Mobile IP Working Group David B. Johnson
INTERNET-DRAFT Carnegie Mellon University
Charles Perkins
IBM Corporation
13 June 1996
Mobility Support in IPv6
<draft-ietf-mobileip-ipv6-01.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
1.1. Design Requirements . . . . . . . . . . . . . . . . . . . 2
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 2
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 2
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.6. Specification Language . . . . . . . . . . . . . . . . . 5
2. Overview of Mobile IPv6 Operation 7
3. Message and Option Formats 9
3.1. Binding Update Option . . . . . . . . . . . . . . . . . . 9
3.2. ICMP Binding Acknowledgement Message . . . . . . . . . . 13
4. Requirements for IPv6 Nodes 15
5. Binding Cache Management 17
5.1. Receiving Binding Updates . . . . . . . . . . . . . . . . 17
5.2. Requests to Cache a Binding . . . . . . . . . . . . . . . 17
5.3. Requests to Delete a Binding . . . . . . . . . . . . . . 18
5.4. Sending Binding Acknowledgements . . . . . . . . . . . . 18
5.5. Cache Replacement Policy . . . . . . . . . . . . . . . . 19
5.6. Receiving ICMP Error Messages . . . . . . . . . . . . . . 19
6. Mobile Node Considerations 21
6.1. Movement Detection . . . . . . . . . . . . . . . . . . . 21
6.2. Forming New Care-of Addresses . . . . . . . . . . . . . . 23
6.3. Sending Binding Updates to the Home Agent . . . . . . . . 24
6.4. Sending Binding Updates to Correspondent Nodes . . . . . 25
6.5. Sending Binding Updates to the Previous Default Router . 25
6.6. Rate Limiting for Sending Binding Updates . . . . . . . . 26
6.7. Receiving Binding Acknowledgements . . . . . . . . . . . 26
6.8. Using Multiple Care-of Addresses . . . . . . . . . . . . 27
6.9. Returning Home . . . . . . . . . . . . . . . . . . . . . 28
7. Home Agent Considerations 29
7.1. Home Agent Care-of Address Registration . . . . . . . . . 29
7.2. Home Agent Care-of Address De-registration . . . . . . . 31
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7.3. Delivering Packets to a Mobile Node . . . . . . . . . . . 32
7.4. Renumbering the Home Network . . . . . . . . . . . . . . 32
8. Correspondent Node Considerations 34
8.1. Delivering Packets to a Mobile Node . . . . . . . . . . . 34
9. Authentication and Replay Protection 36
10. Routing Multicast Packets 37
11. Constants 38
Acknowledgements 38
References 39
A. Open Issues 40
A.1. Session Keys with Local Routers . . . . . . . . . . . . . 40
A.2. Source Address Filtering by Firewalls . . . . . . . . . . 40
Chair's Address 42
Authors' Addresses 42
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1. Introduction
This document specifies the operation of mobile computers using
Internet Protocol Version 6 (IPv6) [6]. 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, known as Mobile IPv6, allows transparent routing of IPv6
packets to mobile nodes using the mobile node's home IPv6 address,
regardless of the mobile node's current point of attachment to the
Internet.
The most important function needed to support such routing to mobile
nodes is the reliable and timely notification of a mobile node's
current location to those other nodes that need it. Correspondent
nodes communicating with a mobile node need this location information
in order to correctly deliver their own packets to a mobile node;
Mobile IPv6 allows correspondent nodes to learn and cache a mobile
node's location, and to use this cached information to route their
own packets directly to a mobile node at its current location. The
mobile node's "home agent", a router on the mobile node's home
network, also needs this location information in order to forward
intercepted packets from the home network to the mobile node, for
correspondent nodes that have not yet learned the mobile node's
location, and indeed, for correspondent nodes that do not even yet
know that the mobile node is currently away from home.
A mobile node's current location is represented as a "care-of
address", an IPv6 address assigned to the mobile node (in addition
to its home IPv6 address) within the foreign network currently being
visited by the mobile node. The association between a mobile node's
home address and its care-of address, along with the remaining
lifetime of that association, is known as a "binding", and the mobile
node notifies other nodes about its current binding using a new
destination option called a Binding Update. IPv6 correspondent nodes
then use a Routing header to deliver subsequent packets to the mobile
node's care-of address. All IPv6 nodes and routers MUST be able to
cache mobile node bindings received in Binding Updates; this leads to
dramatic simplifications in the required protocols, compared to the
methods required for IPv4.
In this document, "movement" is considered to be 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 IPv6 subnet (network prefix) as
it was previously. If a mobile node is not currently link-level
connected to its home IPv6 network, the mobile node is said to be
"away from home".
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1.1. Design Requirements
A mobile node must continue to be able to be addressed by its home
IPv6 address, and to be able to communicate with other IPv6 nodes
using its home address, after changing its link-level point of
attachment from one IPv6 subnet to another.
All messages used to update another node as to the location of a
mobile node must be authenticated in order to protect against remote
redirection attacks.
