate its packet so that the outer IP header was addressed
to the firewall and used the mobile node's care-of address as the
source address. When the firewall decapsulates, it would be able to
authenticate the inner packet based (correctly) on the mobile node's
home address. After the authentication is performed, the firewall
could forward the packet to the correspondent node as desired. This
simple procedure has the feature that it requires the minimal amount
of encapsulation, no assistance by routers or other agents, and that
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the firewall can establish a security relationship with the mobile
node based on its home (i.e., permanent) address.
A.3. Dynamic Home Agent Address Discovery
It is useful for a mobile node to be able to send a Binding Update
its home agent without explicitly knowing the home agent's address.
For example, since the mobile node was last at home, it may have
become necessary to replace the node serving as its home agent due
to the failure of the original node or due to reconfiguration of the
home subnet. It thus may not always be possible or convenient for a
mobile node to know the exact address of its own home agent. Several
methods of allowing a mobile node to dynamically discover the address
of a router in its home subnet are currently under consideration.
A.4. Replay Protection for Binding Updates
Some transforms for use in conjunction with the IP Authentication
Header [1] provide support for replay protection [9, 6]. Ideally,
such transforms would directly support the needs of Mobile IPv6
for providing replay protection for Binding Updates and Binding
Acknowledgements. However, this does not currently appear to be
the case. These transforms provide optional support for accepting
packets out of order, through use of an "out of order window" in the
receiver, and it does not currently seem to be specified how the
size (or presence) of such a window can be controlled. For Binding
Updates, it is important that any packets containing a Binding
Update that arrive at the receiver do so strictly in the order sent
(although some may harmlessly be dropped, as long as a later Binding
Update does arrive). Without control of the window at the receiver,
this ordering requirement on Binding Update delivery cannot be
supported directly by these transforms, although these transforms do
use a sequence number to support their own replay protection.
The Identification field in the Binding Update (and Binding
Acknowledgement) is currently specified in this document for use
in sequencing Binding Updates at the receiver, and in matching
returned Binding Acknowledgements with outstanding Binding Updates
at the sender. The use of this field in this manner, together with
the use of the current IP Authentication transforms that supports
replay protection, seems to support the necessary replay protection
requirements for Mobile IPv6, although it seems that the need for two
sequence numbers in the packet (one for IP Authentication and one for
Mobile IPv6) could be simplified.
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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-dhcpv6-07.txt, August 1996. Work in progress.
[4] Alex Conta and Stephen Deering. Generic packet
tunneling in IPv6 specification. Internet-Draft,
draft-ietf-ipngwg-ipv6-tunnel-02.txt, June 1996. Work
in progress.
[5] Stephen E. Deering and Robert M. Hinden. Internet Protocol
version 6 (IPv6) specification. RFC 1883, December 1995.
[6] Shu jen Chang and Robert Glenn. HMAC-SHA IP authentication with
replay prevention. Internet-Draft,
draft-ietf-ipsec-ah-hmac-sha-04.txt, November 1996. Work in
progress.
[7] David B. Johnson and Charles Perkins. Route optimization in
Mobile IP. Internet-Draft, draft-ietf-mobileip-optim-04.txt,
February 1996. Work in progress.
[8] Thomas Narten, Erik Nordmark, and William Allen Simpson.
Neighbor Discovery for IP version 6 (IPv6). RFC 1970, August
1996.
[9] Michael J. Oehler and Robert Glenn. HMAC-MD5 IP
authentication with replay prevention. Internet-Draft,
draft-ietf-ipsec-ah-hmac-md5-04.txt, November 1996. Work in
progress.
[10] J. B. Postel. User Datagram Protocol. RFC 768, August 1980.
[11] J. B. Postel, editor. Transmission Control Protocol. RFC 793,
September 1981.
[12] Joyce K. Reynolds and Jon Postel. Assigned numbers. RFC 1700,
October 1994.
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[13] Fumio Teraoka. Mobility support in IPv6. Internet-Draft,
draft-teraoka-ipv6-mobility-sup-03.txt, April 1996. Work in
progress.
[14] 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
IBM Corporation
T. J. Watson Research Center
Room H3-D34
30 Saw Mill River Rd.
Hawthorne, NY 10532
USA
Phone: +1 914 789-7350
Fax: +1 914 784-6205
E-mail: perk@watson.ibm.com
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