1.2. Goals
The number of administrative messages sent over the link by which
a mobile node is directly attached to the Internet should be
minimized, and the size of these messages should be kept as small
as is reasonably possible. This link may often be a wireless link,
having a substantially lower bandwidth and higher error rate than
traditional wired networks, and many mobile nodes are likely to
operate on limited battery power. By reducing the number and size
of administrative messages required for mobility support, network
resources and mobile node battery resources are conserved.
1.3. Assumptions
This protocol places no additional constraints on the assignment of
IPv6 addresses. That is, a mobile node may acquire its addresses
using stateless address autoconfiguration [12], or alternatively
using a stateful address configuration protocol such as DHCPv6 [3] or
PPPv6 [7].
This protocol assumes that any mobile node will generally not change
its link-level point of attachment from one IPv6 subnet to another
more frequently than once per second.
This protocol assumes that IPv6 unicast packets are routed based on
the Destination Address in the packet's IPv6 header (and not, for
example, by source address).
1.4. Applicability
Mobile IPv6 is intended to enable nodes to move from one IPv6 subnet
to another. It is just as suitable for mobility across homogeneous
media as it is for mobility across heterogeneous media. That is,
Mobile IPv6 facilitates node movement from one Ethernet segment to
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another as well as it accommodates node movement from an Ethernet
segment to a wireless LAN, as long as the mobile node's IPv6 address
remains the same after such a movement.
One can think of Mobile IPv6 as solving the "macro" mobility
management problem. It is less well suited for more "micro" mobility
management applications -- for example, handoff amongst wireless
transceivers, each of which covers only a very small geographic
area. As long as node movement does not occur between link-level
points of attachment on different IPv6 subnets, link-layer mechanisms
for mobility management (i.e., link-layer handoff) may offer faster
convergence and far less overhead than Mobile IPv6.
1.5. Terminology
This document uses the following special terms:
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.
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 node 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 is a conceptual
data structure used in this document, which may be implemented
in any manner consistent with the external behavior described
here, for example by being combined with the node's Destination
Cache as maintained through Neighbor Discovery [9].
Binding Update List
A list, maintained by each IPv6 mobile node, of the IPv6
address of each other node to which this node has sent a
Binding Update giving its binding, such that the lifetime of
the binding sent to that node has not yet expired. This is a
conceptual data structure used in this document, which may be
implemented in any manner consistent with the external behavior
described here.
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Care-of Address
An IPv6 address associated with a mobile node while visiting a
foreign network, which uses the network prefix of that foreign
network. Among the multiple care-of addresses that a mobile
node may have at a time (with different network prefixes), the
one registered with its home agent is called its "primary"
care-of address.
Correspondent Node
A peer with which a mobile node is communicating. The
correspondent node may be either mobile or stationary.
Foreign Network
Any network other than the mobile node's home network.
Home Address
An IPv6 address that is assigned for an extended period of
time to a mobile node. It remains unchanged regardless of the
node's current link-level point of attachment to the Internet.
Home Agent
A router on a mobile node's home network that, while the mobile
node is away from home, intercepts packets on the home network
destined to the mobile node's home address, encapsulates them,
and tunnels them to the mobile node's current care-of address.
The home agent maintains a registry of the current binding for
mobile nodes whose home address is on the home network routed
by the home agent.
Home Network
A network, which may possibly be a virtual network, having a
network prefix matching that of a mobile node's home address.
Standard IPv6 routing mechanisms will deliver packets destined
for a mobile node's home address to the mobile node's home
network.
Link
A facility or medium over which nodes can communicate at the
link layer. A link underlies the network layer.
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Mobile Node
A node that can change its link-level point of attachment from
one IPv6 subnet to another, while still being addressable via
its IPv6 home address.
Node
A host or a router.
Tunnel
The path followed by a packet while it is encapsulated. The
model is that, while it is encapsulated, a packet is routed
to a knowledgeable decapsulating agent, which decapsulates
the packet and then correctly delivers it to its ultimate
destination.
Virtual Network
A network with no physical instantiation beyond a home agent
(with a physical network interface on another network). The
home agent generally advertises reachability to the network
prefix of the virtual network using conventional routing
protocols.
1.6. 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, in some
circumstances, valid reasons may exist to ignore this item, but
the full implications must be understood and carefully weighed
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before choosing a different course. Unexpected results may
result otherwise.
MAY
This word, or the adjective "optional", means that this item is
one of an allowed set of alternatives. An implementation which
does not include this 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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2. Overview of Mobile IPv6 Operation
In addition to its (permanent) IPv6 home address, a mobile node
while away from home will have assigned to its network interface(s)
a "primary care-of address" and possibly other "care-of addresses".
A care-of address is an IPv6 address assigned to a mobile node only
while visiting a particular foreign network, typically acquired
through stateless [12] or stateful (e.g., DHCPv6 [3]) address
autoconfiguration. The decision about which manner of address
autoconfiguration to use is made according to the methods of IPv6
Neighbor Discovery [9].
Each time a mobile node moves its link-level point of attachment from
one IPv6 subnet to another, it will configure its primary care-of
address at its new point of attachment, and will send a Binding
Update containing that care-of address to its home agent. The
care-of address for a mobile node registered with its home agent is
known as the mobile node's "primary" care-of address, and the mobile
node may also have additional care-of addresses, one for each of the
network prefixes that it currently considers to be on-link. Each
time it changes its primary care-of address, a mobile node also sends
a Binding Update to each other (correspondent) node that may have an
out-of-date care-of address for the mobile node in its Binding Cache.
A mobile node attached to the Internet can always be reached by
sending packets to its home IPv6 address. If the mobile node is not
present on its home network, any packet arriving there for it will be
intercepted there by its home agent, which will tunnel the packet to
the mobile node's current primary care-of address. The home agent
uses IPv6 encapsulation [5] to tunnel the packet.
A correspondent node sending a packet checks its Binding Cache for
an entry for the Destination Address of the packet, and uses a
Routing header (instead of encapsulation) to route the packet to the
destination mobile node's care-of address if a cached binding is
found. Otherwise, the correspondent node sends the packet normally
(with no Routing header), and the packet is then intercepted and
tunneled by the mobile node's home agent as described above. When
the tunneled packet reaches the mobile node, the mobile node returns
a Binding Update to the correspondent node, allowing it to cache the
mobile node's binding for future packets.
Since correspondent nodes cache bindings, it is expected that
correspondent nodes usually will route packets directly to the mobile
node's care-of address, so that the home agent is rarely involved
with packet transmission to the mobile node. This is essential for
scalability and reliability, and for minimizing overall network load.
By caching the care-of address of a mobile node, optimal routing of
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packets can be achieved between the correspondent node and 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
network. In addition, the impact of of any possible failure of the
home agent, the home network, or intervening networks leading to the
home network is drastically reduced, since these components are not
involved in the delivery of most packets to the mobile node.
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3. Message and Option Formats
3.1. Binding Update Option
A Binding Update is a new IPv6 destination option, used by a mobile
node to notify a correspondent node or its home agent of its current
care-of address. As a destination option, it can appear in a
Destination Options header in any IPv6 packet [6], and thus can be
included in any normal data packet or can be sent in a separate
packet containing no data. The Binding Update contains the mobile
node's care-of address, an identification for the Update (to sequence
Updates and to protect against attempts to replay it), and a lifetime
for the binding. The mobile node's IPv6 home address MUST be the
source address of the packet containing the Binding Update, since
the option does not contain space to separately represent the mobile
node's home address.
Binding Updates should be considered a form of routing updates;
handled incorrectly, they could be a source of security problems and
routing loops. Therefore, packets which include Binding Updates MUST
also include an IPv6 Authentication header [1]; sequencing and replay
protection is then achieved by use of the Identification field in the
Binding Update.
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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
16
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 28.
Acknowledge (A)
The Acknowledge (A) bit is set by the sending node to request a
Binding Acknowledgement message be returned upon receipt of the
Binding Update option.
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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 IPv6 header of the packet
carrying this option MUST be that of a router sharing the same
network prefix as the mobile node's home IPv6 address.
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 mobile 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 64-bit number used to sequence Binding Updates and to match
a returned Binding Acknowledgement message with this Binding
Update. The Identification field also serves to protect
against replay attacks for Binding Updates.
Care-of Address
The current care-of address of the mobile node. When set equal
to the home address of the mobile node, the Binding Update
option instead indicates that any existing binding for the
mobile node should be deleted; no binding for the mobile node
should be created.
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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 network. This field
in the Binding Update is optional and is only present when the
Home Link-Local Address (L) bit is set.
As with all IPv6 options, the highest-order three bits of the Option
Type Field (16) of the Binding Update option specify the following
properties of the option:
- The highest-order two bits are 00: Any node receiving this
option that does not recognize the Option Type MUST skip over
this option and continue processing the header.
- The third-highest-order bit is 0: The Option Data does not
change en-route, and thus, when an Authentication header is
present in the packet, the entire Binding Update option MUST be
included when computing or verifying the packet's authenticating
value.
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 28 octets,
the remaining octets are interpreted as extensions. Currently no
extensions have been defined.
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3.2. ICMP Binding Acknowledgement Message
A Binding Acknowledgement message is an informational ICMP message
used to acknowledge acceptance of a Binding Update (Section 3.1)
option, if that Binding Update has the Acknowledge (A) bit set.
Upon receipt of a Binding Update requesting an acknowledgement, the
receiving node returns a Binding Acknowledgement message addressed to
the care-of address in the Binding Update.
If a mobile node fails to receive an acceptable Binding
Acknowledgement message within INITIAL_BINDACK_TIMEOUT seconds
after transmitting the Binding Update, it 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 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 message or the timeout period reaches the
value MAX_BINDACK_TIMEOUT.
The ICMP Binding Acknowledgement message has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
133
Code
8-bit unsigned integer indicating the disposition of the
Binding Update. Values of the Code field less than 128
indicate that the Binding Update was accepted by the receiving
node. The following such values are currently defined:
0 Binding Update accepted
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Values of the Code field greater than or equal to 128 indicate
that the Binding Update was rejected by the receiving node.
The following such values are currently defined:
128 Reason unspecified
129 Poorly formed Binding Update
130 Administratively prohibited
131 Insufficient resources
132 Home registration not supported
133 Not home network
134 Identification field mismatch
135 Unknown home agent address
Checksum
The checksum of the message calculated as specified for ICMP
for IPv6 [4].
Identification
The acknowledgement Identification is derived from the Binding
Update option, for use by the mobile node in matching the
acknowledgement with an outstanding Binding Update.
Up-to-date values of the Code field are to be specified in the most
recent "Assigned Numbers" [10].
Extensions to the Binding Acknowledgement message format may be
included after the fixed portion of the Binding Acknowledgement
message specified above. The presence of such extensions will be
indicated by the ICMP message length, derived from the IPv6 Payload
Length field. When the Option Length is greater than 16 octets,
the remaining octets are interpreted as extensions. Currently no
extensions have been defined.
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4. Requirements for IPv6 Nodes
Mobile IPv6 places some special requirements on the functions
provided by different IPv6 nodes. This section itemizes 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 of a mobile
node, all IPv6 nodes MUST support the following requirements:
- Every IPv6 node MUST be able to process a received Binding Update
option, and to return a Binding Acknowledgement message if
requested.
- Every IPv6 node MUST be able to maintain a Binding Cache of the
bindings received in accepted Binding Updates.
- Every IPv6 node MUST be able to maintain Security Associations
for use in IPv6 Authentication Headers [2, 1, 6]. An IPv6
node receiving a packet containing a Binding Update option
MUST verify, using the Authentication Header in the packet,
the authenticity of the sender (the mobile node for which this
binding applies) before modifying its Binding Cache in response
to that Binding Update option.
Since any IPv6 router may at any time have a Binding Cache entry
for a mobile node, all IPv6 router MUST support the following
requirement:
- Every IPv6 router MUST be able to use its Binding Cache in
forwarding packets; if the router has a Binding Cache entry for
the Destination Address of a packet it is forwarding, then the
router SHOULD encapsulate the packet and tunnel it to the care-of
address in the Binding Cache entry.
In order for a mobile node to correctly operate while away from
home, at least one IPv6 router in its home network must support
functioning as a home agent for the mobile node. All IPv6 routers
capable of serving as a home agent MUST support the following special
requirements:
- Every home agent MUST be able to maintain a registry of mobile
node bindings for those mobile nodes for which it is serving as
the home agent.
- Every home agent MUST be able to intercept packets (e.g., using
Neighbor Advertisements) on the local network addressed to
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a mobile node for which it holds a binding in its registry
indicating that the mobile node is currently away from home.
- Every home agent MUST be able to encapsulate such intercepted
packets in order to tunnel them to the care-of address for the
mobile node indicated in its binding.
- Every home agent MUST be able to issue Binding Acknowledgement
messages in response to Binding Updates received from a mobile
node.
- Every home agent MUST be able to maintain Security Associations
for the mobile nodes from which it will accept Binding Updates.
Finally, all IPv6 nodes capable of functioning as mobile nodes MUST
support the following requirements:
- Every IPv6 mobile node MUST be able to perform IPv6
decapsulation [5].
- Every IPv6 mobile node MUST support sending Binding Updates,
as specified in Sections 6.3, 6.4, and 6.5; and MUST be able
to receive and process Binding Acknowledgement messages, as
specified in Section 6.7.
- Every IPv6 mobile node MUST maintain a Binding Update List in
which it keeps track of which other IPv6 nodes it has sent a
Binding Update to, for which the Lifetime sent in that binding
has not yet expired.
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5. Binding Cache Management
The Binding Cache is the central data structure in Mobile IPv6.
All IPv6 nodes MUST support maintenance of a Binding Cache, and
MUST support processing of received Binding Updates. This section
describes the management aspects of a Binding Cache common to all
nodes.
5.1. 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 length of the option specified in the Option Length field is
greater than or equal to 28 octets.
- 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 differs from the Home Address,
this is a request to cache a binding for the mobile node.
Processing for this type of received Binding Update is described
in Section 5.2.
- If the Lifetime specified in the Binding Update is zero or the
specified Care-of Address matches the Home Address, then this is
a request to delete the mobile node's binding. Processing for
this type of received Binding Update is described in Section 5.3.
5.2. 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 5.1, then the node
MUST examine the Home Registration (H) bit in the Binding Update
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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's Home Address (or update its existing Binding Cache Entry for
this Home Address) to record the Care-of Address as specified in the
Binding Update, and begin a timer to delete this Binding Cache entry
after the expiration of the Lifetime period specified in the Binding
Update.
5.3. 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 5.1, 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, and if a node receives a
valid Binding Update requesting it to delete a binding for a mobile
node, as specified in Section 5.1, then it MUST delete any existing
entry in its Binding Cache for this mobile node's Home Address.
5.4. Sending Binding Acknowledgements
When any node receives a packet containing a Binding Update option,
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
Code 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 Code field in
the Binding Acknowledgement MUST be set to a value greater than or
equal to 128. Specific values for the Code field are described in
Section 3.2 and in the most recent "Assigned Numbers" [10].
The Destination Address in the IPv6 header for the Binding
Acknowledgement MUST be set to the Care-of Address copied from the
Binding Update option. 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.
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5.5. Cache Replacement Policy
Any entry in a node's Binding Cache MUST be deleted after the
expiration the Lifetime specified in the Binding Update from which
the entry was created. Conceptually, a node MUST maintain a separate
timer for each entry in its Binding Cache. When creating or updating
a Binding Cache entry in response to a received 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 MUST delete 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) SHOULD 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 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 Code 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 the 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 network, where it
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 will result in the mobile node sending a Binding Update to this
sending node, allowing it to add this entry again to its Binding
Cache.
5.6. 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's
care-of address, and then to the mobile node's home address. Any
ICMP error message caused by the packet on its way to the mobile node
will be returned normally to the correspondent node.
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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 network, where it will be intercepted by the mobile
node's home agent, encapsulated, and tunneled to the mobile node's
care-of address. Similarly, 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). Any ICMP error message
caused by the packet on its way to the mobile node while in the
tunnel, will be returned to the node that encapsulated the packet
(the home agent or the previous default router, respectively). By
the definition of IPv6 encapsulation [5], however, this encapsulating
node MUST relay certain ICMP error messages back to the original
sender of the packet (the correspondent node).
Thus, whether the correspondent node has a Binding Cache entry
for the destination mobile node or not, the correspondent node
will receive any meaningful ICMP error message that is caused by
its packet on its way to the mobile node. If the correspondent
node receives an ICMP Host Unreachable or Network Unreachable
error message after sending a packet to a mobile node using its
cached care-of address, the correspondent node SHOULD delete its
Binding Cache entry for this mobile node. If the correspondent node
subsequently transmits another packet to the mobile node, the packet
will be routed to the mobile node's home network, 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. Mobile Node Considerations
6.1. 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 IPv6 subnet to another. The primary movement detection
mechanism for Mobile IPv6 defined here uses the facilities of
IPv6 Neighbor Discovery, including Router Discovery and Neighbor
Unreachability Detection. The description here is based on the
conceptual model of the organization and data structures defined by
Neighbor Discovery [9].
Mobile nodes SHOULD use Router Discovery to discover new routers and
on-link network prefixes; a mobile node MAY send Router Solicitation
messages, or MAY wait for unsolicited (periodic) Router Advertisement
messages, as specified for Router Discovery [9]. Based on received
Router Advertisement messages, a mobile node (in the same way as any
other node) maintains an entry in its Default Router List for each
router, and an entry in its Prefix List for each network prefix,
that it currently considers to be on-link. Each entry in these
lists has an associated invalidation timer value (extracted from the
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 6.2. The mobile node registers
its primary care-of address with its home agent, as described in
Section 6.3.
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 any
correspondent nodes attempting to communicate with the mobile node
can still reach it.
To detect when its default router becomes unreachable, a mobile
node SHOULD use Neighbor Unreachability Detection. As specified
in Neighbor Discovery [9], while the mobile node is actively
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sending packets to (or through) its default router, the mobile node
can detect that the router has become unreachable either through
indications from upper layer protocols on the mobile node that a
connection is not making "forward progress" (e.g., TCP timing out
waiting for an acknowledgement after a number of retransmissions),
or through the failure to receive a Neighbor Advertisement messages
form its default router in response to retransmitted explicit
Neighbor Solicitation messages to it. No exceptions to Neighbor
Unreachability Detection are necessary for this aspect of movement
detection in Mobile IPv6.
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 IPv6
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 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
from the router by listening for packets from the router as described
above, and thus, such extra Neighbor Unreachability Detection probes
should rarely be necessary.
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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 IPv6 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.
6.2. 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 (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
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
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one link at a time. For more information on using more than one
care-of address at a time, see Section 6.8.
As described in Section 2, in order to form a new care-of address,
a mobile node MAY use either stateless [12] 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 some cases, a mobile node may already know a (constant) IPv6
address that has been assigned to it for its use while visiting this
network. For example, it may be statically configured with an IPv6
address assigned by the system administrator of the new network. 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 network.
6.3. Sending Binding Updates to the Home Agent
After changing its primary care-of address as described in
Sections 6.1 and 6.2, a mobile node SHOULD register its new primary
care-of address with its home agent. To do so, the mobile node sends
a packet to its home agent containing a Binding Update option with
the Acknowledge (A) bit set, requesting the home agent to return a
Binding Acknowledgement message in response to this Binding Update.
As described in Section 3.2, 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.
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 network. It thus may not always be possible or convenient for a
mobile node to know the exact address of its own home agent.
Mobile nodes can dynamically discover the address of a home agent
by sending a Binding Update to the anycast address on their home
network. Each router on the home network which receives this Binding
Update MUST reject the Binding Update and include its address in the
Binding Acknowledgement message indicating the rejection. The mobile
node is assumed to know a proper anycast address on its home network
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before making use of this method for determining a particular home
agent's address.
6.4. Sending Binding Updates to Correspondent Nodes
A mobile node MAY also include a Binding Update in any normal data
packet sent to a correspondent node. For each correspondent node
to which it has sent a Binding Update, the mobile node MUST keep
information to determine whether or not the correspondent node has
been sent a fresh Binding Update since the last time the mobile node
switched to a new primary care-of address. When a packet is to be
sent to a correspondent node that has not been sent a fresh Binding
Update, the mobile node SHOULD include the Binding Update within the
packet. Thus, correspondent nodes are generally kept updated and
can send almost all data packets directly to the mobile node using
the mobile node's current binding. Such Binding Updates are not
generally required to be acknowledged; however, if the mobile node
wants to be sure, an acknowledgement can be requested, although in
this case, the mobile node SHOULD NOT continue to retransmit the
Binding Update once the retransmission timeout period has reached
MAX_BINDACK_TIMEOUT.
A mobile node MAY also send a Binding Update in any otherwise empty
packet, whenever the mobile node wishes to update a correspondent
node as to its current binding. This is normally done only if
the mobile suspects that its home agent is not operational or is
too far away, a correspondent node is not sending the traffic to
the proper care-of address, or there is an immediate need for the
correspondent node to obtain the binding. A mobile node can detect
that a correspondent node is not sending packets to the proper
care-of address because in that case the packets arrive at the mobile
node's care-of address by encapsulation instead by inclusion in a
routing header within the packet.
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 3.1. No other IPv6 nodes are authorized to send Binding
Updates on behalf of a mobile node.
6.5. 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
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Update message to its previous default router, giving its new care-of
address. If it sends such a Binding Update, the mobile node MUST set
the Home Address field to its old primary care-of address (that it
used while using this default router), and set the Care-of Address
field to its new 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.
The mobile node's previous default router then, in effect,
temporarily act as a home agent for the mobile node's old primary
care-of address. 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 and tunnel it to the
mobile node at its new primary care-of address. Moreover, the
previous router should issue Neighbor Advertisement packets for the
previous care-of address, so that on-link neighbors will send packets
destined to the mobile node's old primary care-of address to the
previous router for encapsulation and tunneling to its new care-of
address.
6.6. Rate Limiting for Sending Binding Updates
A mobile node MUST NOT send Binding Update messages 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 finally be able to process a Binding
Update and begin to route its packets directly to the mobile node at
its current primary care-of address.
6.7. Receiving Binding Acknowledgements
Upon receiving a packet carrying a Binding Acknowledgement message,
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 ICMP Checksum is valid.
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- The length of the ICMP message (derived from the IPv6 Payload
Length field) is greater than or equal to 16 octets.
- The Identification field is valid.
Any Binding Acknowledgement not satisfying all of these tests MUST be
silently discarded.
If the Binding Acknowledgement is valid, the mobile node MUST examine
the Code field as follows:
- If the Code field indicates that the Binding Update was accepted
(the Code field is less than 128), then the mobile node MUST
update the corresponding entry in its Binding Update List to
indicate that the Binding Update has been acknowledged. The
mobile node SHOULD thus stop retransmitting the Binding Update.
- If the Code field indicates that the Binding Update was not
accepted (the Code 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 6.6.
6.8. Using Multiple Care-of Addresses
As described in Section 6.2, 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 6.1). 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 Acknowledge (A) bit set, as described
in Section 6.3.
To assist in smooth handoffs, a mobile node SHOULD retain its
previous primary care-of address as a 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
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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.
6.9. Returning Home
A mobile node detects that it has returned to its home network
through the movement detection algorithm in use (Section 6.1),
when the mobile node detects that its home network prefix is again
on-link. The mobile node SHOULD then send a Binding Update to its
home agent, to instruct its home agent to no longer intercept or
tunnel packets for it. In this Binding Update, the mobile node MUST
set the Care-of Address 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.
The mobile node MUST also send out the appropriate Neighbor
Advertisement packets with the Override flag set, so that its
neighbors on its home network will update the relevant information
for the mobile node in their Neighbor Caches. The mobile node
MUST do this for both its link-local address and its home address.
The Neighbor Advertisement packets can be repeated a small number
of times to guard against occasional loss of packets on the home
network.
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7. Home Agent Considerations
7.1. Home Agent Care-of Address Registration
General processing of a received Binding Update that requests a
binding to be cached, is described in Section 5.2. 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 generall procedure specified in
Section 5.2.
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 Code field is set to 132 (Home registration
not supported).
- Else, if the Home Address field in the Binding Update 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 Code field is set to 133 (Not home network).
- 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
Code 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 5.2. 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" SHOULD be excluded from the normal cache replacement
policy used for the Binding Cache (Section 5.5) and SHOULD 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 the
Home Address specified in the Binding Update (the home agent did
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not already have a Binding Cache entry for this address marked as
a "home registration"), then the home agent MUST multicast onto
the home network (to the all-nodes multicast address), a Neighbor
Advertisement message on behalf of the mobile node, with the fields
in the Neighbor Advertisement set as follows:
Router Flag (R)
1 -- the sending node (the home agent) is a router.
Solicited Flag (S)
0 -- the Neighbor Advertisement message is unsolicited.
Override Flag (O)
1 -- the advertisement SHOULD override any existing Neighbor
Cache entry at the receiver, updating the receiver's cached
link-layer address for this Target Address.
Target Address
The mobile node's home address, copied from the Home Address
field of the Binding Update.
Options
The home agent MUST include at least a Target Link-layer
Address option in the Neighbor Advertisement message, in which
the Link-Layer Address gives the link-layer address of the home
agent itself.
Any node on the home network 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 a small
number of times to increase its reliability. It is still possible
that some nodes on the home network will not receive any of these
Neighbor Advertisements, but these nodes will eventually be able
to detect the link-layer address change for the mobile node's home
address, through use of Neighbor Unreachability Detection [9].
In addition, while this node is serving as a home agent to any mobile
node (it has at least one entry marked as a "home registration" in
its Binding Cache), it SHOULD act as a proxy for each such mobile
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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.
7.2. Home Agent Care-of Address De-registration
General processing of a received Binding Update that requests a
binding to be deleted, is described in Section 5.3. 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 generall procedure specified in
Section 5.3.
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 Code field is set to 132 (Home registration
not supported).
- Else, if the Home Address field in the Binding Update 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 Code field is set to 133 (Not home network).
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's Home Address, as specified in the Binding
Update.
In addition, the home agent SHOULD 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 a small number of times to
increase its reliability, and any nodes on the home network that miss
all of these Neighbor Advertisements can also eventually detect the
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link-layer address change for the mobile node's home address, through
use of Neighbor Unreachability Detection [9].
7.3. Delivering Packets to a Mobile Node
Home agents cannot use Routing headers to deliver packets to the
mobile node, because they can't modify the packet and add to it
in flight. They must always use IPv6 encapsulation [5] for this
purpose.
When a home agent encapsulates a packet for delivery to the mobile
node, the home agent uses the care-of address as the destination
address in the outer IPv6 header. Since the mobile node is presumed
to be receiving packets at the care-of address, the delivery path
from the care-of address to the mobile node's home address is then
trivial.
Note that the home agent cannot insert a routing header, or
modify the destination address of the mobile node, because of IPv6
authentication mechanisms [1]. The home agent is expected to be
involved only rarely with the transmission of data to the mobile
node, because the mobile node will send Binding Updates as soon as
possible to its correspondent nodes.
7.4. Renumbering the Home Network
Neighbor Discovery [9] specifies a mechanism by which all nodes on a
network can gracefully autoconfigure new addresses, say by combining
a new routing prefix with their existing MAC 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 insuring this is for the home agent to encapsulated and
tunnel the multicast packets to the care-of address of the mobile
node as necessary. The rules for this are as follows:
- A mobile node assumes that its routing prefix has not changes
unless it receives authenticated router advertisement messages
from its home agent that the prefix has changed.
- When the mobile node is at home, the home agent does not tunnel
router advertisements to it.
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- When a home network 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 network, 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 [9]
Note that a mobile node is guaranteed that its home address is unique
and used by no other mobile node. However, in some circumstances it
may nevertheless be true that other nodes on its home network form
the same link-local address as the mobile node during the time when
the mobile node is away from its home network. Thus, there is the
requirement above that the mobile node perform Duplicate Address
Detection when it returns again to its home network.
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8. Correspondent Node Considerations
8.1. Delivering Packets to a Mobile Node
The routing infrastructure of the Internet will normally route a
packet destined to a mobile node to the mobile node's home network,
if the Destination Address in the packet's IPv6 header is the mobile
node's home address. Once the packet reaches the home network, it
will be intercepted by the mobile node's home agent if the mobile
node is away from home, and will then be encapsulated using IPv6
encapsulation and tunneled to the mobile node's current primary
care-of address. Using this delivery mechanism, the sender need not
know that the node is mobile.
Correspondent nodes that have received and cached a Binding Update
for a mobile node, MAY instead route packets directly to that mobile
node's care-of address. To do so, the correspondent node includes
a Routing header in each packet to the mobile node, to cause the
packet to be routed to the mobile node's care-of address as the last
intermediate routing point before reaching the final destination
of the mobile node's home address. When the packet arrives at the
care-of address (which the mobile node has associated with its
network interface), normal processing of the Routing header by the
mobile node will result in delivery of the packet to the mobile node
as the final destination of the packet.
For example, assuming no other use of the Routing header in the
packet, the sender initializes the Destination Address in the IPv6
header to the mobile node's care-of address, and includes a Type 0
Routing header [6] in the packet initialized as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type=0|Segments Left=1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Strict/Loose Bit Map |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Next Header
8-bit selector. Identifies the type of header immediately
following the Routing header.
Hdr Ext Len
8-bit unsigned integer. Length of the Routing header in
8-octet units, not including the first 8 octets. For this use
of the Type 0 Routing header, Hdr Ext Len is equal to 2.
Routing Type
0
Segments Left
8-bit unsigned integer. Number of route segments remaining
before reaching the final destination. For this use of the
Type 0 Routing header, Segments Left is initialized to 1 by the
sender.
Reserved
8-bit reserved field. Initialized to zero for transmission;
ignored on reception.
Strict/Loose Bit Map
24-bit bit-map, numbered 0 to 23, left-to-right. For this use
of the Type 0 Routing header, bit 0 of the Strict/Loose Bit Map
is set to 1, indicating strict routing from the care-of
address to the mobile node's home address (both addresses are
associated with the mobile node itself).
Home Address
The home address of the destination mobile node.
If a correspondent node receives an ICMP Host Unreachable or Network
Unreachable message after sending a packet to a mobile node using
its cached care-of address, it SHOULD delete the cache entry from
its Binding Cache until information about the mobile node's current
care-of address becomes available (via a Binding Update).
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9. Authentication and Replay Protection
When sending Binding Updates, a mobile node uses the Identification
field in the option, in conjunction with the IPv6 Authentication
Header, to protect against replays of the Binding Update. The style
of replay protection specified for the IPv6 Binding Update involves
the use of a timestamp as the Identification data. Accordingly the
mobile node and the target of its Binding Update have to roughly
agree on the current time. Stale Binding Updates MUST be rejected.
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10. Routing Multicast Packets
A mobile node that is connected to its home network functions just
like 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 network.
In order receive multicasts, a mobile node must join the multicast
group. Mobile nodes MAY join multicast groups in order to receive
transmissions in one of two ways. First, they MAY join the group
via a (local) multicast router on the visited subnet. This option
assumes that there is a multicast router present on the visited
subnet. The mobile node SHOULD use its dynamically acquired care-of
address (if it has acquired one) as the source IPv6 address of its
multicast group membership control message packets. Otherwise, it
MAY use its home address.
Alternatively, a mobile node which wishes to receive multicasts can
join 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 dynamically acquired care-of address,
or, the packet must be tunneled first to the mobile node's home
address and then recursively tunneled to the mobile node's care-of
address.
A mobile node which wishes to send packets to a multicast group also
has two options: (1) send directly on the visited network; or (2)
send via a tunnel to its home agent. Because multicast routing in
general depends upon the IPv6 source address, a mobile node which
sends multicast packets directly on the visited network MUST use a
dynamically acquired care-of address as the IPv6 source address.
Similarly, a mobile node which 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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11. Constants
INITIAL_BINDACK_TIMEOUT 1 second
MAX_BINDACK_TIMEOUT 256 seconds
MAX_UPDATE_RATE 1 per second
SLOW_UPDATE_RATE once per 10 seconds
Acknowledgements
We would like to thank Thomas Narten for contributing valuable
discussion and reviewing this draft, and for helping to shape some of
the recent changes relevant to the operation of Neighbor Discovery.
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References
[1] R. Atkinson. IP Authentication Header. RFC 1826, August 1995.
[2] R. Atkinson. Security Architecture for the Internet Protocol.
RFC 1825, August 1995.
[3] J. Bound and C. Perkins. Dynamic Host Configuration Protocol
for IPv6. draft-ietf-dhc-dhcpv6-05.txt -- work in progress,
June 1996.
[4] A. Conta and S. Deering. Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6). RFC 1885,
December 1995.
[5] A. Conta and S. Deering. Generic Packet Tunneling in IPv6.
draft-ietf-ipngwg-ipv6-tunnel-01.txt - work in progress,
February 1996.
[6] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. RFC 1883, December 1995.
[7] D. Haskin and E. Allen. IP Version 6 over PPP.
draft-ietf-ipngwg-pppext-ipv6cp-03.txt - work in progress, June
1996.
[8] David B. Johnson and Charles E. Perkins. Route Optimization
in Mobile-IP. draft-ietf-mobileip-optim-04.txt -- work in
progress, February 1996.
[9] T. Narten, E. Nordmark, and W. Simpson. IPv6 Neighbor
Discovery. draft-ietf-ipngwg-discovery-03.txt -- work in
progress, November 1995.
[10] Joyce K. Reynolds and Jon Postel. Assigned Numbers. RFC 1700,
October 1994.
[11] Fumio Teraoka. draft-teraoka-ipv6-mobility-sup-02.txt.
Internet Draft -- work in progress, January 1996.
[12] S. Thomson and T. Narten. IPv6 Stateless Address
Autoconfiguration. draft-ietf-addrconf-ipv6-auto-06.txt
- work in progress, November 1995.
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A. Open Issues
A.1. Session Keys with Local Routers
In the IPv4 route optimization proposal [8], 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 [11] to the firewall problem
based on hiding the non-local source address outside the source
address field of the IPv6 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 IPv6 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.
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Chair's Address
The Working Group can be contacted via its current chair:
Jim Solomon
Motorola, Inc.
1301 E. Algonquin Rd.
Schaumburg, IL 60196
Work: +1-847-576-2753
E-mail: solomon@comm.mot.com
Authors' Addresses
Questions about this document can also be directed to the authors:
David B. Johnson
Computer Science Department
Carnegie Mellon University
5000 Forbes Avenue
Pittsburgh, PA 15213-3891
Work: +1 412 268-7399
Fax: +1 412 268-5576
E-mail: dbj@cs.cmu.edu
Charles Perkins
Room H3-D34
T. J. Watson Research Center
IBM Corporation
30 Saw Mill River Rd.
Hawthorne, NY 10532
Work: +1 914 789-7350
Fax: +1 914 784-6205
E-mail: perk@watson.ibm.com
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